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'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

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.

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.

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


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).

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.

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 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.

Are your differentials operating properly?

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 (?)

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?

A locked diff - to much limited, not enough slip - can cause understeer. The OE diff is not adjustable on the RX-8, and no one would accuse the OE diff of having to much lock up.

A bad diff is evident by a significant increase in one-legging, and is typically accompanied by oversteer, when you light up the inside tire the back end sees a loss of traction.

Heh, thread resurrection.

I have the time before our next event...I might just increase the negative camber in the front and see what happens...

... no one would accuse the OE diff of having to much lock up.

What about too soon?

I'm not sure. It seems to me it even happens when I have the throttle neutral or closed. I'll try to pay more attention this weekend and see if I'm right.

[edit] Oops, this was in repsonse to Mikes' observations about weight transfer.

Jason, that's what I thought.

Charles, pushing on exit under power is a different problem, and that can happen at most any speed with the RX-8 (in BS trim, anyway) if you have too much rear negative camber relative to the front.

My understanding is that can be lessened with a different diff that engages more progressively, like the OS Giken, but I have not personally experienced that.

My understanding is that can be lessened with a different diff that engages more progressively, like the OS Giken, but I have not personally experienced that.
That's my understanding as well. That's why changing the rear lsd is on my list of things to do for STX.

Now THIS is a great thread.

OK....Here's my take.

Myself and 3 other very fast and talented drivers have all concluded that the car has issues when it's under roll for longer periods of time in slower corners. We can make the cars handle wonderfully at just about everywhere we go except I'm still haunted for a particular style of corner---longer, slower corners where the car is in roll for a longer period of time. Our cars exhibit strong tendancies to U-steer at mid corner with ANY throttle inputs and can/will O-steer at track exit on these same corners.

We found this when we constantly battled the longer right hand International Horseshoe at Daytona and then the 2nd right hander horseshoe that follows (West Horseshoe??). The first horseshoe is a full 180 degree rt hander run in 3rd gear with our 5.12 ring and pin (most likely 2nd g in a stock 4.44 gear). We would have to wait soooo long to get on the gas that you'd get totally smoked on exit. We tried everything. Finally got it to be so-so with 3 mm's toe-in rear (and it didn't hurt our straight away speed because we were getting off the corners faster). The car was wicked quick in the faster stuff everywhere except those two LOONGER, SLOWWER turns. Honestly I was wo rqw5 through the bus stop and you could KILL anyone there (I offer this because it appears that other experienced drivers of our cars offer this as a major strength of these cars----yes? correct?). The bus stop is a high speed chicane type of turn with predictable oversteer at entry and mid and goes to rock solid when power is applied. These are the type of corners that I believe our cars are very strong.

Typical suspension setup that we run just about everywhere.
-2.7 FL, -2.5 FR, 1mm Frnt total toe out, 1 mm Rr total toe in. -1.9 LftRr, -1.7 RtRr. Big front bar. No rear bar. 550 fronts. 450 rears. Obviously we change lots of this setup once we get there but for the most part the car hovers around the numbers above (give or take).

There are only a few other corners similar to this where the car has revealed these same tendancies (strong sensitivity to u-steer with tiny throttle application at mid-corner) and they are few. Since I've driven this car on a zillion tracks we were surprised and perplexed at Daytona. Turns out that this type of turn is unique to the many tracks we run and we never experienced anything like this before.

A suspension engineer suggested that the roll center might be in the depths of Hades and that is what we're experiencing. Currently I have a suspension analyzer program on order (Performance Trends) and plan to dabble in the black arts of pickup point locations and find out what's technically going on. I know when we run the car's ride height low the angle of the upper control arm is all whacked out funky and this must have something to do with this issue.

Has anyone gone down this technical road? This is new ground for me and the team.

Cheers,

We use the stock diff and it has proved itself rock solid. Used an aftermarket diff before which went to full lock on accel and we chased our tails on that one for a long while. Cusco diff. Terrible for us. Went to stock diff and the data for inside vs. outside rear wheel looks normal. We also run the car as low as possible in the rear to reduce the rear roll center to eliminate any inner rear wheel spin. We pretty much never experience this anymore.

He lives!!

That was a really good post up there^^ Im scary near those alignments already and the rest im probably going to steal and test next season.

I wish this site had more users like Eric Meyer!

Thanks for the posts, Eric.

Your post does serve as an good reminder about the difference between "low speed" for track use vs. autocross. For Solo duty, my car (in STX trim) does just fine in second gear corners - I have understeer problems in first gear corners. Of course, this is probably due to our setups being optimized (hopefully :)) for different contexts, but we both have the same problem - get far enough below the most common cornering speeds and we both get understeer.

