Good thread with some balanced feedback.

A couple points to add:

1. Shimano knows that the ICE Tech spiders do not transfer heat away from the rotor surface (at least not well enough), which is why the 2017 Dura Ace rotors have been redesigned for road use and have an integrated spider designed to pull heat instead of insulate against it. They finally figured out that road cycling with disc brakes is different than mtn bikes. yippee.

2. Rim material is a huge factor in deciding on brake type. If you want to run carbon rims on a tandem, then don't run rim brakes. Carbon rims are too easy to melt, which at least one member in this list has done.

Oldacura,

I certainly don't have the objective comparison that you have discussed, but I do have a couple of datapoints that are noteworthy (in my opinion).

We ride both mountain and road tandems. Our first two road tandems were equipped with rear disc (Avid BB7 203 rotor). All of our mtb tandems have been hydraulic disc 203 rotors (front and rear).

I've NEVER had brake failure on our mtb tandems. I've turned the rotors blue but yet to see complete fade or failure. We've done some long fast descents. That said, the speed that we reach on the mtb is not as great as the road tandem.

We did have rear brake failure on our road tandem on Mt. Ventoux (TRP HY/RD with Shimano 200mm Icetech rotor). Cooked the rotor.

These are very difficult to compare, but I do believe that the amount of speed (long straight-aways into a tight corners) can be the biggest differentiation. I frankly won't let myself get up to the same speed levels on on dirt as I do on pavement. So that could be the biggest difference.

Maybe having two large (200mm) rotors is the differentiation?
Maybe I just brake differently on the mtb tandem?
Maybe we had a fluke experience with the Shimano Ice Tech Rotor?
Probably I don't get up to the same speed AND then HARD braking into a corner, on the mtb.

Again, too many variables to really draw conclusions.

At the end of the day, we are riding rim brakes on our road tandem. I utilize a deep section rim (NOT carbon - which I believe to be better for dissipating heat). I have toyed with the idea of going full disc on the road tandem, but have yet to go that route.

I'm pretty sure you are not going to get the objective test that you are looking for...

Yes - I can see that one would automatically select disc brakes if 1) frequent rides in wet weather and/or 2) wanting carbon rims.

Colotandem - you bring up another valid point regarding on road -vs- off road riding: If a team is comfortable descending a road at 40+ mph, much of the kinetic energy can be dissipated by wind drag - especially sitting up and maximizing drag. This is almost never the case off-road.

So set up the calculation, with a few assumptions, it is practical to get within a few percent of reality.

Need to know:
* mass of the aluminum rim; then can look up heat capacity.
* determine maximum increase in temperature allowed for the aluminum rim - assume tire increase is the same, use PVT equation with allowed pressure increase to determine max temp increase.
* estimate the surface area of the rim exposed to airflow.
* mass of the stainless rotor; then can look up heat capacity.
* determine maximum increase in temperature allowed for the rotor.
* estimate the surface area of the rim exposed to airflow.

Use something like this to calculate heat transfer: Convective Heat Transfer

Two major factors are missing from this discussion - team weight and tandem use. Ever wonder why an SUV has bigger brakes then a Miata? Or why a car with a much higher top speed or is used for more speed oriented events needs bigger brakes with better cooling?

Also - teams than can easily descend a technical descent at 50mph vs the same team that isn't comfortable going down that same descent higher than 30mph will have different braking needs.

We're using ENVE Classic rims, and caliper brakes, and haven't had an issue.


Admittedly most of our riding is relatively flat, but we have taken it on the Blue Ridge Parkway and descents around Asheville.


We did set the bike up to take a rear disc for the reason you state. However, we haven't found a need for it yet.

That is why my hypothetical situation was setup to remove those variables.

I started to think about nfmisso's suggestion on my lunchtime ride. I did look up typical weights of rims (approx 900 gm) and discs (approx 116 gm). But then I realized that this is a very non-linear calculation that would likely require calculus (haven't done an integral since college), some assumptions about the average spinning speed, assumptions about the average forward speed, and the time-dependent temperature profile of the rim & disc. I gave up for now.

