Wishbone suspension design questions

[olason]

Hello,

I have been posting about different design questions a while ago, well I am finally getting back to it. School has kept me busy besides anywhere from 15 to 40 hours a week of work as well.

From what I understand Caster (longitudinal pitch angle) of the upright (spindle) is set, and you need to do your best to keep that. So your mounting points for your wishbone suspension are at that same angle on the frame mounting locations. Is this a correct assumption?

If that is true, next I am going with Long Arm (lower), and Short Arm (top) double wishbone setup. The arc path of these should be such that if anything I get some negative camber while cornering. Where the top of the tire tilts inward towards the chassis while cornering.



If this is correct, then do I want my arms to be as parallel as can be by design from upright to frame at ride height?

Also since I don't have any analysis background for Finite element analysis on strength of components (class is offered fall 2013), do I want my arms to be symmetric about the balljoint on the spindle? If so what is an approximate alright length between chassis mounting points on the same A-arm?

[exwestracer]

If you are talking about rear suspension, then yes; caster can be fixed by wishbone inclination angle. You want the lower arm level (ball joint center to mounting bolt) at ride height. Upper arm angle will be determined by the amount of camber gain desired for the tire used. For a modern wide, low profile tire, the upper arm will also be pretty close to level...

The arms do not have to be symmetrical, especially on a non-steerable upright. Keep in mind that the forward thrust force is carried through the links, so pushing the front leg of the wishbone forward will help transfer force to the frame more efficiently. How much will be limited by wheel width and offset. The rear leg should be parallel to axle centerline or angled rearward slightly. I personally wouldn't go less than 60deg included angle between the legs of the wishbone.

If you are talking about a front (non-driven, steerable) suspension, there are some very different design priorites.