Sooooo, everyone understands the weight of larger tires isn't the real issue right???

[tomcruise]

Wheel and tire weight affect braking more than acceleration correct? I had heard somewhere that over 70% of the brakes stopping power was used to stop the rotational mass of the wheels and tires, where as it takes a much smaller percentage of the braking force to stop the interia of the vehicle.

 

[IHAVETWO]

OLD RAPTOR GUY IS 100 % RIGHT

 

[HighHP]

A tire is a "reverse lever". A tire/wheel is driven from the center (at least in automobile applications). It is the exact opposite of getting a longer ratchet to get more leverage on a bolt. Imagine if the bolt was spinning the ratchet, the longer ratchet would have less torque at the end.

lets say we have an engine that makes 500 ftlbs of torque, with a transmission first gear ratio of 3:1 and a rear differential of 4:1

500 ftlbs into trans gets you 1500 out
1500 into the differential gets you 6000 ftlbs applied to the axle shaft.
now apply that to different tire/wheels
32" (has a 16" radius) would get that 6000 ftlbs down to the glorious number of 375 ftlbs applied to the ground
33 would be 363.6363
35 would be 342.857
37 would be 324.32

Now if we regear the differential to 4.2 we get 6300 ftlbs applied to the axle shaft.
which with 37's would give you 340.54 ftlbs to the ground. basically matching 35's

This is why people regear after getting bigger tires, they lost a bunch of torque and want it back. Think jeep going from 31's to 37's. it is also why you have to go to bigger axles if you make such a large change, factory jeep axles can't handle the amount of torque going from 3:73's to 5:30's

I have built a few drive trains. This is exactly how you successfully match engine/tranny/axle/tire combinations.

 

[Old-Raptor-guy]

Wheel and tire weight affect braking more than acceleration correct? I had heard somewhere that over 70% of the brakes stopping power was used to stop the rotational mass of the wheels and tires, where as it takes a much smaller percentage of the braking force to stop the interia of the vehicle.

Well to some degree that is true, same as someone else said a heavier tire takes more energy to get rolling.

But consider this, when decelerating, power is not being applied to the tire. Now the road is applying force to the tire and the tire is a normal lever in that case. The road has more "leverage " on a taller tire.

This is easier to visualize if you imagine the vehicle stationary and the road moving (like a tread mill).

 

[New recaros]

Gear Ratio Calculator

Easy to use gear ratio calculator. Use our gear ratio calculator to find the correct gear ratio to run when changing tire sizes to keep your stock gearing.

tiresize.com

 

[Dr Racecraft]

weight of the rims and tires does in fact matter for acceleration, so do gears...

here is the math behind the unsprung weight, it is quite interesting

http://hpwizard.com/rotational-inertia.html

 

[Bugzuki]

I can't seem to write anything without rambling on.

My two thoughts are:
* Tire diameter should be considered part of the gear ration calculation since in effect it is a ring and pinion relationship between the tire and the road.
* Tire weight increases or decreases the inertia of the rotating mass and should be part of all calculations and minimized when possible.

 

[ogdobber]

I think it’s a toss up what makes the difference on the 35 vs 37 package. Weight generally makes a bigger impact, especially when you are only talking 2 inches. But the weight difference is 4lbs per tire so in this case I’m thinking a 50-50 split.
If your tire/wheel weight was 20 more at each corner then it definitely would be the weight.
Same goes for if you were jumping from 31s to 37 then it would be the tire diameter.
16lbs vs 2 inches… flip a coin