front to rear rim size comparison...
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Joined: Mar 2003
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From: Markham, ON
front to rear rim size comparison...
Was thinking that an advanced look at this might be useful because i dont have the knowledge to be able to wrap it around in my brain.
If you had hypothetically speaking 20" rims out back and 17" rims out front and both the fronts and backs had the same tire patch, what effect would there be from the fact that a complete revolution from the front would happen quicker then at the back.
If you had hypothetically speaking 20" rims out back and 17" rims out front and both the fronts and backs had the same tire patch, what effect would there be from the fact that a complete revolution from the front would happen quicker then at the back.
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Joined: Oct 2002
Posts: 2,931
From: Upstate NY
IMO, the difference in revs per mile, less than 5% if you choose 245/45-17 and 285/30-20 tires which are available, won't make any difference exept to the ABS and ASR which can be programmed for those sizes. Even in open wheel Formula or CART cars, the difference in tire sizes (and revs/mile) aren't really any problem.
The tire contact patches wouldn't be all that equal if you used the larger rear tire section. If you used the same tire section, in most cases the 20s would have more rotating inertia, and would look very "ricey".
Other than appearance, the reason for the larger rear tires on a Vette, for example, which is close to 50-50 static weight distribution with driver on board, is the "power to the ground" factor. In steady state cornering, you probably want approximately the same cornering force front and rear. IOW, you'd like to have about the same slip angles front and rear.
However with lots of power, you need more rear grip when you add power during a turn because the rears are trying to generate "side bite" and "forward bite" at the same time. This brings up Traction Circle Theory which is a great sidebar if anyone wants to go there.
IMO, the reason the Z06 Vette has larger rear tires than the standard Vette is to better balance the torque available at the rear wheels for acceleration. There is about 14% more first gear ratio in the Z06, and more engine torque above 4000 or so rpm. The goal was 4 secs. flat to 60 (or better) with a good driver. That takes a balanced system. If you've driven a Z06, you'll notice that it works.
During hard braking, the fronts are doing 70-80% of the work, so you'd like to have more front tire. During acceleration you want the opposite. Tire for max performance are always a compromise, except maybe in a drag only car.
My $.02
The tire contact patches wouldn't be all that equal if you used the larger rear tire section. If you used the same tire section, in most cases the 20s would have more rotating inertia, and would look very "ricey".
Other than appearance, the reason for the larger rear tires on a Vette, for example, which is close to 50-50 static weight distribution with driver on board, is the "power to the ground" factor. In steady state cornering, you probably want approximately the same cornering force front and rear. IOW, you'd like to have about the same slip angles front and rear.
However with lots of power, you need more rear grip when you add power during a turn because the rears are trying to generate "side bite" and "forward bite" at the same time. This brings up Traction Circle Theory which is a great sidebar if anyone wants to go there.
IMO, the reason the Z06 Vette has larger rear tires than the standard Vette is to better balance the torque available at the rear wheels for acceleration. There is about 14% more first gear ratio in the Z06, and more engine torque above 4000 or so rpm. The goal was 4 secs. flat to 60 (or better) with a good driver. That takes a balanced system. If you've driven a Z06, you'll notice that it works.
During hard braking, the fronts are doing 70-80% of the work, so you'd like to have more front tire. During acceleration you want the opposite. Tire for max performance are always a compromise, except maybe in a drag only car.
My $.02
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