On the other hand, Ritterview (Will) rides a lot of steep hills in CA and has melted a ENVE rim using a front rim brake. It may have been excessive conditions, but does underscore the issue is there nonetheless.

FWIW, we've been riding in and around Calistoga, CA this week. Just finished a semi-wet after work road ride with a fast descent after close to 4 miles climbing. Hit just over 50 mph in the diminishing light and slightly dicey conditions on unknown roads. The Shimano hydraulic brakes sure like to make noise after some hard braking until they cool.

This is interesting.
A couple additional points:
The rim has more metal mass and more surface area. It will dissipate heat better.
The melting point of rubber is ~350F <-- the tire will fail before this.

For disc brake failure points:
The boiling point of DOT5 is ~500F (if not degraded) <-- for hydraulic disc
The melting point of aluminum is ~1200F. <-- but will become soft and fail before this.
I'm not sure what temperature the pads glaze over.

So the rim is better at dissipating heat than a disc, but the rim's temperature failing point is lower than disc.

[QUOTE=nfmisso;19122691]So set up the calculation, with a few assumptions, it is practical to get within a few percent of reality.

Need to know:

* determine maximum increase in temperature allowed for the aluminum rim - assume tire increase is the same, use PVT equation with allowed pressure increase to determine max temp increase.

/QUOTE]

You are assuming here that it is purely increased pressure that blows tires. But I've also seen it suggested that tires will fail from heat when the tire slides on the rim, causing the valve stem to sheer off. I don't know if this is really what happens, but given the margin that is built into the "max pressure" ratings on many tires, it does seem plausible that something else besides purely high pressure is going on. More than doubling the pressure in a tire purely due to heat takes a lot of heat when you do the calculations.

[QUOTE=bwebel;19125906]Never heard of a valve stem shearing off. I think the secondary cause you are looking for is that the rubber at the bead, the point of contact with the rim, softens.

One of the issues that keeps coming up about rim brakes vs. disc brakes is the concern about using rim brakes and building up enough heat to cause the tire to explode and blow off the rim. Under normal conditions, non-loaded touring, team weight under 350 has anyone actually had a tire blow off the rim from excessive heat?

You asked if anyone has had a tire blow off a rim. My brother has had a flat with rim brakes on a mountain bike. I don't know if the tire blew off the rim, but it flatted instantly (rear) due to the heat. This was at the end of a 7000 ft downhill on Barr Trail (Pikes Peak). This incident did not cause any injuries (due to a crash). He weighs ~140lbs.


My experience with disc brakes is that they have the same issue with heat (resulting in brake fade and/or damage). They need to be monitored so they don't get too hot. Either way, I'm stopping so that my brakes can cool.

Yes. 286 lb. team. 4 times, all with Aerohead rims, a light pretty much box section rim. Once with 23mm Tricomp tires, twice with 25mm Conti 4000s, the ones with the reflective sidewall.

Two times were after a very steep winding descent where there is no opportunity to use speed for wind-checking. Both times, the rear tire blew off after we were off the descent, TG. Could have been bad. Once with the Tricomp, once with the Conti, i.e. different tire didn't fix the problem.

Twice when braking very hard for lights at the end of different 50 mph descents, running the Contis.

Looking at the reflective Conti tires, it seemed to me that the reflective fabric went under the rim bead and was not as sticky as plain rubber would have been.

Looking at the rims, it seemed to me that there wasn't enough area for heat dissipation.

I threw the tires away and changed to Deep V rims and non-reflective tires. Never had another problem, though we've not descended the steep winding road again. We've descended the other two roads and all the major passes in Washington State with no problem with various other tires and either Deep V or Kinlin XC279 rims.

When we tour, we put on the spare wheel with the Arai drum.

