oldacura

The discussions on this forum about brakes overheating causes me to wonder: Which system is fundamentally better at dissipating heat: rim brakes or disc?

A thought experiment: A tandem team descending a steep mountain road with tight switchbacks. The switchbacks are tight enough to cause the team to repeatedly have to slow way down to make the turn. This situation is severe enough to cause at least one of the brake systems to experience some failure.

They have two identical bikes - except that one has the "best" caliper rim brakes and the other has the "best" disc brakes. They descend this road on each bike under exactly the same
conditions.

Which system is likely to fail?

Brakes basically turn kinetic energy into heat and dissipate the heat to the air. A disc is steel and can get much hotter before it fails. Heat transfer occurs faster at a higher temperature differential. A rim is aluminum, much larger and likely moving at a higher rate of speed through the air. If the rim gets too hot, the tire is likely to blow off.

Under the most taxing conditions, which works better?

CliffordK

I haven't been using disc brakes, but I've heard that they can get really hot, but tolerate heat much better than rim brakes (and tires).

Some older tandems also came with drum brakes, often used as "drag brakes".

If one could get it all adjusted right, then one might be able to tie pairs of brake levers together, and have both disc and rim brakes, perhaps using discs for primary stopping and dragging, and rim brakes as a backup.

FBinNY 

The general rule in braking is that the size of the wiped area is what matters most. So we can disregard the size of the shoes and focus on the difference in the rim and disc, where the heat will end up.

OTOH rims have far more ability to uptake the heat energy, by virtue of the the material, surface area, and volume. They also have more ability to transfer that heat back to the surrounding air. But here's where it get complicated.

Rims are limited in the maximum temps because they are in contact with rubber tires, whereas discs can operate at far higher temps. So you'd need to compare the energy needed to heat the rim to X, to what's needed to heat the disc to the far higher temp. Then because higher temps increase the rate of cooling, that has to also be factored.

Tossing all that into a hat, and comparing based on some experience with brake heating on hills, I suspect, but can't confirm that rim brakes will fare better. In any case, I suspect that both systems will be inadequate over a long descent on a tandem, so riders will have to either accept riding closer to terminal velocity or, as truck drivers do, stop and cool their brakes from time to time.

etasch

If you make a disk brake large enough, machine most the metal from the inside you can mount a tire… FWIW we have bikes that have both types of brakes. And the Velocity Chucker rims on the triplet can take a lot of heat.

zonatandem

Have been riding 'in tandem' since 1975 and pedaled over 250,000 (quarter million) miles as a duo.
Have used all sorts of brakes; cantilevers, center pulls. side pulls, rear U-break and even drum and disc brakes (front and rear).
All have their advantages, all let you stop . . . . some better than others.
Our personal preference for the past 40,000+ miles is Dura Ace sidepull front and and mini V-brake on the rear.
We are a rather light duo (sub 250 lbs total).
Have descended Kitt Peak in AZ (a 6%+ grade) and 11 miles long with lots of switchbacks in the late 1970s with Mafac cantis front and rear and NO 3rd brake. Worked great!
Did have to make one stop as pilot's fingers were cramping up from continuous front/rear braking lever use. Made a 5 minute stop and felt both rims. They were NOT hot, just warm. Continued our descent safely.
Have descended more steep/long hills (Example: Yarnell hill between Prescott and Congress AZ and topped off at 50 mph near the end) and in many other states, using many other rim brakes.
Test rode disc brakes on a custom tandem when discs first appeared and felt that they were overkill.
Stories of tires blown off overheated rims and plastic disc parts melting are legion.
Have also used the old drum brake setup and felt that for us, it was not necessary.
Just our experience and input. Your needs/wants may very from ours. Do what is good and comfortable for your team.
Pedal on TWOgether!
Rudy and Kay/zonatandem

oldacura

I think that all of us have experience with various forms of brakes on a tandem. Most of us have experience with only one type of brake. My question had more to do with an objective comparison between rim and disc brakes under the most demanding conditions. I doubt that such a comparison exists. Short of that, the best comparison might come from a bike brake engineer who truly understands the trade-offs between the two. I'm sure that the heat transfer dynamics are extremely complex.

However, because rim and disc brakes are so different, I would bet that one of them would be the clear winner under such conditions.

There is almost no doubt that disc brakes perform better in wet conditions but we almost never ride in the wet so that is not a concern for us.

