Unequal Leg Length A-Arm Front Suspension
Unequal Leg Length A-Arm Front Suspension
(OP)
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?





RE: Unequal Leg Length A-Arm Front Suspension
FAQ800-768: Software for suspension analysis
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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Unequal Leg Length A-Arm Front Suspension
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.
RE: Unequal Leg Length A-Arm Front Suspension
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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Unequal Leg Length A-Arm Front Suspension
RE: Unequal Leg Length A-Arm Front Suspension
Since he is specifically concerned about semitrack gain that seems an odd observtion.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Unequal Leg Length A-Arm Front Suspension
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
RE: Unequal Leg Length A-Arm Front Suspension
RE: Unequal Leg Length A-Arm Front Suspension
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Unequal Leg Length A-Arm Front Suspension
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
RE: Unequal Leg Length A-Arm Front Suspension
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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.