IRS Angles

[540ZCar]

I've searched but didn't find anything. I was wondering what kind of effect that angles such as SAI and caster have for the rear suspension. I've heard that they don't really matter but I still want to know if they have an effect at all. If you have any other information go ahead and add it, you can't have too much information.

 

[GregLocock]

The suspension I am used to runs massive negative castor, so I'm on board there.

Not too sure I agree about SAI, what do you mean exactly?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[540ZCar]

SAI, the same thing as Kingpin Inclination. Basically from a frontward's veiw a line drawn going from the top ball join through the bottom ball joint. I don't know whether having a larger or smaller angle there would help rear end grip or not.

As for the caster, is whether it's positive or negative the same veiw as with the front of the car (positive to the rear) or does it change to where positive is towards the cabin? How does the negative caster react with your car Mr. Locock?

 

[GregLocock]

Oh, I would have called that KPI. SAI is usually swing arm inclination

IRS KPI doesn't matter, that provides camber from toe, and there is so little toe that the KPI doesn't make much odds.

rear suspension kinematics ARE important, as is the compliant behaviour, but the angle in both views of the spindle steering axis is relatively insignificant.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BrianPetersen]

The terms "caster" and "steering axis inclination" imply a suspension design with upper and lower ball joints (the flexible upper mount of a MacPherson strut counts as a ball joint). Many (most?) rear suspension designs don't really have anything analogous to a "steering axis". The camber and toe and compliance behaviour matter, but it's not done by using upper and lower ball joints and a steering arm. For examples, look at any solid-axle or twist-beam or semitrailing-type design - although the rear wheels can "steer" depending on the linkage design, it's not done by defining a steering axis between ball joints. Multilink designs often have arms pointing in various directions but not necessarily in any places that correspond to an upper and lower ball joint.

 

[540ZCar]

So then if I had a 15* line going through the pivot points on the rear hub, it wouldn't have any different effect than one with a 0*? And since the rear hub doesn't toe in and out (hopefully, depending on bushings) caster doesn't play no role at all?

 

[GregLocock]

That's about it. More accurately there are effects, but you need to do a compliant analysis to find them, and the only ones that really affect the car at all are camber/Fy stiffness and toe/Fy stiffness and toe/Mz stiffness.

For the kinematics think about your bump steer curve, GRCH, camber curve, semitrack gain. Everything else is second order stuff.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[WolfHR]

Greg, am I correct in assuming the most important factor re. KPI geometry on the rear would be mechanical trail? And would it be worthwile to play around with ball-joint placement to try and 'optimize' ball-joint loads for that geometry?

I'm toying with a problem where I have to use same wishbones both on front and rear suspension, and am a bit at loss as what I should try to achieve.

 

[GregLocock]

Same spindles as well?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[WolfHR]

Yes, but I'll try to circumvent that by using bolt-on mounts for UBJ and toe-link/track rod, like Lotus Elise front hub carriers. That way I could at least toy with a thing or two.

 

[hemipanter]

KPI affects indirectly wheel constant through changes in scrub distance, provided the instant center distance is short. It will be easier to keep track of the balance between front and rear axle if you use the same geometry.
Roughly put...
as for the load of the suspension, it happen in different ways depending on the direction in which it occurs, like acceleration, braking, cornering, etc. But in general, we may reduces the forces on the joints and upprights by giving them a more centered possition in the wheel.
It is also important that the load describe a straight line within the tubes of the A-arms and ball joint bearing centre.
Goran

 

[WolfHR]

Thanks, Goran.