[QUOTE=RED MX5,Sep 1 2007, 02:20 PM] LOL, I wasn't YELLING, I was just trying to make the important part stand out from the rest of my drivel.

 

You might do better getting that kind of information from a good book on chassis design, but I'll give it a shot.

Without getting into the details of implimentation, roll steer simply means that the rear wheels steer as the car rolls. There are two kinds of roll steer. There is roll oversteer, and roll understeer. When the back wheels steer toward the outside of the corner as the car rolls, it is roll oversteer, and when they steer toward the inside of the corner as the car rolls, it is roll understeer. If you think about what is taking place, the reasons for the names should be obvious. When the rear wheels steer to the outside of the corner, the back end wants to go that way, and when the back end goes out, that's oversteer. When the rear wheels steer to the inside of the corner, the back end wants to go that way, tucking itself in tightly, in some cases to the point of producing understeer. All cars with solid axles and leaf springs employ roll understeer, because it is safe and stable, and in general, roll oversteer is nasty and must be avoided at all costs.

The AP1's have roll understeer. The rear tires steer into the turn (slightly) as the car rolls, and this reduces oversteer. If the rear anti-sway bar were softer it would actually produce understeer, but the rear anti-sway bar on our cars is massive, so all the roll understeer does is reduce the tendency of the car to oversteer. In action, it works like this ... When you first turn the wheel, the rear wheels are not steering at all, so the car initially has maximal oversteer. This affects the cars initial turn in. As the car actually starts to turn, the body starts to roll, and the rear wheels start to steer into the turn a little, reaching maximum turn-in when you reach the peak cornering forces, so that at the limit the car has less oversteer than it had when you initially turned in. IOW, the cars handling bias actually changes as it rolls going into the turn. As you unwind the steering on corner exit, the cornering forces drop, the roll decreases, and the bias shifts back toward more oversteer.

Some drivers and chassis designers love the approach and some hate it, but the one advantage that is not debatable is that roll understeer increases total grip. This is one of the reasons that a stock AP1 generates 0.91 G's, same as the AP2, even though it has smaller tires and wheels.

One downside (if it is really a downside rather than just a personal preference) is simply that when any kind of roll steer is used, the cars handling changes as it rolls, and some drivers and designers feel that a linear response is easier to drive and more predictable. Since many drivers readily adapt to non-linear handling (F1 drivers do it for every turn ) this objection doesn't hold water in my book, but it is an objection some will raise.

The other downside is that to achieve roll steer you have to design the suspension so that the wheels steer as they move up and down, and roll during cornering is not the only time the wheels move up and down. The also move up and down going over bumps and road irregularities, and as they go up and down due to these irregularities, they also steer. When both wheels hit a bump at the same time the steering has no effect on stability, because both wheels will steer in or out by the same amount. There will be a momentary change in total toe, but if the static toe setting is correct (sufficient toe in) there will not be any toe out or any instability. However, when one wheel hits a bump, one wheel steers and the other one doesn't. The amount of steering is so small that you won't feel it when traveling in a straight line; Every car with a solid rear axle and leaf springs steers this way every time one wheel hits a bump, but you never notice it because the amount of steering is so small. Same same with the S2000 and other IRS setups that use roll understeer. However, if you hit a dip with the outside wheel, mid corner, the wheel drops and steers to the outside of the turn, and if the bump steer is excessive the car will be unstable over dips at speed. Some AP1 owners have reported experienceing this problem, but I haven't, so I suspect it is a function of the particular racing surface and probably pretty rare. In bump the car hits the bump stops and loses grip for that reason rather than due to bump steer, and I think some people have confused the two issues (but of course there is no way for me to be sure).

The AP2 rear suspension does not incorporate roll understeer. Toe does not change as the wheels move up and down. This means that the handling bias is consistent throught the roll in, steady state, and roll out phases of a corner. To avoid excessive oversteer in steady state cornering, the handling bias has to be moved toward understeer throughout, so the car resonds somewhat differently to driver inputs. Some grip is lost due to the lack of roll understeer at the back, but the AP2 makes up for it with larger tires and wheels.

As for how much of the difference (due to the presence or absence of roll steer) can actually be felt, it's really hard to say, because along with the geometry changes there have also been changes to the spring, shock, anti-sway bar rates, bushing stiffness, and so on, over the years. I can't actually feel any difference between the MY00-01 and MY02-03 cars until they start to slide, and at that point the earliest cars give themselves away because the front tires are so glued to the road that the back tires always slide more (unless you force the car into understeer, which of coursre is always possible, with any car). Below the cornering limit I really can't tell much of a difference. The MY04 cars don't feel the same on initial turn in, feel more forgiving, and are easier to push on corner exit. I also think the chassis response to throttle changes is a little slower, but it's really hard to be sure, because all the S2000's are so responsive, and the extra torque of the F22C really clouds the issue.

To each his own. I like the sharp dynamic handling of the early AP1, and I like the (debatably) excessive oversteer of the early cars. For me, it works perfectly on the autocross courses and is a blast on the street. The LSD I'm running now makes the handling even more dynamic and the responsiveness to throttle inputs has been substantially increased, so the guys who think the stock AP1's were unstable would probably find my car undrivable. I don't have any argument with this, becuae it's really a matter of preference, driving style, and setting up the car for a particular venue. My car is NOT easy to drive at the limit, but it's amazing when driven well, and that's the way I like it. Others like other things, and that's OK by me.

LOL, that's why I always tell people to make their own decisions and target their mods toward fixing specific things they don't like (or want to improve). If you follow in someone else's footsteps, you'll end up with a car that suits them.

 

[QUOTE=boofer,Sep 2 2007, 08:28 AM]you know on how some crappy cars there's a lot of play with the steering wheel at-center?

 

I agree with MX5, if you have play with the on-center steering there is probably something wrong with the car. It might be tires with really soft sidewalls (like Toyo T1-S/T1-R), bad alignment, worn out bushings, wheel bearings, loose lug nuts, etc.

As for steering feel, almost any RWD car will have purer and more precise steering than FWD or AWD car just because the RWD has no drive shaft spinning the front wheels.

And I'm one of the guys that prefer not to have the roll steer on the AP1. Sure it is great of perfectly flat roads, but in the real world having the rear end swim or bunny hop in corners (which has always bugged me) is not confidence inspiring or optimal for grip. I for one (and 99% of other S2000 drivers) is not limited by the car, but limited by talent and/or confidence in the car. Having the rear of the car swim in little circles while going 80+ mph sweeper doesn't make me fast This is where I find that the sterotypical European suspension tuning is better than the sterotypical Japanese tuning that seems to assume perfect road surfaces.

 

hahah, i have a MY07 with 493 miles on it. maybe i'm over-exaggerating or something in regard to the "slop" on-center. it really is a responsive car, with very little body roll, and it generally goes wherever i point it. maybe it's all in my head.

 

Yeah, that sounds like new car anxiety. Hearing every creak, rattle and what not.

If you really want to have poor turn in, take your TSX turn the wheel and hit the gas. It'll eventually turn