Lower control arm position

[colinmseries]

I'm furiously reading Staniforth et al and have a specific question I can't find explained.
All the examples for unequal A frames I see, set the lower arm position as horizontal at neutral bump/droop when viewed from the front. I've search photos from F1 etc. and always see the lower arm level with the ground.
For a road car would this always be the best position, or are there instances of an inclined lower arm, at normal ride height, being a better compromise?
Thanks, Colin.

 

[NormPeterson]

Apparently it's part of a better compromise in the independent rear suspensions found on SUVs. Lower links are clearly at an upward angle toward the diff in all the ones I remember seeing. I'm guessing that the back seat / third seat ride quality for most "normal" driving is better that way than with a lower geometric roll center and stiffer rear suspension tuning.


Norm

 

[Fabrico]

Most serious off-road race trucks and buggies are also kept at or very close to level. For one thing, it's probably the only way you can go camber/straight/camber. It also provides lower overall suspension scrub and steering bias.

 

[NormPeterson]

Maybe I should have typed 'roadgoing SUVs", as all of my observations have been made while stuck behind one or the other of those things in traffic. Since under those conditions I can't see much else that I'm generally and intentionally headed toward, I might as well let my curiosity make the most of it. Where I live, most SUVs go no further off road than a gravel driveway. Intentionally, anyway. [/mild rant]

 

[GregLocock]

The enagle of the lower arm at 'design' has a dominant effect on how neutral the toe and camber curves are. You can certainly design suspensions around various levels of droop at design - some high ground clearance designs do exactly this.


However, it makes the vehicle very sensitive to jacking - if you apply a lateral force at the CP then the inner end of the arm will be pushed up. Not good.


Cheers

Greg Locock

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

 

[colinmseries]

Ok thanks. My thoughts were that only bump, as on the loaded side in a corner, really matters. The droop side I assumed did so little work that it didn't matter.
My readings so far seem to teach that it is the top arm length that controls negative camber gain through roll and so I wondered if the level neutral position for the lower arm really mattered that much.

 

[hemipanter]

I usually start of calculating with the lower A-arm parallel to ground. However, depending on CGH, downforce,
A-arm lenght, wheel travel lenght and a number of other
things, I may compromice using different cambercompensation curves, static camber settings and end up with a non-parallell to ground lower A-arm.
What I am trying to say is that it is a question related to circumstances.
Goran Malmberg

 

[colinmseries]

Thanks for the replies. Colin.

 

[Yawing]

F1 cars now days have the lower arm cranked up on he inside...nothing to do with roll centres/bump steer/camber gain. its to get it out of the way of the air from the front wing!!! their suspemsion travels less than 2 inches so F1 isnt a good role model for kinematic design. its all about the aero

 

[2SlowJoe]

The relative angle betweeen the upper and lower control arm controls the slope of the camber curve. The relative length of the upper to lower control arm controls the curviture of the camber curve.

You can have no change in camber with parallel, but not horizontal, upper and lower control arms of equal length.

It is difficult to evaluate the angle of only one control arm and learn much, however most of the production cars I have worked on ended up with the LCA going up as it went inboard.

-Joe