The measured difference in maximum drive force is only 20% lower for the plastic
wheel. Since a tyre is probably required anyway on an aluminium wheel at high Sun
levels this difference in friction only suggests that a tyre would be required on a plastic
wheel at a slightly lower Sun level than for the Aluminium wheel.
There is no really significant drive force gain from using an aluminium wheel,
certainly not sufficient to enable running without a tyre in all conditions.
Consequently a plastic wheel which is lighter, cheaper and easier to make seems the
obvious choice. Just fit a tyre at a slightly lower Sun level than required with an
aluminium wheel.
The next question is when should a tyre be fitted? This can be answered by using the
Mathematical Simulation, simply run the simulation set up for your car with and without
a tyre at different Sun levels and note the race time. The Sun level cut off point for using
a tyre will be obvious from the results.
To make an accurate determination of the Sun level at which to use a tyre it is necessary
to know the friction characteristics of the actual race track. I have not conducted tests on
the Victorian track but expect its frictional properties to be similar to the Box Hill track
as they are both constructed from plywood and painted with a flat water based paint. This
suggests the results of the testing detailed above can be used for the Victorian track.
The NSW track which is normally used at the National event is also constructed from
plywood but has a form of plastic coated surface not paint. We can expect this surface to
have significantly different frictional properties to paint.
To determine the difference if any, testing was undertaken during the 2009 National
event held in Melbourne. On a flat section of track a car (Syndal South Primary School
car “Lean Green Speed Machine”) was towed with and without a tyre fitted and with and
without the single drive wheel locked. The results are recorded below. Note: This car was
fitted with standard R & I acetal wheels, consequently an aluminium wheel was not
tested. Total car weight was 2580 gm. with a load of 663 gm. on the wheel being tested.
Test conditions Towing force gm. Coefficient of Friction
No Tyre wheel locked 47 0.070
Tyre wheel locked 170 0.256
No tyre no wheels locked 1 N/A
Tyre no wheels locked 2 N/A