[richcrocker79]

I know this question has been done to death historically, but all of my recent googling brings up links and horror stories from four or five years ago.

 

My question is reasonably simple, I have a set of Fulcrum quattro carbons which i love. I am heading to France in a couple of weeks, staying near Mont Ventoux and riding up and down it a few times. Will I die on the descent if I use the Fulcrum rims? they have the 3Diamant braking surface, which is supposedly fine, but the best I got from Fulcrum was ‘we’ve never had any complaints about the braking surface’

 

Has anybody had any real world experience of a current technology set of carbon clinchers on long steep descents? I know they will get hot, I’m aware of not dragging brakes etc. but will they fail?

 

Thanks

 

Rich

Topic

[joeegg]

     Myself and a mate did

     Myself and a mate did the Maratona a few weeks ago. He had some brand new Giant carbon rimmed wheels and had no problems at all. We also descended Campolongo in the wet before the event and he said braking performance was very good.On our way to Italy we did some riding in France at temperatures close to 30 degrees with  steep descents and again no problems.

[richcrocker79]

Well, having done three

Well, having done three ascents via the three routes in three days I can honestly say that ventoux is one of the best places I have ever cycled. Could I have used carbon rims? Possibly. Was I glad not to have to worry about it? Absolutely! 

 

Glad I listened to the advice given, and if anybody else finds this while trying to answer the same question, the answer, in my mind is simple. Take the aluminium rims!! 

 

Oh, and if you do go to ventoux, make sure you ride the gorge de la nesque – incredible scenery! 

 

Cheers,

 

Rich

[Carton]

madcarew wrote:

Chrs. Looking at it from a physics point of view, regardless of speeds attained, at the end of the day you are getting rid of the same amount of potential energy (from top of the hill to bottom) so it would seem to me that the slower you need to disperse that energy (i.e. taking longer to get down the mountain) the slower the transfer of energy to the rim, hence allowing more time for dissipation (and the differences are pretty big. On ventoux an averag of 60 kph -which from strava seems ‘normal’- transfer time is 20 mins, at 40 kph it’s 40 min, at 20 kph it’s 1 hour). I would have thought that is more than enough time to dissipate the heat, but it would appear  the convective  effects are substantial (which on a thermodynamic basis I’m struggling to find a reason for as there’s no evaporative agent, so the heat dissipation should be straight line-ish). Thinking as I go, I guess maybe the tyre acts as a radiator…. 

Just few points on this, to hopefully keep you from going down the rabbit-hole:

a) I doubt heat dissipation is straight line-ish. For one, even before evaporative effects, I doubt the airflow effect on heat transfer is insignificant (or linear). Also, consider heat dissipation in the pad vis-a-vis the rim. When you’re pressing the calipers together you’re cooling the pad with the hot rims and viceversa, instead of cool air. But, again, ¯_(ツ)_/¯ 

b) (edit: this has been said, but) While conservation of energy establishes that kinetic energy must equal potential energy loss, not all of the potential energy is lost the same way. So the air resistence on the rider does part of the work without giving off heat on the rims. When you’re going 80kph, even on a relatively steep descent you’re being almost completely airbraked. Mechanical equilibrium is reached through that, and not the heat producing friction on pads on rims.

c) Again, this is somewhat of an appeal to authority, but I think that the fact the Zipp engineers, whose thermodynamics knowledge surely far, far, far outweighs mine, chose to explain it on the basis of experimental data seemingly untethered to a theoretical framework might be a sign that they might be at least a bit unsure about it as well.

[madcarew]

Carton wrote:

madcarew wrote:

Love to know the engineering basis for the result 🙂

I’m more of a desk engineer, but if you’re asking for a theoretical model, I’m afraid that just the thermodynamics are well beyond me. For starters you’ve got some kind of a convection-diffusion situation where the relationships between the variables are expressed in differential equations and nothing is easy to tease out.  And then there are so many other things. Material-wise, you might also have something like a positive feedback where inter-surface adhesion between pad and track might also increase as a function of temperature,  just to name another possible non-linear interaction to consider. Aerodynamics are never linear. Friction is linear but gravity causes acceleration, which then impacts the aerodynamics.

I’d venture a guess that gerally the faster you go the more the air both brakes you and cools your rims, but that might be up to a point where other things come into play if you pick up plenty of speed and then brake too hard.  But ¯_(ツ)_/¯ 

So yeah, “Chariot of Fire”. It may seem like an appeal to authority, but for my money good old-fashioned experimentation is a very sound engineering basis.

