I would describe it as a physical reality - so call it an immutable principle of RWD if you prefer.

To explain... the fundamental aim of driving fast through a corner is to drive on an arc with the largest possible radius at the highest speed (oversimplifying, but good for the current disucssion). So you start by predicting your corner speed. To do this you describe the relationship between the size of the arc and the maximum speed your vehicle can attain on that arc. The maximum speed is determined by the radius and the cornering force. This can be expressed as the equation 15GR = MPH(squared) where G is equal to the maximum cornering force expressed against the force of gravity and R represents the radius of the arc in feet and MPH is the maximum cornering speed.

My memory is that the S2K (on stock tires) is usually considered to have a cornering force of 0.91G or 0.92G. That (however) is with warm tires on a warm day (which will come into play later). Let's imagine (for the sake of argument) that in optimal conditions the corner in question could be taken at 50MPH in the S2K. This would yield a radius of around 183 feet.

Now imagine that the cold tires' cornering force is 0.60. At that point the maximum corner speed is 40MPH.

Sudden throttle lift will reduce cornering force at the rear of the car and increase it at the front due to deceleration. In addition, the cornering force is further reduced at the rear wheels by compression braking. At 35MPH you are now past the maximum speed for the radius of the corner in question for your rear wheels. This will result in your rear wheels sliding out into a bigger radius arc. Once your wheels begin to slide and contact with the ground is lost while you are in the corner, other laws of physics enter the equation and the results are equally predictable.

The reason this applies most often to RWD is that, of course, in RWD the wheels in question are your drive wheels. I suppose that it's theoretically possible to experience trailing throttle oversteer in a FWD car as well (though I, for one, have never managed to make it happen).

 

2 eurocentsworth...
Straight-line skid caught in time
I had read the threads about the SO-2's lack of grip in the cold; today I found this out myself. The air and ground temp was pretty near freezing, no exact figure available. I blasted it out of a turn in first, a little rotation but I was ready for it. The trick is to squeeze the throttle, and this gives you time to feel the drift and catch it with a little counter-steering. The "fun" started when I changed into 2nd at 45mph. I think I did not apply enough right foot in time, immediately I felt the rear go out. You would never believe how quickly I put in right then left, as quick as you say the underlined words. When I stopped the car I felt the tyres, they felt not at all cold, sort of warmish; I had driven the first 2 miles as hard as you can from a cold engine. Was trying to impress this customer, another petrol head. He was impressed. Whether by me or the car or my good luck who knows?
On reflection I think I like the thrill of driving my S2000, but not the danger. Beware out there this cat BITES! In hindsight I would only want to repeat this little experiment on a big wide runway, not on a road.
BTW 10500 miles, and 33psi checked 36 hrs previously.

 

yup. gotta warm those babies up.
good thing you were gentle on the throttle (grin). otherwise your customer might have had a bit more excitement than they were prepared for.
oh - and 45 in 2nd is right around VTEC engagement I think (?). can contribute to the "excitement" if you're not careful.

 

Malachi. Your formula is facinating and I'd like to know more about it. Very theoretical and (I'm assuming) it assumes a constant radius, but interesting nontheless. Anyplace I can do some more reading on this?

I'm also interested in your estimation that the S-02 loses one-third of its cornering force in cold weather. The butt might confirm this but it is an astounding number. Did you guess and is there any source of how this compares with other tires? Is this your estimation at 68 degrees? If it were linear, they'd have less traction than a scooter at thirty degrees. I'd only be guessing but I wouldn't even think that racing tires are THAT sensitive to warming. Perhaps this IS the whole answer.

My question though, was how you would classify this observed (or at this point theorized) propensity of the S2000 for meaningful throttle-induced oversteer at such low speeds. It's certainly not typical of most other high performance cars, I don't think. Of course, we haven't done tests yet but I really don't remember it being so bad (that it requires care not to wreck.) I could be intuitivlely responding, I suppose.

 

i am now having second thoughts about my s2k, i'm scared now. is it only the s2k that has this problem. i don't think i ever heard of an m roadster or a boxster having this problem.

