<VV> Another Brake Question

[AngryCorvair at aol.com]

In a message dated 2/9/2005 10:14:43 AM Eastern Standard Time, Grover Atwood <pnyatwd@myexcel.com> writes:

>If you change to larger wheels,  20 or 22 inch wheels, what does this 
>add to the braking distances. 
>Does it require more brake force to slow the larger diameter wheels?

these are really independent questions, so i'll answer them separately, in reverse order of your asking.

first, does it require more brake force?   YES.

the brakes produce a clamping force, which acts at some distance from the center of rotation (ie the braking surface inside the drum or on the rotor in the case of disc brakes) to produce a brake torque.   this brake torque, divided by the rolling radius of the tire, provides the retarding force at the tire/road interface which actually slows the vehicle.

so, let's ignore wheel weight for a second and only consider rolling radius changes.   taller tires require more brake torque ( = more line pressure = more pedal force) to achieve the same vehicle deceleration.

second, what does this do to braking distances?   that depends on the strength of your leg. :-)

as an example, if you have stock tires about 24" diameter, and your maximum achievable decel is 1g, and it takes 100 lbs of pedal effort to achieve this decel, then you install a set of tires with diameter 26.4" (a 10% increase), to pull 1g decel will require 110 lbs of pedal effort (a 10% increase).

this assumes that the tall tires have the same grip as the short tires.   what if your old tires weren't as sticky as the new ones, then theoretically you could pull even higher decel, and therefore have shorter stopping distances, with the taller tires just because they're stickier.   it all depends on whether or not you can build enough pressure to achieve maximum decel on the new tires.

something else to consider is that taller tires will raise your CG, hence causing a higher percentage of the rear weight to transfer forward for a given decel.   if your car is perfectly balanced with the short tires at a given decel, it will be slightly rear-biased with the tall tires at the same decel.

it's clear to me, but is it clear to you?   please let me know if you understand my explanation or not.

patrick aka <AngryCorvair@AOL.com>