Originally Posted by
JohnL
Any of the SLA ('double wishbone') front suspensions will in principle be superior to the Mac Strut. Having said this, a sensibly modded Mac Strut will be superior to a poorly modded (or designed) SLA suspension, but all else being equal the SLA will be better out of the box (typically, but the Devil is in the detail), and have more scope for improvement, especially for a FWD car.
Mac Struts are a very compromised design, but do have advantages in that they are compact (easier to package into the chassis due to lack of upper wishbone), they are robust, they are lightweight, and they are relatively cheap to make.
Did I say they were cheap?
On the other hand Mac Struts don't change camber enough with suspension motion (which makes them very sensitive to body roll causing substantial unwanted camber change relative to the road surface), the caster angle unavoidably changes with suspension motion, the damper is subject to quite substantial internal loadings that are the result of lateral forces and braking forces being reacted within the damper body (between the piston and the damper tube wall).
This latter problem isn't an issue for SLA suspensions because all forces (other than vertical ones which the damper is well suited to dealing with) are reacted through the upper part of the suspension 'upright' (aka 'knuckle' etc) and in the upper wishbone (not in the damper), but with a Mac Strut the damper body and rod are forced to act as if they were the suspension upright....
Mac Struts are not well suited to wider tyres due to the limited camber change, and tend to require excessive static neg camber angles to compensate for this (more so with wider rubber), but this causes other problems...
Even with substantial static neg camber this does not really make up for the geometric deficiencies of a Mac Strut, just alleviates them somewhat by maximising the lateral grip of the outside front contact patch (i.e. pre existing static neg camber helping to keep the OF wheel near vertical when cornering hard) at the expense of the inside front contact patch, which ends up being very small with a very substantial dynamic camber causing the tyre to be running on it's very inner edge.
Sure, Porsche use Mac Struts on the 911, but on these cars not much is really asked of the front end, all (most of) the lateral and longitudinal traction action occurs at the back end due to the heavily rearward weight bias and the need for rear traction dictating a relatively very roll stiff front end that unloads the inside front wheel very substantially when cornering (thus the dynamic camber angle and grip of the IF is laregely irrelevant...).
Now for a FWD car we need to keep the inside front contact patch planted heavily on the ground (i.e. low front roll stiffness relative to rear roll stiffness) and presented as 'squarely' to the road as we can manage in order to maximise it's grip in order to minimise understeer and maximise traction exiting corners. This is not acheived if we allow the IF to lean over at drunken angles with only it's inner edge in road contact, yet this is what we are more or less forced to do when setting up a Mac Strut for 'high performance' applications.
When trying to get the most out of a Mac Strut design, we can't really make a silk purse out of a pig's ear (Mac Strut = pig's ear), at best we can make a purse that looks a lot less pig's ear-like than it actually is...
Note that for all purpose designed modern race cars (not racers made out of modded road cars), SLA is the suspension of choice at every corner of every car...
