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I presume that if going uphill the lighter the bike, the better, but what about downhill? Is it possible that say a very lightweight rider could find he or she goes faster downhill on a bike that is light vs one that is super light?

Sorry if this is a foolish question, but it's something I've often genuinely wondered.

Like probably many new bike riders out there one of my favourite things is going on long downhill descents where you can feel yourself slowly but surely getting faster and faster reaching speeds that seem like they would infinitely grow the longer the descent is.

When it comes to downhill descending and reaching the highest speeds, is there a sweet spot with a bike and a particular person when it comes to the weight of the bike and the weight of the person?

Thanks.

42 comments

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dave atkinson [6440 posts] 4 weeks ago
1 like

as you go faster aero is much, much more important than weight. look at downhill speed record bikes: they're all industrial components and big fairings. i wouldn't suggest something like that, but light weight doesn't help going downhill. my fastest downhill speeds have been on aero road bikes, which I doubt is a coincidence.

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kil0ran [1184 posts] 4 weeks ago
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A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

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vonhelmet [1338 posts] 4 weeks ago
2 likes
kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

How do you figure? A heavy rider will have more frontal area than a light one, so will experience more air resistance. Other things being equal, they’ll be slower as a result.

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FluffyKittenofT... [2272 posts] 4 weeks ago
9 likes

Surely this is relevant?

 

https://en.wikipedia.org/wiki/Galileo%27s_Leaning_Tower_of_Pisa_experiment

 

If you dropped a light bike and a heavy bike (plus light and heavy riders...say Clarkson and Hammond...yeah, definitely make it those two) off the tower of Pisa at the same time, other than differences due to air resistance they would hit the ground at the same time.

 

(To be sure, should probably repeat the experiment with Eric Pickles and Matthew Parris)

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Yorkshire wallet [2321 posts] 4 weeks ago
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vonhelmet wrote:
kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

How do you figure? A heavy rider will have more frontal area than a light one, so will experience more air resistance. Other things being equal, they’ll be slower as a result.

This doesn't seem to play out quite as expected in my experience. My bigger, heavier mate seems to be able to pull away from me on freewheeling descents, despite having more frontal area, wider tyres and being on a flat bar bike. This is bend free as well.

 

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jaysa [98 posts] 4 weeks ago
1 like
vonhelmet wrote:
kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

How do you figure? A heavy rider will have more frontal area than a light one, so will experience more air resistance. Other things being equal, they’ll be slower as a result.

I'm 4kg heavier than last year and descending measurably faster on all my local hills as a result.

But on sharp corners, I'm not as fast, and sprinting out of the corners is slower.

On balance, more pies = descend most hills faster !

Pantani was a fearless descender - Eurosport covered the 95 World Champs at 2500m in Colombia, and there was a helicopter shot of him descending past loads of riders at least 5mph faster.

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hawkinspeter [2666 posts] 4 weeks ago
1 like
vonhelmet wrote:
kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

How do you figure? A heavy rider will have more frontal area than a light one, so will experience more air resistance. Other things being equal, they’ll be slower as a result.

However, other things aren't equal.

Extra weight would actually help when going downhill, providing that the weight doesn't hugely increase your frontal area (unlikely to make much difference unless you're carrying dustbin lids or something).

Assuming that you're just coasting and not pedalling, then the forces driving you forwards will be a component of gravity i.e. the steeper the slope, the greater the force. NB. The force increases with increased mass, although in a vaccuum, the acceleration would stay the same as more force is required to accelerate more mass (F = M x A).

The forces acting to slow you down will be mainly the air resistance against you (approximately proportional to your frontal area and the square of your speed) and the rolling resistance. Rolling resistance is proportional to your weight and speed, but is generally quite low compared to the air resistance, so we can forget about it at higher speeds.

So, increased mass will produce a larger downhill force which will be balanced (at terminal velocity) by the air resistance at a higher speed - hence heavier riders will go downhill quicker unless they are bizarrely shaped.

