understeer/oversteer definition

[Harry R Glesner III <hglesner@ast.lmco.com>]

GANG:
     To understand understeer and oversteer, we first must look 
at the contact patch, i.e. were the tire is meeting the road, 
and not think of that meeting in static terms. It is in three 
dimentional motion, I'm sure even that is a over simplification, 
but not only is it rolling over the ground, but deflecting 
left/right, and twisting at some angle relative to the steel 
wheel and suspension components, this twisting is known as "slip 
angle". Don't be confused by the turm slip angle, the tire is 
still twisting, not so much "sliping", if we have gone to really 
slipping over the ground we have exceedied the coefficent of 
friction, and the tire is no longer doing its job. 
     As a car go through a corner, centrifugical forces transfer 
the center of mass around the center of gravity generating more 
load on the outside tires. As the driver inputs steering input 
by turning the steering wheel the desired change of direction is 
tranmitted to the road wheel (through whatever linkage the  
manufactuer designed) and generates twisting forces to the tire.
     This difference of angle between the road wheel, and the 
rubber tire is slip angle. The greater the vertical load, the 
higher the coefficient of friction the greater the slip angle. 
Now understand this is a very simple model we are building here, 
believe me the permeatations of this situation, will send mere 
mortals for the schnapps bottle.
     If the front tires generate more "slip angle" than the rear 
tires i.e. the front tires are on a larger radi than the rear, 
the front of the car will be on a arc leading to the outside of 
the turn i.e. understeer, if the rears have the larger slip 
angle the rear tires are on a larger radi and so we have 
oversteer. 
     Now simple tuning of the system with "sway bars", or anti 
sway bars. The larger bars increase vertical loading of the 
contact patch, increasing coefficient of friction, making the 
slip angle larger. So which ever end of the car is miss 
behaving, i.e. you want to cure understeer, you would put a 
larger dia. bar in the rear, or shorten up the attachment point 
and adjustment links on your fancy Weltmeister rear bar.
     Shocks (kinetic energy absorbers), and spring are other 
ways of tuning the car, even to the point changing bound v 
rebound to control weight transfer through out the chassis at 
different phases of a turn (enterance, middle, exit) Power down 
thru the exit phase will produce a change in weight distribution 
changing the slip angles again.
     Tire pressure has some control over slip angle also, the 
more pressure the higher the spring rate of the tire itself and 
the more resistant to twisting it becomes, atleast to a point. 
Then we run the risk of really elevating the tires temp and have 
the potental performance degrade.
     I can't recommend any of Carol Smith books to highly. They 
are the best, most of this data can be found in "Tune to Win". 
His other books are "Engineer to Win", "Prepare to Win". You 
would not believe at SCCA race weekends how many tool boxs have 
copies of these books in them.
     
     Sorry this was long, just not a subject to lend itself to 
being short.

Rick Glesner
Littleton, Colorado
'91 90q20v
'82 911 SC
'83 Spec Rx-7 (for sale, anyone want a turn key race car?)
looking for a 944 for PCA group I