[Land-speed] Car Spins

Ed Weldon 23.weldon at comcast.net
Wed Jun 10 22:22:13 MDT 2009


Mayf -- Angular acceleration (about a yaw axis) is equal to the summation of
the torques divided by the polar moment of inertia.  A perfect car that
exactly equal drag and lift on each side of a yaw axis through the mass
centerline and equal driving and/or friction forces from each wheel will
have all the torques balance out to zero; so it will stay in a straight line
regardless of the traction of tires on salt.  But let anything upset that
torque balance such as different traction in the driving wheels or a
difference between the aerodynamic forces on each side of the car (like from
an offset driver position like a street roadster an inline engine header out
one side of the car) or even a driver over correction in steering and now
the traction of each of the 4 wheels on the salt is all that resists the
acceleration torque the rotates the car into a yaw position.
Once the yaw starts if the unbalance of aero forces (which depend on body
side profile) causes enough increasing torque about the apparent yaw
centerline and a rapid increase in angular acceleration the angular momentum
will carry the car into a prolonged spin.  Our experience is that this
happens too fast for driver reactions to control the movement once it
starts.  I'm a mechanical engineer, but I don't pretend to have any deep
understanding of this problem.  The mathematics is scary to me; I think it
would make a good thesis subject for a phD mechanical engineering candidate.
But looking at the problem I see some issues that stand out.  One is that we
know little or nothing about the aerodynamics of cars once their angle
attack leaves the zero degree (of yaw) position.  Relative positions of
center of gravity and crude estimates of center of pressure is the best we
have at this point.  Some wind tunnel test data for increasing yaw angles
would be great; but I doubt anyone has ever done that.
Another interesting issue is what happens to the actual yaw axis once any of
the wheels break traction.  My instinct tells me that once the front wheels
break traction the yaw axis moves toward the rear wheels.  This would make
the "arrowhead" configuration of some streamliners and lakesters and
modified roadsters less effective once a spin starts.  Does this make any
sense?
What effect does a misalignment of front and rear axles do to this? I would
think that if the body is 'crabbed" at a degree off angle to the direction
of travel that would create a bias in one direction.
How about cross winds?  A 15mph cross wind is like the air coming at the car
from 4 degrees off straight ahead.  But how much does that affect a car
because of ground effects?  I think I may find the answer to that on in my
bookcase.
How about "torque lift" on the rear axle?
Could the direction of prevailing winds at Bonneville produce a "grain" in
the salt surface?
Could left handedness or right handedness in drivers have an effect?
Hmmmmm............ I'm not in good shape here.  I took me 2 hours just to
figure out some of the questions to ask....
Ed Weldon

----- Original Message ----- 
From: "drmayf" <drmayf at mayfco.com>
To: "LSR" <land-speed at autox.team.net>
Sent: Wednesday, June 10, 2009 3:19 PM
Subject: [Land-speed] Car Spins


> Been thinking more about spins on the salt. Are there reasons, theory,
> guesses, why a particular car would spin one way and consistently spin
> that direction?   Just curious as it is not my intention to do this on a
> regular basis, lol...
> Any thoughts that can tie the wheel base, track, weight, what the driver
> had for breakfast, etc is appreciated.
> mayf


More information about the Land-speed mailing list