The Win Tunnel: Which is More Aero? Rim Brakes or Disc Brakes?

Written by Tyler Benedict Dec 15, 2014 7:50 a.m. ET

Support us! Bikerumor may earn a small commission from affiliate links in this article. Learn More

Specialized continues putting its in-house wind tunnel to good, entertaining use with the latest test, determining whether rim brakes or disc brakes are more aero. They tested at various wind angles and did find about an 8 second savings over 40km in a mild cross wind, but for which? And would that really matter considering the improved braking one might offer? Watch and see!

Tyler Benedict

11,200 articles

Tyler Benedict is the Founder of Bikerumor.com. He has been writing about the latest bikes, components, and cycling technology for almost two decades. Prior to that, Tyler launched and built multiple sports nutrition brands and consumer goods companies, mostly as an excuse to travel to killer riding locations throughout North America.

Based in North Carolina, Tyler loves family adventure travel and is always on the lookout for the next shiny new part to make his bikes faster and lighter.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

30 Comments

Oldest

Newest Most Voted

Inline Feedbacks

View all comments

SamSkjord

9 years ago

Does this mean people will stop whining in comments about how unaero discs are? No.

Does it mean I can get an S-Works Venge with discs? Hopefully.

Dave B

9 years ago

Well, when it to their commercial benefit, companies will claim an 8 second disadvantage in 40K is nothing. When their product gives an 8-second advantage they will claim it’s HUGE.

anonymous

9 years ago

It’s 8 seconds with a crosswind.

Peter

9 years ago

@Dave B: exactly. Imagine if they found that the Evade was 8 seconds faster than the Air Attack. Holy Moses, would :08/40K be an AMAZING time savings.

As for the cross vs. headwind: I don’t know how many times I’ve ever ridden into a 0-degree headwind. Maybe like 3 in my entire 30+ years of cycling. Normal wind conditions are almost always at some degree of yaw. Virtually identical in a 0-degree headwind is pretty well meaningless for real world application.

Skip

9 years ago

Do you think the results might be a tad different if they closed the rim brake lever? probably not, just a thought.

Jo-slow

9 years ago

What about the weight penalty? When you have multiple corners in a descent, it usually implies you have to go up another hill. What would the extra weight would add to you time going uphill? Considering the costs and the very small potential gain (demonstrated loss on flat land), I will keep my rim brakes…

chuckster

9 years ago

I’ll give them credit. Without testing, theory is useless. I’ll also say that disc will beat rim on any wet day, but the differences are small in dry conditions. But there’s also a lot left unsaid (and worth some 3rd party testing… hint to Velonews or others…). I’ll also guess that engineering and R&D will close the gap at least most of the way, but we all will pay a pretty stiff entry fee for the technology for a while.

– The rim brake used is about the most unaero sort currently available – the worst case scenario. Hidden rim brakes on truly aero frames (in the fork, under the BB, etc) could likely improve the rim brake numbers more than an extra 8 seconds worth. Maybe even some of those new direct mount models. On the other hand, it may be tough to make a disc setup much more aero than currently until it’s really integrated into the frame which isn’t happening yet, and then there’s still that disc hanging out in the wind which probably (?) can’t be shaped easily like aero rims have been. So it would seem that disc is at “best case scenario” and rim at “worst case” for this test (plus the release lever!). This is also on a Tarmac, which is far from the most aero frame (but yes, a great race-proven frame), and may mute the differences.

– It looks like they only tested one yaw angle. It would be interesting to see some bigger yaw angles to see if that disc really starts to churn up some turbulence.

– It would be cool to see the testing on something like a Cervelo P5 (yeah, I know it doesn’t have disc brakes) or something cutting edge with a disc rear wheel and deep dish front, plus a rider fully decked out in aero gear. This would really bring out the differences between disc and rim brakes, and would be interesting to see at different yaw angles. But to be honest, there’s probably an already obvious aero reason that there’s not a P5 Disc Brake on the market already which may answer the question.

– Finally, I get that the rider on the bike is part of the equation, but that’s gotta add a lot of static to wind tunnel results (even if he tries to ride precisely the same way) when you’re talking about +/- 8 seconds over an hour or so.

Oh, to have a wind tunnel and team of engineers at my disposal lol!

The other Andy

9 years ago

There are a couple of problems here:

First and foremost, no one really makes disc-specific aero rims. Disc specific rims don’t need a brake track, which could have significantly implications for rim shape and for general aerodynamics.

Aong with that, you have rim weight. If you don’t have a brake track, you can further reduce weight. Which would move weight away from the rim, and towards the hub.

And, total weight, as far as I can tell, is not an issue. You can build a 6.8kg disc-equipped bike pretty easily with modern components. With the previously mentioned changes to bike design (rim shape and weight), you could end up with a disc-equipped bike that is actually more aerodynamic, at the same weight, with less rotating mass.

