09-04-2014, 02:24 PM
Slip Angle
Slip Angle.pdf (Size: 212.02 KB / Downloads: 19)
Slip angle is the angular difference between the direction the tyre contact patch with the road is
pointing and the direction of the wheel (figure 1, below).
This sounds odd - you might expect that the tyre tread would point in the same direction as the
wheel wouldn't you? It doesn't because being made of rubber, the tyre sidewalls deform, and the
tread pattern itself can 'squirm' when the wheel is turned from the straight-ahead.
In fact, modest slip angles are 'good' as tyres generate progressively more grip with increasing
slip angles (figure 2, opposite), albeit up to definable limit where after no further grip is
generated. Thereafter, increasing slip angles are 'bad', and the tyre will tend to loose grip.
Therefore, it should be apparent that if the slip angles for the front and rear tyres are the same
(the tyres front and rear are generating similar levels of grip), the car will steer essentially as if
there was no slip angle at all (neutral cornering).
Now let us consider what happens when the slip
angles exceed our optimal grip generation limits.
If the rear slip angle is larger than the front one
you have a condition known as over-steer while if
the front slip angle is larger, the condition known
as under-steer results (also see the explanation
on the tracking page).
You will recall that a slip angle results from a
combination of tyre sidewall flex and tyre traction.
Note that if there is no traction (on ice, for
example), then the slip angle will become
essentially zero. On the other hand, if a wheel
travels in a direction other than the one its contact
patch is pointing, then you have a SLIDE angle
rather than a slip angle.