My motto is "Just say NO to MacStruts."
Struts require a fairly high Steering Axis Inclination (SAI) to get the strut out of the wheel - in the neighbourhood of 10 to 15°. While this alone isn't bad, when the wheel is turned in (torward the centre of the car), the spindle will go from a horizontal position, to pointing down - producing positive dynamic camber. This is not good. Another option is to move the entire strut further inboard to reduce the SAI, at the cost of Scrub Radius and the resulting high steering effort. ("Cut" struts turned into uprights have the same problem as full struts with respect to SAI and camber loss).
One way to offset this positive camber gain is to add more caster. Generally run have the caster that you have SAI. So, if your struts have 10° SAI, you want to run about 5° caster.
Using camber plates, as so many "tuner" folk do, only exacerbates the problem as sliding the top of the strut inward increases SAI, thus requiring more caster to compensate.
Caster "lifts" (or "unloads") the outside wheel on a turn, which may cause the inside rear to pick up or at least unload to a degree. This may increase wheel spin and other issues. You will want a very compliant rear suspension on a RWD vehicle.
Another issue with struts is on roll there is little to no negative camber gain. If a car is lowered and the lower control arms are no longer horizontal, the suspension can even go positive camber in roll. This prevents the car from using the full contact patch, thus reducing traction.
The solution is twofold: 1) Increase static negative camber such that the wheel is vertical in roll (my FSP Nissan Sentra ran 3.5° negative camber in order to keep the tires flat in a corner) - this costs you in acceleration and braking power. 2) Stop the strut suspension from doing what it shouldn't by running very stiff springs and/or very stiff sway bars. Of course, this can cause other issues. There is no free lunch.
Having said that, struts do work. BMW, Porsche and even Lamborghini use struts. BUT, neither NASCAR nor F1 use them. They are simple, and cheap to produce.
Double wishbone, specifically short-long arm suspensions, allow much more desireable control of the wheel in roll. By using different length arms (short upper, long lower), you can cause the top of the wheel to tuck in in roll, such that the tire remains vertical in a turn, maintaining contact patch for acceleration, turning, braking, smiling, etc.
After having tried to get a MacStrut car (B13 Sentra) perform better than a double-wishbone car (EG Civic), I'll never go strut if I don't ~have~ to. Just say no.
Does that help at all?
G
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