Low-speed understeer
 

I have an STX RX-8, and I'm experiencing mid-corner understeer only on tight corners. On faster corners, my car is beautifully balanced. If I encounter a kink or decreasing radius, I can get the car to rotate with a lift or touch of the brakes. On the real tight hairpins, however, my car will turn in optimistically, but then start to push after that initial turn-in.

We run almost entirely on concrete. Actually, I've encountered little asphalt at all this season and it'll probably stay that way.

My car is on Stance coilovers with 10k springs in front and 6k in the rear. I'm on the stock bars with Bridgestone RE11s.

If anybody has any suggestions, I'd be glad to hear them. It's unlikely I'll make a drastic change so close to Nationals, but I have three (possibly four) more days of local events before then. Thanks!

Nick,

What are your current alignment settings?

Duh. I should have included that to begin with.

Front:

-2.0deg camber
6.0deg caster
1/8" toe out

Rear:

-1.5deg camber
1/8" toe in

Maybe poke the rear toe out a bit?

Nick,

Any idea (from pics, bystanders and/or video) how much are your tires rolling over in these low speed corners? It's possible that a tire pressure increase and/or slight increase in front negative camber could improve this issue.

I'd also have to agree with NotAPreppie, in that taking out some of your rear toe in clould also improve the handling of the car in these low speed corners.

I'm going to try a little more air in the front tires. If that doesn't do it, I'll give the camber a shot. Those are unlikely to change my car's manners in high-speed sections.

I have the same issues with understeer on low speed corners, but feels great and rotates very easily on higher speed corners. My alignment settings are similar to yours but I'm on stock springs and 40th Anniversary Bilsteins, but I have a racing beat front and rear sway bar. I'm running RE01R's and I'm running 35psi front and 37psi rear - what pressure's are you running? If you're running an ideal pressure for maximum grip in the front and you increase psi in the front, it seems to me that you'd be more likely to get understeer. To get more rotation I typically increase rear tire pressure, but if I go too high the back end starts to get a little unpredictable.

Also I have a feeling that if you dial out all of the understeer completely on the car then it might become more of a handful on the high speed corners, but I'd love to get an update from you if you're able to get it dialed in.

What are you doing with the gas / brakes during the hair pins? Are you braking in a straight line then accelerating as you enter the turn, or are you trail-braking into the turn and slowly adding throttle only after you pass the apex?

My car does this too. I do not know what causes this, but I suspect the tires are being pushed to such ridiculous slip angles that the fronts just give up.

I basically drive around the problem - slow down, being sure that a lift or soft dab of brakes will fix it if you do go in a little more quickly than you should. I do trail brake if the corner is decreasing radius. This worked very well for me today at a PCA event. We had sweepers of all radii, and my car was sweet as can be on the faster ones, and stubbornly pushed on the slow stuff. Again, as long as you don't dive in way past the car's limits on the tight sweepers, it does OK.

I also wonder if damping can affect this. Perhaps too much rebound is keeping the inside from transfering load to the outside? :dunno:

I once had a conversation with the driver of a competitively setup STU car (STi) who had a very impressive rear wing. I was told this wing helped by allowing the car to be setup such that it pushes less at low speed, but at high speed the wing provides enough down-force to keep the rear planted.

This is interesting considering the rules state that wings & spoilers should "have no significant aerodynamic function at solo speeds." I brought this up and was told this wing was approved in a specific ruling.

So, perhaps wings are worth looking into. Perhaps next year for me.

beautifully balanced and achieving maximum potential G loading are two different things entirely

I do believe that this setup (aero wing) is the ultimate solution - dial out all low speed understeer, and add downforce in the rear for high speed stability. But the problem is that autocross speeds are not generally high enough to produce lots of downforce, so you'd have to have one heck of a big, ugly wing on the back of the car and you still might end up chasing around the back end for most corners.

Looking at my MaxQData data logs, my initial impression is that the differences in lateral g's between the larger and smaller radius corners is much smaller than I would have guessed based on car behavior (i.e., pushing in the tight corners).

Understeer feels slower than it actually is. By changing the bias to the rear you are only lowering the limits of the rear of the car, but not increasing the limits of the front tires at all - so lateral g's would probably not change much even though the feel of the car might.

I learned this when the fastest guy in my region hopped in my car while I was having understeer problems and showed me how to go fast despite how the car felt - he beat me by .8 sec.

The only thing a little oversteer will do is help keep you pointed towards the inside of the turn and help you get on the power a little earlier, but many road racers set their cars up with a little understeer bias so that they can be consistent.

I don't think that's always true - it depends on how you go about changing the bias. Tire pressure or alignmnent change; sure I agree. Increasing rear sway bar/spring rate, or decreasing front sway bar/spring rate, you are simultaneously increasing the load on the outside rear tire and decreasing the load of the outside front tire. Make the right change (assuming glaring understeer to begin with) and you should be increasing the overall limit of the car. At least that's the theory.

But yes, I agree the car is more predictable with a bit of understeer and that's how I have my car setup. Just don't like it when it transitions to lots of understeer in the slow stuff, although I am less worried about it now that I have looked at the logs.

being oversprung will exacerbate high steering angle conditions

not setting up for the corner properly, overdriving, etc. will exacerbate it regardless

1st gear is your friend too and can often be used in an RX-8 more often than most people realize, learn to downshift properly i.e practice, practice, practice

Are your differentials operating properly?

here's an old STU video, think I was in 2nd gear for only 2 or 3 turns, four 1st gear downshifts and five 2nd gear upshifts on my last run once I figured it all out:

http://www.youtube.com/watch?v=IJHcj4kpueA



.

Thanks for that Mark. No doubt, 1st gear is important, particularly when competing on street tires. I've gotten better at downshifting, just need to remember to use it...

I definitely have an aversion to downshifting to first. I heel-toe all day long normally, but I can't get it done on an autocross course. My problem is that I have long legs, but not long arms. In order to stay close enough to the wheel, I have to crowd my legs a little. That means I have no room to fit my knee in the normal heel-toe position (toes on brake pedal, heel on throttle).

On the street, I split my foot length-wise across the gas and brake. I find that very difficult to do in competition because I press the brake pedal so much further down than I do normally. The Hawk pads have helped a little, but they also require much more careful pressure on the pedal.

I freely admit it's a problem I need to solve, and it's possible I just need to keep trying it. I've had a real block against it, I guess.

I may be way off but someone told me it also can be cause by incorrect settings in the rear LSD housing.

That was why I posed the above question.......

I had the same problem, last trackday I changed my wheel alignments to -3 Degree Camber and 1mm Toe out in front and -3 Degree Camber and 1mm toe in in the rear. This cured my understeer in slow corners alot.

All I've done to my LSD is add synthetic oil. As far as I know, there are no settings in the factory diff - am I wrong? Or, does the diff wear over time in a way that can induce understeer?

AFAIK, there have been several warranteed differentials, one of which was on Easy_E1's car. Might be worth considering. :dunno:

The situation you're describing is something that I think many of us have complained about with the RX8.

It seems to me that, given the lower speed/larger throttle opening, this push occurs when maximum weight is being transfered to the rear...I've wondered if there is something in the rear geometry that causes some detrimental toe change and resultant rear steer in these conditions (?)