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Things that 'everyone knows' aren't necessarily so

There are things every cyclist seems to believe, pieces of cycling lore passed down from rider to rider through the ages like holy writ. Problem is, an awful lot of them are either completely wrong, or based on a grain of truth that’s been mangled beyond recognition. Let’s pick a few of them apart.

Aluminium frames only last five years

Frame crack (CC BY 2.0 garycycles7|Flickr).jpg

Frame crack (CC BY 2.0 garycycles7|Flickr).jpg

Yes, aluminium frames can  fail; this crack was almost certainly caused by hanging a rack and bags from teh seatpost (CC BY 2.0 garycycles7|Flickr)

Or two years, or whatever. There’s a grain of fact in this one and it’s all about metal fatigue. If a piece of metal is repeatedly flexed it will eventually break, as anyone who has idly bent and unbent a paperclip knows. This happens even if you don’t flex the metal enough to permanently bend it.

This is metal fatigue, and it’s an odd phenomenon because not all metals behave the same way. If you repeatedly flex a piece of steel by a large amount, it will eventually break. But if you only flex it slightly, it won’t. The load below which a piece of steel doesn’t break from metal fatigue is called the fatigue limit.

This kind of cyclical loading and unloading is exactly what happens to bike frames, so you can design a steel frame that will essentially last forever, as long as it’s not crashed and it’s protected from corrosion. (Bike designer Brant Richards has pointed out it's not quite that simple. "To actually hit true fatigue limit stress levels frame would be very heavy indeed," he says. Nevertheless, the relationship between stress and lifespan for steel is such that you can build frames that last literally decades.)

Aluminium is different. If you repeatedly load and unload a piece of aluminium it will eventually break, however small the load. However, the smaller the load, the longer this takes.

Having more material spreads the load around, increasing lifespan, and the shape of the piece makes a difference too. That’s why aluminium frames have fat tubes, because a larger and therefore stiffer tube has a longer fatigue life.

Using these design techniques it’s possible to make an aluminium frame that will last many years, which is why there are still plenty around from the 1990s.

Steel frames go ‘dead’

The Light Blue Kings 853 - lug detail

The Light Blue Kings 853 - lug detail

You don’t hear this one as much as you did when steel was the dominant frame material. It was rubbish then and it’s rubbish now. As discussed above, a properly-designed steel frame can last forever, and that’s been obvious for decades.

How did this one get started then? A cynic might say that it’s good for bike shops to have people believe that you need to replace something you don’t, but I think there’s more to it than that.

On a new bike, everything works perfectly, and there’s a certain excitement about getting used to the differences in feel between your new and old rides. Your old bike, whatever it’s made from, feels familiar. Familiarity can easily become boredom. It’s not that an old bike feels ‘dead’ (whatever that even means) but that the unfamiliarity of a new one is exciting.

There’s one saddle height rule that works for everyone

sadle height.jpg

sadle height.jpg

Read half a dozen general books on cycling and you will find as many recommendations for ways to set the distance between your saddle and pedals. Saddle height nostrums will be based on your inside leg multiplied by a certain number (1.09 from pedal to saddle is common; 0.883 from bottom bracket to saddle is a suspiciously precise other); the angle of your knee; or placing your heel on the pedal with your leg straight, among others.

These methods produce a wide variety of saddle heights for any particular rider, which should ring alarm bells. Not only that, but they variously fail to take into account flexibility, shoe size and the sole-axle distance of your shoes and pedals.

At the very best these methods give you a starting point for where your saddle should be, though they can be off by quite a bit, especially the “inside leg times 1.09” rule, which tends to produce high saddle positions.

A bike fit expert will be able to help you fine tune things, though you can do this by feel as well, making small adjustments to saddle height. It’s hard to know what’s perfect, but aches and pains in hips. knees and ankles will soon tell you if something’s wrong. Carry an Allen key and make adjustments on the road, especially if you’re doing a long ride. 100Km of hills on a wrongly-adjusted bike can do damage that takes weeks to heal.

Tyres must have a tread pattern

ritchey tom slick tread

ritchey tom slick tread

This one’s simple. Car and motorcycle tyres have grooves in the tread to disperse water, otherwise they can aquaplane. Bicycle tyres, being much narrower, can’t aquaplane at typical bike speeds. In fact, you’d have to be doing over 200mph aquaplane a bike tyre, in which case Dave Brailsford probably wants to hear from you.

But tyre company marketing departments remain wedded to grooves, even though they can actually degrade tyre performance. That’s because the sections of rubber between the grooves can flex and squirm into them, and that increases the tyre’s rolling resistance.

It’s telling that when a tyre manufacturer wants to make a tyre for those situations where every second counts, such as a time trial, they make slicks. Look at the Continental Grand Prix Supersonic, for example, or, for a slightly less extreme example, Michelin’s new Power Competition tyres.

Carbon frames go ‘soft’

Colnago V1-r - seat clamp.jpg

Colnago V1-r - seat clamp.jpg

Sound familiar? It’s the modern version of ‘steel frames go dead’ and ‘aluminium frames only last five years’. And it’s almost as daft.

As long as it’s not crashed, a carbon fibre frame won’t become weaker in use. In fact, many carbon fibre frames hugely exceed standard tests for fatigue life, to the point where manufacturers get bored and turn off the testing machines.

It doesn’t seem like they get more flexible either, at least not in ways riders can tell. The first widely-available carbon frames appeared in the early 1990s, and some have been in continual use ever since. They’d be seriously floppy by now if this was a real issue.

However, while the fibres themselves are almost infinitely durable, you can imagine that the resin might degrade over time with repeated flexing. It turns out this is what happens.

