Seattle Forrest

I asked in the Clyde forum, because there's a lot of inertia in there ... but I'd like a second opinion from the 41. Have you ever had any trouble, or come close to it, from your rims heating up during a descent?

I'm going to be getting a new set of wheels in the next few weeks, and narrowing down which ones. They'll almost certainly be carbon rim tubulars. I realize these have less trouble with heat than clinchers, but I'm about 200 lbs ( and 6'1", not just a tub of lard! ) and I like taking my bike into the mountains for long climbs. What goes up must come down. It's rare to see grades steeper than around 10 % for very long out there. Hurricane Ridge is 5,240 feet of elevation over 17 miles. Within the city, the worst they'll deal with is about 1/2 to 2/3 miles at 19 %, with cross streets that traffic could come out of, so I ride the brakes coming down this one. But I don't do it often - there are other placed I prefer to ride.

Should heat dissipation be a reason for me to prefer one wheel set over another?

DScott

I do a fair bit of climbing here in So Cal, using alum. rimmed wheels, and have never had a problem with that. But then, I'm not sure how I would know.

HokuLoa

Sure, but I guess "overheat" would be the debatable term. When I was over 200lbs and riding long, steep Rocky Mtn descents at really fast speed the rims and pads would get REALLY hot and would smell a bit. To me that was overheating and a sign I should try to use my body more to slow down and feather the brakes more. Frankly I'm not familiar enough with aluminum rims and heat to tell what "overheat" actually means in a structural integrity sort of way. It is enough to me that those signs indicated I could be doing a better job of descending and braking without allowing my speed to creep even higher.

DScott

^^^ Hmmm...

I guess there is the bicycling urban myth that over-heated wheels will explode tubes and blow tires off the rims, but I think the smoking/melting/stinking brake pads is the more likely problem. Haven't yet experienced that with stock Campy brakes/pads on alum rims.

Commodus

I've heard that carbon, tubulars, and high heat are are a bit of a bad mix. The carbon can do funny things, the glue can melt, etc.

That said, the pros seem to do okay on them...of course they aren't 200 lbs.

It seems that your weight and riding preferences indicate an alloy rim would be most suitable.

Seattle Forrest

I've been warned about this a few times, and it's what I was worried about. No one in the Clyde forum reported having any trouble with this. From what I hear, brake pads can melt onto the rim and cause all sorts of problems, but none of them are catastrophic. At this point I feel like heat probably isn't something that should be a big factor in my decision, but, like I said, the wisdom of the 41 will be nice to have.

tanguy frame

During Cycle Oregon 2007 we crossed the cascades from the high dessert to the willamatte valley. the road we descended on was a poorly maintained steep winding forest service road with no cars. I was at the back of the pack because I started late and rode slow to make up for it. At the top of the pass they blocked all the riders and were sending down a series of ambulances. Eventually they ran out of ambulance capacity and sagged all the remaining riders down the hill.

People were crashing because they didn't know how to control their speed and they missed the hairpin turns off both sides. some hit the rock walls, and others fell off the cliffs. The road was torn all to hell and there was loose gravel everywhere (ever run ito tight gravel?). There were lots of blown tires which people were attributing ot overheating due to constant breaking.

That was the steepest and twistiest road I've ever seen and I wished I could have descended on my bike, but when I saw it up close, I'm certain I would have crashed too. I don't like to control my speed. I would have crashed due to failure to apply the breaks, not brake malfunction due to overheating.

Shimagnolo

Lennard Zinn's article on CF rim braking performance: https://www.velonews-digital.com/velo...103?pg=74#pg74
You may need to fiddle with the controls at the top of the page to make it large enough to read.

I'll stick to aluminum rims.

topflightpro

There have been several reports, on forums and in the trade press, about the issue of heat, due to excessive braking, causing problems with carbon clinchers. The heat can cause the resin to melt and the rim to deform.

It does not seem to be much of an issue with tubulars as there are supports behind the braking surface. Clinchers have a much thinner wall behind the braking surface.

Newer carbon clinchers have been designed to reduce the likelihood of such heat-related failures. Additionally, most manufacturers report that the problem is mitigated by using their specific brake pads.

This is not a widely reported problem on wheels with aluminum braking surfaces.

Andy Somnifac 

banerjek

I'm small, but I still worry about heat.

Once the pads start melting and they glaze up, your stopping power is significantly reduced. Under normal circumstances, that's no big deal, but you really want full braking power on the type of descents that heat your rims hot enough to burn you and melt pads. Plus your hands get really tired.