[QUOTE=Carbonfiberboy;19126131]Yes, the recollection I have is of someone indicating that the softening of the rubber caused the tire to slide on the rim, and the tube stayed stuck to the tire, which caused the valve stem to sheer as the tube moved. I've had a blowout like this myself, but it wasn't from heat, it was from something jamming between the tire and the fork. In any case, I do think there is likely more than just increased pressure caused from heat buildup that is causing a heat related blowout.

I think that bwebel is slightly mis-remembering something I posted.

I was the race pilot for a heavy team+tandem (220kg). We had a heat related incident on the way to the start of a race that is particularly unfriendly toward tandems because it finishes with a steep 9km climb
https://www.strava.com/segments/628376
The descent is not used for racing, because it is deemed to be 'too dangerous' because it is both steep and technical, so the race starts at the bottom of the hill and eventually finishes at the top, which is where most people park.

The race wheels on that bike are fitted with tubulars, and on that day we were running with rim brakes. One of the characteristics of tubular glue is that it is temperature sensitive and when it gets sufficiently hot it melts and loses all adhesion, and we experienced just that 3/4 of the way down the hill resulting in the valve shearing incident. We were fortunate that the we weren't cornering right at that moment, so the tyre stayed on the rim and we managed a controlled stop.

2 years later we did the same descent on the same bike fitted with disc brakes and managed the descent without incident.

Ultimately this form of failure is pretty rare since it is unusual for tandems to have tubulars, however there are similar stories from single bikes with tubulars and alpine passes.

I have heard that clinchers blow off rims more readily when the rims are hot, as evidenced by the observation that tyres can be pumped to much higher pressures in the workshop than could be achieved as a result of increased pressure as the result of temperature alone. The speculation has been along the lines that the heat made the rubber at the bead more slippery.

I have no idea what temperatures are required to either melt tubular glue or to cause a blow-off.

Cheers,

Cameron

Thanks for tales of your experiences. I learned several things:

1) For a team to have a blow-out once could be due to the captain not being aware of the heating issue. For a team to have several incidents gives credence to the limits of rim brakes.

2) I didn't know that one could (or should) use rim brakes with carbon rims. Seems like wear could be a concern. Also, I don't know how well carbon rims can absorb & dissipate heat. Also, since carbon fiber is a composite of carbon fabric & resin, I assume that when a carbon rim melts, it is the resin that softens (correct?).

I currently riding on the Giant SLR1 carbon wheels on my TCR single. They brake well with the special carbon pads on my Dura Ace BR-9000 brakes (and are decent in the wet too). Tthe SRL1 and SLR0 carbon wheels are supposed to have a Tg (Glass Transition Temperature) rating of 245-degress Celsius (higher than the industry standard of 160). This means that the resin will weaken at this temperature and potentially allow a rim blowout As a point of reference, 100 deg C is the boiling point for water. Giant claims that in a series of braking tests, both in the wet and dry, their wheels are better than similar wheels from Reynolds, Bontrager and Zipp in heat dissipation.

Nevertheless, I would never use carbon wheels on a tandem with rim brakes. It's far better to use alum wheels with rim brakes or carbon wheels with disc brakes.

After a full season of riding these Shimano rs785 discs I finally figured out the problem with the front caliper... sticky pistons, especially on one side. During the Calistoga rides, I thought we had delaminated the 180mm Freeza rotor, but that was not the issue. The same piston problem has been plaguing us for a year, but at this point it was just about seized.

We're now riding in the Los Gatos and Morgan Hill areas with plenty of coastal steep pitches to test the limits of tandem brakes. Our front caliper was dragging so badly the noise was getting quite embarrassing and the drag noticeable. The one side is a bit stubborn to work in and out, so I suspect it will resume behaving badly soon. I'm not sure if my maintenance routine (clean & lube with mineral oil) on the pistons will permanently solve the issue, so I went ahead and ordered a replacement caliper. On the plus side, no other brake related problems. Anyway, the carbon rims + discs survived another extreme test (Summit Rd amazingly narrow - like 12ft? - twisty and undulating... reminiscent of Mallorca hills and narrow roads) and the Hwy 9 descent with a top speed of > 56mph.