 

Carik:

Rest assured all high performance RWD cars can get you in deep crap if you don't treat them with respect. The power with an S2000 comes from 0- 9000 revs as smooth as squirting a hose. It's exhilarating but demanding, to harness that potential and be safe in varying driving conditions. It's a serious performance car aimed at enthusiasts. Don't be scared of the car, be scared of your own abilities to cope with it. I've seen 71mph on the speedo at the end of 2nd just changing up with 4 gears to go!! Just don't drive beyond your own abilities it's that simple!!

Look, to put this in perspective think about an AC Cobra, massive BHP and RWD and the dream of many an enthusiast. You can spin one of those at ANY speed in ANY gear just with the throttle at ANY time. No-one condemns the car for that, they stand in awe of it!!! There's nothing wrong with the S2000 either, which has the same handling characteristics but somewhat less acute. The S2000 isn't for everyone and should be a very carefully considered purchase!!

 

Toyota MRs Spyders also exhibit the same behavior with the same results. Go over to spyderchat.com and do a search on this topic (rain/wreck/spin/crash/etc) in the forums. Those cars also come with max performance summer tires as stock fitment: either Bridgestone RE040s or Yokohama Advan A043s. The dialog on this topic mirrors that on S2KI.com very closely.


(fwiw - i'm sure this has almost nothing to do with is, but the MR2 also comes with differing sizes front/rear.)



Does anybody know how the polar moment of the S2000 compares with that of the MR2 and Miata?

 

Hmmm... I'd guess any book on the basics of racing would cover it (probably in a section on line management or cornering). I'd suggest looking on Amazon - there are probably a stack of books there. And it's hardly a formula i can take credit for - it's from my notes from Skip Barber race school.

While it does, in fact, assume a constant radius you can apply it to the radius at any point in time so it can apply to changing radius as well (a good way to explain the difficulties in a decreasing radius corner and why that sort of corner so often results in trailing throttle oversteer or missed apexes).

[QUOTE]Originally posted by jschmidt
[B]I'm also interested in your estimation that the S-02 loses one-third of its cornering force in cold weather. The butt might confirm this but it is an astounding number. Did you guess and is there any source of how this compares with other tires? Is this your estimation at 68 degrees? If it were linear, they'd have less traction than a scooter at thirty degrees.

 

[QUOTE]Originally posted by carik
[B]i am now having second thoughts about my s2k, i'm scared now. is it only the s2k that has this

 

I've had the S2000 now for 11 months and I've experienced what Lawdog mentions (though without spinning or crashing). Sometimes you go at an 'average' speed around a corner, not remotly pushing it, but the back comes out. For me it was always progressive and eazy to catch though. I've had it happen after 2 mins driving as well as 20 mins driving (ie. with cold tyres as well as warm tyres). I've had it happen on cold days as well as warm days (not hot days though). Also, I've had it happen in the wet as well as in the dry. Just an estimate - I've had this happen good 10 times altogether ... here's some of my reports:
https://www.s2ki.com/forums/showthread.php?...p?threadid=7986
https://www.s2ki.com/forums/showthread.php?...p?threadid=9279

Though, I have a very different theory as to 'why?' this happens. It has very little to do with the tyres as I had this happen with new tyres as well as worn tyres. Conditions do have some relevence as well as the tyre temteratures (and pressures) but the main thing that I notice is OFF-CAMBER corners. S2000 does not like OFF-CAMBER corners and has a tendency to drift the back even at medium speeds. Throw in some bumps in there (even small ones) and it doesn't take much to get the back sliding. S2000 is not very well tuned for off camber corners and I think that is the main culprit. You can compensate for this with 'right' tyre pressures as well as ALLIGNEMT though. I can't remember exaclty when I had the back step out like Lawdog's last time but it has been a while. The car is a lot better after I've had 'custom' allignemt put on and played around with the pressures for a while.

So Lawdog, two questions for you:
a) Was the corner off-camber (even mildly)?
b) any bumps (even small ones)?