Originally Posted by
SoSmellyAir
I thought the 105% rule compares (a) the widest part of the mounted tire to (b) the external width of the rim at the edge of the rim, such that (b) should be about 105% of (a), because the point is to minimize further disruption of the airflow at the transition from the tire to the wheel, where "transition" refers not to where the tire touches the wheel, but where the airflow comes off the tire and (at least partially) attaches to the sides of the rim.
I think it's the widest part of the tire to the widest part of the rim. And yes, doing this there is a narrower pocket where the tire meets the rim, but I think this is effectively a sort of dead space and doesn't matter. If the 105% part were where the tire met the rim, than this would be the widest part of the tire and would therefore define the width of the tire. Unless of course the lip of the rim were much thickness than the tire. But this would create a step, which would likely be undesirable unless it acted like a trip strip on a wing.
The point is you want the rim to be slightly wider than the tires so the air doesn't break away from the rim. I think for the most part the tire forms a circular leading edge and the air is coming off at a angle such that if the rim is 105% wider than the tire the air will continue to stay attached to the rim.
Here's an image capture from a Silca video on the 105% rule. It's clear to see that where the tire meets the rim, is narrower than the widest part of the rim. But, have a look at the entire video to see this in context.
