Rear castor

[Christopher Crowe]

Any thoughts about this entire area would be appreciated. Theoretical or practical. Other than curing bumpsteer, etc., I'm unclear as to what can be gained by puting a lot of castEr in the rear. And yet, I see cars with some pretty extreme rear upright angles. What am I missing?

Thanks,

Chris

[RobLav]

I think coconut oil is more healthy than castor oil.

[Christopher Crowe]

!!!

(it took me a moment to get it, Rob; then I re-read my post!)

[RobLav]

I use rear caster to simply attain the the desired bump steer Chris.

[Jnovak]

Rear caster is the simple way to adjust bumpsteer. However there are a couple of other factors.

In the rear (or the front) there are 2 kinds of wheel steer in the car (not counting driver input).

1. Bump steer or roll steer as some call it.
2. Compliance steer. Compliance steer is the steer effect caused by the compliance (bending, deflection etc) of the mechanical components of the suspension and the chassis. If the chassis and the suspension is exceptionally rigid then compliance steer is a mnor effect. If not it will drive you nuts to try to solve the problem. All cars have some level of compliance steer and the questions are:

1. How much
2. Is the compliance steer understeer or oversteer.

If (on the rear suspension) the center of the contact patch is in front of the compliance axis of rotation then the compliance steer will be understeer. If the center of the contact patch is behind the compliance axis of rotation then the compliance steer will be oversteer.

Do not think that this is a minor issue. I have tested many race cars all the way from NASCAR to F1 in our compliance lab at Ford. It can be a very dominant effect on a race cars transient handling.

Thanks ... Jay Novak

[Paul LeCain]

Rear caster is the simple way to adjust bumpsteer. However there are a couple of other factors.

In the rear (or the front) there are 2 kinds of wheel steer in the car (not counting driver input).

1. Bump steer or roll steer as some call it.
2. Compliance steer. Compliance steer is the steer effect caused by the compliance (bending, deflection etc) of the mechanical components of the suspension and the chassis. If the chassis and the suspension is exceptionally rigid then compliance steer is a mnor effect. If not it will drive you nuts to try to solve the problem. All cars have some level of compliance steer and the questions are:

1. How much
2. Is the compliance steer understeer or oversteer.

If (on the rear suspension) the center of the contact patch is in front of the compliance axis of rotation then the compliance steer will be understeer. If the center of the contact patch is behind the compliance axis of rotation then the compliance steer will be oversteer.

Do not think that this is a minor issue. I have tested many race cars all the way from NASCAR to F1 in our compliance lab at Ford. It can be a very dominant effect on a race cars transient handling.

Thanks ... Jay Novak

I'm not quite understanding this.

I'm assuming that on most formula cars the 'compliance axis of rotation' on the rear will be about the axle. The a-arms will bend that way.

If that is right, then you'd want the rear caster to be positive (upright tilted backward at the top) to prevent oversteer.

Do I have this right?

[Christopher Crowe]

And I too am still a little confused. And this is an area I've never seen found on in the usual available literature.

Most formula race cars seem to have the top of the upright leading the bottom (the upright is tilted forward) to get out the bump-steer. But after that, there's suspension deformation -- and I had no idea that could be substantial enough to even consider. How do you measure it? I assume it may even be a dynamic thing, changing with speeds, wing-loadings, all kinds of things...

Is this where you toss out the book and just play around with the car's caster?

Again thanks

Chris

[Rick Ross]

Compliance steer is mentioned briefly in Gillespie's vehicle dynamics text, but without any diagrams or detailed discussion.

[Jnovak]

I will try to put some diagrams together to explain this. It is all about the forces and torques generated by the lateral forces at the tire contact patch. These forces cause EVERYTHING to deflect. This includes the control arms, the frame etc. Many different geometries can be very functional WRT compliance steer as long as the appropriate structural components are stiff enough.

Think of it this way, lets say that you have a high Df car that is capable of cornering at 3+ Gs in higher speed turns (very reasonable and possible) Let's say the rear end of the car weighs 600 lbs. That means that you will be seeing approximately 1800 lbs of total lateral force at the contact patchs not including any bumps or curbs etc. Try to estimate the forces on the various components of the system including the arms, the frame, the uprights etc. Things are deflecting guys and this can cause significant steering effects on the car.

On a TOP Indy car we measured the compliance steer was 3 times the bump steer. Guess which was more important.

Thanks ... Jay Novak

[Christopher Crowe]

Compliance steer 3 X the bump steer. Well, throw out the static measurements.

And I'm guessing this gets into trial and error settings. Thanks Jay -- and yeah, a drawing would be great.

Truly, thanks alot. I knew nothing of, and have read nothing on this phenomenon.

C

[S Lathrop]

Years ago, I had an experience of the effects of rear caster. I have never satisfied myself as to what I actually was seeing.

The track was Charlotte road course. The cars were 2 Zink Z10s. I had assembled both cars. The drivers were Bruce McGinnis and I.

The Zink Z10 could be bump steered to zero in at 2 different caster angles: one with the top of the upright forward and the other with the top of the upright rearward. The designed position would have the top of the upright forward because of the rake in the car. Bruce's car was setup with the upright rearward and mine was with the top or the upright forward. The bump steer of both cars was almost identical over the same range of movement. On the track, Bruce's car would step-out on corner exit and mine would track out normally. After we reset the rear caster on Bruce's car as my car was, his car tracked out properly.

Needless to say, I enjoyed being faster than Bruce that day.

The only explanation I could give had to do with the difference in the anti-squat characteristic of the car as it was rolling out of the turn. But I have never been satisfied with that explanation. In those days we used very soft springs and the cars had a lot of motion.

[R. Pare]

In the compliance arena, I'd be willing to bet that the "old-fashioned" method of using 2 trailings arms attached to the chassis would produce less compliance than the same design with a-arms attached only on the bellhousing and transmission as per "modern" methods.

[Christopher Crowe]

Interesting...

And I'd bet this is absolutely right.

Haven't put mounts on my "spar" as yet. Those forward arms might better feed the for-aft loads into the actual chassis tubes north of the spar, I'm thinkin'. Think I'll visit my Sus-prog software once again.


Thanks again, Richard!

[jerry freeman]

I let the world see my turd that I will polish for the next few years. We want to see some pictures.

Jerry

[Christopher Crowe]

-- if I were to show it now, I'm afraid I'd dissipate some energy needed to finish it! As you all-too-well know, it's one huuuuge undertaking. We are getting there... soon, soon -- I hope!

Chris