Wind Tunnel Test of Discs v Rim Brakes, pt 1

On June 24th, we had a good idea. We had a trip to test a bunch of stuff at A2 planned out, the results of much of which we've published previously. Since we'd seen a rise in road disc wheel activity, we thought the time was right to include Rails with discs. It's such a small step from ignorant guessing to knowing, all it takes is a test.

Well said, sir!

The good idea was when we thought "you know, no one's really published anything remotely definitive about the aerodynamics of disc versus rim brake wheels - maybe some bigger media outlet wants to work with us on the story?" Before lunch, we were hooked up with Velo, with the proviso that the story would be about the whole package - wheels in the bike. 


Since we didn't have "except for brake format" race-oriented rim and disc brake frames of our own at the time (we do now), Caley Fretz at Velo offered to arrange getting two frames to A2, and off we go. 

For those keeping track at home, June 24th is a challenging date on the cycling calendar. Life is in FULL swing. Caley was off to France to cover a bike race, we're going like mad trying to keep on top of orders, but we were behind ourselves getting to the tunnel and didn't want to delay it anymore.  We scheduled the tunnel for July 28th (which is already a lot less hectic for us than June), which would give everything plenty of time to happen even in light of the TdF and everything else. 

The only thing more expensive than paying for testing in a wind tunnel is paying for not testing in a wind tunnel. Despite confirmation that frames had shipped, frames hadn't shown up and there was no good info on where they were or when they might arrive at A2. Since I was driving down and had made a bunch of arrangements to do other stuff in concert with the trip, plus our desire not to delay the trip anymore, we kept the schedule even as the frame component started to look a little questionable. We rearranged schedules by a day to give the frames an extra day to arrive, and kept on. Fortunately we were able to be super productive during the day and a half that we were there when the frames were supposed to hit, because the frames never showed. We never doubted the Velo component, but it won't be a surprise when I say it felt like not everyone involved was playing it straight up. What are you going to do? We did our testing, made our contingencies, and when the frames never actually showed up, we went home without that piece accomplished. 

Thanks to Caley's persistence, the frames eventually showed up at A2. Plan A was just to have A2 run the tests on our tab, with me "present" by remote connection- basically Facetime. Plan A never works. When A2 unpacked the bikes, there was a lot of work to do in order to net out differences in the frames. The disc bike was Di2, the rim brake was mechanical. Seats were different. Bars were wrapped differently. Too much noise. Another high and hard fastball, the degree to which this was within the pitcher's control is up for debate. So I saddled up a jumbo jet and flew back down to A2 to equalize the bikes as much as possible. When the bikes went into the tunnel, they were as equal as they possibly could have been - the only differences were the differences elemental to disc versus rim brake bikes. A2 sent the data files directly to Velo, and I shipped a bunch of photos off.

As alike as they can be made

With this accomplished, the only difficult part was keeping mum about what we had done and learned. You spend that kind of dosh to make that kind of a leap in your understanding of things, and your instinct is to start shouting about it post-haste pronto. Nope. Gag order until the December issue dropped, which was scheduled to happen in early to mid November. Tough, but worth the eventual exposure. Patience is not my strong suit, this was agonizing. 

Then the December issue came out, and it was the awards issue, no sign of our test. After my coronary event subsided, I learned that there had been a shuffle in the editorial calendar and it would be in the January issue. Not ideal, but okay, just a couple of weeks tacked on. And then, Monday, this beauty landed in my inbox. 

Of course nothing is ever even that straightforward, right? Of course not. Flipping through Twitter last night, I see a "how much do discs really slow you down?" tweet.




Hmm, we didn't take a video so what's with the YouTube link? Oh. Specialized decided to publish their own test, from their tunnel. Coincidentally, one day after Velo drops the issue with our story. That sure is one heck of a coincidence, huh? They've got the resources do it, and we're certainly in favor of more info being out there for consumers. The problem is that their test sucks - they left sloppy differences in place between the two bikes, and they only tested with wind from one side. If you guessed that the differences from one side to the other are absolutely nothing alike - congratulations, you win! 

