Carbon rims and descending big hills...........

wycombewheeler][quote=madcarew wrote:

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.

Jimmy Ray Will wrote:

it all depends on how you descend and how heavy you are, and how hot it is. 

I'd also imagine that tyre and rim width will play a factor. 

If you are relatively heavy, and like to drag a brake, then yes, there is a real risk of you blowing tubes on long, hot descents. if however you don't letting the bike go, before hitting the anchors  for turns etc. then you shall probably be fine and dandy. 

The problem with carbon rims is that they don't shed braking heat quickly. Different resins, and using more effective braking surfaces can reduce heat build up a bit, but fundamentally a carbon rim is going to get hot and stay hot when being braked. 

There is no fool proof solution to this sadly... which to me is another reason why the industries are pushing discs so hard. 

 

I agree with all that you've said bar the disc aspect. How often do the pros, well actually anyone crash on high speed wet descents due to actual inadequate braking power on carbon rims?

Aren't incidents a combination of not understanding the limitations of your kit in X conditions and failing to adapt to that, AKA pushing the boundaries and going beyond and then most of all find that the limiting factor is in fact the human in their judgement and the tyres not the brakes themselves?

Yes, in the wet, discs have a clear advantage (over rim brakes) in ultimate braking power and how much later you can brake to stop at the same point, also for some who don't bother to learn how to control the amount of braking will be helped out by discs in that respect too (modulation).

However similar with motorvehicles, you'll end up with people pushing the new 'envelope' same as before, gain more confidence so braking much later and finding that they no longer have sufficient thinking time in certain instances and braking later means the tyres are going to lose traction more often due to much harder, later braking from higher speeds in the wet thus we will still have the same amount of crashes if not more due to these factors.

The 'solution' is not go with a system that is cumbersome/ugly and adds weight/changes where the stress points on the frame and forks are, and loses a significant amount aerodynamically (all negatives IMHO) but to learn when you need to brake in x conditions at x speed, learn how to navigate descents and bends better/safer without having to be on the brakes all the time, learn how to modulate your braking and that if you can't descend to a reasonable standard and be safe with respect to yourself and other road users then maybe think long and hard about where you cycle and if the conditions/terrain is beyond your and your bikes capability just stay away and choose something less risky.

Just because one has a bike similar or even better than a pro doesn't mean you have any business being in certain environs trying to copy them, particularly in not so great conditions. Doing so has catastrophic outcomes all too often and discs will not resolve this at all.

If discs were so great why do we still have millions of crashes in motorvehicles every year in the UK alone with 185,000 killed or injured despite all the protective systems in modern vehicles? it's not the brakes themselves but the higher speeds, later braking, loss of traction of tyres and smaller window of thinking time for the human.

The ONLY reason manufacturers have come up with discs is to make money, they can try to sell it as a safety feature all they want but it's totally bogus.

If the OP is concerned and doesn't have the required experience/skill (it sounds to me they don't) then I suggest they simply stay away from the descent mentioned and go ride somewhere where they are within their experience/skillset until they are sure they can do so safely with the kit they have, this should apply to all. I know and have seen even in the pro ranks of too many people riding beyond their capabilities and coming a cropper, great athletes but not so good at understanding X, Y and Z.

BehindTheBikesheds wrote:

Jimmy Ray Will wrote:

it all depends on how you descend and how heavy you are, and how hot it is. 

I'd also imagine that tyre and rim width will play a factor. 

If you are relatively heavy, and like to drag a brake, then yes, there is a real risk of you blowing tubes on long, hot descents. if however you don't letting the bike go, before hitting the anchors  for turns etc. then you shall probably be fine and dandy. 

The problem with carbon rims is that they don't shed braking heat quickly. Different resins, and using more effective braking surfaces can reduce heat build up a bit, but fundamentally a carbon rim is going to get hot and stay hot when being braked. 

There is no fool proof solution to this sadly... which to me is another reason why the industries are pushing discs so hard. 

 

I agree with all that you've said bar the disc aspect. How often do the pros, well actually anyone crash on high speed wet descents due to actual inadequate braking power on carbon rims?

Aren't incidents a combination of not understanding the limitations of your kit in X conditions and failing to adapt to that, AKA pushing the boundaries and going beyond and then most of all find that the limiting factor is in fact the human in their judgement and the tyres not the brakes themselves?