 

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Mungecrundle [1137 posts] 4 weeks ago
1 like

I used to think think this. You know the experiment: drop a feather and a lump hammer in a vacuum and they will fall at the same rate, reaching the ground at the same time. Do the same in air and you get complications due to air resistance. Do the same on a bicycle on a slope and it gets even more complicated.

Upshot is that there are plenty of explanations via your search engine of choice as to why a heavier rider will accelerate faster downhill than a lighter rider (with all other variables being controlled).

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alansmurphy [1925 posts] 4 weeks ago
2 likes

I think it's fair to say that as an amateur cyclist the best gains are to lose weight and increase power to get you up the hill quicker, train at descents including cornering, aero position etc. and enjoy yourself. As for the bike, n+1 iinit!

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StraelGuy [1588 posts] 4 weeks ago
2 likes
hawkinspeter wrote:

However, other things aren't equal.

Extra weight would actually help when going downhill, providing that the weight doesn't hugely increase your frontal area (unlikely to make much difference unless you're carrying dustbin lids or something).

Assuming that you're just coasting and not pedalling, then the forces driving you forwards will be a component of gravity i.e. the steeper the slope, the greater the force. NB. The force increases with increased mass, although in a vaccuum, the acceleration would stay the same as more force is required to accelerate more mass (F = M x A).

The forces acting to slow you down will be mainly the air resistance against you (approximately proportional to your frontal area and the square of your speed) and the rolling resistance. Rolling resistance is proportional to your weight and speed, but is generally quite low compared to the air resistance, so we can forget about it at higher speeds.

So, increased mass will produce a larger downhill force which will be balanced (at terminal velocity) by the air resistance at a higher speed - hence heavier riders will go downhill quicker unless they are bizarrely shaped.

 

 

Does this affect how quickly fat and thin squirrels can run up and down trees, too?

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hawkinspeter [2666 posts] 4 weeks ago
6 likes
StraelGuy wrote:
hawkinspeter wrote:

However, other things aren't equal.

Extra weight would actually help when going downhill, providing that the weight doesn't hugely increase your frontal area (unlikely to make much difference unless you're carrying dustbin lids or something).

Assuming that you're just coasting and not pedalling, then the forces driving you forwards will be a component of gravity i.e. the steeper the slope, the greater the force. NB. The force increases with increased mass, although in a vaccuum, the acceleration would stay the same as more force is required to accelerate more mass (F = M x A).

The forces acting to slow you down will be mainly the air resistance against you (approximately proportional to your frontal area and the square of your speed) and the rolling resistance. Rolling resistance is proportional to your weight and speed, but is generally quite low compared to the air resistance, so we can forget about it at higher speeds.

So, increased mass will produce a larger downhill force which will be balanced (at terminal velocity) by the air resistance at a higher speed - hence heavier riders will go downhill quicker unless they are bizarrely shaped.

 

 

Does this affect how quickly fat and thin squirrels can run up and down trees, too?

Most definitely.

The larger-boned squirrel will quite often mistakenly climb onto a branch that isn't quite strong enough, so they end up plummetting to the ground. The more svelte squirrel will choose to clamber down the tree trunk which will be slower.

 

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alansmurphy [1925 posts] 4 weeks ago
3 likes

These are treemendous insights but I need a root and branch review of the data in the absence of graphs, or we could just leaf it!

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madcarew [863 posts] 4 weeks ago
1 like
vonhelmet wrote:
kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

How do you figure? A heavy rider will have more frontal area than a light one, so will experience more air resistance. Other things being equal, they’ll be slower as a result.

"Other things being equal"... they aren't, the mass is greater.