Those of you who get Velo will have seen or will soon see the data for yourselves, and we'll be able to talk a heck of a lot more about it soon. At this point, we're glad that objectivity is starting to displace conjecture, and happy to be at the forefront of the discussion.

Back to blog


Dave, thanks for all the time and effort that you put into providing objective data like this. As an engineer your methods and approach only further validate the data you provide. (And I'm sure that your accountant is beating you silly for all the money you are spending on this stuff!) That being said, the Specialized vid sure raises the hair on the back of my neck. Their data is presented with a crayon at best. Chris is obviously reading a script that was prepared for him by the marketing guru's. I guess it is hard to debunk their data when they don't present any!Thanks again for all of your work.


Chris is pretty young, but he's do dummy. I thought he'd gone to MIT, but just checked – CalTech undergrad, Stanford Phd in Aeronautics. He's smart, and with his training, this is not his first trip around the mulberry bush. This does, however, illustrate just how blatantly wrong it is to compare results from different tests in different tunnels to each other. I have no doubt that A2 is an absolutely premier tunnel, but I can't say anything about the Specialized tunnel.

Dave K

Not that age is THE determining factor in experience, but the guy in the Specialized video looks like he's about 23. If Specialized has their own tunnel you'd expect these guys to be running every possible config every day and know what the hell they're doing. I lost a lot of faith in the Specialized results when they published their results on leg hair using the Chewbaca scale. These "engineers" need to go back and relearn the art of experimental design and statistical analysis. When doing an experiment, nothing is ever as simple as it seems.


Thank you for the thorough testing and all the information you shared. You mentioned that the disc wheels were the same wheels as the rim brake wheels with a disc attached. It is fair to test this way as it gives an aerodynamic measurement that is apples-to-apples. However, the test results were expressed in Watts required under various conditions. But this is not a complete picture of watts consumed under different configurations by the rider.A carbon clincher that is truly optimized for disc brakes should have less rim weight and have a better shape because the rim does not have to support rim brakes. Weight in the rim is one of the worst weights to have as it requires more centripetal force and continual watts to keep it moving. You mentioned this issue and I appreciate that you were out front about this. It would be even more informative if we could truly understand the Watts required by the rider on a disc based wheel set vs a rim brake wheel set. I speculate that a bike and wheel set that are fully optimized for disc brakes would require fewer watts. Thank you


Hey Dave and November crew,We're seriously sorry on the timing. That was my fault – we wanted to answer the questions we'd all gotten about disc brake Tarmacs and thus did the test. Had no idea the Velo issue was a day away. We launch our videos on Tuesdays and that was the next one to run.I can assure you after 5 years of testing bikes at the A2 Wind Tunnel as well as developing laboratory experiments for aero testing that both test protocols are sound and the facilities produce fantastic results. You can compare the results, not apples to apples exactly but in a rough context to see if the independent test results align. We published a video showing a different test than the one you ran and both show disc brakes to not be substantially slower than rim brakes, though there is a delta.We did have bikes that we're as perfect as your setup – our rim brake bike was mechanical braking and shifting while our disc bike only had a Di2 wire exposed for shifting. Thus we expect the disc bike in our test had a bit of an unfair advantage. However, it's not much of a difference and instead of canceling the video, we just put the transparent results out there that matched the bikes in the video.We're glad to provide your team our raw data from the more elaborate testing. We've spent a lot of time on disc versus rim testing and regardless of the details of the bike, have come to the same conclusions many times. This is true with and without rider but of course the flow field changes with a rider off of the bike.Lastly, you're welcome to come tour our facility next time you're in California. We designed our Win Tunnel from the ground up to be the best testing ground for aero testing and I think you'll agree that both our team and tools are scientifically valid. Please reach out to me directly for any follow up. Mark

Mark Cote

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