Mac Strut and SLA wheel rate

[meb]

I PM'd one of our members but thought I might throw the question out to the group.

I understand that motion ratio is limited the suspension's geometry, and that, wheel rate is what the tire sees. If track width is increased, should I expect wheel rate to change with either a SLA or Mac Strut? Part of my intuition make me think that the answer is not so black and white with a Mac strut...that perhaps geometry can affect whether or not a change occurs...and that spring and damping rates may be so high as to make any minor difference meaningless.

The arugment I posed was that if I flipped my 05 Mini over and pressed down on the front tire's centerline with enough force to displace the tire one inch, it would take less force to achieve the same range of motion if the tire was say 20mm farther away from the lower ball joint.

A primitive picture - using a pry bar to lift a 100lb stone with the fulcrum in the middle of a 10' long or 20' long pry bar...the 20' bar requires half the force to left the stone - assuming no deflection.

Michael

[Al Weyman]

I was always of the understanding that with a McPherson Strut arrangement the wheel rate was as near as damn it to the spring rate, maybe 5 to 10% different. I do know that any calculations have to be squared for example if you had a 1000lb spring sitting in a wishbone at exactly half way the wheel rate would be 250lbs.

[meb]

I was about to try a calculation or two when a few in my circle said there no wheel rate change with a Mac strut.

My argument is, if I take two hypothetical tire locations - changes in track width - and simulate a 1" bump for each I should expect the strut to compress less as the tire moves farther way from the strut's LCA attachment point. And since linear spring rates are determined based upon lbs/inthe effective wheel rate should be less...yes?

When you wrote wishbone did you mean an SLA set up? I think, for a mac strut, if the strut attaches to the LCA half way between the inner pivto and the ball joint and wheel rate is essentially 50%...but this also depends upon the angle of the strut - question, not a statement???