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Slip Angle
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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 (neutralcornering).
Figure 1. Definition of slip angle. Note how the contact patch of the tyre need
not be in the same orientation as the whole wheel, often lagging a few
degrees behind.
Now let us consider what happens when the slip angles exceed our optimal
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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 flexand 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.
Slide angles and slip angles are VERY DIFFERENT. Inertia determines the
direction the car will travel if it slides (centripetal force). If you lose traction
you will slide in the direction you were travelling at the time the slide starts.
So long as you have traction you will travel in the direction the contact patchpoints, not the tyre. Slide angle is the angular difference between the
direction inertia sends you and the direction your tyres are pointing while slip
angle is the angular difference between the direction your contact patch is
pointed (thus, the direction the tyre moves) and the direction the tyre is
pointed.
Decreased traction reduces slip angles and increases slide angles!
Figure 2. Lateral force versus slip angle. Note that lateral force progressively
increases from 0 degrees slip to a peak at about 8 degrees. Thereafter, the
adhesion is lost, and the lateral force that the tyre is capable of generation
progressively deteriorates.
What else affects slip angles? Acceleration and braking, tyre profile size and
tyre camber.
Acceleration and braking affect traction primarily because of weight transfer
and toe-angle changes.
The greater camber, the greater the camber thrust. Camber thrust attempts
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to turn your wheel into a turn in the direction of lean. Thus, greater camber
thrust yields smaller slip angles.
Given that the rear camber of the MGF is greater than the front, you should
expect that the rear tyres are almost always operating with a lower slip angle
than the front tyres. (I.e., your car tends to under-steer.)
What changes a slip angle into a slide angle? Excessive slip angles!!! That is,
a slip angle is so called because the part of the contact patch that is to the
outside of your turn is moving faster than the wheel itself is in the direction it
(the contact patch) is pointing while the part on the inside is moving more
slowly. (Exactly like camber thrust.) Since the outside part is moving faster
than the tyre it must be slipping. The inside part is gripping better than it
would if moving in a straight line. For this reason the contact patch 'walks'
itself into the turn.
The greater the slip angle, the larger portion of the contact patch that is
slipping. At some point there is so little part of the contact patch that is not
slipping that traction is lost and the tyre begins to slide. Until shortly before
then, slide traction increases. Note, however, that traction is generally not
lost all at once. Rather than an abrupt loss of traction, it tends to be lost
gradually. (Thankfully!!!)
For more info on slip angles and handling, click here.
http://autozine.kyul.net/technical_school/handling/tech_handling_4.htm
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