 A rider is (kind of) a sphere, so their volume (roughly equivalent to mass) increases as a 3rd power relative to a given dimension, (4/3*pi *r^3) whereas their surface area (think frontal area ) increases relative to the same dimension squared (4*pi *r^2), so a 100kg person all things being equal will only have 30% more surface area than a 70kg person, but 42% more mass. (the difference is accentuated when they both get into an aerodynamic tuck, the larger person's frontal area is less than 10% greater than the smaller person, but they have 40 % greater 'motive force') So their gravitational potential energy (which is converting to kinetic energy) is greater  and will equate to greater speed as they are both losing the same height.

Fluffy kitten's tower of Pisa experiment assumes that air resistance is minimal, which it clearly isn't in the case of a cyclist. So your heavier cyclist will both accelerate faster, and attain a higher final speed. So, if you want to go fast(er) downhill, ride a heavy, aero, bike.

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Griff500 [307 posts] 4 weeks ago
1 like

Back to the OP,which was about bike weight, not rider weight, the answer is no. Whilst you could argue, as some have here, that a heavier bike might be marginally faster downhill, and the handling marginally worse due to the higher combined c of g, these effects are trivial.My alloy bike is 3kg heavier than my carbon bike, which makes it 40% heavier, but when all up weight is considered (circa 77kg versus 80kg), the bike only makes a 4.5% difference. That 4.5% equals an extra half a gear uphill where weight matters, but makes sfa difference downhill where drag is dominant.

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hawkinspeter [2666 posts] 4 weeks ago
2 likes
alansmurphy wrote:

These are treemendous insights but I need a root and branch review of the data in the absence of graphs, or we could just leaf it!

.

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StraelGuy [1588 posts] 4 weeks ago
3 likes

So the upshot of this long and quite interesting debate is that if you want to improve your downhill speed, the solution is basically an aero road bike and a rucksack full of half-bricks ?

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bikezero [25 posts] 4 weeks ago
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Thanks for all the interesting, informative (and sometimes humourous!) replies (enjoyed the laughs).

For me, losing weight or gaining it for that matter is not really an option. I don't know if I have an underlying health problem of some kind but in my adult life (and I am pushing 40 years old) my weight has never fluctuated more than the tiniest tiniest bit however much I eat or excercise. I am 6'1 and skinny (not super skinny but you could probably say very skinny) have weighed pretty much exactly the same as I did at age 20 as I do today. "I have a superfast metabolism" I suppose is the comforting way of looking at it, though I don't know if that's the real reason.

Anyhow, I have a cheap Btwin Triban 500 bike...the weight is probably about 10.2kg (I made a couple of small modifications to drop a tiny bit of weight from the bike in stock form).
It's easy to notice that on flat stretches the bike with me on it isn't so fast. Cyclist fly by be constantly. However on long downhill descents it seems insanely fast.

It is going to be interesting next week when I get my recently acquired somewhat lighter Btwin Ultra AF frame fitted with the Triban wheels and new components groupset (105) I have coming. I'm am going to take the bike (once I am sure it is safely set up) on these particular very long steep downhill descents I know of and see if I now seem to go even faster.

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bikezero [25 posts] 4 weeks ago
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...Well, I will spend a few days getting to know the bike first, but following that i won't be able to wait!

One thing I did notice with the Triban over my long time riding it is that it seemed to get faster the more I cycled it. I presume that is down to getting to know the bike, subconsciously learning to position myself better and perhaps stronger power in my legs..

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vonhelmet [1338 posts] 4 weeks ago
2 likes

Thanks all. I’d not thought about the relative impact of air resistance compared to weight, as I was merrily modelling the cyclists as point masses. Damn you A-level physics.

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JMcL_Ireland [10 posts] 4 weeks ago
1 like
StraelGuy wrote:

So the upshot of this long and quite interesting debate is that if you want to improve your downhill speed, the solution is basically an aero road bike and a rucksack full of half-bricks ?

