Unequal Leg Length A-Arm Front Suspension

[MStiller]

I work at a company designing our own lightweight vehicle, and we're planning on having a double wishbone suspension with pullrod-actuated inboard shocks for the front suspension. The vehicle is all composite, and due to the lack of space in the engine compartment, we were planning on mounting the A-arms directly to hard points on the exterior. As a result, due to the taper of the nose, the front a-arm pivots on the body are about 6" closer to the centerline of the car than the rear pivots. Also, the ground clearance is high, which unfortunately means that the lower a-arm, which would ideally be horizontal, slopes down to the wheel from the body. Any helpful hints on how to minimize the track change during jounce/bump steer/change in camber during jounce/etc. etc. that results from the fact that the suspension geometry is a nightmare?

 

[GregLocock]

Well, if you can mount the rack exactly between the inboard upper or lower arm positions you will be in a decent place to start, but when the geometry is bad the best bet is software

faq800-768

at least one of the free options will do what you need, perfectly.

I realise that you are making a fashion statement by your choice of suspension, but did you perhaps consider that a bad double wishbone might be a lot less satisfactory than a good alternative layout that actually suits your hardpoints?


Cheers

Greg Locock

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

 

[MStiller]

Greg-

I've been using the Lotus software, and I tried OptimumK as well, and both have gotten me close, but I agree, something else that works better but isn't as sexy would be preferable....The problem is, I'm not really sure what other options we could use. Given the situation I'm in, what would another option be that would be feasible, practical, and would work well? Also, this is going to be a front wheel drive vehicle, so it needs to get power to the wheels too, which adds some complexity in running the driveshafts to the wheels.

 

[GregLocock]

So far as software goes, wishbone.bas or whatever it is called is perfect for this job, if you stick with the SLA. With some cunning it can be modified to form the basis of a powerful goal seeking geometry generator.

If you abandon the SLA I don't know what would work for you, exactly, but a multilink of some sort seems more likely to be a path to success. Design it as a mechanism to do what you want, rather than thinking in terms of styles of suspension.

The downward sloping arm certainly increases the degree of difficulty in meeting a sensible semitrack gain target, which I suppose leads me on to how you set your targets. Make sure that you have a good idea of what your targets are before you start. Again there's an faq on that.











Cheers

Greg Locock

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

 

[mishar]

Problem with 6” difference between pivots could be solved by introducing a fixed piece of 6” to support front pivot. It can be incorporated to the body and actually look very good. Sloping lower A-arm is not a problem as long as arms are parallel (or close). If you are making it from scratches you can move wheel side A-arm attachment points up in order to make arms parallel to the ground. Camber should change to more negative when wheel is pushed up. That helps with handling.

 

[GregLocock]

"Sloping lower A-arm is not a problem as long as arms are parallel (or close). "

Since he is specifically concerned about semitrack gain that seems an odd observtion.

Cheers

Greg Locock

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

 

[MStiller]

So, here's another issue. When you're doing the fvsa calculation, typically the body side pivots are at the same level, but if you actually do out the calculation for each pivot, due to the fact that our forward pivot is closer, its actually technically higher on the line to the IC, which would theoretically make it so that we would have the front pivot higher than the rear pivot. I'm assuming this would be even more disastrous than it is already, but I'm not 100% on it.

In terms of the multi-link, whats the best way to go about designing those? Is it similar to designing a double wishbone, or are there different calculations, etc. to take into account?

Thanks for the help

 

[mishar]

Since he is specifically concerned about semitrack gain that seems an odd observtion.

I sincerely hope that he is not going to run it on rails. Double wishbone suspension can’t provide semitrack and constant camber at the same time anyways. At least not with any serious suspension travel and high clearance indicates need for extensive travel.

 

[GregLocock]

I do not know how to design a multilink of any complexity by hand. It can certainly be done. Roughly speaking I'd draw it in three views and draw some motion paths and then try and find 4 bar links that would satisfy them.



Cheers

Greg Locock

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

 

[MStiller]

Thank you everyone for the help, it is invaluable.

Another question: We need to have some positive kingpin offset to allow for steering through differential braking of the front wheels, but obviously we dont want to set it too high. What is a typical value of positive kingpin offset that would be worth starting with that wont cause the handling to go wild? I've been searching online/in the forums, and I havent found values anywhere....is this the sort of thing you just guess at?

Thanks

 

[GregLocock]

I don't have a feel for KPO, numbers around 70 mm ring a bell.

No, we don't guess at the relationship between the steering axis and the wheel centre, we try and simulate whatever effect it is we want. Things it affects include stability under braking, returnability, sensitivity to vertical inputs, and so on.

I use KPI, castor, mech trail and scrub radius to 'describe' this relationship, but other people do use the geometry at the wheel centre height to do the same thing.

So I think you are proposing large amounts of negactive scrub?





Cheers

Greg Locock

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