Tour magazine flex-tested carbon fibre forks and found that after 100,000 cycles they became less stiff. Chuck Texiera, a senior engineer at Specialized told CyclingTips.com what happens: “The epoxy matrix will at some point start to form little cracks, and then over time it will just have the connectivity of the fibre.”

As with so many of these beliefs, there’s a disconnect between what the engineering says is happening and what a rider can actually feel. A frame might be less stiff, but Texiera doesn’t think a rider could tell.

He said: “Over really extended periods of time, you can expect the stiffness of the frame to change ever so slightly but it’s such a small number. We can measure it but I really wouldn’t think it would be perceivable.”

Rotating weight is crucial

Lightweight Meilenstein wheelset Detail

Lightweight Meilenstein wheelset Detail

“An ounce off the wheels is worth a pound off the frame,” goes the old saying, implying that rotating weight, especially on the wheels, is supremely important. The claim is sometimes laid out in less hyperbolic terms that weight on the wheels counts twice because when you accelerate you have to get it both spinning and moving forward.

Problem is, it’s not true. In 2001 bike engineer Kraig Willett analysed the forces on wheels and concluded:

“When evaluating wheel performance, wheel aerodynamics are the most important, distantly followed by wheel mass. Wheel inertia effects in all cases are so small that they are arguably insignificant.”

The idea that rotating mass is important comes from the belief that wheel inertia matters, because it’s inertia that has to be overcome to accelerate a wheel. But Willett clearly demonstrates that wheel inertia doesn’t matter, so rotating weight is also relatively unimportant.

Why not? Well, you don't do much accelerating when you ride a bike, and even when you do the acceleration is relatively low, so the power expended accelerating a bike with ‘heavy’ wheels is only fractionally higher than that needed for light wheels. Overall weight matters when you’re climbing, but even that’s not as big a factor as people imagine and it’s a lot cheaper to save weight off your middle than the bike.

In fact you spend most of your time, and therefore effort, shoving the air out of the way, and that’s a far better basis for choosing wheels. The roughly tenfold difference in the effect of aerodynamics versus total mass means you’re far better off with a pair of good aero wheels than a pair of light ones.

Narrow tyres are faster

Tyre close up for pressure.jpg

Tyre close up for pressure.jpg

You can see where this one comes from. In cycling, smaller things are lighter and lighter things make you go faster, right? Well, no, not for tyres. Countless measurements have demonstrated beyond doubt that rolling resistance of tyres is lower if the tyres are wider, as long as the construction — carcass thickness and materials, tread rubber and depth etc — is identical.

But is that the whole story? What about weight and aerodynamics?

As discussed above, weight, even rotating weight, has a much lower effect on performance than people think, so the few grams difference between 23mm and 25mm tyres is immaterial.

We’re not aware of any detailed modelling of the aerodynamic effects of fatter tyres, but let’s have a bit of a stab at it. Aerodynamic drag arises from an object’s frontal area and its drag coefficient.

Drag coefficient depends on an object’s shape and how air flows over its surface. A very aerodynamic shape such as a smooth wing might have a drag coefficient of 0.005, while a brick’s is more like 2.0.

Multiplying the drag coefficient by the frontal area gives you the aerodynamic drag, so drag force increases as, say, a tyre gets wider.

According to CyclingPowerLab, the frontal area of a cyclist in the drops is about 0.36m². The change from 23mm to 25mm tyres adds 0.001436m², an increase of 0.4%. That’s the increase in power you’ll need to maintain any given speed. It takes 102 watts to maintain 18 miles per hour in this scenario, which increases to 102.5 watts with the fatter tyres.

According to BicycleRollingResistance.com, there’s a 0.3 watt difference in rolling resistance per tyre at this speed between 23mm and 25mm versions of Continental GP4000s II tyres at 120psi. The half-watt increase in aerodynamic drag is therefore almost exactly countered by the decrease in rolling resistance.

The problem here is that you’re not going to get the other benefit of fat tyres – a softer ride – if you keep the pressure the same. If you do reduce the pressure, then the rolling resistance goes up too, and you end up with slightly more total resistance.

With 28mm tyres it turns out you have a bit more leeway and can drop the pressure a little. At 100psi our 28mm GP4000s IIs have 0.5 watts per tyre less resistance than 23mm tyres at 120psi, and one watt more aerodynamic drag.

Narrow tyres, then, faster or slower? The answer, it turns out, is “it depends.” The total aerodynamic and rolling resistance depends on tyre size and pressure, and which is faster changes with how you fine-tune those variables.

An extra complication we haven’t mentioned yet is speed. As you go faster aerodynamic drag increases more than rolling resistance. At finishing sprint and time trial speeds, you’re almost certainly better off with narrow tyres.

If you don’t race, though, you might have noticed that we’re talking about small differences in resistance. A 28mm GP4000s II at 80psi has the same rolling resistance as a 23mm at 120psi. Does the extra watt of air resistance matter? It’s definitely not a difference you can feel (the threshold for that is 5-10 watts depending on the individual) and it’s going to make a tiny difference to your ride time even on a long ride. You might well decide the comfort is more than worth it.

Kirkpatrick Macmillan or Leonardo Davinci invented the bike

The bicycle sketch allegedly drawn by a student of Leonardo Da Vinci (Wikimedia Commons).jpg

The bicycle sketch allegedly drawn by a student of Leonardo Da Vinci (Wikimedia Commons).jpg

It’d be nice to believe the bike was invented by a Scottish blacksmith, but the evidence is very thin indeed.

