E-Fans

[Jackpine]

Personally, I think the added mass has only a modest effect on the acceleration when you go to larger tires. Yes, there's more weight to get moving, but that's a small increase. Using my tire as an example, increasing the diameter by two inches adds 15# per tire, 60# for four (about the weight of a child in the truck). Remember, a tire is mostly air.

Of course, there's more rotating inertia to overcome with a larger tire but:

My tires (about 31" with "squish" factored in) spin at about 650 rpm at 60 mph, hardly a dizzying rotational speed. 33" tires spin at 611 rpms and 35" tires spin at 576. So even though there is a small increase in rotational inertia, a larger tire doesn't even have to be brought up to the same rotational speed, there's a "cancellation" effect. Once the tires reaches that speed, it wants to stay there.

But, let's assume you see 250 ft-pounds of RW torque at the real axle. This translates to a "pushing" force of 193.5# at the road contact surface of a 31" tire. If you increase the tire diameter to 33", that force drops to 181.8#. With a 35" tire, you are down to 171.5#. The change from 31 to 33 inch tires gives you a 6% reduction in accelerative force, and a change from 31 to 35 inches reduces the accelerative force you feel by 11%.

Those percentages are significant and are the result of a loss of "mechanical advantage" caused by the larger tire. It is not the weight of the tire.

- Jack