April 18, 2007

Wheel and Tire changes

Bigger rims and stickier tires
There is not enough room inside our 14 inch wheels to fit bigger brakes, so we must increase the wheel rim size. There is the question of how big to go in wheel size. To decide this, we must look at what tires are available.
In most ICE racing, the more rubber you can put on the car, the faster you go. The gain in grip around corners and under braking outweigh the negatives of more rotating mass and air drag.
We do not know if this will be true in an electric car. More speed in the corners should compensate for the slightly slower acceleration and lower top speed.
The other consideration is efficiency. Running wider tires will use up more Watts per mile. Since we are already forced to run the controllers at lower power levels to finish 30+ mile races, any loss of efficiency will mean turning the power down further in long races. In short races and qualifying runs, we expect wider tires will make us faster.
We decide to try running the widest tires we can fit without flaring the fenders. Flaring the finders would make the car wider and create even more air drag.
We can fit a tire with about a 10 inch section width.
We look at the Hoosier race tires selection and the Goodyear choices. The 15 inch rims offer the best tire choices for our needs.
Until now, we have been running the Hoosier DOT P205-50ZR14. These 'DOT' tires are designed to meet two goals:
1. . to meet the US Department of Transportation rules that define what is legally required for street use;
2. . and still be the fastest possible racing tire.
The result is a racing slick with two small grooves.
The tire companies warn against using these tires on public roads. It would be like driving on a bald tire when the road is wet. The tire is light and not very stout, and its main purpose is to allow race series to run 'street' tires at racing speeds.
There were two main reasons we started the Electric Imp out on 'DOT' tires.
  • First...
    the 'DOT' tires came in a little harder tire compound than the full racing slicks. This means that the tire grip stays consistent for a longer time at the expense of having the most grip possible. This is useful for testing a new car since it is possible to do back to back testing without the tire grip changing significantly.
  • Second...
    the tire engineers felt the 'DOT' tires, because of their radial belt design, might have less rolling resistance than the Bias Ply racing slicks which should give us greater range.
Hoosier DOT specs
On the other hand, the tire engineers suggested that we would probably go between one to two seconds faster per lap with full race tires. Goodyear and Hoosier are unable to provide much guidance in how much range going to the bias ply will cost us. Eventually we will have to spend a test day comparing. For now, we decide in favor of trying for maximum grip.
Hoosier & Goodyear SLICKS specs
We order a set of Goodyear 23.0X8.0-15.
We need a set of 15 inch rims. Since we are trying to fit as much tire as possible into the wheel wells, we must take a close look whether we can move the tire any further in without rubbing on the suspension. It looks like we might have about an extra 15 mm.

The Rota Slipstreams 15 X 7 5 X 100 are fairly light at 12.9 lbs and reasonable priced. They come with a 35 mm offset, 15 mm less than our 14 inch rims. This is as close as we think will work.

Roto Slipstream with Goodyear Slick
According to Goodyear's recommendations, the bias ply tires will work best with less negative camber than we ran for the radials. We change the camber settings and mount the tires on the car. Less camber puts the top of the tires further out, closer to the inside edge of the outer fender. We move the suspension from full droop to full bump. The tires do not rub.


Tires flex up and down and in and out under load so we must test them on the track. We mark the side walls with white paint.

tire marked to check for rubbing
We take the car to Homestead-Miami Speedway. We check the tires after one lap and there is no obvious rubbing. I go out and do four more laps breaking in the tires and bedding the new brake pads and rotors.
When we remove the wheels, there is a clear rub mark on the inside of the right front tire.
tire rub mark

The culprit is the lip of the floor board which can be easily fixed.

tire rub on metal lip
We are ready for Roebling Road.


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