The other Andy

9 years ago

Significant* implications.

satisFACTORYrider

9 years ago

S needs to get this kid to wind test eating gels vs bars. gotta help as many stravatards as possible.

skip

9 years ago

gels are way more aero brah. you can tell when you throw them to the side of the road and they land on the asphalt with a nice satisfying thunk. None of that fluttering around forever and drifting in the wind like a cliff bar wrapper. But the cliff bar wrappers are lighter….

satisFACTORYrider

9 years ago

ok but which electronic contraption is perched on a stravatard’s bar is more aero? garmin or iphone?

Mike

9 years ago

The most un-aero thing on my bike is me. I’d say even with the 8 seconds that the 5’6″ 160lb’er is going to beat my 6′ 200lb frame every time.

Author

Tyler Benedict

9 years ago

satisFactory – Neither. They’re more aero when placed in your pocket. Proof is about 1:20 in:
https://bikerumor.com/2013/03/26/patrick-seabase-his-47×17-track-bike-versus-galibier/

satisFACTORYrider

9 years ago

why link to hipster tool, tyler? haha. fair play. it’s monday..

chuckster

9 years ago

Andy… I wonder how much of what you’re saying would make a significant difference. Devil’s advocate here – I don’t know the real answers of course… Does a current Zipp (for example) brake track shape vary much from the aero ideal in the first place? I don’t think you have to modify the cross section of a brake track much for braking purposes to make it work well for aero purposes, and the surfaces don’t have to be precisely parallel for braking anyways. I think Zipp did some sort of “caulking strip” at one point to fill the tire/rim gap (which could be done for disc or rim brakes), but I think that starts to get impractical, or annoying pretty quickly (and adds grams again).

How much weight does a carbon tubular brake track really add, especially when you figure it’s gotta have some impact resistance anyways (at least for cross use or pothole crack prevention), and the fact that a disc up front adds torque at the hub that means a spoke pattern and strength that wouldn’t be needed in a rim brake.

Finally, 6.8 kg UCI rule may soon be gone. If so it would be easier to get a bike down to, say, 5kg w/ rim brakes for a Pantani-sized climber and aero would again have to be balanced against weight.

Bob

9 years ago

So you’re saying #aeroisnteverything?

chuckster

9 years ago

… and apparently I missed the VN article that just came out and addressed a few of these issues. All interesting stuff, especially the yaw angle differences that make a crosswind from one side insignificant, but pretty huge from the other side. I guess we’ll see which way the industry heads over the next few years with this, thru axle, hydro, electronic and everything else. Back to my fixie.

Alex @ Hermes Sport

9 years ago

It is a bit telling when a company who has a vested interest in pushing disc brakes is compelled to admit they’re slower.

EagerBeaver

9 years ago

Cam is SO dreamy!

Antipodean_G

9 years ago

This reminds me of all those silly ‘lab tests’ Giant did to try and tell me 27.5 is faster and better and… whatever than 26″.

End of the day, they did what they wanted, 26 is dead and I am now wondering if this will be the same for road brakes?

9 years ago

People, discs stop better in all conditions. This is safer and allow you to ride more confidently. For all but a very very few of us the slight aero and weight differences are inconsequential. I know, change is hard–When I started MTB in 88 I loved my Canti’s. I resisted switch to v-brakes for too long. Same from V to disc. I was wrong. If you can swing it, upgrade to disc on the road, ride forward, and don’t look back. Rim brakes will not be around on performance bikes within a handful of years. This is for the same reason drum brakes are not used in cars anymore–because they don’t work as well.

Craig

9 years ago

I use down tube shifters with a coaster brake, they are way more aero than integrated shifters/brake levers and rim callipers or discs. I can get home at lunch time 3 minutes faster
with this setup.

Gillis

9 years ago

And I use a fixed gear…no brakes to slow me down (or stop me) and no derailleurs to get in the wind. I get home 5 minutes faster with this setup. 😉

Brad

9 years ago

10 degrees of yaw might not sound like much but it is actually a heap, almost a worse case scenario. Normal riding is usually no more than 7 degrees. Not sure why they tested so far out?

This article explains yaw very well.
http://aerogeeks.com/2014/08/28/aeromail-what-is-yaw/

Mortimer

9 years ago

For extreme aero using wire spokes one can go 12 spokes min rim braked. Prob 24 spokes is min dished. If the wheel acts like a fan less blades is more aero. Then with discs bigger is better. 140mm on the back is a tad small. How about 160mm? How big could one go, 200mm? Nah , let’s go really big. Why not integrate the disc into the surface of the rim? Yeah big discs are best (aka rim brakes).

satisFACTORYrider

9 years ago

yawN

Blair Ellis

9 years ago

From someone who owns a 15 S-Works Tarmac Disc and has real world experience with it, the felt aero difference is negligible compared with my 13 Tarmac. This is right in line with VeloNews and Specialized testing.

Braking is altogether different. It is VASTLY improved. Since I live in the mountains and love the technical descents, I’ll take the disc over rim brake any day of the week. Coming from a motorsports background, a lackluster braking system is the first thing to get upgraded on a car. There’s nothing like confidence inspiring brakes.

The entire peloton WILL be on hydraulic disc brakes in the coming years. It’s just a matter of when, not if.