RJM

Are discs a drag? Wind tunnel testing disc brake road bikes - VeloNews.com

Good article.

bt

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RJM

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I doubt that the amount of wind drag a disk brake brings really makes much of a difference at all. It certainly wouldn't to me.

BigJeff

At least this one has the article linked and some data.

this looks like positive first results

350htrr

It's funny how that test seems to dismiss another test that got a lower number for the disc brake resistance as not as meaningful because there was a rider on the bike... Wouldn't there always be a rider on the bike when it's being ridden?

RJM

I would think sticking a rider on it would bring way to much noise into the test to be of much use. If you want to find out what bike is more aero, there really is no reason to use a rider. To make a determination about how much time is saved or speed is gained, then you would need a rider I would imagine.

November Dave

This particular test was done on August 13. I don't know when the article was actually written, but it hit mailboxes last week. There's no reference to any other test in the article, because there were no other tests to dismiss when this got done.

Having a rider or not is always something to consider. I have tried on a couple of occasions to be the person riding the bike in the wind tunnel, and you can't believe how small a movement it takes to throw a big spike into the data. To counter that, you could do a whole buttload of runs and get the volume of data points to smooth the data, but in order to do that you are looking at an expense that's into the 5 figures by the time all is said and done.

350htrr

It is worth noting, too, that Specialized itself recently performed a similar test and saw a smaller difference between the disc and non-disc Tarmac. That test was performed with a rider, and ours was not; adding a rider adds another variable, but it also changes the way the wind hits the back end of the bike. Neither protocol is a perfect one.
Read more at Are discs a drag? Wind tunnel testing disc brake road bikes - VeloNews.com

My point was trying to be that with a rider it seems that it's a smaller difference because of airflow differences caused by the rider, thus maybe not as important as the test without the rider seems to show...

hueyhoolihan

good points.

it leads one to question whether or not aero tests that can be done just as well with or without a rider serve a purpose if done without one. as has been mentioned, is it significant to know how aerodynamic a bike is when not being ridden? i suppose it could be if one were to use it as a projectile...

sort of reminds me of the question concerning falling trees, forests, noises, and the presence or absence of observers.

grolby

No, you can't draw that conclusion. Not being able to detect as large a difference when you've got a big source of noise in the data (the rider in this case) doesn't mean that the difference is smaller. It COULD mean that. Or it might just mean that the difference is harder to see. That's why tests without a rider are so important. Ultimately, there will be more and more testing done over time and we will accumulate a clearer picture of the aerodynamic effects of disc brakes vs. calipers.

grolby

This is the issue at the heart of the decision to include a rider or not. It's very difficult to get as good of a sample from a test with a rider on the bike, if not impossible. It's true that bikes are ridden by people, and that's important, but remember that even with a rider, bikes don't get ridden in wind tunnels. There are limits to any test. Getting good answers with a rider on the bike, as November Dave says, takes a lot more time and money. That will eventually be spent, but these tests are a good start.

hueyhoolihan

Hey! that's not fair. you didn't quote my whole post. you left all the folksy, humorous (IMO) stuff out.

November Dave

I hadn't read that in the online update, I was going from my print copy.

It would be unprecedented if the presence of a ride reversed directionality of data. There have been tests that have shown, for example, a close correlation between wheel alone -> wheel in bike -> wheel in bike with rider tests. The rider could serve to decrease differences one to the other, but for the presence of a rider to erase the penalty on the drive side while leaving it completely intact on the non-drive side would certainly be unusual.

The goals of this whole exercise were to inject some objectivity into what had been a subjective and often emotional debate, and to provide a starting point against which progress can be measured. I think it's clear that both have been done. Subjectivity and emotion will never leave the debate - they never do.

350htrr

I think a more "important" number for brakes than aero-ness in a race is does it work better/longer/consistency in different conditions allowing the rider to brake later more confidently to keep up speed longer, that kind of testing is what would get disk-brakes onto race bikes/or not... JMO