Current Projects chat and miscellany

Odd things happen in that gap, which is why Look designed the new 596 Triathlon bike to have a gap of about 12mm, a counter-intuitive decision in the light of a decade of ultra close wheel cutouts. The reason for the frame to follow the line of the tyre as closely as possible is to make the seat tube act as the leading edge of an aerodynamically integrated body including the rear disc wheel - this works really well as long as the wheel isn't turning, which is why the likes of Cervelo etc. provide horizontal dropouts and adjusters to allow you to optimise the gap. Even with the disc spinning, the bulk flow over the whole system stays nice and smooth, and you get low drag, as long as you only measure the drag force pushing backwards on the bike. Traditional wind tunnel tests don't measure how much power is used to actually keep the wheel spinning, and this is where it gets interesting, and where Look made a discovery which has influenced some track teams to start running bigger gaps between the rear wheel and the curved seat tube.

Essentially, what happens is that the rear tyre boundary layer gets pulled into the tight gap and is then acting as a thin shear layer; the bit by the tyre wants to keep up with the tyre, the bit by the tube wants to stay still, and as any fule kno, the friction in a layer a fluid being sheared between two surfaces goes up as the thickness decreases.

As a result, increasing the gap a little bit makes the drag pushing on the whole system a tiny bit bigger, but the relatively large increase in the thickness of the shear layer (about a 4 or 5 fold increase in thickness compared with typical "tight" clearance) makes the power required to spin the wheel a tiny bit smaller. One of those tiny bits turns out to be a little bit bigger than the other if you judge the increased gap just right, and the design becomes an act of balancing one set of drag against another. The same effect also explains why some of the latest time trial frames have more clearance around the tyre as it go through the fork crown than was fashionable as recently as a few years ago.

Yeah I was going to say that.

:)

Interesting, thanks.