LCA angle towards rear question
LCA angle towards rear question
(OP)
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
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





RE: LCA angle towards rear question
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: LCA angle towards rear question
There are no space issues.
RE: LCA angle towards rear question
RE: LCA angle towards rear question
Norm
RE: LCA angle towards rear question
RE: LCA angle towards rear question
Norm
RE: LCA angle towards rear question
RE: LCA angle towards rear question
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.
RE: LCA angle towards rear question
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.
RE: LCA angle towards rear question
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.
RE: LCA angle towards rear question
RE: LCA angle towards rear question
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: LCA angle towards rear question
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?
RE: LCA angle towards rear question
Apologies to all.
RE: LCA angle towards rear question
RE: LCA angle towards rear question
Lincoln LS is probably along the same lines.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: LCA angle towards rear question
Maybe there's a whole lot of designed-in understeer somewhere else, but I rather doubt it.
RE: LCA angle towards rear question
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
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: LCA angle towards rear question
RE: LCA angle towards rear question
The LCA's, however, do seem to be canted out more.
Also check out #22 in the series.
RE: LCA angle towards rear question
Maybe i should mention that one can adjust camber with the corvette suspension by adding shims to the rear bolt of the upper LCA's and this creating more angling of the upper arms in plan view.
RE: LCA angle towards rear question
RE: LCA angle towards rear question
The 1974 Chevy Shop manual says to add/remove shims equally at the front and rear of the upper control arm shaft to adjust camber. Swapping shims from front to rear or vice versa is more of a "pure" caster adjustment. Changing shims at just one end of the shaft would seem to change caster and camber at the same time.
RE: LCA angle towards rear question
RE: LCA angle towards rear question
RE: LCA angle towards rear question
All that said, the main reason plan view splay is added to control arms is package. There is often a pesky fuel tank in the way of the ideal LCA fwd pivot ideal location.
-Joe
RE: LCA angle towards rear question
RE: LCA angle towards rear question
RE: LCA angle towards rear question
RE: LCA angle towards rear question
The rate of change of Side View Swing Arm Angle (SVSAA) definately is effected by the F/A path of the upper and lower ball joints. The F/A path of those joints is effected by the plan view angle of the control arm pivot axis.
I did use a little symbol speak when I said SVSAA, and noticed that you responded SVSA (Side View Swing Arm). The SVSA is not an actual arm, it is a thought aid based on the instant center of motion of the wheel center. As the wheel moves up and down the instant center moves as well.
The wheel motion is controlled by the ball joint motion as they are connected via the knuckle. The more plan view splay the control arm pivots have, the more the control arms act like semi trailing arms.
RE: LCA angle towards rear question
I don't know if my attached diagram is what is being discussed here. Please share your thoughts, comments, opinions..
RE: LCA angle towards rear question
That is what is being discussed here. I agree with your analysis.
The plan view angle of the control arm pivot axis will effect the F/A motion of the wheel center with vertical wheel travel. The SVSA concept is a visualization tool that helps a person see the motion of the wheel in the side view. On an SLA, there is no physical side view swing arm and the calculated SVSA angle and length will change as the suspension articulates.
Since the plan view angle of the control arm pivot axis effects the F/A tragectory of the ball joints and therefore wheel center, the plan view angle of the control arm effects SVSA and therefore anti's.
Taken at an extreme (making the plan view axis 90º to F/A, and attaching the knuckle directly to this arm) you would end up with a trailing arm suspension. The SVSA would then be the instantaneous angle of the trailing arm. The instantaneous angle of the arm would clearly change with suspension travel, likely going from recession to procession with travel as in many compound cranks and 4 link rear suspensions...