Chrs. Looking at it from a physics point of view, regardless of speeds attained, at the end of the day you are getting rid of the same amount of potential energy (from top of the hill to bottom) so it would seem to me that the slower you need to disperse that energy (i.e. taking longer to get down the mountain) the slower the transfer of energy to the rim, hence allowing more time for dissipation (and the differences are pretty big. On ventoux an averag of 60 kph -which from strava seems ‘normal’- transfer time is 20 mins, at 40 kph it’s 40 min, at 20 kph it’s 1 hour). I would have thought that is more than enough time to dissipate the heat, but it would appear  the convective  effects are substantial (which on a thermodynamic basis I’m struggling to find a reason for as there’s no evaporative agent, so the heat dissipation should be straight line-ish). Thinking as I go, I guess maybe the tyre acts as a radiator…. 

[Carton]

madcarew wrote:

Love to know the engineering basis for the result 🙂

I’m more of a desk engineer, but if you’re asking for a theoretical model, I’m afraid that just the thermodynamics are well beyond me. For starters you’ve got some kind of a convection-diffusion situation where the relationships between the variables are expressed in differential equations and nothing is easy to tease out.  And then there are so many other things. Material-wise, you might also have something like a positive feedback where inter-surface adhesion between pad and track might also increase as a function of temperature,  just to name another possible non-linear interaction to consider. Aerodynamics are never linear. Friction is linear but gravity causes acceleration, which then impacts the aerodynamics.

I’d venture a guess that gerally the faster you go the more the air both brakes you and cools your rims, but that might be up to a point where other things come into play if you pick up plenty of speed and then brake too hard.  But ¯_(ツ)_/¯ 

So yeah, “Chariot of Fire”. It may seem like an appeal to authority, but for my money good old-fashioned experimentation is a very sound engineering basis.

[madcarew]

Carton wrote:

Just a little FYI for all engineers in here (armchair,  desk chair, and control chair): 

“At Zipp we built a machine we called the ‘Chariot of Fire’ to try and understand all of this during our carbon clincher development, and it used a 400-lb. flywheel driven by an electric motor and could simulate most any type of downhill situation, different rider mass, and could even be programmed with real-world descending situations. It could also simulate environmental conditions like heat, cold, or rain. We found that the worst thing you can do during descending is prolonged, constant braking; the rim never has time to cool so the temperatures continue increasing at some pretty unbelievable rates. Even giving the wheel 2-3 seconds of rest during a test could drop temperature by almost 100 degrees F.”

Read more at http://www.velonews.com/2015/02/bikes-and-tech/technical-faq/technical-faq-hot-tires-rims-oman_361634#lhT44vVg3RvqjDxS.99″

There’s a bit more there. But my takeaway is that for those really big mountain days (ie. attempting to join the club des cinglés), alloy is a great choice, particularly if it gets hot. Have fun!

Thanks for that. I said originally I’d be happy to change my view on receipt of good engineering evidence. 

View cautiously changed.

Love to know the engineering basis for the result 🙂

[Carton]

Just a little FYI for all

Just a little FYI for all engineers in here (armchair,  desk chair, and control chair): 

“At Zipp we built a machine we called the ‘Chariot of Fire’ to try and understand all of this during our carbon clincher development, and it used a 400-lb. flywheel driven by an electric motor and could simulate most any type of downhill situation, different rider mass, and could even be programmed with real-world descending situations. It could also simulate environmental conditions like heat, cold, or rain. We found that the worst thing you can do during descending is prolonged, constant braking; the rim never has time to cool so the temperatures continue increasing at some pretty unbelievable rates. Even giving the wheel 2-3 seconds of rest during a test could drop temperature by almost 100 degrees F.”

Read more at http://www.velonews.com/2015/02/bikes-and-tech/technical-faq/technical-faq-hot-tires-rims-oman_361634#lhT44vVg3RvqjDxS.99″

There’s a bit more there. But my takeaway is that for those really big mountain days (ie. attempting to join the club des cinglés), alloy is a great choice, particularly if it gets hot. Have fun!

[madcarew]

richcrocker79 wrote:

wow, loads of information to take in, thank you all.

…

As for my skill set, well I’ll be there next week, so I will find out – having nothing to compare it to, I have no idea, but confidently cautious is my usual approach, so that should work out ok.

Thanks for all the input – it made for interesting reading!

Rich

Enjoy 🙂

 

[Anonymous]

Good decision, as you said

Good decision, as you said prev, you’ve no real experience of riding on long, really fast descents where you’ll likely need a shed load of braking and you don’t know the terrain and how much actual braking you’ll need. Your riding enjoyment won’t be lessened any by using the Mavic’s but you’ll have a little more to fall back on and you’ll come away with a better idea as to what works, what doesn’t (for you) and if in the future you could get away with using carbon rims on similar.

better to learn with a side of caution and move forward than destroy an expensive pair of wheels at best, at worst, well, you know how things can go wrong when you have no braking on high speed descents.

