Does increasing rim size affect the acceleration?

[thebarber]

it'll depend on the weight of the 16" rim/tire combo vs the 15's.

[CrankyOldMan]

Originally Posted by thebarber

it'll depend on the weight of the 16" rim/tire combo vs the 15's.

To some extent, yes, but even if you get a 16" combo that matches the weight and diameter of the stock tires on stock steelies, the distribution will be different. The distance from the axis of rotation has a much larger impact on acceleration than overall mass alone. In most cases, going with a 16" rim means a wider rim and tire, increasing the amount of mass at the outer radius of the tire and wheel.

If we move away from the thin-walled (read: zero thickness and width) cylinder to a thick-walled model with a height (width), the width and radius become even more significant:




Where p is the density of the material, h is the axial length of the cylinder, r2 is the outer radius and r1 is the inner radius. The width is a linear scalar quantity, but the radius is "quad-ed", meaning that even a small increase makes a big difference in the moment of inertia.

I do recognize that we may be talking about fractional quantities of torque or acceleration in all cases, but the principle stands: radius changes the values faster than width or even weight. That said, the same diameter wheels/tires with lower weight will of course have less negative impact on acceleration.

Just watch a figure skater do a spinny-thing where they start with their arms out and then bring them in to their chest. No change in mass, only radius. Makes me queasy just thinking about spinning that fast.

I've been meaning to try and get some wheel manufacturers to cough up actual numbers, but I suspect that their engineers will be guarding that kind of information very closely.