Smart questions
Smart answers
Smart people
Join Eng-Tips Forums

Member Login

Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

Yves40 (Automotive) (OP)
12 Feb 11 11:47

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 ?


GregLocock (Automotive)
12 Feb 11 16:46
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.


Greg Locock

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

Yves40 (Automotive) (OP)
12 Feb 11 16:58
Its a 79 corvette suspension

There are no space issues.

dfoxengr (Automotive)
11 Mar 11 10:31
so from side view the front pivot is lower than the rear pivot on the LCA? sorry i dont understand the description.
NormPeterson (Structural)
11 Mar 11 10:36
Think plan view.

BrianPetersen (Mechanical)
11 Mar 11 15:09
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 (Structural)
11 Mar 11 15:54
But does that particular configuration (with the upper arm pivots essentially parallel to centerline in plan view) make for anti-dive or "pro-dive"?

dfoxengr (Automotive)
11 Mar 11 16:54
anti-dive is a side view geometry, no?
BrianPetersen (Mechanical)
11 Mar 11 23:14
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 (Automotive)
12 Mar 11 11:51
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 (Automotive)
12 Mar 11 11:54
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 (Automotive) (OP)
12 Mar 11 15:30
Dfoxengr is correct. That is what I was trying to discribe
GregLocock (Automotive)
12 Mar 11 18:50
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.


Greg Locock

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

autogyro46 (Electrical)
20 Mar 11 11:44
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 (Electrical)
20 Mar 11 11:50
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 (Automotive)
20 Mar 11 14:12
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 (Automotive)
20 Mar 11 18:03
If weird angles on wishbones appeal to you check out the Mustang Cobra IRS.

Lincoln LS is probably along the same lines.



Greg Locock

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

autogyro46 (Electrical)
20 Mar 11 18:37
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 (Automotive)
20 Mar 11 19:57
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.  


Greg Locock

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

dfoxengr (Automotive)
20 Mar 11 23:16
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.
autogyro46 (Electrical)
21 Mar 11 7:54
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 (Automotive) (OP)
21 Mar 11 9:59
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 (Automotive)
21 Mar 11 22:31
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 (Mechanical)
22 Mar 11 6:36
"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 (Automotive)
24 Mar 11 12:44
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 (Automotive)
24 Mar 11 18:48
no, bump steer /= ackerman steering
2SlowJoe (Automotive)
6 Apr 11 14:50
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.

dfoxengr (Automotive)
6 Apr 11 19:57
SVSA to my knowledge is not controlled by ball joints.
dfoxengr (Automotive)
6 Apr 11 20:07
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 (Mechanical)
7 Apr 11 7:45
Here's some info allegedly about the original stingray geometry.  Everything BUT the plan view.
2SlowJoe (Automotive)
11 Apr 11 18:47

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 (Automotive)
29 Apr 11 15:49
Hello guys

I don't know if my attached diagram is what is being discussed here. Please share your thoughts, comments, opinions..
2SlowJoe (Automotive)
10 May 11 20:06

     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...

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Back To Forum

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close