OneIsAllYouNeed

I'd pick disc brakes for my next switchbacked descent on a loaded tandem. They fade before anything catastrophic happens. When you feel the fade, come to a complete stop and let 'em cool.

That said, I've only stopped to let rim brakes cool on the tandem once in 8 years of riding it. We were on a dirt mountain pass with 4 full panniers. I think the GVW was 400lb. The descent was Gibbons Pass, heading north into Sula, MT -- 2000ft over 7 miles with a whole bunch of switchbacks. I wasn't worried about a blowout, but the brakes were fading a little bit before we pulled over. We had cantilever brakes and well-worn Bontrager Clyde rims.

A few years ago, we got a new fork and "upgraded" to a BB7 road with 203mm disc in the front. The rear is still canti with a Dyad rim. I haven't felt it fade yet. None of the descents here in New England are long enough to justify brakes with more heat capacity.

If you're stuck with rim brakes and want something that can handle the gnarliest descents, pick a heavy rim, such as the Velocity Chukker.

bwebel

I think your last line is significant in that different teams have different demands, so I don't know that your search for the objective "best" brake can be answered. For example, another thing to consider might be the difference in failure modes. I've never ridden either type of brake to failure, but my understanding is that disks will basically fade from the heat, while overheated rims will cause the tires to blow off. These are very different failure modes. So if it turns out that deep dish rims could take more heat than disks, but that the failure mode is more sudden and catastrophic, which is "better" will be a personal judgment.

oldacura

We've climbed & descended most every paved mountain pass in the Colorado Rockies. Also climbed & descended Mt Evans & Mt Haleakala on Maui - all with rim brakes. Never had an issue. On a couple of occasions I became concerned that the rims might overheat (steep, winding road, hot day) and stopped to check the rims. They were hot but I don't think we were close to a blowout. While this is just our experience, there is no way to compare how this might have gone with disc brakes. I don't know if any of the teams out there have a more apples-to-apples comparison.

Leisesturm

The thing with rim brakes is, even if they could ultimately take more heat input than a disc... the way you find out that you have exceeded your tires limit of heat input is when it fails. I personally would rather the warning of impending brake failure caused by spongy braking of disc brakes. I also doubt that there is anything on the rim brake side that can compare to Ice Tech type rotors in 203mm gripped by hydraulic calipers. Seriously doubt that there is a tire made that can cope with the kind of heat input that the disc brake setup I just mentioned can.

mtseymour

In dry weather, rim brakes or disc brakes will not fail if used with proper technique and in good working order. I've used cantis, sidepull, and V-brakes on fairly demanding descents without any failures. It's just a good idea to check for rim/pad wear and excessive heat.

However, I have no doubt that hydraulic brakes work better in the most taxing conditions. Everything else being equal, a tandem with hydraulic disc brakes should descend the same route much faster with more reliability. With disc brakes, it's easy to make multiple steep descents (several miles at 6% or steeper) without worrying about brake fade or worn pads. The same team will be slower and more cautious when descending with rim brakes. No one has bothered to test this objectively because there is so much downhill or enduro mtn bike results to show that disc brakes offer more power, better modulation, and better reliability (rain or shine). It's no surprise that disc brakes are also the preferred choice for motorcycles and cars.

Ludkeh

I have disc brakes front and rear on my DaVinci Design tandem. Our old tandem was a Comotion Java with rim brakes. Here in Central New York we encounter some very steep hills. With my old bike I was always worrying about stopping ability after continued use. With my DaVinci and disc brakes I have complete confidence in it's braking ability. No matter how much I use them, they continue to stop us with strong braking and no fade. I don't have to pull hard on the brake levers and the stopping power is easily modulated. Highly recommend disc brakes. You won't be sorry.
.

oldacura

Sounds like many here have moved from rim brakes to disc and like the discs. However, no one can answer my original question with either objective evidence or well researched analysis. Also, many have tales of overheating disc brakes. I'm not aware of as many tales of rim brake failures. We have used only rim brakes with no failures. I know that this is not evidence of their superiority.

tkramer

There's no objective answer because there are too many factors to consider.