The claim that Kirkpatrick Macmillan built a treadle-powered two-wheeler in 1839 didn’t emerge until after his death in 1878. A relative of Macmillan, James Johnston, made the claim in the 1890s, but was unable to produce any documentary evidence that what Macmillan had built was a two-wheeler.

The story goes that Macmillan built the first bike, but over the following years others copied the design. Cooper Gavin Dalzell was supposed to have built one in 1845, but again there is no contemporary evidence.

Treadle-drive tricycles and quadricycles were not unusual in the middle of the 19th century and it seems likely that the late-19th century recollections of velocipedes that formed the basis for Johnston’s claims were actually of three-or four-wheeled vehicles. Cycling historian David Herlihy covers the Macmillan claims extensively in his book Bicycle: The History and points out that none of the claimed accounts of Macmillan’s bike or others derived from it actually say it was a two-wheeler.

This, Herlihy points out, is remarkable, given what a novelty a two-wheeler would have been. When the French front-drive bicycles emerged in the late 1860s they were a sensation, because riders were able to travel on them without touching the ground. That Scottish newspapers of the time made no mention of this is remarkable.

“After all,” Herlihy writes, “a single French-style bicycle in the United States in 1866 led to both a clear-cut description of the article in a local newspaper and a patent application. It seems highly improbable that an arbitrary number of equally eye-catching machines could have operated in and around Scotland's largest cities—or anywhere else for that matter—for nearly thirty years without leaving the slightest paper trail.”

Herlihy doesn’t even bother to mention Leonardo da Vinci’s alleged invention of the bike. A sketch of a bicycle-like device emerged in 1974, claimed to be part of Leonardo da Vinci’s Codex Atlanticus. The sketch was attributed to Gian Giacomo Caprotti, a pupil of da Vinci, and was claimed to be a reproduction of a lost drawing of a bicycle by da Vinci himself. It was later established to be a forgery, though da Vinci’s reputation and that of literary historian Augusto Marinoni were powerful enough that it took until 1997 for the forgery to be unveiled.

According to a 1997 paper by Prof. Dr. Hans-Erhard Lessing, the Codex Atlanticus was examined by another da Vinci scholar in 1961 and the bicycle sketch was not present, though there were some geometrical doodles that the forger incorporated into the bicycle.

Lessing writes: “the bicycle sketch is definitely a recent forgery that can be dated between 1967 and 1974”.

But why would anyone forge a drawing of a bike? The short answer seems to be ‘national pride’. The bicycle was a seminal device that laid the basis for many vital technologies of the 20th century. Karl Benz’ Patent Motorwagen — the first internal combustion engined vehicle — was essentially a tricycle with an engine, with roller chains for power transmission, tension-spoked wire wheels and a tubular steel frame. The Wright brothers, who flew the first heavier-than-air plane in 1903, were bike mechanics and like Benz used bike technologies to save weight on their Flyer.

There’s a certain kudos, then, to being the country that invented the bicycle, which is why the strongest proponents of the Da Vinci drawing were Italian, Macmillan advocates were Scottish and so on. Marinoni never conceded the Da Vinci sketch was a forgery and as recently as 2009 his followers were still defending it, albeit rather incoherently.

Our official grumpy Northerner, John has been riding bikes for over 30 years since discovering as an uncoordinated teen that a sport could be fun if it didn't require you to catch a ball or get in the way of a hulking prop forward.

Road touring was followed by mountain biking and a career racing in the mud that was as brief as it was unsuccessful.

Somewhere along the line came the discovery that he could string a few words together, followed by the even more remarkable discovery that people were mug enough to pay for this rather than expecting him to do an honest day's work. He's pretty certain he's worked for even more bike publications than Mat Brett.

The inevitable 30-something MAMIL transition saw him shift to skinny tyres and these days he lives in Cambridge where the lack of hills is more than made up for by the headwinds.

68 comments

Avatar
festina [58 posts] 1 year ago
5 likes

Oh here we go, the old fatigue chestnut.  Fatigue is such a stoopid subject full of mumbo jumbo that I don't even believe it tuely exists.

Aluminium frames aren't oversized for fatigue; aluminium is not as stiff as steel so, in order for it not to ride like a noodle (or an old Vitus), the tubes need to be 'fatter' which increases the second moment of area by placing the extreme fibers further away from the neutral axis.  This reduces stress.

Also the tensile properties of aluminium are significantly less than steel so you need more material in order not to fail under a static load. More material means larger cross sectional area and therefore lower stress.  So basing a comparison of materials on fatigue curves is only half the story as stresses in an oversize frame will be less than those in a skinny steel tubeset.

Aluminium is lighter than steel so you can get away with adding more material via fatter tubes and still have a light weight frame, you can't do that with steel, it was already optimised before aluminium arrived on the scene (it's limited by wall thickness and therefore buckling).

To be honest the biggest difference, in the early days of aluminium frames, was in manufacture.  Where do all frames fail? In the weld.  Heat affected zone (which leads to a loss of grain structure and a change in material properties), inclusions etc. make that the weakest point however at that time the majority of steel frames were brazed; so they were effectively 'glued' together rather than the TIG welds of aluminium frames (think cannon dale were the only people brazing aluminium).

Aircraft are mostly made from aluminium and some of those are still flying from the 50's with few problems.  Unlike bikes they tend to avoid welds in stress critical areas though.  Bikes, like most things are designed by static operating loads.