Enjoy.

[richcrocker79]

wow, loads of information to

wow, loads of information to take in, thank you all.

 

I have decided that discretion is the better part of valour, and I’m fitting my kysriums so I have an alloy rim. As was said ealrier in the post, if it is playing enough on my mind to ask a bunch of strangers on the internet then it would probably make the whole descent more worrying than need be!

 

As for my skill set, well I’ll be there next week, so I will find out – having nothing to compare it to, I have no idea, but confidently cautious is my usual approach, so that should work out ok.

 

Thanks for all the input – it made for interesting reading!

 

Rich

[part_robot]

Regarding the tech:

Regarding the tech:

Pro wheels don’t fail at 120kmh because they use tubulars which have much thicker material around the braking area which can handle and conduct away considerably more heat. Meanwhile the clinchers used by the rest of us are thin because the brake track is (with very few exceptions) in the same region as the clinching. Don’t use pro bikes as a reference point; carbon clinchers can and do fail and no brand is immune. Plus the pros are better than you at handling their bikes and already know the courses.

Sorry to say but carbon clinchers really are dumb and nothing more than a gimmmick to part us from our money. Get a lightweight aluminium aero pair like Hunt Aero Road, one with a carbon fairing like the mid price Mavics or just do the sensible thing and get a bike with disc brakes 😉

End of.

[madcarew]

wycombewheeler][quote

wycombewheeler wrote:

[quote=madcarew]

Incidentally, my basic engineering tells me that Energy is proportional to speed squared, so if braking late and hard as suggested by many (to allow the rims time to cool down) I think is probably not good advice, as ultimately the rims have to get rid of more heat (as speed will have built up more in the interim). A safer approach would be to not let your speed get so high in the first place, but still corner as quickly as you can (to reduce the amount of speed loss required). I’m quite happy to be put right by someone with good engineering knowledge. I imagine it is possible (but I think unlikely) that the rate of heat transfer through the carbon may affect the answer.

You are neglecting the air braking energy dissipation. The more energy lost to wind resistance the less that has to be lost through the brake pads. By braking as little as possible speed is kept up and less energy heats the wheel. After all at the top energy is proportional to elevation. This becomes kinetic and then becomes heat either in the air (dispersed and negligible) or in the brake pads and wheel. The worst thing to do would be drag the brakes and descend the entire mountain at 20kph.[/quote

There’s an element of truth in what you say, but there’s no way it takes less (braking) energy to slow from 90 kph to 40 than it does to go from 70 to 30. (the total energy dissipated is 60% more)

We could develop the argument of dragging the brakes all the way down at 20 kph. The issue, so far as heat build up on the rims, and heat dissipation is, of course the time factor, because the total amoun of energy lost is the same (M x g x h) Do you think that if you dragged the brakes all the way down but only allowed yourself to get to 3 kph the brakes would over heat? of course not, because (in the case of mont ventoux) it would take 7 hours to get to the bottom. At 20 kph it’s going to take you an hour. At an average of 60 pkh it’s going to take you 20 mins, and because the temperature rise is a function of work, the faster you slow down, the more work is being done, and so the temperature rises higher. When you’re dragging the brakes slowly the work is spread over a greater time giving the heat more time to dissipate, so your wheel temp is going to be lower. Your rims will stay at a lower temperature if you maintain a lower steady speed down the hill. The issue will only get clouded if you averaged the same speed but maintained a steady speed of (for example) 40 kph compared to averaging 40 kph but achieveing high speeds and braking hard for the corners. I suspect the average wheel temperature would be similar, but your peak temps would obviously be higher when braking from higher speed which clearly makes the rims more likely to glass or the brakes to melt.. With only a basic engineering knowedge (but reasonable physics) I think this takes us back to my original conclusion that to get to the bottom safest (lowest rim / brake temp) you brake as evenly as possible while still taking the corners as quickly as you can.

[madcarew]

freebsd_frank wrote:

 

madcarew wrote:

Actually CFC has a higher specific heat capacity than alloy (1.1J /gdegC on average cf  0.9 J/gdegC for alloy).

 

You seem to have quoted a figure for aluminium which is not necessarily the
same as when it has been alloyed with whatever.

I quoted for 6062 aluminium, the most common alu alloy used on bikes and rims in particular

You are right, for fibres it varies wildly, but for the entire product it is remarkably consistent, +/- 10%, I checked. 