I ride three tandems: a Ventana MTB with two pot Magura hydraulics, a Santana with V-brakes, and a Paketa with the Shimano road hydraulics. Do I miss the disc brakes on the Santana when I ride it? Absolutely

... not. In fact I'd say I'm more carefree with the older technology. I can carry extra cables to backup if there's a failure. They are quiet (no "schwing-schwing-schwing" after heavy braking), I don't have to worry about bending rotors if I need to get the wheels off and on in a hurry or on the side of the road. The pads last almost at least 2-3X longer, are cheaper and available anywhere. They're cleaner - they don't leave grimy brake dust all over my fork and chainstay. With 32mm tires inflated to 85-88 PSI there is plenty of thermal expansion buffering in the air chamber to not worry about blow outs.

The Shimano hydraulics are fiddly and can be a pain to keep up and maintain (Bleeding on initial setup. You have to pull the pads to check wear. Oh, did I mention the dust?).

The Maguras have been worry free, but they're ~2004 vintage (before weight savings were a factor leading to compromised design) and they don't get a fraction of use as the Shimanos.

Yes, the Shimanos work and feel great and require less pressure at the lever. I've ridden them in the wet including two downpours. But, I didn't ride the bike any faster or less cautiously than I would have with rim brakes. So I honestly don't think they gave me any advantage whatsoever over rim brakes. Mud clearance, grit and other extremes of off road riding are a better argument than wet performance.

They're just two slightly different modes for doing the exact same thing. One is only "better" than the other insofar as it meets your real-world needs, desires and expectations of performance, reliability, aesthetics, maintenance, cost, weight, aerodynamics and on and on.

Any "objective" heat data would have to come out of a laboratory environment, (i.e., not applicable to real-world use, since so few of us ride our bikes in a laboratory). Some acceptable level or benchmark of heat capacity for all components involved would have to be established which of course is much easier to do with the disc brake systems. You can be sure that Shimano, SRAM, Magura, Et al. have done that with their products. But what is the baseline for rim brakes? Again, just lab coat data collection - and to what end when you finally compare the two?

Carbonfiberboy 

There's an old thread about descending Ventoux that's quite interesting:
https://www.bikeforums.net/tandem-cyc...t-ventoux.html

I recall only one mention of a rim-braked tandem w/o drum. It had no problems, while all the discs had troubles. Older tech, but maybe something the OP might want to peruse. When we tour, we put on our spare rear with the Arai.

act0fgod

Just upgraded to a tandem with dual disc and sold the arai on ebay. I guess I bought into the hype of the discs dissipating heat and being superior to rim brakes. Guess I should have considered keeping the arai.

Paul J

We've used rim brakes exclusively for years and were a little skeptical of disc brakes especially with some of the stories heard here. We purchased our Co-Motion Speedster this past spring which has the TRP Spyres. We are nearing 2,000 miles on this bike and have had really good luck with the disc, no squeal that seems to be a problem with some of the other cable actuated discs. We live in a fairly hilly area of Pennsylvania and have found them to be a little stronger then the rim brakes. We are on the OEM brake pads which we've adjusted in once since new to dial-out some of the play in the levers. We are a heavier team, a little over 400 and I'm now a fan. I do wonder if we could get even better braking with the installation of compression-less cable housing? I'm not sure how the OEM housing compares to after market compression-less housing. Our other bike has the rim brakes with 2 sets of wheels one with an Arai drag and the other without. We've not had the drag brake wheelset on for a couple of years now though this is the bike I'm planning to convert to Trekking gearing and maybe bar-end shifting for future loaded touring. I do like the comfort of the drag brake with a loaded touring bike, but I'm still a little old school.

Bostic

Do you recall how the descent on Haleakala was from the lower switchbacks (6000' to 3600')? That's the section of the marathon descent that would have me concerned the most regarding rim brakes on a tandem. Looking over my ride from 2012 I was averaging right about 30mph with a max of 39mph on the rental Trek I was using.

fietsbob

Heat Sink .. if you have discs get the ones with an aluminum spider, the more aluminum the better..

It will draw heat from the stainless steel disc, to keep it, relatively, cooler..

merlinextraligh

I think the issue of how fast you descend cuts the exact opposite. Both rim brakes and disc brakes have plenty of stopping power.

The issue is heat, and if you descend fast, you use less brake, with more time to cool the brakes between applications. So rim brakes don't raise a tire blowoff issue for a team that descends aggressively.