Avatar
festina [58 posts] 1 year ago
4 likes

I'm gonna call bogus on the wheel weights too.  I'very ridden the same bike with a 50% change in wheel weight and the difference was night and day.  No one is paying me to say that and I built them wheels me self.  I agree it's only an issue when you are accelerating but I think there is a tendency to forget that the wheel acceleration is different to rider acceleration.  Yes aerodynamics play a part too but it's very dependant on what you are doing.  A nice flat TT at a constant pace then it's best to go aero.  A crit race where you spend an amount of time in a bunch (and therefore in 'dirty' air where aerodynamics have lesser effect) or a hill climb then wheel weight is important.  If you don't believe me add some weights inside your rim and test it.  There is a reason pros use light weight aero wheels and it isn't just to get the rest of us to part with cash, otherwise they'do use alloy ones as they brake better.

If the math doesn't match empirical data then the math is wrong not the empirical data.

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bigblue [21 posts] 1 year ago
0 likes

How fast do you have to be going for aero wheels to make a perceivable / significant difference, compared to non-aero wheels ? 20 mph ? 30 mph ?

OK, depends on what perceivable / significant means, but you get the idea. Maybe compare it to the actual numbers quoted for varying tyre width ? Or more intuitively, if I'm doing 20 or 30 mph on aero wheels, how fast would I go for the same effort on non-aero wheels ? I would imagine the difference is really small at 20mph, but I don't actually know for sure.

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BBB [461 posts] 1 year ago
4 likes

Guys, you've left out a crucial part of the rolling resistance equation -"suspension losses" and excluded the latest research that covers it pretty well. Without it, evaluating effect of width/volume and pressure on tyre performancere is pointless.

In order to understand why wider tyres (at lower pressure) are usually faster, you need to ask yourself what the purpose of pneumatic tyres is...

Vehicles with pneumatic tyres roll faster than those on solid wheels...

Riding on chipseal will be always slower than on a smooth tarmac...

Tarmac will be always slower than a velodrome...

Why do you think this is?

 

 

 

Avatar
rjfrussell [414 posts] 1 year ago
1 like
bigblue wrote:

How fast do you have to be going for aero wheels to make a perceivable / significant difference, compared to non-aero wheels ? 20 mph ? 30 mph ?

 

 

According to Wheelsmiths, "it depends".  They suggest the more aero (ie deeper) the wheels, the lower the speed at which they start to make a difference.  But, essentialy, it is high teens and above.

Not sure what the scientific back up,  if any, for this is.

 

http://www.wheelsmith.co.uk/#!aero-in-plain-english/viwub

 

But anyway, it doesn't really matter, because whatever the performance effect, ENVEs look great.  (I was going to say "sick"- but does that now mean good or bad-  it is so hard to keep up.)

 

Avatar
drjohn [46 posts] 1 year ago
1 like

Yes I agree with the comment above, fatigue is a very design dependent, and also manufacturing dependent process.  

Mind you there aren't any Comet airliners still flying are there? They were from the 50s IIRC. Dismissing fatigue as an issue is ignoring the fact that when you do get a fatigue failure it may be rare, but could also be life threatening. There is the rub - low chance of failure but serious consequences makes fatigue a big issue. The closest I came was having a pair of aluminium handlebars snap as I crossed a busy main road. When it happens it is often unexpected. Only a small % of 50s aircraft are flying today because you have to inspect against fatigue or accept high risk of death. Vulcan and Concorde will not be flying again because of this.

SO how do you know if the frame design is good? Have you done any FE analysis? Have you audited the manufacturing process? 

Did you get an X-Ray of your CF frame when you bought it? 

Usually, CF tends to last forever in fatigue or has defects that make it s**t, but proving your frame has defects is very difficult. How many of us have had a carbon fibre squash racket and returned it because of internal delamination? Yup, nobody. You would have to use same same model of racket over and over, or be the guy stringing it to notice.

But no roadie has much to fear. If you were riding a BMX in the eighties, you will know someone who broke a frame or two through normal abuse. If you break a road frame it's because it was taking a load it wasn't designed for.

e.g. my chain jammed after  the 11sp Ultegra lower jockey wheel drifted off the bearing. The steel mech hanger ended up rotating about 100 degrees (I bent it back). Thanks Shimano. That jockey wheel has now been replaced.

e.g. my mate's Trek Madone seatstay snapped freakishly after a plastic carrier bag got caught in the transmission. Bad luck. An ally or steel frame would have survived maybe.

e.g. a colleague closed his garage door and dented the top tube of his Klein Bottle (with colour flip paintwork -ouch!)

Fatigue is not on my list of top worries - that would be accidental damage, corrosion and an awful paintjob rendering the bike practically unrideable in public. The corrosion affects steel more. Bad paintjob more likely an issue somehow with aluminium frames!!!

FWIW - Sean Kelly also kicked ass on noodly Vitus frames.

 

Avatar
festina [58 posts] 1 year ago
3 likes

Oh whilst we're on myths what about the old debate about whether a wheel hangs from the top or is proper up from the bottom.  I read an article in cyclist last year where some university professor stated that the spokes act as columns and that the wheel is supported from below.  Oh how I laughed.

Avatar
festina [58 posts] 1 year ago
1 like
drjohn wrote:

FWIW - Sean Kelly also kicked ass on noodly Vitus frames.

 

 

Ha. Kelly could have kicked arse on a kids bike.  I did have one of those Vitus frames though and they weren't all that bad. Light for the time too. 

Incidentally concorde lost it's type certification due to the fuel tanks being susceptible to accidental damage and not fatigue but I accept the point about comet, bad design practice.

