rear suspension kinematic influence of scrub,Caster etc.

[Ben A]

Hello,

i´ve a question of the kinematic fort he rear axle.

What are the most important things?

In my opinion:
Big caster around 6-8° for good stability
Not to much KPI ?
Zero bump steer

What are important to know about Scrub radius and trail on the rear Axle.
I now for the front it is important to have a trail for Steering feedback and a scrub radius for Steer und brake feeling. But on the rear I can´t imagine what is here important to know.
And how the thing influence the car.

Hope you could tell me a few things to start in the ball park and to analyse the effects.
Perhabs a little discussion on the topic too.

Thank you.

[exFSAE]

I'd like to hear how and why caster on the rear end is going to give you stability.

[Ben A]

Positive caster tends to straighten the wheel when the vehicle is traveling forward, and thus is used to enhance straight-line stability? Or not? But I read from your answer that this refers only to the front axle?

Why i´ve seen lot´s of car with much caster poitive and negativ.

So the Caster on rear gives me only more space for package the Toe link to make long to get it stiff enough? So should the scrub radius be small in order to reduce the torque in the link. right?

Thank you

[js10coastr]

Originally posted by exFSAE:
I'd like to hear how and why caster on the rear end is going to give you stability.

Stability from caster is heavily dependent on the ackerman. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[EPMPaul]

What caster will influence in the rear is compliance. More caster all other things equal will give bigger caster trail, hence more moment about the rear steering axis. From there depending on what your geometry and your link stiffness' are, you may get compliance understeer or oversteer.

Also, it'll affect how much gyroscopic effects you get. Again that'll change your compliance. However, gyroscopic effects are fairly minimal compared to the forces generated by your tire.

[Ben A]

Ok, thank you for your answer. It draws another view on that topic to thing about.


So summarized:
In general i can say the rear axle musst be stiff enough to get no self steering effect caused by any force and moments. Thats easy and cleary ;-)

So zero scrub radius is better because of the Moment around the steering axis during braking?
A compromise is to chose of caster (try to get small) and get a big toe link and a good position to achieve zero bump steer.

Thank you to all.

Ben

[Buckingham]

the rear steering axis.

There is no rear steering axis, as none of the three spherical joints used to constrain the plane are actively "steered".

[NickFavazzo]

what about toe link compliance? toe adjustments? I believe there is a rear steering axis.

(note; I'm not a vd man..)

[Mbirt]

Originally posted by NickFavazzo:
(note; I'm not a vd man..)
+1. Not a fan of VD here either.

[Buckingham]

what about toe link compliance? toe adjustments? I believe there is a rear steering axis.

If your "Upper Balljoint" is more compliant than your "Toe Link", then won't the upright pivot about the axis defined by the "Toe Link" and "Lower Ball Joint", then isn't that your "Steering Axis"?

Note that I use quotes because you could call any of the spherical joints your "Toe Link". We just call the most adjustable one the Toe Link out of convenience, it has nothing to do with the physics.

Yes there is an "axis about which the wheel complies", but it is likely an arbitrary axis in space since all three of your spherical joints deform in some direction some amount.

If your "toe link" is 20x more compliant than your ball joints to the point where you can confidently say that the wheel pivots about the axis defined by the other two points, then you have bigger problems (unless it's by design).

[BillCobb]

But wait, there's more: Caster is a front suspension primary design factor first as an augmentation of steering wheel torque gradient so that limit performance is not compromised by a negative value resulting from decreasing tire Mz. Also, another way to approach caster is as the rate of change of camber with steer. So, for large steer angles (as in low to moderate speed cornering venues), it can provide some extra side bite. Obviously tire properties direct the way caster values should go.

In the rear, lateral force and aligning moment deflection steer need to be accounted for and some negative caster could be considered for attenuating these effects. Scrub radius (front or rear) is NOT a fixed, rigid value set by dimensional specification. If you have the tire data, divide the Mx by Fy to get an important scrub radius modifier. in heavy cornering and under large Fx power application, your effective scrub radius can/will be very different than some value picked out of the air based on a 'rule of thumb' dimension. The sign of this dynamic value will probably be different, too. But it depends on the tire, its rim width, pressure and sidewall construction. Again, you really need the tire data for this end of the car, too, before you set hardlines down for a build.

One more thing, a K&C test usually shows that the frame and attachment hardware is often the spongy part of the compliance recipe, not the links, joints or fasteners. Yes there have been real, laboratory K&C tests that I have seen done on FSAE cars over the years on MTS, Anthony Best, and Lotus testers by companies who make production cars and tires.

I wonder how many of you have ever asked for this to be done as a 'favor' as part of a sponsorship ?

The design goal should not be compliance elimination, but compliance management. It can never be eliminated because part failure is sure to result.

[BillCobb]

That would be Mx / Fz. And this as a gradient with Fy gives lateral spring rate.

[Zac]

Originally posted by BillCobb:
That would be Mx / Fz. And this as a gradient with Fy gives lateral spring rate.

I think it's important to point out that this will give you the location of the center of pressure of the contact patch, not the centerline of the tire.