Conversely, if descend cautiously regularly modulating speed, then you may want disc brakes to avoid rim heating issues.

oldacura

I don't recall the particulars of the descent. I (we) were very tired from the ascent and I wasn't paying a lot of attention to anything but getting down safely. I do remember that it was pretty cold near the top and I wasn't thinking about overheating the rims. I do remember that it took 9 hours to ascend (7 hours of wheels turning) and 2 hours to descend. Everything hurt when we finally got back to Paia.

oldacura

Thanks - tkramer, paulj, carbonfiberboy & merlinextrligh for your perspectives. I think this is what I was looking for.

For most of us we either accept what we have (as we did on our first bike) or make a choice to go rim -vs- disc (as we did on our new bike) and learn to live with the compromises.

My guess is that if Shimano or any of the other companies migrating to discs ever did an objective test or calculation to determine the heat capacity of discs -vs- rims and it came out that rims had a higher heat capacity, they would likely bury the results because the company had already decided to migrate to discs (as long as the heat capacity of discs was adequate).

Darth_Firebolt

not really. the interface between the stainless rotor and aluminum spider isn't large enough to transfer the kind of heat you're talking about. that's why ice tech freeza rotors have the fins directly attached to the stainless disc, instead of to the spiders.

mtseymour

Since you asked about a comparison of the "'best' caliper rim brakes" vs the "'best' disc brakes" under the "most taxing conditions", let's look at specific options.

For rim brakes, your best options are the Santana long-arm V-brakes (or Shimano Dura Ace BR-9000 dual pivot brakes). You may also throw in a boutique rim brake like the THM Fibula.

Based on my personal experience, this is my ranking of tandem disc brakes:

1. Shimano R785 11sp shifters (mechanical or Di2 version) with the Shimano Saint 4-piston hydraulic calipers (w 180mm or 203mm rotors). The 2017 Dura Ace ST-R9170 shifters would be even better.
2. Shimano R785 11sp shifters (mechanical or Di2 version) with R785 2-piston calipers. The R785 caliper is essentially an older XT design.
3. Shimano/SRAM/Campy mechanical shifters with TRP Hy/Rd 2-piston caliper. The Hy/Rd uses a cable to actuate the hydraulic caliper.
4. Shimano/SRAM/Campy mechanical shifters with Bengal M700T or Avid BB7 single-piston mechanical calipers..

Since disc brakes have been widely used on mtn bikes since the 1980s, there are thousands of results from sanctioned races and timed events. Everyone from World Cup racers to weekend warriors choose disc brakes because they offer more power, modulation, and reliability in all conditions. Shimano has discontinued XTR/XT v-brakes because of low demand, and only makes v-brakes at the Alivio/Acera level. SRAM/Avid also limit their v-brakes to the entry market.

There is no conspiracy because Shimano and SRAM continues to offer both types of brakes. The competitive reality is that no racer or serious rider would choose an Alivio v-brake over a XTR disc brake on a grueling downhill when money, ranking points, or pride are at stake.

Before you say "it's different for road bikes", read this article about disc vs rim brakes on a 17km descent of Mount Etna. I think it's fairly representative of most disc brake reviews:

https://roadcyclinguk.com/gear/shima...UwVv6o8S3LY.97

Still not convinced? Why don't you do your own road test. Do several big descents on two tandems, one with rim brakes and the other with hydraulic discs. Borrow a 2nd tandem if you have to and do 3-5 shuttle runs in quick succession. Bike fit is not important because you're not doing much pedaling. Use Garmin or Strava to measure time & distance. Find out which brake is faster and more reliable for your toughest descents.

A rim brake is fine for most century rides and tandem rallies, but we would go with dual hydraulic discs for loaded touring or epic descents (eg. Mt Ventoux).

oldacura

Thanks for the link. I've never raced and we don't attempt to beat other riders in a descent. This article focuses on the modulation of disc brakes and describes rim brakes as all or nothing. That has not been my experience with rim brakes. Most of them have seemed to me to be able to modulate - but maybe not as well as a disc. I've never thought of rim brakes as binary. The disc brakes on my mountain bike work quite well. I use them much more than I do on a road bike. However, they do tend to make noise after repeated application.

I do appreciate all of the thoughts on this. However, all of this does not answer my original query: If you dump an equivalent amount of energy into a set of disc brakes and a set of rim brakes (until at least one begins to fail), which one fails first and what is the nature of that failure?

I was just wondering out loud. I am too lazy to set up and run this experiment myself.