It depends on the brand of carbon wheels. Some can withstand heat longer than others. Also, SwissStop yellow pads produce more heat than SwissStop Black Prince.

Where did you get this? Interesting data.

does that mean the yellow's have better braking than the black prince? or do they conduct heat away better?

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after seeing your graph I found another from wheelbuilder.com - can't get it to post, but it's stopping distance in wet conditions from 60 km/hr - 0

black - 90 m
yellow - 121 m
competitor - 133 m


Not really related to my question, but for comparison if you're looking at this thread. It's weird b/c their own website gives the yellow a 9 in stopping power and the black gets an 8. Though the modulation favors the blacks

Does the cork buildup on the brake track help or hurt the braking performance? Does it help provide more friction/bite or is it making them more slippery? thanks

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Great things never come from comfort zones.

Might want to check out this vid: https://www.youtube.com/...Eq96jcELpB302qa2U3nA

If you are not using cork pads why does your rim have cork buildup? Is the buildup even over the entire rim or is it uneven and the pads pulse as the rim rotates?

If the buildup is uneven I would think it could hurt braking performance. There might also be other impacts like heat buildup.

How easy it is to destroy a carbon rim will depend on the wheel. Many of the newer rims use different resins that handle heat better than older designs. I have blown out a brake track on an old set of reynolds rims maybe 6-7 years ago on a long descent. Since then I have moved away from carbon rim brake wheels. I'm not knocking reynolds they replaced my rim and rebuilt my wheel for free, I just am more comfortable on aluminum brake tracks with rim brakes.

Good braking technique also helps. Releasing the brakes long enough to allow the brake track to cool can help.

You don't have any cork residue on your rims.

If you have Zipp or Campag rims, no real problems.
Anything else, put your head between your knees and pray.

SwissStop yellow seems to have a pretty low melting point. HED used to only recommend cork for their Stinger wheels and I found out the hard way that it is because the yellow pads work great, until they start to melt. At that point the braking gets really unpredictable/grabby and you end up with nasty brake goo melted along the brake track. I would guess that the yellow pad will melt long before the resin on the carbon rim gets so hot that the rim will have any structural problems.

I would use cork if your rim manufacturer doesn't recommend a specific pad.

I was riding a set of my newest caron rims recently, bontrager xxx2 and black prince pads, and was doing some exploring in my new area and got on a hill that was about 2 or more miles down at 6-7% average with a lot of twists. Having never been on the road, and hitting high speeds quickly, I used the brakes a lot, more than I ever have on a set of carbon rims. They seem fine, but it made me nervous enough that I just ordered a set of hed ardennes wheels today so I can keep exploring without concern.

While I am sure my rims are fine, it was a quick reminder that I don't need to be in the position to worry and there are very easy ways to mitigate the risk...I realize I really didn't answer the question since it depends on the rim, but am telling you I decided to NOT find out how hard/easy it is to kill carbon rims ;)

maybe that is the buildup on the track, melted yellow cork. Seems like I'll be switching to the black prince for a higher melting point and lower temperature for my next set. Giantex rims are pretty well made, not the generic Chinese ones that are popular, but they are likely close to 10 years old. I've never had to true them or any other service, but I have noticed more build up this year compared to other years on the brake track.

thanks to all for the info. I just have to make these last as long as possible, cause I'll likely not be buying another set of deep wheels. I don't need a new bike now and the next one will likely come with disk brakes, changing everything.

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Great things never come from comfort zones.

It isn't yellow cork.

How heat is transferred/dissipated between the rim and brake pad varies a lot. The residues on the rims indicate you are melting the pads. Given the decent you are talking about isn't extreme I suspect the wheels are actually staying cool and the pads are taking the brunt of the energy. Many manufacturers designed wheels in this manner to protect the wheels but it can give unpredictable or generally bad breaking as pad manufacturers haven't optimized the pads to work when super hot. You see wheel companies making specific recommendation for pads to get around this incompatibiltity.

You should definitely be cleaning the residue off the rim. This residue will be picking up bits of grit off road and turning into sand paper when you brake. It will grind down the braking track and significantly reduce the working life of the wheels. The same principle applies to alloy rims and disc brakes. Dirty surface=bad braking and unnecessary wear.

