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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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