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Video: Disc v rim brakes: which is more aero?Specialized Win Tunnel Disc rim brakes

Video: Disc v rim brakes: which is more aero?

There’s only one way to find out: FIGHT!
by Mat Brett
Thu, Dec 11, 2014 10:06
33

Disc brakes and rim brakes offer similar levels of aerodynamic performance in a head-on wind but rim brakes are slightly more aerodynamically efficient in a crosswind, according to research conducted by Specialized.

Researchers from Specialized trialled two Tarmac road bikes in their wind tunnel in Morgan Hill, California, one with rim brakes and one with disc brakes.

The differences between the two bikes were the brakes themselves, the hoods, and the rims. The testing was conducted with the same rider aboard each of the bikes.

“At zero degrees, or a head-on wind, there was actually no difference between the two setups,” says Chris Yu from Specialized’s Aero R&D.

“When we introduced [a] 10° yaw angle to simulate a crosswind to expose those disc brakes, we started to see a little bit of a difference. Over our standard 40km distance we saw that the disc brakes were about 8secs slower than the rim brakes.”

Those were the only angles that Specialized tested: head on and 10°, so you couldn’t say that this is the most comprehensive research ever conducted.

According to Specialized, the aerodynamics doesn’t give the whole picture.

“If you’re doing any rides with a lot of descending or tight corners, that little bit of extra confidence that the disc brakes will provide may wipe out those 8secs over the 40km. So it’s really the whole package and not just the aero that you have to look at.”

We’ve still not seen any research into the relative descending speeds of riders with disc brakes and riders with rim brakes. Most people probably suspect that discs will save you time on a descent by allowing you to brake later, but we’ve not seen any research that quantifies this. Point us in the right direction if you have.

wind tunnel
Specialized
aerodynamics
Aero
Mat Brett

Mat has been in cycling media since 1996, on titles including BikeRadar, Total Bike, Total Mountain Bike, What Mountain Bike and Mountain Biking UK, and he has been editor of 220 Triathlon and Cycling Plus. Mat has been road.cc technical editor for over a decade, testing bikes, fettling the latest kit, and trying out the most up-to-the-minute clothing. He has won his category in Ironman UK 70.3 and finished on the podium in both marathons he has run. Mat is a Cambridge graduate who did a post-grad in magazine journalism, and he is a winner of the Cycling Media Award for Specialist Online Writer. Now over 50, he's riding road and gravel bikes most days for fun and fitness rather than training for competitions.

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33 comments

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DrJDog replied to KiwiMike | 9 years ago
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KiwiMike wrote:

Maybe the question should be: how much slower do you go on long descents due to concern over the performance of rim brakes? Going down a soaking-wet Furka pass a few years back I couldn't let myself go above about 30kmh, as the rim brakes were so useless in the wet. It wasn't that discs would have been an advantage, it's that rim brakes are such a disadvantage.

Is there an echo in here?

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KiwiMike replied to DrJDog | 9 years ago
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DrJDog wrote:
KiwiMike wrote:

Maybe the question should be: how much slower do you go on long descents due to concern over the performance of rim brakes? Going down a soaking-wet Furka pass a few years back I couldn't let myself go above about 30kmh, as the rim brakes were so useless in the wet. It wasn't that discs would have been an advantage, it's that rim brakes are such a disadvantage.

Is there an echo in here?

Not really. The current conversation is mostly around performance benefits. That you can brake later, go harder, stop quicker. This wind tunnel test is about proving to TT fiends that the critical 16 seconds they gained by changing to a different type of jersey back-pocket zip won't be completely negated.

For most people who don't ride at the ragged edge due to a) not being that fit b) having a day job and c) not loads of cash to spend repairing broken bikes, this 'enhanced performance' sell passes them by. No wonder they think it's all about marketing for the sake of it.

But if you tell someone they can maintain their normal, comfortable speed down a long hill in pretty much any weather and not worry about crashing due to poor brakes, or not get sore hands - that's a reliability/safety sell - which is much more relevant to the masses.

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earth replied to KiwiMike | 9 years ago
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KiwiMike wrote:

But if you tell someone they can maintain their normal, comfortable speed down a long hill in pretty much any weather and not worry about crashing due to poor brakes, or not get sore hands - that's a reliability/safety sell - which is much more relevant to the masses.

Thing is that doesn't really sell it to me either because I will drag the brakes when descending regardless of the conditions and type of brake. I just wouldn't fast descend a narrow country lane that is covered in potholes, gravel and grass up the middle however powerful the brakes are.

Besides they are only more powerful in the wet. My rim brakes on carbon rims are as powerful if not more than my BB5 discs.

The argument that works better for me is the lack of rim wear and no chance of the rim overheating.

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