is 4 wheel steering a better option????

[Tom Wettenhall]

I believe you'll find that roll steer will perform in a largely similar fashion to 4WS in high speed corners, i.e. it's evil; and in any case where you're not generating maximum lateral g, you'll have a car which doesn't really turn at all.
You could perhaps reverse the direction of the roll steer, start with some toe out which makes your turn-in plenty good, but then recovers when you get some body roll going. I may test that on our car, actually...
The RX-7 system mentioned earlier on strikes me as being much the same thing, a g-sensitive toe control.

[Tom Wettenhall]

I should say, our 2010 car ran like that. Once we got it turning a bit better after comp, it just got nasty, difficult to recover from slides. But that car had other handling issues.
It's really easy to build the possibility of roll steer in to a car, just space your tabs a bit further apart than you usually would, and have different sets of spacers which you can swap to move the pickup up or down.

[JDS]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Z:
JDS,

Err, half right. "As the car rolls" one wheel toes in, the other toes out, so both steer in the same direction. Hence the term "roll steer" for this behaviour. You are right in that it is a disadvantage during rear squat or lift.

Z </div></BLOCKQUOTE>

Whoops. Where was my mind yesterday! Righto!

[Adambomb]

My $0.02 after examining both roll steer and rear "active" steering on fairly comprehensive whole car vehicle dynamics model for the vehicle dynamics class they used to offer here. Sort of a re-hash of what many smarter people have said already, perhaps I can add a little something

For roll steer, since it is on the "reactive" axle and is a reactive effect driven by roll angle, it will not help turn in. Instead, it is simply another way to add steady-state under or over steer. In fact, I prefer to think of it as no more than a fancy application of bump steer (kinematically at least). OK, now here's some fun food for thought: Solid axle cars often have roll steer! For a coil-spring car with trailing arms, think about the "steady state" inclination from a side view of the trailing arms, and how their effective length from a top view changes as the car rolls. Now with leaf spring cars, imagine which end is fixed and which end pivots, and how the spring mounting location moves fore and aft in roll. OK, enough for the random tangent of the day...

Active steer! For our assignment we were to examine the behavior of a system that mimics the control system used in a late '80s Honda Prelude, which sounds quite similar to the one in the RX-7 (and probably most of them in those days). The rear steer angle was determined solely from front steer angle. At low angles it "crabwalked," ie rear wheels turned the same direction. I guess the thought was this would work well for fast lane changes or dodging things on the highway, as you could alter your trajectory and just skip the whole yawing the car business, all without waiting for the rear tires to build up lateral forces. On a steady state corner it would add understeer...because yet again you're avoiding the whole yaw thing. But hey, if one were so inclined, I suppose they could also build in some roll steer to re-cancel that out, right?

Then as steering angle increased the rear steer gain function would change directions and go opposite, for excellent mall parking lot maneuverability and tighter parallel parking. I suppose this may happen at the steering angles you would see on a tight autocross corner; I personally have no idea how well this setup worked in autocross. It would add a steady state oversteer component, but again at steer angles high enough that you would really only see in a driveway or a parking lot (whether at the mall or autocross!). Not only that, but to get to this level of gain the rear steer has to change directions; don't imagine that would do good things for transients.

OK, so with that system, the biggest things you gain are the abilities to dodge things on the highway and make the car easier to park. Sounds like a system well suited to a minivan to me! Not to say that a different control scenario couldn't be devised for racing.

[JDS]

Just to be clear, the RX-7 system was not an active rear-steer. It used flexible bushings, so the amount of rear steer was proportional to the lateral force acting on the rear tires, and not related to steering angle.