Slip Angle Confusion

[djones]

I've just been looking into slip angles and have ended up very confused!

I have read on the internet a slip angle is:

"Slip angle is the angular difference between the direction the tyre contact patch with the road is pointing and the direction of the wheel"

Then on a DVD I've just bought (I will most a youtube link later on) he explains it as the difference between the angle of the tyre and direction the vehicle is traveling.

So which is it??

[twoshots]

As I understand it the two definitions essentially expressing the same angle. Direction of tyre contact patch is the same as the direction vehicle is travelling, and the angle of the tyre and the direction of the wheel are also the same thing.

[xpensive]

I agree with twoshots, but I like the first definition better.

[Belatti]

The first definition is right.

the difference between the angle of the tyre and direction the vehicle is traveling

This definition is badly writen. What is "the angle of the tyre" ???
If you ask me, the angle of the tyre is the direction the vehicle is traveling, so according to that definition, slip angle will always be 0

[DaveKillens]

http://www.youtube.com/watch?v=W8UiE7yvO_M

[djones]

The DVD definition seems to completely fall down (it's not just badly written) when you take into account a slide.

e.g you get massive understeer, the slip angle will still be the same as the contact patch stays at the same angle but the direction of the car is now going straight on.

Is that right?

i.e the DVD definition is wrong but then confuses things even more by thinking slip angle is slide angle.

Or am I just making all this up!!?

[zoru]

As I understand, the definition is as follows:

"The tyre x-axis is an the intersection of the tyre
centre plane with the road plane, assuming a flat road.
A tyre slip angle is an angle between the tyre x-axis
and a direction of wheel travel."

The Internet definition is vague.
You can not measure "the direction the tyre contact patch
with the road is pointing", really.
Even when the tyre is free rotating with no slip angle,
most part of the tread in the tyre contact patch is moving
laterally. (The tyre contact patch is not rectangular.)
So only a small part of the contact patch is travelling
in the same direction as the wheel.

The DVD definition may have mixed a tyre slip angle
and a vehicle slip angle a bit.
It is correct when the yawing velocity is zero so that
all the tyres are travelling at the vehicle velocity.
(e.g. big corner R, or a massive final understeer when
the car goes dead straight, no yaw velocity... )

[West]

Take slip angle as the difference, in degrees, of the tire heading (where the tire is pointing) and the angle of the footprint (where it's actually going)

[Scuderia_Russ]

Surely though as soon as you introduce one minute of toe angle then you cannot say that the tyre is going in a particular direction because it is technically angled to scrub.
EDIT: Sorry, just read West's post.