Grand Prix Racing   The Science of Fast Pinewood Cars 
Once again, the answer must be a definite, MAYBE! As with many of these potentially timesaving techniques, the result depends on many factors. Here are some of the factors to consider:
 Wheel moment of inertia  it is reduced by cutting the tread and that's good!
 Wheel weight  it is reduced by cutting the tread and that's good!
 Wheel tread friction  it may get worse if you can't smooth the wheel down again
 Wheel edges  may contribute to drift if left sharp
 Wheel balance  may cause vibration or wobble
 Wheel torquing  the wheel may get jammed up against the bottom of the axle hub
The plan of attack here is this: First, we'll run a computer simulation of a car with cut treads and one with out to illustrate the possible advantage. That will take in the discussion of factors 1 and 2 while finding the right parameters to use in the simulation. Factors 3, 4 and 5 are covered elsewhere in this manual. So after some brief comments about them, we'll conclude with a discussion of factor 6.
In order to stage a virtual race of a tread cut car and a normal one, we need to find the moment of inertia of a cut tread wheel. First, how much tread is to be cut off? From the models it is clear that the more that is cut off the better. But there still has to be enough to support the car! Let's use 0.1 inch of tread left after cutting.
Now, we can apply the method used to calculate the moment of inertia for an AWANA kit wheel and just change the tread cylinder width from 0.315 inches to 0.1 inches. The worksheet looks like:
Cylinder  Radius  (in)  Width  Volume  %Mass  Ii/mi  Ii/m 
outer  inner  (in)  (in3)  (in2)  (in2)  
1  0.118  0.049  0.276  0.01001  13.953  0.008163  0.00113898 
2  0.266  0.118  0.016  0.00284  3.959  0.042340  0.0016762 
3  0.309  0.266  0.236  0.01835  25.578  0.083119  0.0212602 
4  0.531  0.309  0.033  0.01939  27.028  0.188721  0.0510075 
5  0.591  0.531  0.1  0.02115  29.481  0.315621  0.0930482 
Totals  V=0.07174  %m=100  I/m=0.16813108 
to get the weight of the cut wheel, we need to assume the tread plastic has the same density as the rest of the wheel. It's volume was reduced from 0.11721 in3 to 0.07174 in3. The original weight was 0.08333 oz, so it is now 0.08333(0.07174)/0.11721 = 0.05100 oz.
Weight(_0.05100_ oz)/386.088 = Total Mass (0.0001321027 ozs2/in).
I/m (0.16813108) x Total Mass (0.0001321027) = I
Total Wheel Moment of Inertia, I = 0.000022211 ozins2.
An uncut AWANA kit wheel has a moment of inertia of 0.000048637 ozins2. so this is about 46% of that original value. With four wheels on the track, a car with tread cut like this should behave like one that has two uncut wheels lifted off the track (a tricky but possible balancing act). If one of these is lifted, the car should have even more advantage  as long as the other factors listed above prove beneficial in your situation.
Using the "typical" parameters for friction discussed in the question on the worst source of friction, we can run our race! With all four wheels on the track, the result is:

That's fast!
Cutting a wheel doesn't reduce tread friction on a smooth surfaced track. However, it can reduce it on a bumpy one.
If the edges of the narrowed wheel are sharp the wheel may tend to follow the wood grain in the track if it is not smoothed out. A rounded edge on the wheel may improve this tendency, but probably won't eliminate it.
A wheel with more weight on one side than another will wobble. Cutting a wheel perfectly is not easy without the right high speed rotary tools. Wobbling raises and lowers the center of mass a minimum of a hundred times in the course of a race, multiples of that if there is more than one irregularity on your wheel. Most of the energy used during the up motion is returned on the downward motion. But some is lost to additional air movement and friction.
Wheel torquing happens when the wheel is no longer able to keep the axle on the bottom of its bore. One end tips up and the axle touches the wheel bore on the top at one end and the bottom at the other. Torquing is generally not good. It always increases axle friction by a factor of (n+1)/(n1) but can be worth it in some cases.
A kit wheel cocked at 27 degrees from vertical will experience about three times more axle friction (n = 2). Axle friction reduces to the "normal" amount as the slanting angle decreases (n > 30 or so, O is less than 1 degree).
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Grand Prix Racing   The Science of Fast Pinewood Cars 
Copyright © 1997, 2004 by Michael Lastufka, All rights reserved worldwide. 