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

 

[autogyro46]

Okay, here are two more, but given the distortion in photo lenses, we can only guess at the hardpoints. This is the best I can come up with.
The LCA's, however, do seem to be canted out more.

Also check out #22 in the series.

 

[Yves40]

In an old book regarding an old Mercedes 200 suspension design. I read that they also used this feature. Apparently it enhances the anti-dive feature, but it didn't explain why in detail.

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.

 

[dfoxengr]

please tell us what book that is. I might have it. also the camber change should be explained not as an angle change of the wishbone in plan, but the outward movement of the upright's balljoint, for clarity

 

[Tmoose]

"adjust camber with the corvette suspension by adding shims to the rear bolt of the upper LCA's "

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.

 

[indyracer]

Longer lower arms give you a more consistent arc through your wheel travel, thus the bottom axis of the wheel stays about the same. The longer arm requires a mounting point farther away from wheel than the shorter upper A. The short upper A has a tighter arc through travel; it will pull the camber inward as suspension compresses, which is desired in cornering. Caster change feeds more into the hands of the driver. All angles can be changed to fit the desired performance. Your mounting points, caster and camber change can all be tuned to fit your needed performance: Bump steer, camber and caster gain. you might want a bit of toe added on compression, assuming you're going into a corner, it would provide steering Ackerman, yes?

 

[dfoxengr]

no, bump steer /= ackerman steering

 

[2SlowJoe]

The plan view angle of the LCA inner pivots will effect the rate of change of SVSAA. If the inner pivots were located in the same line laterally, the F/A motion of the LBJ with vertical travel would be a straight line perpendicular to the side view inclination of the inner pivot axis. By adding the plan view splay the LBJ path with vertical travel becomes curved in side view. Adjusting the rate of change of SVSAA will allow tuning the rate of change of antisquat. More antisquat with squat is generally prefered.

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

 

[dfoxengr]

SVSA to my knowledge is not controlled by ball joints.

 

[dfoxengr]

Oops is there a delete, lol. I believe I mistyped. What i was getting at is that yes the ball joint moves in side view slightly with respect to the LCA axis. but by and large SVSA is controlled by angles of the LCA and UCA axes to eachother, but passing through the ball joints. So I dont see much of a measurable difference in SVSA going on here with such small caster angle changes and the small wheel travel.

 

[Tmoose]

Here's some info allegedly about the original stingray geometry. Everything BUT the plan view.

 

[2SlowJoe]

dfoxengr,

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.

 

[mvf4sp]

Hello guys

I don't know if my attached diagram is what is being discussed here. Please share your thoughts, comments, opinions..

 

[2SlowJoe]

mvf4sp,

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...