Can't find a reference at the moment, but I remember reading that was Sean Kelly's training solution to going uphill faster - up this hill in particular https://www.strava.com/segments/623748 (on a 70s era bike with no doubt a horrendous gear ratio)

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PRSboy [344 posts] 4 weeks ago
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Is there a point at which mechanical drag also has a bigger influence... so bearing quality, tyre resistance/pressure etc start to have a proportionally bigger impact as speed increases?

For example, there is a descent near us where if I am riding my old faithful MTB I struggle to get anything like the same speeds that I will on my road bike, even crouched low in an 'aero position' and in normal roadie clothes.

The bike frontal area can only account for so much drag, and the MTB is around 5 KG heavier than my road bike.

 

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vonhelmet [1338 posts] 4 weeks ago
2 likes

Mountain bike tyres will have huge resistance compared to road tyres.

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hawkinspeter [2666 posts] 4 weeks ago
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vonhelmet wrote:

Mountain bike tyres will have huge resistance compared to road tyres.

Yes, that resistance will be proportional to your speed but the air resistance will be proportional to the square of your speed, so the rolling resistance will only be significant at slow speeds.

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StraelGuy [1588 posts] 4 weeks ago
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That's a similar principle to ballistics. If you double the weight of a bullet for a given velocity, you double the energy carried by the bullet. However, if you double the velocity of a bullet for a given bullet weight, you roughly quadruple the energy it carries (sorry, used to be an FAC holder ).

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fukawitribe [2601 posts] 4 weeks ago
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hawkinspeter wrote:
vonhelmet wrote:

Mountain bike tyres will have huge resistance compared to road tyres.

Yes, that resistance will be proportional to your speed but the air resistance will be proportional to the square of your speed, so the rolling resistance will only be significant at slow speeds.

True, but the rolling resistance can be significant yet still only proportional to speed - it's the value of the coefficient that can knacker things. You can easily get 20+W per tyre difference* between decent road tyres and decent, but pretty unsuitable, MTB tyres depending on pressure and speed - Kenda Small Block 8s seem popular but particularly shite in this regard. Tubeless and latex tubes seems to be able to help the MTB tyres significantly that said 

https://www.bicyclerollingresistance.com/specials/tubeless-latex-butyl-t...

 

* quick look-up at 25-30km/h at lowish pressures.

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hawkinspeter [2666 posts] 4 weeks ago
2 likes
fukawitribe wrote:
hawkinspeter wrote:
vonhelmet wrote:

Mountain bike tyres will have huge resistance compared to road tyres.

Yes, that resistance will be proportional to your speed but the air resistance will be proportional to the square of your speed, so the rolling resistance will only be significant at slow speeds.

True, but the rolling resistance can be significant yet still only proportional to speed - it's the value of the coefficient that can knacker things. You can easily get 20+W per tyre difference* between decent road tyres and decent, but pretty unsuitable, MTB tyres depending on pressure and speed - Kenda Small Block 8s seem popular but particularly shite in this regard. Tubeless and latex tubes seems to be able to help the MTB tyres significantly that said 

https://www.bicyclerollingresistance.com/specials/tubeless-latex-butyl-t...

 

* quick look-up at 25-30km/h at lowish pressures.

Yes, absolutely. Tyre pressure also makes a surprisingly large difference to rolling resistance, but again, once you get over a certain speed, it's the aero effects that dominate (it'll just be at a slower speed than with optimised tyres/pressure).

I thought I'd test out the whole "rucsack of bricks" on my commute home this evening by carrying a 2Kg bag of unroasted coffee beans (along with my usual work clothes, mini computer, squirrel suit etc). However, I wasn't very scientific about it and unfortunately I overtook a cyclist and then had him on my rear wheel for a couple of miles. That wasn't a problem except that I usually try to drop people off my rear wheel and end up knackering myself in the process. I think the extra weight might have helped on the downhills, but I think he might have turned off rather than me losing him. Still, I'll count that as a success.

