Barrettscv 

I know that hydraulic disc brakes offer the ultimate in safety on long steep descents. overheated rims, tubes and tires are not an issue with disc brakes.

While I have multiple disc brake road bikes, I'm still using a rim brake bike road bike occasionally. Two questions come to mind:Are high quality carbon rims that use modern high temperature resins just as safe an aluminum rims while descending steep mountain roads?

Are tubeless tires always safer than clinchers that use tubes?

Koyote

I've got no idea on your first Q; will leave that to others.

Regarding your second Q: on a long, fast, steep descent, I would prefer tubeless tires with adequate sealant. In my experience, even when they get cut so badly that the sealant won't completely seal up, they lose air more gradually than a tubed tire that suffers a nasty puncture.

datlas 

It's up to your comfort level, but I do believe there is a consensus that for long descents, avoid carbon fiber rim brake wheels due to possible overheating. I expect that goes for tubed or tubeless.

We went to Whiteface a few years ago, which is 8 miles at 8%, and I purposefully did NOT take my CF wheels over an abundance of caution.

Zaskar

Any rim/tire/brake setup can be perfectly safe or very dangerous. It's the rider (technique) not the gear.

You have front and rear brakes - use them.

Don't ride the brakes to stay at a given speed - use the brakes intermittently to scrub off enough to go below your target speed - hard on, totally off, hard on, totally off. I'll explain. If you want to average 35 mph on the decent, coast to 40, brake hard enough to slow to 30... coast to 40, repeat. Don't ride the brakes at 35 to stay at 35. That's how things overheat.

Iride01 

Is one just as safe as the other? I think that probably depends! <grin>

Carbonfiberboy 

Depends on which alu rims. Deep alu rims, say 28mm at least, lose heat quickly to the atmosphere. Box rims not so much, but either will lose heat more quickly than carbon which is not a good conductor of heat.
"the thermal conductivity of the C/C composites remains almost constant at the value of 5.28 ± 0.42 W/mK in the direction perpendicular to the fiber axis."
"The thermal conductivity of aluminum and its alloys, which is 88 to 251 W/m K,"

Getting the heat off the brake track and into the atmosphere is the whole deal.

On our tandem with tubed tires on 28mm deep alu rims, we air brake as best we can while letting the bike run to whatever speed, then brake down hard for the corners. If it's a winding descent, I alternate front and rear brakes. We've blown tires off alu box rims. It's not the puncture risk, it's blowing the tires off that's the issue. I doubt tubeless is any better than tubed in that respect and might very well be worse, as they lack the tube holding the bead in place on the rim. I would never run hookless rims on a rim brake bike. Non-folding tires with steel wire in the beads are also not good as the wire retains heat and can eventually melt the bead on a long descent.

You're not completely out of the woods with discs, as discs can definitely overheat. A riding buddy of mine went off into the blackberries when his rear tandem disc got red hot and melted the pad. The precursor was that his front brake had failed and he was trying to bring the speed down before a sharp corner. There's also the relatively well-known story of the single bike descender who was dragging his disc brakes to hold speed constant until they both overheated and totally failed. Somewhat terrifying. Someone had told him that you can't overheat discs.

koala logs

If you're worried about big punctures that cause sudden loss of pressure during descents, just use protected "puncture resistant" tires with bead-to-bead puncture protection. They can come in both tubed and tubeless versions. A tubeless puncture resistant tire will offer you the best protection possible.

This is an example of puncture resistant slick road tire. But the tubeless versions is only available in >32mm wide tires.

https://www.panaracerusa.com/product...vel-tires#info

rm -rf

That's my technique, too, on my aluminum rims. Hard brake, let go. I alternate front and back wheels if possible.

But-- that really doesn't work on steep downhills. I don't know the cutoff point, but somewhere around 10% grade, the bike regains it's speed "instantly". The brakes are working almost continuously.
Probably, stopping for a cooldown is the only good solution here.

I do find that keeping my speed slowed way down does help. The braking force in my fingers is lower. That has to affect rim heating.

Barrettscv 

Great answer, thanks

redlude97

In addition to the tubeless potentially saving a flat from a puncture, they tend to not come off the rim if you do it a puncture that doesn't seal so you can at least ride on a bit of rubber as you slow down due to how tight the beads are on the rim bed.

terrymorse 

Personal data point:

I have been using a single pair of carbon wheels with rim brakes since 2011. I often descend mountain roads that require aggressive braking.

I have never once experienced overheated rims or tire blow offs, and the braking surface of the rim shows very little wear.

Then again, I know how to brake correctly. It's not that difficult.

smashndash

this is a pretty unhelpful comment. Not only is it condescending, it doesn't contain any information we can use. If you've never encountered even an ounce of brake fade on carbon rims, you're not actually doing very sketchy descents.

To OP, aluminum is safer than carbon, simple. If you're really afraid of overheating your rims, the peace of mind with alloy is worth it. Not that alloy rims won't fade at all. It's just nearly impossible to have the rim itself fail from heat.

tubes vs tubeless: definitely avoid latex tubes with rim brakes. However, I've personally never heard of a butyl tube failing from braking. Plus, a tube is more likely to handle overpressure from heat than a tubeless setup. That being said, I think it's really not worth choosing one over the other for this reason. Other things, like ease of setup or flat prevention are much better reasons.