The law of conservation of energy, all of the kinetic energy from the bike is transfered to heat. some of the heat is transfered to the rims, and some of the heat to the pads, therefore, a brake pad that heats up less is either less effective, or is better a dissipating the heat

A freind of mine, who descends significantly slower than me, has fried 3 rims. He descends running the brakes the whole way down. On a 1000m (3300ft) descent, the carbon rims do not dissipate the heat as fast as the pads are building heat, as a result they get hotter and hotter to the point that they eventually delaminate. On the other hand,I tend to brake short and hard for the corners, and avoid running the brakes. Even on big descents, my rims are only warm to touch

On the topic of Swiss stop yellow pads, I use them as they have the best braking performance in the wet, however, they don't last very long, a few big days riding mountain passes and the are completely worn. Corima cork pads work well in the dry but are truely terrible in the wet

Honest question - how does braking hard/short allow for more heat to dissipate than braking soft/long? Isn't the same amount of kinetic energy dissipated for both braking styles by the time you get to the bottom?

I guess it could work if your rims cooled faster than they heat up so not braking lets them cool faster than they heat?

Its something I have wondered about myself, and I am sure that a more scientific person can explain better. This is my explanation: When you descend dragging the brakes, the temperature builds up slowly, with each revolution of the wheel, the rim cools a little less than the amount of heat generated by the brake blocks, as a result, over a very long descent the wheels can get very hot. On the other hand, braking hard into a corner, increases the temperature quite rapidly, early on the descent, the maximum temp generated will be higher than running the brakes, however, the rims and brakes get time to cool before the next spike in temperature.... so on average, the temperature is the same as running the brakes. Running / dragging the brakes starts off cool and gets progressively warmer, while sharp short braking creates a high temperature spike, but returns to normal temp before the next spike. So long as the temp spike does not exceed the melting point of the resin on the wheels all is fine

There are a lot of things going on and I barely remember any thermodynamics, but I think the big difference is a) you are braking less b) aerodynamics is playing a big factor.

I am guessing you get to the bottom of the hill faster than your brake-dragging buddy, so you are dissipating less heat because your brakes are doing less work to resist gravity. The second big factor is aerodynamic drag. If you are going 40 mph on the straights, your wind resistance is probably 500 -1000 watts. If your buddy is doing 20 mph his aero drag is probably 200-250 watts. Those 4 little pieces of rubber are doing a heck of a lot of work.

When you brake sharply the temperature is very high at the surface and this creates a larger thermal gradient which leads to a higher rate of heat loss to the surrounding air.
This not only sheds heat more effectively but prevents the heat from being drawn deeper into the wheel structure.

Descending the Alpe d'Huez a few years ago, I caught up to some cars near the midpoint and had to scrub my speed the rest of the way, until about turn 3 when my rear wheel expanded so much I had a blow out. The rim was totally ruined. AdH is about 16km so my wheel gave out after dragging over 11-12km.

Jack

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You're missing the biggest factor which is dissipation of kinetic energy via aero drag. The best way to save your rims is to descend faster and brake only when necessary, i.e. for corners.

This! Exact reason why braking like you mean it is better for your wheels than riding the brakes. Applies to cars too if you want your brakes to last.

good analogy, riding the brakes will warp your rotors and kill your brakes. It's much cheaper to replace brake pads, bike and car, if you brake hard and release and repeat, than it is to replace rotors or carbon wheels because you ride the brakes.

As for Alpe d'Huez, I'm not gutsy enough to ride carbon rims there. I think even the big hill near me that prompted this thread will be an alloy wheel and pad setup from now on.

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Great things never come from comfort zones.

You are right, I forgot about aero drag. Since drag increases exponentially with speed, the faster you go down, the larger the effect of drag, at 60kph, drag is more than double compared with 40kph

Also really good point someone made about speed, if the average speed down the descent is faster (ie braking only for corners), then the amount of kinetic energy transfered to the brakes and wheels is less

As for descending AdH or other big descents in the Alps, I have zero issues with rim brakes. I find that shorter steeper descents in the UK with crap road surfaces are much harder on the brakes. I get the point about being stuck behind traffic on a big descent, it's one of the reasons I avoid riding mountain passes at weekends

There are a number of elements to consider here and Josh Poetner's response here: https://www.slowtwitch.com/...nce_Part_2_7152.html. Highlights a lot of issues. The take home message is many of the key components in the systems behave in a temperature dependent manner. As temperatures shift several critical aspects of the system change which lead to problems and ultimately failure at a weak point. As Josh points out mass is incredibly important as the more mass you have in the effective brake track the more buffered you are. Rim pads are also critical but the ideal but as he notes the best compounds are expensive and wear quickly.

Steve Hed also thought and wrote about the processes involved. I can't find the article off hand but he strongly argued that for 99.99% of uses the Jet + construction of an alloy rim with a carbon fairing was a superior solution to a full carbon construction on the road. In pretty much everything wheel related it seems Hed was at least a decade ahead of this time so I really don't think this was just sales hype.