Avatar
BBB [461 posts] 1 year ago
0 likes
festina wrote:

I'm gonna call bogus on the wheel weights too.  I'very ridden the same bike with a 50% change in wheel weight and the difference was night and day.  No one is paying me to say that and I built them wheels me self.  I agree it's only an issue when you are accelerating but I think there is a tendency to forget that the wheel acceleration is different to rider acceleration.  Yes aerodynamics play a part too but it's very dependant on what you are doing.  A nice flat TT at a constant pace then it's best to go aero.  A crit race where you spend an amount of time in a bunch (and therefore in 'dirty' air where aerodynamics have lesser effect) or a hill climb then wheel weight is important.  If you don't believe me add some weights inside your rim and test it.  There is a reason pros use light weight aero wheels and it isn't just to get the rest of us to part with cash, otherwise they'do use alloy ones as they brake better.

If the math doesn't match empirical data then the math is wrong not the empirical data.

No one is saying that you're not going to feel any difference at all, it's just that what you feel doesn't translate even closely to what really happens in the reality. It's just like with tyres  at 120PSI. You may bouncing all over the road feeling very fast but you're slower than you'd be at over 20PSI less.

Your acceleration will be limited almost entirely by several stones  of flesh bones + the whole bike.

All tests, experiments and calculations evidently show that the forces involved are tiny. Even some tyre manufacturers which would be the  first to claim how quickly their lightweight tyres accelerate, admit that.
Something to also bear in mind when comparing the weights of various wheels is that the differences will be mainly in the hubs anyway. Rims in half decent wheelsets are more or less within 50g. from each other.

As for the pros, who gives a ***** what they use. It doesn't consitue evidence.

Again, just because you feel something...

Avatar
Welsh boy [431 posts] 1 year ago
2 likes
festina wrote:

Oh whilst we're on myths what about the old debate about whether a wheel hangs from the top or is proper up from the bottom.  I read an article in cyclist last year where some university professor stated that the spokes act as columns and that the wheel is supported from below.  Oh how I laughed.

Why did you laugh at a well educated person who had done serious research into a subject?  Was it because his findings contradicted what you thought happened (after all, this article is about commonly held misconceptions)?

Avatar
Welsh boy [431 posts] 1 year ago
3 likes
festina wrote:

...wheel acceleration is different to rider acceleration.  

 

How is that exactly?  On my bike, if my wheels accelerate, so does the rider.  Is my saddle bag acceleration different from my handlebar tape acceleration?  Where do I get a fast accelerating bottle cage?  Does a blue frame accelerate differently from a green one?

This is a really interesting point and one I am really hoping that you can clarify with some evidence, I hope it is not another cycling myth with no scientific evidence to back up.

Avatar
Mike T. [15 posts] 1 year ago
2 likes
festina wrote:

Oh whilst we're on myths what about the old debate about whether a wheel hangs from the top or is proper up from the bottom.  I read an article in cyclist last year where some university professor stated that the spokes act as columns and that the wheel is supported from below.  Oh how I laughed.

Just because you can't grasp a scientific principle doesn't mean that it's not true.  Tests have been done to measure the change of load on both upper (under tension) and lower (under compression) spokes of a tensioned wheel - which in engineering terms is a pre-tensioned struture.

This is the same principle as concrete bridge beams encasing pre-tensioned cables.  The beam will be stronger than a non-cabled beam until load (downward flexing) of the beams causes the cables' tensions to reach zero.

The lower spokes, when loaded, will exert a upward force on the hub and will do so until their pre-tension reaches zero (a loose spoke!).  The change in their tension is far greater (measured!) than the increase in tension for the "upper" spokes.

Avatar
700c [1171 posts] 1 year ago
2 likes

This is a more balanced account of the effect of tyre size on speed than previously provided by road.cc. There are some circumstances where narrower is better after all..

As for 'suspension losses' (are you referring to tyre deformation or hysteresis?), I wouldn't say leaving it out makes the article pointless, but pressure is another variable - which is not what that particular 'myth' was about. Yes the two are related of course.. then you've got smoothness of the road surface - no point bouncing along on 28mm 60 psi on a well-surfaced road is there.

Perhaps there should now be a myth buster about 'wider tyres are faster' because just like the opposite argument, it ain't always so. .

Avatar
hawkinspeter [1139 posts] 1 year ago
0 likes
Mike T. wrote:
festina wrote:

Oh whilst we're on myths what about the old debate about whether a wheel hangs from the top or is proper up from the bottom.  I read an article in cyclist last year where some university professor stated that the spokes act as columns and that the wheel is supported from below.  Oh how I laughed.

Just because you can't grasp a scientific principle doesn't mean that it's not true.  Tests have been done to measure the change of load on both upper (under tension) and lower (under compression) spokes of a tensioned wheel - which in engineering terms is a pre-tensioned struture.

This is the same principle as concrete bridge beams encasing pre-tensioned cables.  The beam will be stronger than a non-cabled beam until load (downward flexing) of the beams causes the cables' tensions to reach zero.

The lower spokes, when loaded, will exert a upward force on the hub and will do so until their pre-tension reaches zero (a loose spoke!).  The change in their tension is far greater (measured!) than the increase in tension for the "upper" spokes.

That sounds odd to me - have you got a link to those measurements? If there's a bigger change in tension below the hub than above the hub, it makes me think that the hub must be moving up or down. Surely the tensions must equal out otherwise you've got something strange happening with momentum.

Avatar
joules1975 [485 posts] 1 year ago
2 likes
BBB wrote:

Guys, you've left out a crucial part of the rolling resistance equation -"suspension losses" and excluded the latest research that covers it pretty well. Without it, evaluating effect of width/volume and pressure on tyre performancere is pointless.

In order to understand why wider tyres (at lower pressure) are usually faster, you need to ask yourself what the purpose of pneumatic tyres is...

Vehicles with pneumatic tyres roll faster than those on solid wheels...

