giskard New Reader
12/22/08 9:41 a.m.

I'm talking about this type of curves, but for a wet surface:

I have Haney's excellent book "The Racing & High-Performance Tire" (here's an excerpt) but it has no tire slip angle curves for a wet surface:

As an injuneer, I like to see the curves in order to understand this behaviour: ;)

blaze86vic New Reader
12/22/08 12:09 p.m.

I've never seen a wet slip angle plot before. My guess is that the slip angle will grow faster, and the loss of traction would be much more abrupt. I know that the difference from static friction to dynamic friction is much greater in wet conditions (which is partially why I'm guessing the curve will be sharper).

AngryCorvair Dork
12/22/08 2:45 p.m.

shape is very similar, but peak is lower. tire manufacturers hold this data very close to their chests. if you want to see the curves, you either have to be a big-spending customer (ie an OEM) or you have to generate them yourself.

curtis73 Reader
12/23/08 12:47 a.m.

It would be the same basic curve. You can't predict coefficient of friction on wet surfaces, and even if you could, that's not what those charts are measuring.

If you notice, that charts plots longitudinal force versus longitudinal slip. It calculates how much slip angle the tire provides based on the force provided. It has nothing to do with coefficient of friction between the tire compound and the road. That chart assumes frictive action.

That chart is demonstrating slip angle... not lateral slip. That chart assumes that the tire is frictive to the road. The more lateral load you apply within the frictive realm, the more the tire deflects. Once you exceed the frictive realm, the tire "breaks loose" and the slip angle decreases while the tire slip increases.

So, for wet surfaces, the chart is the same. Its just that you may not reach those peak slip angles before you overwhelm the frictive traction between the tire and road. The slipperier the road, the less likely you are to reach the peaks on that chart.

Slip angle is based on tire deflection, not actual lateral loss of traction. For a more in-depth understanding of slip angle, I might suggest Milliken's "race car vehicle dynamics." Good read.

AngryCorvair Dork
12/23/08 9:06 a.m.

to curtis73:

if you're talking about this chart:

then I'm going to have to say, "not exactly."

also to curtis73: the following definitions are not intended specifically for you, rather they are for the entire reading audience, who may not have the same background as you and might not understand all the terms used.

this chart is showing normalized longitudinal force on the Y axis, versus longitudinal slip on the X axis, for a variety of slip angles. (edit: i originally said "steer" angles)

Normalized means divided by the vertical force on the tire as tested, which is why it says (Fx/Fz) on the Y axis title.

Longitudinal means fore-aft, ie acceleration or braking. Conventional definition is "translation speed minus rotation speed, divided by translation speed", such that +1.0 on the X axis corresponds to zero rotation speed. in other places you'll see this defined as "100 percent slip". So, to the right of the Y axis is braking, and to the left of the Y axis is acceleration.

the data were taken at specific slip angles and each set of data is graphed, which is why there are curves noted "alpha =" 4.0, 8.0, 12.0, 16.0, and 20.0. there is no calculation of slip angle here.

slip angle is the difference between the direction the tire is pointing and the direction that it is actually traveling. in this case they're measuring it in degrees.

the point of this chart is to show that longitudinal force capability decreases as slip angle increases.

the purpose of normalization is to allow both axes to have the same scaling, from -1 to +1.

from the magnitude of the peaks, we can be fairly sure that the data for this graph were generated by testing on dry high-coefficient surface (highest peak is about 0.87, which occurs at the lowest steer angle tested, alpha = 4 degrees). and it does indeed have something to do with the coefficient of friction between the tire and the road, because on ice the peak normalized longitudinal force would be around 0.1, rather than 0.87.

If you're familiar with the friction circle concept, this graph just shows that concept in another way, ie as steer angle increases, maximum longitudinal force capability decreases.

EDIT: I love this stuff. We cover some of this in the Society of Automotive Engineers' excellent seminar "Applied Vehicle Dynamics." You should all take this three-day class, held each May and November at the BMW Performance Center just outside Greenville SC. Learn more about this class here

did i mention that i'm a co-instructor?

curtis73 Reader
12/23/08 10:51 a.m.

Dude... I'm there. How much and where do I sign up. I can't get enough of this stuff. I bought the Miliken book and literally couldn't put it down. I read it cover to cover over a 5-day period.

I had to dust off some of my decade-old calculus, but that kind of stuff just gets me going. Can't get enough.

AngryCorvair Dork
12/23/08 11:56 a.m.
curtis73 wrote: Dude... I'm there. How much and where do I sign up. I can't get enough of this stuff. I bought the Miliken book and literally couldn't put it down. I read it cover to cover over a 5-day period. I had to dust off some of my decade-old calculus, but that kind of stuff just gets me going. Can't get enough.

click on the "register" button next to the dates found here

it's $2350 for non-members of SAE or $2150 for members, for three days of the coolest engineering class you'll ever take. and if you pay the non-member fee they comp you a one-year membership ($100 value iirc)

in this class you'll get to drive new BMW 335i sedans, plus a couple of other BMW vehicles for comparison in certain learning modules. the last couple times i've been there, we drove 650i as the comparison vehicle, and instructors got to sample the new M3 and the X6 on the autocross course.

Our Preferred Partners