speedread

In "Speed Secrets" pages 36-42 it says: "tires have to slip a certain amount to achieve maximum traction...As your cornering forces and speed increase, the tire ends up pointing in a slightly different direction than the wheel is actually pointing. The angle between the direction the tire is pointing and the path the wheel is following is the slip angle".
[So far it sounds like the tire is pointing a slightly different angle than the wheel due to the tire bending under force, but there is no sliding.]
Then it says: "On a dry track, maximum traction - and , therefore, maximum acceleration, braking, and cornering (maximum slip angle) - occurs when there is approximately 3 to 10 percent slippage.
[ Now it sounds like the car slides slightly to obtain maximum traction. Which is it?
On the one hand the tires bend some with no sliding while you feel the car about to slide; On the other hand the tires slide some.
Slide or no slide?]
Thanks for your help.

Bull

Both. Remember, a small amount of "slippage" and noticable sliding are two different things.

Bob Rouleau

Traction peaks just as the tire begins to slide, then it reduces quite a lot. Modern radial tires have slip angles around 7 degrees. That is if the rim is pointed at 0 degrees in a left hand turn, the car will be following a path of up to 7 degrees. This is the reason you car seems to "drift" to the right exiting a fast left hander for example.

Rgds,

DrJupeman

Slip, when used to refer to slip angles, does not relate to sliding as I always understood it. Its more of a twist between the wheel and tire as Bob points out.

Bob Rouleau

Exactly, DR J has it. Consoder if tires were made out of solid steel, the skip angle would be very very small. Since they are made of rubber (and things) they distort under load. This happens in all directions, cornering, braking and accelerating.

Rgds,

Bull

Bentley does treat this topic in detail in the original Speed Secrets, including a discussion about the progressivity of a tire. Part of his summary states:

"On a dry track, maximum traction - and, therefore, maximum acceleration, braking, and cornering - occurs when there is approximately 3 to 10 percent slippage, depending on the type of tire. This means that a tire develops the most grip when there is actually a certain amount of slippage".

He goes on to talk about the losses due to excess "sliding".

speedread

It sounds like you all are saying: if I am slightly drifting to the right in a hard left hand corner, but not sliding, I am probably close to maximum traction, and the trick is to notice the difference between a slight drift and begining to slide?

speedread

Is there any tire squeal at max traction? I've assumed there is, and drive with a some squeal but little to no sliding as a goal.

kingleh

really all depends on tires...

the less grip u have... the more slip angle there is... as you can see in rally

and in f1.. u dont see anyone sliding at all..... they are sliding.. but just TINY TINY bit..


supercup.. the guys slide pretty good... 996 cup cars like a good bit of slip angle/sliding... but then your tires go away fast...



in a street car on street tires.... a good bit of slip angle... 10 degrees or sumtin.. is usualy the fastest way...

Bull

Also depends on the tires. Street tires generally sound pigs at feeding time. Some R-compounds make some squeal, others very little to none.

JCP911S

Ok, I've got my flamesuit on, but I do not think "slip angle" as anythign to do with the wheels. THis is just sidewall flex.

I think it is the difference between the angle of the tire and the vector angle of the motion of the car. going straight ahead, the car and tire are pointing in the exact same direction. Slip amgel is zero.

As you turn the wheel, the tire's slip angle increases, and that forces the nose of the car to begin to rotate in... but never at the exact angle of the tire. If the car and the tire are at the same slip angle, the car will not be turning in any more it will be steady state.

The tire must maintain some degree of slip angle relative to the car to overcome the car's tendancy to want to go straight. so there must be a constant accellerative force pulling the care around in a turn... this accelleration is caused by the tire's slip angle.

shiners780

This is an interesting thread.

JCP911S, it sounds like you're combining slip angle and yaw angle together. Slip angle refers to wheels, while yaw angle refers to the car itself.

As I understand it, slip angle is the difference between the direction the wheel is pointing (draw a centerline down the center of the wheel), and the direction the wheel is traveling. The tires both flex and slip to achieve a wheel's slip angle. Too much flex and slip and the tires slide, too little flex and slip and the tires (and car) are too slow.

coryf

Slip angle as described in most (if not all) books refers to the angle between the direction the tire is pointing and the direction the tire is actually going. It has nothing to do with tire deflection on the wheel. A tire must slip (either in a lteral angle or rotationally) in order to produce any noticeable traction in any direction.

Driving at slip angles well beyond maximum coefficient of friction is "sliding." But even 7 degrees slip feels like a lot when you are accustomed to the 1 or 2 degrees that you'll get driving agressively on the streets. Consequently, many novice drivers will think they are "sliding" well below the optimum slip angle.

As Leh said, ideal slip angle will change drasitcally depending on application.

TD in DC

I am no expert on this, but this is my understanding.

As some others have pointed out, reaching a slip angle of 7 degrees does not necessarily mean that you are sliding. In fact, you may not be sliding at all. The only thing that has happened is that the tread (rather than the sidewalls) has twisted a small amount. The twist in the rubber of the tread increases the cornering force you can handle before you start perceptibly sliding.

Although probably not technically accurate, it does help to visualize if you think of a gecko climbing a slick window and twisting its feet for a little "extra" grip. Without that little "twist," the gecko might slide down the slick window. With too much "twist," the gecko's grip will break and it will slide down the window. With just the right amount of twist, the little "hairs" on the bottom of his feet just stick better, which is similar to what the rubber in the tread does when twisted slightly.

In practice, you probably will never notice it if you are not developing sufficient slip angle because you are probably going too slowly to slide. If you develop too much slip angle, your traction will start to decrease and you eventually will start perceptibly sliding. In theory you could exceed the optimum slip angle before you start sliding, but I think in practice most people aren't that subtle: you exceed and you start sliding.

If you are perceptibly sliding, your tires are scraping across the surface of the track, which could theoretically be at any slip angle (e.g., flatspotting your tires by braking too heavily in a straight line without ABS, in which case your slip angle would be 0 degrees).

In the end, the slip angle is an interesting fact to learn, but I don't think it changes much about how you drive your car. I mean, AFAIK, you don't really go into a corner thinking hmmmmmm I am hitting a slip angle of 3 so I need to go to 7. Rather, you try to be as fast as possible without losing control: the slip angle will work itself out naturally.

To answer your question more directly, slip angle and sliding are not directly related, although you may well start sliding if you exceed the optimal slip angle for the tire (but of course you could also start sliding even at or below the optimal slip angle).

Doc V.

The direction that the tire's wheel points differs from the direction that the tire travels. The angle which is generated between the tire's and the wheel's directions is the slip angle. Carl Lopez notes that a car entering a corner is changing is direction of travel and rotates around its center as its yaw angle increases. The aim, Lopez adds, is to slide the car to its optimal yaw angle and then to hold it there for the corner's duration. By holding the car at its optimal yaw angle, the driver will operate the car in a range where its tires deliver their maximum grip, and more grip allows the driver to raise his or her car's cornering speed.