Riding on chipseal will be always slower than on a smooth tarmac...

Tarmac will be always slower than a velodrome...

Why do you think this is?

 

THIS!

The energy loss through vibration of the bike, and more importantly the body, can be fairly significant, and it changes depending on body mass and fat percentage. I have seen few rolling resistance tests that include this. A wider tyre at a lower pressure reduces these vibrations, and thus reduces the overall rolling resistance effect. Obviously the smoother the road the benefit is less.

Here are interesting articles on the matter (from the same site admitedly).

https://janheine.wordpress.com/2012/08/12/suspension-losses/ 

https://janheine.wordpress.com/2010/10/18/science-and-bicycles-1-tires-a...

and I've seen a few different places where suspension loss is connected to rolling resistance in relation to motor vehicles.

As for wheel weight, I agree that the rotating weight has little effect on overall speed, but it most certainly has an effect on handling. An easy way to test this is to

1. grab two front wheels, one heavier than another (the difference doesn't need to be much),

2. grab first wheel by the spindle

3. get someone else to start it spinning at a decent speed, and then try and twist the wheel left and right.

4. Repeat with second wheel (making sure spin speed is similar) and be amazed at the difference.

Less of an issue on a road bike perhaps, but on a mountain bike the difference is very noticiable.

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festina [58 posts] 1 year ago
3 likes

Ha! I do enjoy sparking controversy.

Regarding wheels; if the spoke takes compression what stops the spoke from coming out of the rim? Loosen off all the spokes in your wheel and then tell me it doesn't hang from the top.

Regarding wheel weights; yes feel is one thing but the strava KoMs on the same bike with different wheels don't lie (and yes rider were in similar conditions within days of each other).

Regarding wheel acceleration; the top of the wheel is going twice as fast as the bike, the bottom of the wheel isn't going anywhere. I don't think rotational acceleration is the same as linear acceleration. I haven't looked into the equations (yet) I'm just saying that the difference in wheel weight is noticeable on acceleration. I have deep section wheels but I would say the difference (on feel) is less tangeable. Part of this will be due to the way they exert their benefits. In the grand scheme of things none of it really matters much.

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festina [58 posts] 1 year ago
3 likes
Mike T. wrote:

Just because you can't grasp a scientific principle doesn't mean that it's not true.  Tests have been done to measure the change of load on both upper (under tension) and lower (under compression) spokes of a tensioned wheel - which in engineering terms is a pre-tensioned struture.

This is the same principle as concrete bridge beams encasing pre-tensioned cables.  The beam will be stronger than a non-cabled beam until load (downward flexing) of the beams causes the cables' tensions to reach zero.

The lower spokes, when loaded, will exert a upward force on the hub and will do so until their pre-tension reaches zero (a loose spoke!).  The change in their tension is far greater (measured!) than the increase in tension for the "upper" spokes.

OK, so let's agree that all the spokes are in tension. Load is applied through the hub towards the ground. The upper spokes will see an increase in tension. The lower spokes will see a decrease in tension but, and here is the important part, they are still in tension. To support from below they would have to be in compression. This is the difference between a spoked wheel and a cart wheel. In addition a spoke is not designed to handle compression, it's shape is all wrong, it would have to have a larger cross section to resist buckling forces, bit like the spokes in a cartwheel.

As for concrete, it's rubbish in tension but great in compression, hence steelwork helps in just the same way as carbon and resin work together.

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earth [381 posts] 1 year ago
0 likes
festina wrote:

Oh whilst we're on myths what about the old debate about whether a wheel hangs from the top or is proper up from the bottom.  I read an article in cyclist last year where some university professor stated that the spokes act as columns and that the wheel is supported from below.  Oh how I laughed.

 

What I find interesting about these debates is that despite the wheel having been in use for many years, people are still studying how and why they work.  Is there much more to contribute on the subject?

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DaveE128 [955 posts] 1 year ago
1 like
hawkinspeter wrote:
Mike T. wrote:
festina wrote:

Oh whilst we're on myths what about the old debate about whether a wheel hangs from the top or is proper up from the bottom.  I read an article in cyclist last year where some university professor stated that the spokes act as columns and that the wheel is supported from below.  Oh how I laughed.

Just because you can't grasp a scientific principle doesn't mean that it's not true.  Tests have been done to measure the change of load on both upper (under tension) and lower (under compression) spokes of a tensioned wheel - which in engineering terms is a pre-tensioned struture.

This is the same principle as concrete bridge beams encasing pre-tensioned cables.  The beam will be stronger than a non-cabled beam until load (downward flexing) of the beams causes the cables' tensions to reach zero.

The lower spokes, when loaded, will exert a upward force on the hub and will do so until their pre-tension reaches zero (a loose spoke!).  The change in their tension is far greater (measured!) than the increase in tension for the "upper" spokes.

That sounds odd to me - have you got a link to those measurements? If there's a bigger change in tension below the hub than above the hub, it makes me think that the hub must be moving up or down. Surely the tensions must equal out otherwise you've got something strange happening with momentum.

Momentum has nothing to do with it. The reason, I imagine, why the spokes change tension more at the bottom of the wheel than the top, is that the rim actually flexes very slightly, squashing the bottom section ever so slightly flatter. This means that the bottom spokes attached to the flattened area get shorter, so their tension is reduced. This is what is meant by compressed I think - they get shorter under the (external) load.

If you imagine the bottom of the wheel getting squashed, the overall height of the wheel is reduced. (It probably gets wider to keep the same circumference). Therefore the total tension across top and bottom spokes is reduced. This is why the reduction in tension in the spokes at the bottom is bigger than the increase in tension at the top.

