Originally Posted by
asgelle
We're not looking at the effect. but the change in the effect with changes in mass. A 20% change in mass increases the driving force from gravity by 20% and also increases the ******ing force from rolling resistance by 20%. Since the OP wants to neglect this change, I'd say it's incumbent on him to show it's negligible. From the above, it's not obvious that it must be.
120% of small is still small. Rolling resistance is also simply proportional to speed whereas air resistance is a squared relationship. It's pretty clear to me that it's a negligible effect when talking about a 30-40mph descent. If you've ever ridden on large diameter rollers, you know it doesn't take much power to push wheel speeds well higher than 30mph, and a smooth road is the equivalent of a very very very large diameter roller.
Whatever the effect of rolling resistance, I think it is pretty clear that aerodynamics is the much more dominant effect for this problem. Rolling resistance and frontal areas (assuming equivalent tucks) are both second order effects when talking about descending speed.
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