Tom, check this out:

http://cycle.shimano-eu.com/publish/content/global_cycle/en/nl/index/news_and_info/Velotech_Disc_Brake_Test.html


Lots of technical links to test results.


The links show that the discs had been tested up to 1050W without failure by the Germans (for shimano) before. So the question to me then is - how much load does it take to overheat them?

Yeah, I've seen that, but there must be some kind of disconnect between that testing and worst case "real world".

Quick and dirty calculation...if I'm traveling at 35mph (16m/s...typical speeds for a local 8% grade) and I need to come to a stop in ~6s (sounds reasonable for a panic stop) that would require a dissipation of >1800W by my quick figgerin'...and I'm not a huge guy (~85kg bike+rider mass).

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http://bikeblather.blogspot.com/

The reports themselves don't match your calculation - nor do they seem to match their own limitations. The maximum sustainable deceleration rate in the reports is 6 m/s2 whereas your panic stop is 2.6 - so the DIN test knows that it is possible to load the brakes to that level. Their thermal testing was 1050W for 90 seconds (repeated), but if the discs can be subjected to loads that are over twice that high in panic stops, why not test there? Also for circumstances like long descents and pros, the speeds might be close to double what you used.

Maybe the DIN test is faulty, in that it was designed more around what rim brakes could do in the good old days? I think that good, current rim caliper designs can easily get to loss-of-control braking power (just as discs can), so why not test at the threshold?

just guessing wildly here as it's early and I need more coffee, but the IceTech might be the first circumstance where the overload failure that is likely occurring all the time to folks with steel discs (fading) has more dire consequences - pointing back to faulty test protocol? Perhaps the energy threshold for the DIN test needs to be moved?

However if that does happen, it's likely that discs will get heavier and larger...

Yes, but..

Carbon tubular rims have crappy braking surfaces, and caliper loads aren't well supported. In theory they could be made even lighter way out there at the rim. If a disc was made large enough to absorb the 1.0 kW load for sustained braking without fading, it could be an improved option for mountain stages. The rims could be really light.

Also, I believe that carbon rims will wear at a higher rate than aluminum rims in dirty conditions. I have worn through rims on my mountain bike (before discs) and replacements are expensive. That would be even more expensive if the rims were carbon.

I've ridden my road bike down 5-6 mile long dry descents, and the rim brakes faded (started vibrating). I also stopped a few times to let the rims cool down for fear of blowing off a tire. I also rode a few long, very curvy descents in the rain, and they were truly frightening - the wet brake squeeze / caliper chatter / changing friction as the rim dried produced very unpredictable braking. I've ridden my mountain bike (plain steel discs) down similarly long descents and had no fade, but did have massive squealing. If disc brakes can be properly designed for road bikes I do think they are a better solution, especially in crappy weather.

I just think the testing protocol is wrong. Not sure why, but somehow there seems to be the idea that road bikes would need smaller discs than downhill mountain bikers. I think the opposite is true.

Which of course gets back to the original point: they're not likely to be very aerodynamic. I guess at zero yaw the disc would almost disappear, but not after it started to change.

Which all just further illustrates the folly of using carbon braking surfaces due to the misplaced desire for lighter rims.

I find it quite amusing that most people's first reaction to crappy braking with all-carbon wheels is to think about adding a braking disc...thereby adding aero drag and mass (~1lb) when they very simply could get dramatically better braking by just using rims with aluminum braking surfaces for typically less of a mass increase and no worse aero drag than the carbon wheels.

I also find it amusing that the main reason for switching (a LONG time ago) from wooden (a natural "composite") rims to aluminum rims on road bikes was the dramatically better braking, especially in the wet. I guess that lesson was long forgotten when the idea was made to make rim braking surfaces out of manufactured composites :-/

I ran across this article last week and thought it was highly appropriate: http://santanatandems.com/...standingBraking.html

"What’s missing is the simple realization that a bicycle’s rim brakes are, in fact, disc brakes. Rim brakes have always been disc brakes. When cars and motorcycles were fitted with disc brakes, they caught up to the braking efficiency bicyclists had known for a half-century."

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http://bikeblather.blogspot.com/

Thanks Chris! ;-)

Found this on facebook, thought you might like it. Erik Buell has been working for a while on moving the front brake on his racing bikes to the rim, and here's some simplified diagrams explaining the move.

https://www.facebook.com/ErikBuellRacing/photos/a.417900267495.190529.197762537495/10152646531292496/?type=1&theater


In the motorcycle industry, this brake is picked on regularly for not being quite a match for the standard double front disc - but on the other hand his front wheels are very, very light.

That's awesome...and, I bet if high performance motorcycles ran high aspect ratio tires (i.e. narrow width compared to the diameter) like road bicycles, he'd try to incorporate the braking surface into the rim itself and move the caliper to straddle the tire :-) Thanks for that!

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http://bikeblather.blogspot.com/

Funny, I ran into the same article in post 223 on the previous page on this thread.

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It doesn't really matter what Phil is saying, the music of his voice is the appropriate soundtrack for a bicycle race. HTupolev