UrS4

Yes it is a balance, increased grip makes up for the lost time spining without traction so that makes sense obviously. And by the logic posted earlier, in short sprints in the low mph the difference is minimal and only becomes significant in an ideal setting at longer higher speed runs.

Okay but how much does it apply to increased braking distances?

I do understand that it is the friction that the tires make that causes te deceleration but the extra mass of the wheel should require more clamping force which is a constant in stopping as fast as you can.

Taken to the extreme and probably keeping it very simple a heavier ie. larger vehicle will take longer to stop with the same sized brakes and pads, correct. Or taken in context of something we see everyday, SUV that have put 24" rims require more distance to stop.

Norm Peterson

As long as you have enough braking torque available to lock up the wheels, there is zero difference - other than you have to step a tiny bit harder on the brake pedal to get the same results.It's additive as far as the clamping force is concerned rather than constant, since you can look at the torque required to decelerate the wheel/tire/rotor assembly to be in parallel with the brake torque required to develop the deceleration force down at the ground. You can look at it as a parasitic loss unless the driver is physically incapable of applying the extra necessary force. Don't laugh on this last, BTW- the latest version of the FMVSS braking standard called for the same performance from lower pedal force levels, and I'm pretty sure that it wasdone for reasons involving minimum physical capabilities. Hopefully it doesn't apply to too many people here.

I think the 32 lbs vs 24 lbs ratio for the S197 here still holds, and since you're effectively adding the 32 to about 3500 lbs the difference looks like about 0.9%, so it amounts to maybe afew ounces harder in an all-out stop where you aren't going straight to locked-wheel conditions. In the extreme, it *might* affect one end a tiny bit more than the other in terms of brake balance, but other effects are bigger.

In your extreme case, the diameter starts becoming a bigger issue. Rotational inertia is a [mass] x [radius](squared) property, so if you swap a 30 lb 18" wheel out for a 24" 40 pounder the effective weight increase is more like 100 lbs for the 40 that the scale says. And that's just for the wheels. Then, since the radius over which the braking torque acts is also greater, you have less overall leverage from your foot to the contact patches.


Norm

UrS4

Thanks Norm,

And appologies to the OP, didn't mean to hijack your thread to this degree.

Norm Peterson

I should be thanking you. And thanks to Tim for the pictures and giving this expanded analysis a place to get started.

It really hadn't occurred to me to look at this particular combination in that much detail before.


Norm

Stoenr

Nice choice! I almost ended up with those, but went with the CT-R's instead.