LCA angle towards rear question

[Yves40]

Hi,

I have a question about the angle of the lower controle arm when compared to the axis of the frame.

On the car I have here the lower control arm points more inward at the rear than the upper control arm. Besides having an effect on caster, i'm not sure why this is so.

Is there any reason for this ?

Thanks

Yves

 

[GregLocock]

There are all sorts of possible reasions, packaging being one of them. Without a much better description of the suspension it is hard to say.

Cheers

Greg Locock


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[Yves40]

Its a 79 corvette suspension

There are no space issues.

 

[dfoxengr]

so from side view the front pivot is lower than the rear pivot on the LCA? sorry i dont understand the description.

 

[NormPeterson]

Think plan view.


Norm

 

[BrianPetersen]

I'm thinking that it would cause the path of the upper and lower ball-joints to follow a slightly different trajectory, which leads to an anti-dive effect during braking.

 

[NormPeterson]

But does that particular configuration (with the upper arm pivots essentially parallel to centerline in plan view) make for anti-dive or "pro-dive"?


Norm

 

[dfoxengr]

anti-dive is a side view geometry, no?

 

[BrianPetersen]

It can be, but if the control arm axes are not parallel to the centerline of the car, that can also give anti-dive. I know some Honda double-wishbone front suspensions did this. In that case, it was the upper arm pivot that was significantly angled in plan view. Doing this can create a situation where the anti-dive effect gets stronger and stronger as the suspension compresses, but in droop there is not so much effect.

If I understand the original post correctly, the LCA pivot axis is closer together at the rear of the car than it is at the front of the car. If that's the case then the lower ball joint would tend to get pulled forward with suspension compression. When that happens, it tilts the spindle backwards (against the normal braking-torque direction) a little bit. That suggests an anti-dive that increases with suspension compression. Pulling the lower ball joint forward also directly opposes the braking force - again, anti-dive.

The thing about this arrangement is that at whatever ride height where the ball joint is at the same height as the LCA pivot axis, the ball joint will be moving straight up and down, but it will only start getting pulled forward more and more as suspension compresses beyond that point. Thus, no caster change at that ride height, but anti-dive progressively builds (and caster start changing) with suspension compression.

I have a suspicion that it's done this way to have at least some anti-dive, but minimize the adverse effects of changing caster with suspension movement.

 

[dfoxengr]

I was sure that anti geometries were solely side view kinematics.

From RCVD "[the anti effect] It results purely from the angle or slope of the side view swing arm." pg. 617.

Basically it is the reaction that the suspension links take on as a result of braking (and their angle w.r.t. the CG), or in the case of the rear acceleration.

What you are describing is a caster change and the angle (in top/plan view) LCA will probably also result in some toe change with travel as the upright moves and reacts on the tie rod.

 

[dfoxengr]

Front lower
0\ /0

Front upper
0| |0

excuse me but this is a top view of what it seems the OP is referring to. the 0 is the tire while the lines are the axes of the control arms.

 

[Yves40]

Dfoxengr is correct. That is what I was trying to discribe

 

[GregLocock]

The antidive geometry of double wishbone geometry iseasily calculated by wishbone.bas available on the locost website. Once you know the svic location a simple fbd will give you the weight transfer etc which is probably what you really want.

Cheers

Greg Locock


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[autogyro46]

Forgive me for being a bit late, but there is an interesting variation on this theme: the rear arms on the Pagani Zonda R.(See attached)

It appears that both upper and lower arms are canted outward at the front: the top one at~10 deg or so, and the lower ones maybe a bit more. I would think this would turn the virtual swing arms into something like semi-trailing arms, giving a bit of bump steer. Note that the toe control link is fixed in relation to the upper arm.
I'm presuming this is more of an anti squat technique. Has anyone seen this elsewhere?

 

[autogyro46]

Oops! the jpg somehow didn't take. Here's the Pagani photo link. It's the tenth image: the ghosted overhead view.
Apologies to all.

 

[dfoxengr]

again, probably not anti-squat since the zonda's view in plan (top). quickly looking at it would say it is going to toe out on bump, so that the outside wheel in a turn gains toe out. also on launch added toe which could be used to increase traction due to a higher slip angle. many possibilities, but again hard to tell everything without knowing what the SVSA is doing, rear view swing arm is doing, etc.

 

[GregLocock]

If weird angles on wishbones appeal to you check out the Mustang Cobra IRS.

Lincoln LS is probably along the same lines.



Cheers

Greg Locock


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[autogyro46]

Why, oh why, would anyone choose to add toe out to the outside rear wheel on bump?
Maybe there's a whole lot of designed-in understeer somewhere else, but I rather doubt it.

 

[GregLocock]

Perhaps in the great tardition of American sports car design because they don't allow the suspension to move.

Anyway, without a top view and a front view you can't tell whether it'll be toe out or toe in.

Cheers

Greg Locock


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[dfoxengr]

Right I believe what Greg and I are alluding to is that a complete 3 view drawing is required to ascertain more about the design.