Caster Angle for Rear

[kevin17]

Hi,

My name is Kevin. I'm designing the suspension for our university's electric formula student team. I'm new to suspension design so my basic knowledge have come from reading Milliken and Milliken's Race car Vehicle Dynamics, Competition Car Suspension, various online threads and online articles.

From what I've read so far, I understand caster angle helps the wheels to self align when there is no pressure on the steering. I've seen some formula student cars with positive caster angle at the rear aswell as front. What is the purpose of having caster angle at the rear if it's a front wheel drive?

I also understand caster angle has a good effect on camber, so the loaded outer wheel gains negative camber as the steering angle increases. Is camber gain the only reason for adding caster angle at the rear?

[SNasello]

Caster angle on the rear axle is not an important metric by itself, as like you said, the rear suspension does not steer (unless you have rear wheel steering). Start at the contact patch and think how the forces will be reacted into the upright and suspension. Perhaps you have heard of the significance of toe base and its effect on complaint? Think also about the forces and moments acting on the upright and not just on the geometry.

Stop thinking about the kinematics and start analyzing the problem from an elasto-kinematic point of view. Don't underestimate the effect of rear toe compliance on handling.

[Claude Rouelle]

Kevin17,

A. Caster Angle can have an influence on bump steer. Some suspension are designed with this effect on purpose.

Think 3D for a while and let's take the example of a double wishbone.

1. In pure kinematics (no compliance), the outboard (upright) pickup point moves on a plan perpendicular to the inboard (chassis) axis, right? If the inboard (chassis) axis is not parallel to the ground, the outboard point will move on plane which is not perpendicular to the ground, still follow me?

2. If you have some sort of anti and if the top or bottom axis of the inboard pickup points are not parallel to each other, the top and bottom outboard pickup point (on the upright) will have a different longitudinal displacement and therefore a caster variation.

3. If there is a caster variation when the wheel goes up and down Vs the chassis there will be a vertical movement of the toe link outboard point that will be different than the vertical movement of the wheel itself.

4. But at the same time there is that toe link. The outboard pickup point of the toe link will move Vs the chassis on a sphere of which the center is the toe link inboard point and the radius is the toe link red end to rod end center.

5. Put 3 and 4 together and you will see that there is a great chance that while the outboard toe link point goes up and down and also rotates around the wheel center, there will be not only vertical but also possible lateral and longitudinal movement of this toe link... and there fore...bump steer.

Now the question is" is bump steer a good or a bad thing and that is another story. I simply would say that this is not necessarily a bad thing.

B. I am 100 % with Stefan Nasello here. Compliance and compliance effect is largely ignored by most FS teams. In fact because of bad design and / or bad manufacturing, or simply by ignorance of what compliance and compliance effect are, or the absence of compliance numbers in their simulation inputs, the vast majority of FS teams operates cars that have more compliance, much more compliance than passengers cars. Just take pictures of FS cars on skip pad or autocross and you would be amazed how many of them run positive camber on the outside wheel.

C. Again 100 % with Stefan Nasello: the rear toe is the worst compliance that you can have on the car control and stability.

FYI the control is the variation of yaw moment per degree of steering (delta) and the stability is the variation of yaw moment per degree of CG side slip angle (beta). The control and stability are worth to be simulated in the entry of the corner or at the apex.

Professional racing drivers can feel as little as 0.05 degree of rear toe setup in control and stability while 0.5 degree of rear or front camber or 0.5 degree of front toe is not or barely sensed by the same professional drivers. When you realize that, you start to measure the importance of the accuracy of the rear toe adjustment.... and the importance of limiting the rear toe compliance.

D. Last observations. Most students think that rear caster trail and rear scrub radius are not important as rear wheel are not steered. OK but the rear caster trail is the leverage of the rear tire Fy and the rear scrub radius is the leverage of the rear tire Fx...that will create more Mz.... that will create more effort on the suspension elements, especially the toe link.... that will create more toe compliance.

[Tim.Wright]

A very enlightening activity I did in the last few years was to make an excel based "solver" for a generic 5 link suspension in which you apply contact patch forces and it solves for the 5 link + damper forces. It will teach you:

1. Regardless of it's name, the toe link doesn't control the toe angle any more than any of the other links (either kinematically or via compliance)
2. "caster" and "kpi" are merely defining an axis about which the toe link reacts forces applied to the wheel.
2a. Therefore you can also define a "kinpgin axis" for each of the other 4 links in the suspension. At this point the term "kingpin axis" seems pointless. I refer to them as "neutral axes" since any contact patch forces aimed at a link's neutral axis will see a zero reaction in force at the said link.
2b According to 1, the "neutral axis" for the toe link is no more important than any of the others for a rear axle in terms of toe control.
3 Toe compliance is not purely a function of the caster+kingpin geometry. It roughly the sum of the following transfer function for EVERY link: ToeChange/GroundForce = (LinkForce/GroundForce) x (LinkCompliance/LinkForce) x (ToeChange/LinkCompliance)

Then, at the end of the activity you will come to the conclusion that there is no such thing as a toe link on rear axle...

I also reinforce what Claude says about controlling rear toe in general. The driver is highly sensitive to it primarily because most of the slip angle a driver feels is made up of the rear axle sideslip + steer effects and secondly because toe control in general is important when you have a high cornering stiffnes to weight ratio.

[mech5496]

1. Regardless of it's name, the toe link doesn't control the toe angle any more than any of the other links (either kinematically or via compliance)
3 Toe compliance is not purely a function of the caster+kingpin geometry. It roughly the sum of the following transfer function for EVERY link: ToeChange/GroundForce = (LinkForce/GroundForce) x (LinkCompliance/LinkForce) x (ToeChange/LinkCompliance)

Then, at the end of the activity you will come to the conclusion that there is no such thing as a toe link on rear axle...

I also reinforce what Claude says about controlling rear toe in general. The driver is highly sensitive to it primarily because most of the slip angle a driver feels is made up of the rear axle sideslip + steer effects and secondly because toe control in general is important when you have a high cornering stiffnes to weight ratio.

That is really really important stuff, which I believe only a 10% of FSAEers ever thought of....

[Tim.Wright]

It's also commonly misunderstood in the industry.

Its fun watching peoples heads explode when you show them a rear suspension with negative "caster trail" but a toe-out lateral compliance steer response.