Rear wheel cutout question

I would think they overall aerodynamic advantage of a full disc would trump a small space between the tire and cut-out.

Don't know the answer, but my instinct is the same as Bryan's.

I had this same problem as well when I used my Renn disc. I figured I was still better off using the disc than the spoked wheel, but no real evidence to support that tho.

Agree with JK and Bryan.

I don't remember which manufacture but I think I remember at least one of them saying they think there should be more not less room between the rear wheel cutout and tire.

Found it - Look is the company and there is talk that Kestrel's wind tunnel testing found the same thing.

Didn't the youtube video interview have John Cobb saying something along the lines of "away from the cut out is better"?

Might want to check with Cervelo.  I think that either they or someone else implied that some other manufacturers "copied" the wheel cut-out without really understanding all the engineering invoved in the bike and, on those other bikes, it didn't really work as it should.  So not sure others discussing their cut-outs will be applicable to the Cervelo.

I guess now we are talking urban legend but I remember hearing the same thing from Scott – a little bit more space between the frame and the wheel reduces turbulence.

From my understanding of the John Cobb video previously linked in the past couple days, the rotating tire/wheel creates an updraft on the backside of the wheel.  This causes a high-pressure area behind the tire when the downtube is too close in the case of a front wheel.  For a rear wheel, I imagine that the same phenomenon occurs with the tire pulling air around as it rotates.  For the rear wheel though, we are looking at the gap at the front of the rear wheel and the seat tube.  I imagine that a bit of a gap will allow the air being dragged around the rotating tire to flow smoothly and not compress it, causing a small extra power requirement.  Too much gap and the air passing by the seat tube will rejoin the air being dragged by the rotating tire which probably isn't great for aerodynamics.  Ideally, I imagine the gap is set just right so the air being dragged around by the rotating tire can move freely and the oncoming air from the forward movement of the bike slips around the tire/wheel.

Just my $.02.  I could be completely wrong - it's happened before.

UCI guildlines says that a credit card has to fit between the cut-out and the tire cut.

I guess what I'm asking is what that "just right" gap would be.





That's the minimum, and has nothing to do with the question at hand.

Edited by the bear 2009-08-06 2:55 PM

do you have a close up pic?

Not on me. Maybe I'll get one when I get home much later tonight. Really it's not **that** big of a gap, just wondering if anyone had a feel for what was ideal, and what do you lose if you move away from that ideal.

Cobbs seems to say that that's it's not critical, not to worry about it.  Part 2 3:35-4:07

Don't know the answer to the OP, all I know is that I'm stunned that Bear doesn't know the answer to a bike-related question...

FWIW, found some refernces to an older Cervelo discussion indicating 3-5mm was "optimal".  Unfortunately, couldn't locate the actual discussion to see who (and how they) arrived at that conclusion.

Yep, guess he does. Funny thing, I'm the one who posted the links to those videos yesterday. Just, uh, testing y'all to see if you were paying attention.

So we're good to go. Nothing more to see here!

Do now. Wasn't born with all this knowledge, y'know. You learn from asking questions.