I know it's counter-intuitive, but pre-tensioned structures generally are.

It is accurate to say that the bottom spokes are taking more of the load than the top spokes. That doesn't mean they are not in tension.

edit: see this picture for what I mean about the deformation (exaggerated). It also shows the loads on the spokes in the extreme case, I think:

http://i.imgur.com/P9bIPkR.png

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wellcoordinated [206 posts] 1 year ago
0 likes
Welsh boy wrote:
festina wrote:

...wheel acceleration is different to rider acceleration.  

 

How is that exactly?  On my bike, if my wheels accelerate, so does the rider.  Is my saddle bag acceleration different from my handlebar tape acceleration?  Where do I get a fast accelerating bottle cage?  Does a blue frame accelerate differently from a green one?

This is a really interesting point and one I am really hoping that you can clarify with some evidence, I hope it is not another cycling myth with no scientific evidence to back up.

 

Linear acceleration - you, vs angular acceleration - your wheels.

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joules1975 [485 posts] 1 year ago
2 likes
Welsh boy wrote:
festina wrote:

...wheel acceleration is different to rider acceleration.  

 

How is that exactly?  On my bike, if my wheels accelerate, so does the rider.  Is my saddle bag acceleration different from my handlebar tape acceleration?  Where do I get a fast accelerating bottle cage?  Does a blue frame accelerate differently from a green one?

This is a really interesting point and one I am really hoping that you can clarify with some evidence, I hope it is not another cycling myth with no scientific evidence to back up.

 

Becasue you have to get the wheel rotating AND moving forward.

Stick the bike on a stand, turn the pedals or just push the wheel round with your hand. Notice how much effort is required. Some of that will be air resistance but most will be related to rotation inertia.

Add that to the linear acceleration needed to ge the wheels moving forward with the reat of the bike when actually trying to go anywhere, and you can see why wheel weight does matter more than the same amount of weight elsewhere on the bike, and why where the weight is located on the wheel also matters.

The discussion is around how much wheel weight matters in the overall system (you and the bike), and that depends on your riding style, ability and riding location.

In my view the 'x grams on a wheel is worth twice that weight elsewhere' statement still applies, it's just that those x grams may not always be that important overall wherever they are located.

 

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CharlesMagne [88 posts] 1 year ago
0 likes
festina wrote:

Oh here we go, the old fatigue chestnut.  Fatigue is such a stoopid subject full of mumbo jumbo that I don't even believe it tuely exists.

Aluminium frames aren't oversized for fatigue; aluminium is not as stiff as steel so, in order for it not to ride like a noodle (or an old Vitus), the tubes need to be 'fatter' which increases the second moment of area by placing the extreme fibers further away from the neutral axis.  This reduces stress.

I think you might find the stiffness to weight ratio is somewhat in aluminium's favour, otherwise planes would have been made out of thin steel rather than thick aluminium. Young's Modulus however, that's what affects the strength and therefore size of tubes. Material ductility will also have an impact on tube forming shapes. I believe the thinness of current steel tubes plays a limiting role on their useful size. Whith tubes as thin as 0.2mm joining becomes an issue and the likelihood of denting, as well as a needless increase in mass compared to necessary strength when moving to a larger diameter.

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The Family Cyclist [26 posts] 1 year ago
3 likes

Steel does fatigue and go soft. Well that's my claim as to at I need a new bike and im sticking with it

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Yorkshie Whippet [624 posts] 1 year ago
2 likes

With regards to spokes, bike wheels are hung from the top, whilst solid wheels such as cars are held up from the bottom. 

If you take a stationary and upright bike wheel, as mentioned before,  for the spokes at the bottom to be compressed they would need to be in contact with a solid base. If you removed the top spokes those at the bottom would push through the rim holes and provide no support. Anyone whose built wheels will understand. 

Mean time you can cut a stationary solid car wheel in half and it will still support the car of the ground.

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BBB [461 posts] 1 year ago
1 like
700c wrote:

This is a more balanced account of the effect of tyre size on speed than previously provided by road.cc. There are some circumstances where narrower is better after all.. As for 'suspension losses' (are you referring to tyre deformation or hysteresis?), I wouldn't say leaving it out makes the article pointless, but pressure is another variable - which is not what that particular 'myth' was about. Yes the two are related of course.. then you've got smoothness of the road surface - no point bouncing along on 28mm 60 psi on a well-surfaced road is there. Perhaps there should now be a myth buster about 'wider tyres are faster' because just like the opposite argument, it ain't always so. .

Gravity and air resistance aside, suspension losess are all the forces other than hysterisis that slow down the rolling of a bike. You can't just test some tyres in different widths on drums/rollers and claim that that they make xx W difference. On the road the difference will be greater than in a laboratory, as lower pressure will allow wider tyres to roll more efficiently on imperfect surfaces.

Speaking of pressure perhaps we should also add another myth to the list: Higher pressure makes you faster...

I don't claim the article to be pointless as a whole, but only the tiny part of it. I've glad that the the rotating mass nonsense have been covered pretty well  1

 

 

Avatar
hawkinspeter [1139 posts] 1 year ago
0 likes
DaveE128 wrote:
hawkinspeter wrote:
Mike T. wrote:
festina wrote:

Oh whilst we're on myths what about the old debate about whether a wheel hangs from the top or is proper up from the bottom.  I read an article in cyclist last year where some university professor stated that the spokes act as columns and that the wheel is supported from below.  Oh how I laughed.

Just because you can't grasp a scientific principle doesn't mean that it's not true.  Tests have been done to measure the change of load on both upper (under tension) and lower (under compression) spokes of a tensioned wheel - which in engineering terms is a pre-tensioned struture.