Yes, in the wet, discs have a clear advantage (over rim brakes) in ultimate braking power and how much later you can brake to stop at the same point, also for some who don't bother to learn how to control the amount of braking will be helped out by discs in that respect too (modulation).

However similar with motorvehicles, you'll end up with people pushing the new 'envelope' same as before, gain more confidence so braking much later and finding that they no longer have sufficient thinking time in certain instances and braking later means the tyres are going to lose traction more often due to much harder, later braking from higher speeds in the wet thus we will still have the same amount of crashes if not more due to these factors.

The 'solution' is not go with a system that is cumbersome/ugly and adds weight/changes where the stress points on the frame and forks are, and loses a significant amount aerodynamically (all negatives IMHO) but to learn when you need to brake in x conditions at x speed, learn how to navigate descents and bends better/safer without having to be on the brakes all the time, learn how to modulate your braking and that if you can't descend to a reasonable standard and be safe with respect to yourself and other road users then maybe think long and hard about where you cycle and if the conditions/terrain is beyond your and your bikes capability just stay away and choose something less risky.

Just because one has a bike similar or even better than a pro doesn't mean you have any business being in certain environs trying to copy them, particularly in not so great conditions. Doing so has catastrophic outcomes all too often and discs will not resolve this at all.

If discs were so great why do we still have millions of crashes in motorvehicles every year in the UK alone with 185,000 killed or injured despite all the protective systems in modern vehicles? it's not the brakes themselves but the higher speeds, later braking, loss of traction of tyres and smaller window of thinking time for the human.

The ONLY reason manufacturers have come up with discs is to make money, they can try to sell it as a safety feature all they want but it's totally bogus.

If the OP is concerned and doesn't have the required experience/skill (it sounds to me they don't) then I suggest they simply stay away from the descent mentioned and go ride somewhere where they are within their experience/skillset until they are sure they can do so safely with the kit they have, this should apply to all. I know and have seen even in the pro ranks of too many people riding beyond their capabilities and coming a cropper, great athletes but not so good at understanding X, Y and Z.

 

Yar, you may have missed the point I was making about discs. One reason the industry is keen to push them is because there is no ultimate 'fix' for carbon clinchers. Carbon will always get hot, which in turn heats the tubes, which in extreme scenarios causes blow outs. These are common enough to be a recognised problem in ral world riding as; not everyone is under 75kg and not everyone has the ability and desire to let a bike go on a steep / long descent. 

The only definitive fix for carbon clinchers is to remove the braking surface away from the rim... hence the disc requirement.

carbon rims are more expensive, and more desirable than alloy rims, so naturally something the industry wants to keep. 

That said, a carbon tubless set up could get as hot as it likes, it wouldn't blow any tubes.

That said, a carbon tubless set up could get as hot as it likes, it wouldn't blow any tubes.

[/quote]

Assuming you meant 'tubeless' this is not true I'm afraid.  At a certain point the rim will overheat and itself fail structurally.  A tubeless setup has the same lateral forces on the rim wall as a tubed setup and it is this lateral force on weakened rims that causes some of the wheel failures.

Tubs have the edge here, if they get hot there is less stress on the rim as there isn't any force required to hold the tyre on, the glue or tape does that.  Or does that until it melts, which is a different but equally scary failure mechanism...

wellsprop wrote:

No real world experience, but...

Look at the speeds the pros were hitting in the Tour de Suisse, 120kph+, their wheels didn't fail.

Not to say they don't fail - ever seen the video of the guy riding a Canyon down Mount Ventoux and he overheats the pads?

Comes down to how heavy you are and how much breaking you are doing. Personally, I wouldn't be too worried about using a pro level set of carbon wheels on a descent, one would hope that they have been tested and designed against failure.

Not sure Pro experience helps the OP.  Pros don't ride clinchers, they ride tubs. 

Sniffer wrote:

wellsprop wrote:

No real world experience, but...

Look at the speeds the pros were hitting in the Tour de Suisse, 120kph+, their wheels didn't fail.

Not to say they don't fail - ever seen the video of the guy riding a Canyon down Mount Ventoux and he overheats the pads?

Comes down to how heavy you are and how much breaking you are doing. Personally, I wouldn't be too worried about using a pro level set of carbon wheels on a descent, one would hope that they have been tested and designed against failure.

Not sure Pro experience helps the OP.  Pros don't ride clinchers, they ride tubs. 

Yeah, fair point, totally forgot to think about that!