EDIT: I should say that I've been using carbon rims for the last 3 years in the hills of california. Most descents are ez, no stress whatsoever. Some, like mt. Umunhum or metcalf road (sustained -10%, hard turns) require me to be very mindful about how I brake. But I'm a very confident descender, and I still face a good deal of brake fade. I'm probably not really close to my rims exploding, but it's unpleasant. I have yet to attempt quimby road on these wheels for that reason (bumpy, technical, -13% for a mile).

Barrettscv 

I agree with all the points in your post. I've ridden mountain passes in Italy where the descent was 4000 ft in less than 12 miles. I do use the technique of bringing the bike to a 10 mph using either the front or rear brake briefly to help keep the rims from overheating. This technique combined with pausing all use of the brakes intermittently has kept me safe. It's useful to resist using the brakes for a few miles to give the rims a chance to cool before I reach the more technical or dangerous sections further down where I resume braking.

My original post was a purely technical question about materials, but bikeforum's always has a variety of unhelpful opinions for every narrow question.

I have a rim brake climbing bike and both carbon and aluminum rims that fit that bike. I'll switch to aluminum rims and also change the pads when I know that long descents will be on route.

terrymorse 

The other possibility is that you do ride "sketchy" descents, and you have adequate braking skills (and decent brake pads).

Why? I have been using latex tubes for years, with rim brakes. Zero issues. Then there are the decades of pro cyclists on tubular tires with latex tubes and rim brakes. They seemed to descend just fine.

Explain how you think a tube helps reduce the chance that a tire will blow off the rim hook from overpressure.

It's worth mentioning that many of us have been descending these Bay Area roads you mention for many years (except Mt. Umunhum, which was closed until recently), with rim brakes and latex tubes. And decades before that, many of us rode them on tubulars with latex tubes.

GhostRider62

1. Yes

2. No

Hookless and tubeless vs hooked with latex tubes on hooked clincher rims? I'll take latex and have used them forever in hilly and mountainous terrain. Lots of safety margin for a blowoff with hooked clinchers, which is not necessarily the case with hookless and tubeless.

I've abusively braked on high temp resin transition carbon rims and then checked the rim temps for giggles. Not a problem with mine. With proper braking technique and the correct pads, I don't have a problem at all with rim brakes in the mountains. Heavy rain? different discussion.

rsbob 

About a month ago was out exploring and climbed a closed super steep paved single lane road. The grade ranged between 12-17% and my lungs blew up (gasping for air) after 3/4 mile. It was extremely curvy with a guard rail on one side and steep cliff on the other. It continued up god knows how far probably to a cell tower but I bagged it after I hit a false summit. Had never descended anything so mercilessly steep with my disc braked road bike. Had to ride the brakes or would flown off any number of curves and over the precipice. About 1/4 mile down, stopped the bike and the discs were so hot that placing my finger near them felt like heat radiating from a stove burner. So I waited a couple of minutes for them to cool a bit and went another quarter and stopped, rinse and repeat. If my brakes had faded or failed the acceleration would have been so fast that my only choice would have been to put it into the mountain side. Learned my lesson about descending steep climbs that give no run-offs or reprieves, as well as climbing them in the first place. Appears the road was closed for a reason.

spelger

So what exactly is brake fade? I doubt it has ever happened to me or I might know. Assume something related to brakes becoming less effective during use.

I have rim brakes only and do a lot of descending. Never have problems but don't do a lot of braking unless really windy.

Barrettscv 

Both rim and disc brakes convert mechanical energy into thermal energy. If the heat cannot be transferred from the materials to the atmosphere efficiently than the heat build up renders the brakes ineffective. In other words, brake fade.

Brake fade almost always is made obvious by a large increase in effort at the lever.

terrymorse 

"Brake fade" generally means a reduction in braking performance, which can be different things.

Friction fade: If a brake pad gets hotter than its rated max. temperature, the pad can begin to break down, and the friction between the pad and the braking surface will decrease. The cyclist will need to squeeze the brake lever harder to achieve the same stopping force.

Fluid fade: With hydraulic brakes, brake fluid may get hot enough to boil, making the brake feel "spongy" as you squeeze the brake lever. The gas in the piston is the equivalent of a spring that has to be compressed, so the brake lever needs to moved farther to achieve the same stopping force.

CliffordK

I did support for Cycle Oregon a couple of years ago pre-COVID.

I talked to some of the mechanics after the crossover from Steamboat, OR to Culp Creek, OR. Pretty intense pass with several switchbacks.

I think they did have a couple of carbon rim failures due to excessive heat.

As far as tubeless, my tests were cut short when I hit a rock bad enough that it ripped out a chunk of the sidewall big enough to create a leak that leaked down in a few feet, and did not seal even with waiting for a half hour or so with the hole down.

It may have just been bad luck that might have damaged other tires with tubes too. However, it isn't uncommon to encounter a few stones on corners of curvy roads.

Tubeless doesn't mean invulnerable.