It also gives credence to the importance of talking to other teams about rim and tire choices.

This has been my experience also. Especially with the outboard piston - and on just the front wheel. It is most problematic when the pads are worn down past half their lifespan. You could re-position the caliper gradually, but then run the risk of the rotor rubbing the caliper body as it heats and warps closer to the margins of the gap ("schwing-schwing" after hard braking). I even replaced my front caliper with a new one after I could no longer push the piston back in without crunchy sounds/sensations emanating from the caliper body. I have found that replacing pads sooner than the official spec and cleaning the calipers more frequently keeps things running better.

My cleaning steps:

1. Remove the wheel and pads.
2. Use a lot of isopropynol with cotton swabs and Kim-wipes (fancy low-dust tissues) to wash the dust off the caliper and out of the rotor gap.
3. Keep going until you see no more dust residue.
4. Let dry, then daub a bead of brake fluid around the pistons, wetting them by capillary action.
5. Finally, push the pistons back home. (I'm a tool geek, so I use the Park PP-1.2 press).
6. Inspect the pads and rotor and reassemble. Don't adjust the caliper position unless needed.

Mechanical calipers obviously don't have this problem, but have their issues as well.

We live in Switzerland and ride our tandem in the Alps a lot and have taken it to several other mountainous locations. We've nearly always had 3 brakes on the bike, a mix of disc brakes and rim brakes, although the exact models and combinations have evolved over 8 years. I'm happy descending pretty fast and use the best braking techniques I can, alternating between the three brakes as much as possible. However, we've done many extreme descents in the past, some averaging 10% for extended periods on narrow European roads. All descents that we've done in North America were a piece of cake compared to some of these due to the lesser gradients and wider roads over there, but I'm sure that we haven't tried the most extreme ones.

We've twice had inner tubes explode on the tandem due to overheating rims from excessive braking on descents; I don't believe the tire popped off of the rim on either occasion, it instead appeared that the inner tube material itself just failed due to heat before there was enough air pressure to separate the tire from the rim.

I've never overheated a tube on a single bike, but have been on rides with other people on single bikes and twice seen tubes fail due to rim heat on descents. I believe both were on steep descents in heavy fog with a reasonably heavy rider - a bad combination. I've also seen tubular tires roll off of friends' bikes due to the glue melting (Beloki's crash in the 2003 TDF was apparently caused by the same thing), and I've seen tubular tires shifting around the rim after the glue melted until the tire bulge up where the valve stopped it from going further. Fortunately, none of these resulted in serious injuries (except for Beloki).

We've also overheated a disc brake on the tandem to the point where it was barely braking and was smoking/steaming and smelling bad once we finally got the bike stopped during a very wet descent with full touring gear in New Zealand. On other occasions, a disc brake has started to fade but not too significantly, and the discs have been discolored and slightly warped at times (we've always used discs of between 200 and 220 mm diameter). We've only recently tried a disc brake with hydraulic fluid (we'd previous only used cable-actuated discs), and haven't yet managed to boil the fluid, but I've read reports that it is possible.

So, I've experienced nearly all types of brake overheating failures, most of which were on our tandem. I cannot say exactly which is more likely to happen given a certain standard degree of braking force, but I do know that a disc brake failing during a descent is less catastrophic than a tube failing, especially if you have one or two other brakes still working.

Thanks - your accounts seem to be the closest thing to the comparison I was looking for: failures of both systems with different failure modes.

After cleaning and lubing both front and rear calipers, this maintenance routine seems to have done the trick. It's the first time since we started using the RS785 brakes that both calipers are now fully retracting during repeated high load braking, AND NO DISC RUB NOISE AFTERWARD! Yippee! My one remaining noise gripe is with the pad rattle on the front, and I see no solution to it.