Learn Me: How to cure FC RX7 understeer

General rule of thumb:

Understeer- soften front- stiffen rear

Oversteer- stiffen front- soften rear.

That is indeed the general rule. But that rule assumes an ideal platform...like a formula car with a very stiff chassis and double-wishbones at all 4 corners.

Cars with MacPherson struts are far from ideal. Think of an E30 with MacPherson front, semi-trailing arm rear, and relatively flexy chassis. At the front, you don't have much static negative camber to begin with. Under load, you only gain a little bit of additional camber before the tire rolls over onto the sidewall. Meanwhile, the rear has too much negative camber to start with and you get even more under load. That means any significant body roll leads to terminal understeer.

The trick is to maximize roll-stiffness across the front axle using a combination of spring rates, anti-roll bars, and damper settings. That's right: stiffen the front to reduce understeer. This is the exact opposite of what you would do in an "ideal" car, but it works...just ask any BMW CCA club racer!

Something else to check: bump stops. A bump stop is essentially a small very stiff elastomer spring. If the car's been lowered but the stock bump stops are still in place, the top of the strut will contact the bump stop earlier in the suspension stroke than you'd like. Bingo: your wheel spring rate just shot way up.

In endurance racing you don't want to eliminate them because they serve a very important function of cushioning the suspension in a full bump situation. Without that cushion, things bend and break.

There's two ways to address this. First is shorter bump stops. Take them off and cut them down to, say, a half inch long then reinstall. Or (and this to me is a better solution) raise the top of the strut the same amount the car was lowered. Racing Beat used to sell a FB lowering kit which did exactly this; it's a 'cap' which fits over the stock strut mount.

Never seen one for an FC but I don't see where it would be real difficult to fabricate.

wvumtnbkr wrote: What about spring rubbers? Does anybody use them in road racing? Can I just jam a bunch of them (2 or 3) between the front coils and increase my spring rate? Thanks, Rob R.

Increasing the spring rate should be done before bars.