 

madcarew wrote:

The conductivity in alloy is the key to the heat loss. Alloy will cool down
faster.

 

Correct. It’s why the heatsink on my GPU in this computer is made of Al alloy.

Wake me up when they make them from CFC.

The initial point was the specific heat capacity was higher. It isn’t. Noone was suggesting fibre makes a better heatsink

madcarew wrote:

Your described result is very different to the real world experience of most people using carbon composite wheels.

 

Pray tell me: what percentage of these people with CFC rims ride up and down
alpine passes?

That’s immaterial, as you yourself later say you can get up to 60 mph. The point is that cf wheels don’t disintegrate on a wholesale basis as implied in your point. Play the ball, not the man.

Huw Watkins wrote:

Mavic’s coments from one of their recent wheel launches:

[i]”At the 2015 Etape du Tour, for example, Mavic assisted with 100 wheels, of
which 52 were failures and 38 were carbon clinchers that had delaminated,
product manager Maxime Brunand said.”[/i]

 

Mavic’s real world experience seems to considerably differ from yours.

They’re busy flogging CFC wheels too, so it’s hardly in their interest to
mention such damning stats.

It seems to imply that 25% of failures were alloy rims. What %ge of rims in the etape were alloy and what %ge carbon clinchers? Your figures are meaningless quoted in isolation like that. 

madcarew wrote:

The amount pros are paid has nothing to do with the likelihood their wheels will fail as you’ve described.  And they like to do it day after day, their livelihood depends on it so they’re not going to do it on something that fails with the certainty you’ve described.

 

You’re entirely wrong. The pro’s will do whatever is required of them if you
bung them enough money.

I ride with pros every season. I’ve never met one who would agree with that. Bernie Eisel, Cav, Mark Renshaw, so many others are on record as saying they hesitate to ride with some guys because they are too young and too dangerous. Ergo they are very concerned with their safety and ability to support their family tomorrow

Shoot up with whatever the latest performance enhancing drug is without giving
a rat’s arse about the physical damage it could be doing to them?

Sure. They’re young men on the make and every man has his price.

If they were told to do Alpe D’Huez on a Raleigh Chopper having ingested a
truckload of Ritalin, they’d be more than up for it….given suitable
financial compensation and being told by the team manager that [i]”You want to
get along in the team, don’t you?”[/i]

your argument is at the wrong end of the spectrum. Shooting up with ped’s improves their ability to be there to race tomorrow, as do wheels upon whose structural integrity they can rely. They are very interested in that. They have little regard for 20 years time.  Nothing in a pro racer’s lifestyle reflects an acute interest in their old age health. No elite sport is good for your health.

 

madcarew wrote:

As for pros going down faster and not braking as much: they corner at ‘similar ‘ speeds to a good club cyclist (what you’d go round the corner at 20 mph, they’ll do at 25 mph; however they’re slowing down from 60+ mph instead of 45 – 50 mph, which means they actually do 60% more (energy loss from) braking, or allowing for your club cyclist to be 20% heavier (85kg vs 70kg) they are still doing 45 – 50% more braking; so if it can stand up to the pros, it can stand up to a club cyclist (YMMV)

 

You’re pulling figures out of your arse again. There is absolutely no reason
why I can’t get up to “60+ mph” coming down an alpine pass if I choose to.

If you do the physics there, you’ll see the figures are sound.

I can guarantee you, having descended in the elite peloton they hit 60 mph an awful lot more often on one descent than you do, hence their rims are more likely to delaminate if it was going to be a major issue. For comparison at the NZ Nats 2 years ago I hit 105 -110 kph each time on a moderate descent in the bunch. When riding it solo I rarely broke 85 kph.

Clearly the red mist had descended by the point you read that and you were unable to read the disclaimer :”YMMV”.   

At least I can stand assured that my wheels aren’t going to come apart when I
come to brake.

To cover your original point again, alloy rims have a lower specific heat capacity than CF, and this has nothing to do with their ability to not over heat.

[S13SFC]

I’ve carbon Cero RC45 Evo and

I’ve carbon Cero RC45 Evo and alu Fulcrum Racing Zeros.

Experience has taught me that when I go to the high mountains then the Zeros go on.

Better safe than sorry.

[srchar]

I’ve done the Maratona a few

I’ve done the Maratona a few times, EDT, Majorca 312 and a couple of weeks in Girona and Chamonix on a pair of Boras. No issues whatsoever. I’m in the “descend fast, brake hard” camp.

Don’t drag the brakes all the way down and carbon rims are fine.

[Author]

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