Mcpherson 1

[modificater]

What geometries effect the bump camber curve of mcpherson struts?


It looks like control arm length (the radius it travels) in relationship to the top strut mount?

 

[hemipanter]

Spot on!
Goran

 

[modificater]

I don't have any modeling software, so my next question is would the changes be too extreme to effectivly package a change in control arm length or strut mounting location to be a viable option to gain or lose bump camber?

 

[GregLocock]

You can draw it out sufficiently accurately - just draw it at max travel, kerb and full rebound. If you are feeling fancy then do it half way between each of those as well.

If you raise or lower the inboard and/or outboard ends of the arm you can fine tune the shape of the camber vs jounce curve, a bit, for the roll angles of interest (you won't see full jounce on one side and full rebound on the other simultaneously usually).

Note that if you do modify the arm then you'll need to think about your bump steer curve as well.







Cheers

Greg Locock

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

 

[NormPeterson]

A 2-D Excel model should at least give you an idea what's happening, and you could probably generate a plot. Just going through the exercise will likely push your understanding a bit.


Norm

 

[modificater]

I will take some measurements and draw out the car of interest. One thing I'm seeing is that the camber gain isn't linear since it's following the radius path of the control arm. Is that correct?


With respect to camber curves then, how limited is McPherson compared to Double wish?

I understand some of the other limitations.

Thanks for your time guys, this forum is a great resource.

 

[GregLocock]

Yes, it should be non linear. You can make it less curved by changing the length of the arm, change the focal point by tilting the arm.

In theory MacP is horrible.

In practice it works fine.

This is a classic demonstration of the old motto "In theory, theory and practice are identical. In practice they aren't."

That is if you know what you are doing, judging by BMW's success with MacP, you can make them work well.

I think this has a great deal to do with modern tires.





Cheers

Greg Locock

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

 

[hemipanter]

You may notice that when the A-arm are in 90 dgr to the centreline of the leg You have a point where the camber change sign.
There is often a number of cambercurve discussion on different boards, which is all good. But many times the advantage of camber gain is looked at for only one specific situation. In other situations it might be less desired.
We must look att the whole driving situation for the car.
From that point of view Mcp is ok, but the advantage of doubble A-arms is not only camber related.

Goran

 

[GregLocock]

I agree Goran, for someone who has only 1 shot at making something work, or wants the freedom to tune everything, there is a great deal to be said for unequal length wishbones.

But, MacP struts are one of the few compromised suspensions that seem to have survived the test of time, at all levels of performance save perhaps open wheeler circuit work, which may be a function of the first para.

Cheers

Greg Locock

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

 

[modificater]

Other than camber curve, antidive, antisquat, and roll centers, what are some other maybe lesser known performance benifits of double wish?

From what I've gathered they are ignored by manufactures for cost, and packaging reasons. I find it interesting what vehicles you end up finding with SLA suspension, sometimes you even see newer generations of the same vehicle go back to McPherson.

 

[hemipanter]

Doubble A-arms has a great potential for different dynamic
geometry features. Also, if you look at my frontsuspension
on the picture, the uppright is very deep in the wheel. The KPI is amost zero as well as the scrub. Newer minde why this is so, but it is possible using doubble A-arms. The McP leg would have needed to go throught the wheel and tires.

Goran

 

[modificater]

Excellent point on the KPI/Scrub, I shouldn't have missed that.

 

[GregLocock]

Actually you'll find that manufacturers who care about suspensions spend a lot of time finessing double wishbone (and similar) geometries. Unfortunately for road cars their properties are modified by the compliances in the bushes and body, so hand analysis is no longer practical.




Cheers

Greg Locock

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

 

[kikiturbo]

"The KPI is amost zero as well as the scrub. Newer minde why this is so, but it is possible using doubble A-arms. The McP leg would have needed to go throught the wheel and tires. "

not necessarily true... as far as the scrub is concerned...
MacP struits usually have a large offset between the wheel centerline and lower attachment point for the struit, so there is space to move the upper struit bearing out enoug to get the KPI needed and resulting scrub...

my delta integrale, which is very 80's technology, has zero scrub (as far as I can measure) with a "normal" MacP

as for the geometry change... I did some work on a racing car that used MacP struits in the front and we lowered the ball joint to get the better camber curves. But IMHO, you should look at not only camber change in bump, but combined bump + steer, as there is great scope to adjust your geometry with more castor, which will give you more camber in a turn..

also, I do not know how is current MacP technology, but one of the problems with it, IMHO is that the struit has to resist bending forces in a turn, which affects the resistance and stinction in a damper...

 

[modificater]

Thanks for the information guys. This has got me thinking.

Again for a mcpherson stetup:

On a paper model it looks like a horizontal (with respect to the ground) control arm is one that will gain and lose camber at the fastest rate. Am I correct?

How then on a Mcpherson setup do you balance both the most exaggerated camber curve with a good roll couple without a really low CG?

 

[modificater]

Or maybe better said, how do you raise the roll center on mcpherson if trying to maintain a horizontal control arm?

 

[GregLocock]

by reducing the kpi... I think

Cheers

Greg Locock

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

 

[hemipanter]

At vertical leg (zero KPI) and horizontal A-arm we should have ground level Rc.
Goran

 

[modificater]

Was I correct on the parallel to ground A arm having the greatest camber gain/loss of the radius?

 

[hemipanter]

"Was I correct on the parallel to ground A arm having the greatest camber gain/loss of the radius?"

It is the relation between the leg and A-arm that set the camber gain-loss rate. Not really the A-arm to ground situation. The shorter the leg, the greater the effect.
The A-arm to leg angle both set the rate and the + or - camber effect.
Hmmm, I wish I had the strenght to make some drawing here...
Goran