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BehindTheBikesheds [2512 posts] 4 weeks ago
0 likes
vonhelmet wrote:
kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

How do you figure? A heavy rider will have more frontal area than a light one, so will experience more air resistance. Other things being equal, they’ll be slower as a result.

I'm 97kg, not as much muscle to fat ratio as I'd like but looking at me you wouldn't say I look my weight, so I'm not giving a massively larger frontal area and you have to take into account different clothing and actual body position too.

I've always outsripped 'smaller' and lighter riders on the downhill when freewheeling, managed this on a flat bar hybrid with 25mm gatorskins, the difference is always noticeable though I can get a decent aero position on it when needs be.

Managed 46mph coming down the A505 from Luton to Hitchin (about 7-8%) without much bother, again that was on a flat bar.

As for whether a bike can be too light, nope, handling ability of the bike, your familiarity with it plus your confidence to descend quickly and importantly picking your line/knowing when to sit up/brake are massively important factors when it comes to making progress.

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kil0ran [1184 posts] 4 weeks ago
0 likes
vonhelmet wrote:
kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

How do you figure? A heavy rider will have more frontal area than a light one, so will experience more air resistance. Other things being equal, they’ll be slower as a result.

Personal experience. As a larger rider I overtake the climbing whippets on descents, and I don't consider myself particularly brave. Sadly I'm left for dead on the climbs so it all evens out

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madcarew [863 posts] 3 weeks ago
0 likes
PRSboy wrote:

Is there a point at which mechanical drag also has a bigger influence... so bearing quality, tyre resistance/pressure etc start to have a proportionally bigger impact as speed increases?

For example, there is a descent near us where if I am riding my old faithful MTB I struggle to get anything like the same speeds that I will on my road bike, even crouched low in an 'aero position' and in normal roadie clothes.

The bike frontal area can only account for so much drag, and the MTB is around 5 KG heavier than my road bike.

 

In short, no. The mechanical drag remains roughly constant and continuously decreases as a proportion of the total drag, until at 50kph it is less than 1% of drag. At 70 kph it is less than .4% of drag.

On your MTB assuming knobblies you have considerable wind resistance from the tyres, and just generally less aerodynamics in the bike, and it would be very very difficult to get as aero on a MTB as you can on a road bike

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davel [2675 posts] 3 weeks ago
1 like
hawkinspeter wrote:
fukawitribe wrote:
hawkinspeter wrote:
vonhelmet wrote:

Mountain bike tyres will have huge resistance compared to road tyres.

Yes, that resistance will be proportional to your speed but the air resistance will be proportional to the square of your speed, so the rolling resistance will only be significant at slow speeds.

True, but the rolling resistance can be significant yet still only proportional to speed - it's the value of the coefficient that can knacker things. You can easily get 20+W per tyre difference* between decent road tyres and decent, but pretty unsuitable, MTB tyres depending on pressure and speed - Kenda Small Block 8s seem popular but particularly shite in this regard. Tubeless and latex tubes seems to be able to help the MTB tyres significantly that said 

https://www.bicyclerollingresistance.com/specials/tubeless-latex-butyl-t...

 

* quick look-up at 25-30km/h at lowish pressures.

Yes, absolutely. Tyre pressure also makes a surprisingly large difference to rolling resistance, but again, once you get over a certain speed, it's the aero effects that dominate (it'll just be at a slower speed than with optimised tyres/pressure).

I thought I'd test out the whole "rucsack of bricks" on my commute home this evening by carrying a 2Kg bag of unroasted coffee beans (along with my usual work clothes, mini computer, squirrel suit etc). However, I wasn't very scientific about it and unfortunately I overtook a cyclist and then had him on my rear wheel for a couple of miles. That wasn't a problem except that I usually try to drop people off my rear wheel and end up knackering myself in the process. I think the extra weight might have helped on the downhills, but I think he might have turned off rather than me losing him. Still, I'll count that as a success.

So... Are unroasted or roasted beans faster? 

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