Dan--
Front wheel slip? Theoretically there will be some. 2% was an unsupported
guess on my part for what's going on when conditions are less than perfect.
With the car running straight, weight on the wheels, good surface conditions
and low speeds my guess is that you couldn't measure wheel speed difference
relative to ground speed without pretty sophisticated instrumentation. If the
car is pulling left or right, the front is starting to lift, the course is
bumpy then the wheel has less inclination to act like a gear running on a
rack. The other variable is the aerodynamic loading caused by the airflow
over and around the wheel. I'd guess that the load over the top of the wheel
is higher than the bottom resulting in a net braking torque on the wheel. (but
it might be just the opposite) The power to turn the wheel against this kind
of torque has to be transmitted through the contact patch between the salt and
the wheel. Does this make sense?
Maybe this is an area worthy of discussion and inquiry.
I just received the following comment from Rich Fox. Another good data
point.
"The one time I went around, in hindsight, it was long gone by the time I
realized something was not right. Car kept going left. I kept putting in
pressure to the right. Still going left. Realized I was going to run over
the lights. Jumped off throttle. Here we go. Car must have been pretty
sideways by that time. Rich Fox"
I'd theorize that Rich's experience came about as a result of unbalanced
thrust from one drive wheel having better traction than the other.
(weight/torque loading?) As the car speed increases or the car passes through
an area where front wheel traction is reduced the aerodynamic forces acting on
the front side of the car overcome the resistance of the front wheels to
sideslip. The wind pushes the car past the threshold of unstable equilibrium
and around it goes.
Could it be that the 10-15 mph north wing we often see in the afternoon at
Bonneville actually helps?
Is this one of the reasons why modified roadsters are more stable with their
high polar moment of inertia (weight concentrated in the ends of the car
instead of around the middle) and low down front sheet metal?
Ed Weldon
|