Re: understeer/oversteer definition

[David Lola <djl@alanex.com>]

Great explanation.  Thanks.

David
A4

Harry R Glesner III wrote:

> 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