This is the same principle as concrete bridge beams encasing pre-tensioned cables.  The beam will be stronger than a non-cabled beam until load (downward flexing) of the beams causes the cables' tensions to reach zero.

The lower spokes, when loaded, will exert a upward force on the hub and will do so until their pre-tension reaches zero (a loose spoke!).  The change in their tension is far greater (measured!) than the increase in tension for the "upper" spokes.

That sounds odd to me - have you got a link to those measurements? If there's a bigger change in tension below the hub than above the hub, it makes me think that the hub must be moving up or down. Surely the tensions must equal out otherwise you've got something strange happening with momentum.

Momentum has nothing to do with it. The reason, I imagine, why the spokes change tension more at the bottom of the wheel than the top, is that the rim actually flexes very slightly, squashing the bottom section ever so slightly flatter. This means that the bottom spokes attached to the flattened area get shorter, so their tension is reduced. This is what is meant by compressed I think - they get shorter under the (external) load.

If you imagine the bottom of the wheel getting squashed, the overall height of the wheel is reduced. (It probably gets wider to keep the same circumference). Therefore the total tension across top and bottom spokes is reduced. This is why the reduction in tension in the spokes at the bottom is bigger than the increase in tension at the top.

I know it's counter-intuitive, but pre-tensioned structures generally are.

It is accurate to say that the bottom spokes are taking more of the load than the top spokes. That doesn't mean they are not in tension.

edit: see this picture for what I mean about the deformation (exaggerated). It also shows the loads on the spokes in the extreme case, I think:

http://i.imgur.com/P9bIPkR.png

I didn't think about deformation of the rim. I'm surprised that there's any noticeable deformation that would affect the forces on the spokes by a measurable amount.

My point about momentum is that if there's more forces acting on the bottom half vs the top half of the wheel, then wouldn't that produce a net force either upwards? I would think that in a tensioned structure, the forces should balance out, but then I'm no engineer.

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Boss Hogg [107 posts] 1 year ago
1 like

It makes a hell of a difference climbing steep sections (8%, 9%, 10% or more) on lightweight wheels. You can also corner with much more precision - and thus safety - when decending fast on lighter wheels. So I ride the lightest hoops (and tires) I can afford.

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pdw [64 posts] 1 year ago
0 likes
joules1975 wrote:

 

Becasue you have to get the wheel rotating AND moving forward.

Stick the bike on a stand, turn the pedals or just push the wheel round with your hand. Notice how much effort is required. Some of that will be air resistance but most will be related to rotation inertia.

Add that to the linear acceleration needed to ge the wheels moving forward with the reat of the bike when actually trying to go anywhere, and you can see why wheel weight does matter more than the same amount of weight elsewhere on the bike, and why where the weight is located on the wheel also matters.

The discussion is around how much wheel weight matters in the overall system (you and the bike), and that depends on your riding style, ability and riding location.

In my view the 'x grams on a wheel is worth twice that weight elsewhere' statement still applies, it's just that those x grams may not always be that important overall wherever they are located.

The point is that you don't spend much time accelerating, and it's not wasted energy: if you stop pedaling, you'll roll further with heavier wheels.

It's often said that lighter wheels are important for climbing.  Even if you start your climb from a standing start, you've only got to spin it up to your climbing speed once.  Put the front brake on, lift the rear wheel and I reckon you could spin it up to 15mph in a single pedal stroke. That's it: that's the total benefit of lighter wheels for the entire climb.  Beyond that, there's benefit to lighter wheels, but it's the same as weight lost elsewhere.

For a crit where you're constantly accelerating, and then chucking your kinetic energy away by braking then there's more of a case for lighter wheels, although even then I think being aero is usually more important than being light.

Avatar
joules1975 [485 posts] 1 year ago
0 likes
pdw wrote:
joules1975 wrote:

 

Becasue you have to get the wheel rotating AND moving forward.

Stick the bike on a stand, turn the pedals or just push the wheel round with your hand. Notice how much effort is required. Some of that will be air resistance but most will be related to rotation inertia.

Add that to the linear acceleration needed to ge the wheels moving forward with the reat of the bike when actually trying to go anywhere, and you can see why wheel weight does matter more than the same amount of weight elsewhere on the bike, and why where the weight is located on the wheel also matters.

The discussion is around how much wheel weight matters in the overall system (you and the bike), and that depends on your riding style, ability and riding location.

In my view the 'x grams on a wheel is worth twice that weight elsewhere' statement still applies, it's just that those x grams may not always be that important overall wherever they are located.

The point is that you don't spend much time accelerating, and it's not wasted energy: if you stop pedaling, you'll roll further with heavier wheels.

It's often said that lighter wheels are important for climbing.  Even if you start your climb from a standing start, you've only got to spin it up to your climbing speed once.  Put the front brake on, lift the rear wheel and I reckon you could spin it up to 15mph in a single pedal stroke. That's it: that's the total benefit of lighter wheels for the entire climb.  Beyond that, there's benefit to lighter wheels, but it's the same as weight lost elsewhere.

For a crit where you're constantly accelerating, and then chucking your kinetic energy away by braking then there's more of a case for lighter wheels, although even then I think being aero is usually more important than being light.

 

Totally agree, but like many others you fail to mention the effect of wheel weight on bike handling.

See my previous post.

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BikeJon [196 posts] 1 year ago
6 likes

I always thought Da Vinci was too clever to make the cranks that long. They would strike the ground! I then had the thought that maybe he wasn't very good at drawing but I've done some research and actually he was quite handy.

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