If you brake for only a few seconds at a time so heat has some chance to dissipate, I find that it's not a huge issue.

merlinextraligh

IMHO, overheating is not a significant concern on carbon tubulars.

I'm the OP's size, and I have done rides like Six Gap, and Brasstown Bald Buster and raced Everest Challenge on Zipp 303 tubulars without any problem.

Only issue with tubulars and heat is that you can melt the glue. To my knowledge carbon tubulars are not substantially more prone to that than aluminum.

popeye

All the reports I have heard of tires blowing off the rim due to heat were tandems. It happens.

ColinL

Before you buy tubulars you seriously need to find a friend with them and help him install a brand-new tire. Or better yet, do it all by yourself while he watches and chuckles.

Seattle Forrest

^ I'm planning on having LBS do that for me.

rooftest

I've experienced noticeable brake fade on my old bike - '04 Ultegra brakes with aluminum rims. Haven't had that problem with the new Dura Ace ones - paired with aluminum rims.

I don't have any experience with carbon rims. If braking is important to you; you're probably not looking at them anyway.

Seattle Forrest

I put that in there to see if anybody was paying attention...

frisky99

I remember a utube I think of Eddy Mercyx reaching back with his water bottle and spraying his rear brakes on a long fast descent. Anyone else see this ?.

zzzwillzzz

it's pretty infrequent but it does happen more with tandems (2x the weight, same amount of braking area). according to the tandem experts, the cause seems to be not that the heat increases the pressure too high but that the heat affects the rim strip and make it soft and the tube blows from the spoke hole edge of the rim, velox tape works better than the plastic rim strips to prevent this. of course this is with alum rims, carbon's a different story.

coachboyd

Here's how this works.

With carbon tubulars you are braking on the strongest part of the rim. The carbon can absorb some of the heat and in extreme circumstances have been known to melt the glue on the rim. If your tire is properly glued this should never be a problem. I have actually left my wheels in a hot car with the windows closed to let the glue soften to rip off a tire easier. . .but it's still a pain. There are no forces pushing on the rim so prolonged braking does not have a high risk associated with it.

With carbon clinchers, there is a hook holding to tire in place. That hook has all the pressure of the tire and inner tube forcing outward against it. When the rim heats up (just like with the tubular), the outward pressure on the hook can cause the rim to bow outward under high heat. This happens when the carbon reaches it's tg temperature (tg stands for glass transition, used to describe when the solid melts). Ways to combat this are by using brake pads designed with the epoxy on the brake track and by using carbon with higher tg points. With a combination of these the temperature will stay lower and the carbon will remain stronger. This is why almost all of the carbon rim manufacturer's recommend using a specific brake pad with their carbon clincher rims. As long as these instructions are followed you can use carbon clinchers in a very hilly area. This is similar to driving a car in the snow, it can be done very well, we just have to adjust the decisions we make.

chaulky61

**********? - done properly, and if you have one preglued , it is as easy installing a clincher.

Braden1550

Nothing to it. It's not that hard. It's not hard to glue properly either. lol

saratoga

Descending Brasstown Bald in NE GA requires some aggressive braking to set up the turns if you want to bomb it at a decent speed. That's a steep descent with more turns than straight sections and it didn't overheat the brakes to where they faded or wouldn't grab... or even smelled hot for that matter.

The front rim was hot to the touch at the bottom, but no issues otherwise. This was on a set of alu Campy Eurus wheels.

Seattle Forrest

Thanks, Boyd. Great explanation. Simple, very cogent, and immensely helpful.

PS - Your 24 mm carbon tubies are stupidly affordable. $800 for a 1,072 gram wheel set? LBS has a pair of not especially aero hoops tipping the scales at 1,085 grams and $2,900. Too bad I'm a little over the weight limit. But I'll be looking at your CX wheels.

coachboyd

It's not so much a weight limit as a recommendation. For riders over 175 pounds the 38mm tubulars are going to be a much better option than the 24mm, especially now that we have 24/28 spoke options available in tubulars.

With the super light weight of the rim on the 24mm, it doesn't keep momentum that well. That means anytime you are on the flat roads and you coast you'll be slowing down much quicker, leading to more pedaling and a higher overall energy expenditure. By going slightly deeper you save energy on the flat to rolling terrain and the added 100 grams is barely going to be noticeable on the climbs. For all of my hilly races (except pure uphill time trials) I use the 38mm instead of the 24mm.