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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. |
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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. |
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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. |
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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. |
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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. |
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We
can fit a tire with about a 10 inch section width. |
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We
look at the Hoosier
race tires selection and the Goodyear
choices. The 15 inch rims offer the best tire choices for our needs. |
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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. |
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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.
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- 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.
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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. |
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We
order a set of Goodyear 23.0X8.0-15. |
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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. |
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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.
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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. |
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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.
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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. |
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When
we remove the wheels, there is a clear rub mark on the inside of the
right front tire. |
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The
culprit is the lip of the floor board which can be easily fixed.
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We
are ready for Roebling Road. |
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