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.

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Just trying to ensure that both sides are known. There are some big contradictions in their story-like where they say and one venue that they do these tests in batches, months before they are published and one venue, and then in another venue They explain the variance in frame styles that they used in this test as being part of the "last minute rush" to get the test done-So I don't think that suspicion is undue, but I do want to be fair


Dave, with all due respect, be careful of assuming the pedigree correlates with someone knowing what they're doing. This is no different from assuming results from any and all tunnel tests are valid. Look I did my Ph. D at Purdue and a post-doc at Stanford, and what I can tell you is there's some not so bright people from both schools with degrees. In the end, if I can paraphrase Bill Parcells, you're only as good as your experiment says you are. Bad data results from bad design and the design falls on the scientist.


Meng – Yes, Trek's thing does a fairly good job at explicating the concept. Turbulence no doubt has an effect on spinning cranks, but tests with a rider pedaling show a consistent drive side bias on 'regular brake' bikes.


Dave, to follow up on "sail area," I guess that means the extra areas can generate lift at some yaw angles? I read that from here Trek's whitepaper: question about the cranks spinning was whether that behavior might cause extra turbulence around the chainring and waste energy downstream.


Paul – What you are after is referred to as "watts to spin." The most authoritative work on this topic has been done by Andrew Coggan, who estimates that the TOTAL VALUE for this is appx 5w for non-aero wheels and 3w for aero wheels (this is per wheel), with aero being the big delineator. Road disc rims don't have all that much to lose in the weight department because a lot of the rim structure is still devoted to keeping the tire on. Carbon clinchers may lose on the order of 40g per rim over time, but we'll see. As for shapes, the Rail shapes, and most current verified aero rim shapes (in which I exclude anything that hasn't been proven in a wind tunnel, because at this point, come on…) nearly completely ignore the brake track function in shaping anyway. Meng – The drive side of a normal bike is faster. Without going into a long explanation, the shorthand method used to describe what happens on the drive side of a normal bike (because the crank/chain rings are there) or the NDS of a disc bike (because the discs are there) is to say that they have extra sail area. Cranks do not spin in tests of bike without rider. They are placed in a normalized position, as shown in pictures.

Dave K

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