What effect does tire deflection have on camber requirements?

[Dan Nauts]

A member of my team has a modeled suspension that accounts for tire deflection.

One of the assumptions made by the team member is that the deflection is ideal, i.e., there is no contact patch deformation.

He contends that the measured camber in 1.5deg/g of roll per g-force is more than a positive 8 degrees different from the "solid disk" method of suspension modeling. Subsequently, he has designed a scrub radius into the suspension of 1.8 inches to account for the tire sidewall deflection, as well as no intent for caster angle to increase the negative camber on the loaded outside wheel. He believes that due to the 8 degree difference, that he sees as nearly unchanging, that there should be no importance placed on static camber, and decreases the priority of the camber coefficients in bump and roll.

He quotes a lateral sidewall stiffness number for the Hoosier 19.5x6.5-10 R25A compound at 450lb/in from a Hoosier tire tech.

He also intends to have static negative camber of greater than 3-4 degrees negative on an equal a-arm setup (inner control arm mounts are inline in the top view) to attempt to counter act his 8 degrees of positive camber increase.

My questions are,... how far off base is this? In my experience with Hoosier AS03's, they really like alot of negative camber (-1.5 to 2deg), and tire squirm or contact patch deformation was greatly less than any set of Victoracers I ever ran. Do the non D.O.T. Hoosier R25A's have a greater contact patch deformation or sidewall deflection than the D.O.T. race tires I've run? Does the designed scrub radius "use up" the availible slip angle in a real suspension model that includes tire deflection? Does the designed in scrub radius counter act ideal contact patch deflection? real deflection?

[Denny Trimble]

Dan,
Tell your teammate good job for looking at lateral tire deflection in his suspension design. It's on my to-do-list...

I don't exactly understand the "modeled tire deflection" then "no contact patch deformation" comments.

I think of a tire as a "black box" that performs based on temperature, normal load, camber, and slip angle. The tire data gathered was measured at a certain wheel angle, so the tire lateral deflection wasn't measured. In my opinion, let it be what it will be, and control the wheel camber. That's how the data was generated...

Longitudinal and lateral deflection of the contact patch under load will change the force centroid of the contact patch (and so will camber), which changes the effective scrub radius and trail.

The hoosiers we use are bias ply, not like the radial autocross / roadrace tires you're talking about. Ours are less tolerant of excessive camber. But, you're running the really high sidewall version, the 19.5. I think those are designed for F500 cars (bloated karts with no springs). I'd avoid that size like the plague, we used it in '98 and the response time and uncontrolled vertical spring rate / damping of the tire was not to my liking. The 18.0's should be much stiffer in both regards.

I'm not sure what you mean by scrub radius using up slip angles. Scrub radius (front-view offset between the steer axis intersection with the ground, and the contact patch centroid) serves the purpose of giving the driver a steering torque if one wheel locks up under braking. It can also add to weight jacking, which may be a good or bad thing depending on your differential.

I didn't see you guys on the registration list. What are your plans? If you ever feel like bringing your car over here, we can set up a multi-team testing day at Boeing. It would be fun!

[Dan Nauts]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Denny Trimble:
Dan,
Tell your teammate good job for looking at lateral tire deflection in his suspension design. It's on my to-do-list...

I don't exactly understand the "modeled tire deflection" then "no contact patch deformation" comments. <HR></BLOCKQUOTE>

In what I seen with autocross tires is that the inner edge of outisde tire will actually come up off the ground. His model of the lateral tire deflection assumes that the contact patch shape stays constant.


<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>I think of a tire as a "black box" that performs based on temperature, normal load, camber, and slip angle. The tire data gathered was measured at a certain _wheel angle_, so the tire lateral deflection wasn't measured. In my opinion, let it be what it will be, and control the wheel camber. That's how the data was generated...

Longitudinal and lateral deflection of the contact patch under load will change the force centroid of the contact patch (and so will camber), which changes the effective scrub radius and trail. <HR></BLOCKQUOTE>

I guess I'm confused about which direction the force centroid moves in the loaded tire,... If his assumtion is correct it would move inward, and his "actual" camber is based off the centerpoint of the contact patch.

If what actually happens is that there is deflection in the contact patch enough to either pull the inner edge off the ground, or otherwise change the shape of the contact patch, you would think that the force centroid does not move much inward at all.

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>The hoosiers we use are bias ply, not like the radial autocross / roadrace tires you're talking about. Ours are less tolerant of excessive camber. But, you're running the really high sidewall version, the 19.5. I think those are designed for F500 cars (bloated karts with no springs). I'd avoid that size like the plague, we used it in '98 and the response time and uncontrolled vertical spring rate / damping of the tire was not to my liking. The 18.0's should be much stiffer in both regards.

I'm not sure what you mean by scrub radius using up slip angles. Scrub radius (front-view offset between the steer axis intersection with the ground, and the contact patch centroid) serves the purpose of giving the driver a steering torque if one wheel locks up under braking. It can also add to weight jacking, which may be a good or bad thing depending on your differential. <HR></BLOCKQUOTE>

I've only "felt" what added scrub does, I know what it is in the front view, but from what I know of too much scrub is that it will put a kind of dead feeling into the steering, give you the sensation of dragging the inside tire around the turn. My interpretation of what was happening is that the additional scrub placed enough force into the tire when cornering that the availible traction or available slip angle was used to the limit. The team member also believes that jacking forces don't apply in our car (I know better), so there was no consideration to limit jacking forces on the drive end, in fact, I believe his intent was to increase the weight jacking at the back (tubular swaybar).

This guy has made some different assumptions about racing cars than I would have made. I guess I'm tryin to understand where he's coming from by fishing around for input. The big things that are throwing me off are, he intends to setup his Ackerman steering angle in the neighborhood of 135-140deg, jacking forces should be ignored, caster is always bad, static camber doesn't really matter and neither does camber coefficients in roll or bump, less than 1 inch of wheel travel is fine, bump is not important in cornering and a couple other things I'm sure I've forgotten. Nearly everything stated above is opposite from what I know and counter to my experience as a racer.

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>I didn't see you guys on the registration list. What are your plans? If you ever feel like bringing your car over here, we can set up a multi-team testing day at Boeing. It would be fun! <HR></BLOCKQUOTE>

We fully intended to compete this year, but weren't funded due to departmental process, and have had a huge political slowdown in the last few weeks. I am still in shock that 140 teams registered in just over an hour.

Aye,... fun indeed. If we can't fundraise enough money for Student we'll definitely go to a 2 year plan, and try to build in time for as many Nat'l Tours as we can.

There is also the chance that SAE at WSU will be dead after this, last year's team (read: president) failed building a car in time to go to Michigan, they were building major suspension components until 4 days before competition, there was no rolling chassis, the frame didn't satisfy the safety rules, the engine kinda ran, there was no body whatsoever, no wiring,... it is unbelieveable how much they underestimated the project. And it is amazing how much the club has come into question with our ME department as a result.

Dan

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how much driving can you get on tires with 4 deg static neg camber?