FVSAL Decisions

Allright enough screwin around. Time to decide on a few things, namely baseline FVSAL and how it progresses through heave and roll. In case it has been a while since the last time you designed an independent suspension, we'll go through a refresher. Take a look at the following graphic I stole off a Google image search:

You can see the a-arms, and if you virtually extend them out they intersect at a point, A. The distance from A to the center of the spindle (NOT to C) is your "swing arm length." Basically when the wheel moves it's going to act like it's just on a beam that pivots around A. This length defines your camber gain in ride and in roll simultaneously. You cannot change one without changing the others.

Really long FVSAL: Very small camber gain in heave. At FVSAL = +inf, bump-camber = 0 degrees per inch, and roll-camber = 1 degree (wheel) per degree (chassis).

Conversely, short FVSAL (near half your track width): Small or zero camber change in roll, and high camber change in ride.

You can also have a negative FVSAL... it's all summarized here:

The equations to generate the above are very simple, and can be found in RCVD around page 625 (plus or minus).

Essentially how parallel or non-parallel your a-arms are, defines your FVSAL. To make things more difficult though, your FVSAL changes as your suspension goes through travel.

Equal-length a-arms: no change.

Top a-arm shorter than bottom: outside FVSAL shortens in roll, inside FVSAL extends (and can go negative). FVSAL also extends in heave, shortens in squat.

Top a-arm longer than bottom: outside FVSAL extends in roll (and can go negative), inside FVSAL shortens. FVSAL also shortens in heave, extends in squat.

By the way I figure

• I want the integral of the roll-camber curve to be relatively small. I don't necessarily care what it is on-center, so long as when the car takes a set in a corner I haven't lost too much camber.
• Going down a straight, when not grip limited, I can probably stand to have some bump-camber (while still having little bump-steer). The benefit is I pick up some roll-camber control.
  
• When mid-corner I want to take care of the outside tire more than the inside tire, as it is going to be most heavily loaded and dominating grip level.

As such I believe I want a positive, non-infinite FVSAL on center, with the top control arm longer than the bottom so as I go into a corner my outside tire bump-camber control gets progressively better. Additionally, under braking when the fronts are highly loaded my FVSAL will extend for progressively better camber control over bumps... and if I do the same in the rear, under acceleration I will have the same effect for when the rear tires are heavily loaded!