I did some experimentation with softening low-speed compression damping on the front to lessen low-speed understeer, and it seemed like it helped, but it's nothing definitive yet.

As for the roll center question, I think Mark has been investigating this so perhaps he will chime in. On my last car, a 94 Miata, I made a 2D CAD model of the front suspension that helped me understand the relationship between ride height & roll center height, and could of also been used to investigate alternative pick-up points. I've been meaning to do the same for the RX-8 but have not done that yet. Of course, for the model to be usefull we need accurate measurements of control arms, uprights, existing pick-up points, etc. which I don't have.

I would really like to know "how low is too low" on the RX-8. Particularly for the stock pick-up points, since I can't change those under STX rules. I'm happy to collaborate any way I can, and/or am interested in hearing what you discover in this area.

UGH. When is someone going to check for excessive toe in at mid and corner exit with 2 video cameras?

I have always suspected this is happening but have never seem proof.

Maybe bushings would solve it?

FM

UGH. When is someone going to check for excessive toe in at mid and corner exit with 2 video cameras?

I have always suspected this is happening but have never seem proof.

Maybe bushings would solve it?

FM

I remember my guys bump steering the car and not seeing any crazy toe issues. Are you suggesting there is a migration to front compression Toe-In during body roll?

We run a wickedly huge front bar and some 450+ spring rates. I've used up to 750 front spring rates and have been slowly reducing them as I added more front bar.

Momma didn't raise no engineer so this suspension geometry stuff is not second nature. Something I did find interesting is this: With a big front bar and a pretty good size spring (and looking at shock pot data travel) it didn't appear that the low speed, longer corner, body roll u-steer issue was because of toe-in (or out) bump steer. The suspension simply doesn't have that much travel. My guy says it's those crazy forces at work that I've yet to discover.

Will share more data when available.

BTW, the game plan is to run the car as low as possible and change the pickup points to something more optimum.

If the car does have toe issues in cornering, it might not show up in a bump steer measurement if you're just cycling through the suspension in a garage. Pretty much every modern car has tuned bushings to dial in tiny amounts of compliance toe-out up front and toe-in out back to make the car understeer. Additionally, the arm geometry is usually chosen to isolate the compliance effects between vertical ride motions and lateral cornering motion, so it might only become significant in small-radius low-speed corners when small changes in steer angle have a large effect.

I guess the only way to figure out if that is the case is with 2 cameras or a reasonably good model of the suspension with bushing stiffness numbers. 2 cameras might be easier.

You know, I just spent a full weekend on the Summit Point Shenandoah Circuit embarrassing many cars including a supercharged Lotus Exige S 240 (http://stuart5.com/images/exigepass.png) (that's a point-by, video to come... ;)), and my really only complaint is the 2nd-gear corner understeer. (Well, OK, I'd like 500 more RPM too. :yumyum:)

There are three corners there -- Hammer, Hook, and Corkscrew (http://www.mazdadrivers.com/09_trackmaps/shenandoah_circuit_map_3.jpg) -- that are slow and longish. Entry is fine, but the car pushes badly through mid-corner. I found myself dumping the throttle to get the tail to rotate. Left-foot braking is no help. Tire pressures were just-right (BFG R1s), though I might go up a PSI or two. On corner exit the car was glorious, getting the power down and steering easily with the throttle and just a little bit of counter-steer when needed.

I definitely have much more suspension work to do. I have Konis, but still run stock springs. Based on what I've learned here and in the camber thread, I need a lot more camber and may want more rear bar. I put in a MS front bar and drilled the rear bar for more stiffness, but I think more rear bar might be called-for to decrease understeer. I have Progressive street springs in a box waiting for installation.

I think if I can get the springs in, then get -2+ degrees of camber front and slightly less in the rear, and stiffen the rear bar, I can mostly defeat the understeer and make this thing even more of a g-monster. I wonder if I should ditch the MS and drilled stock bars and go with RB at both ends?

As said earlier in the thread, you might want to mount a camera pointing at the rear wheels.
I think there is a toe issue that will show up from mid corner on out while under power.
If so, you might just be able to replace one or two bushings and you are good to go. No more toe change and you could leave the rest of the car alone.
Think of the outside rear tire trying to push the car forward and see which bushing would be good to change.
I wish someone would do this and report back if it works or not.:dunno:
FM

I just went and looked under my car to see where the squishiest looking bushings are. As far as I can tell, the upper camber control rod has a huge bushing carrier in it at the wheel end (at least quite a bit larger than the lower camber link and toe link), so that combined with the high and forward mounted toe-link causes compliance toe-in under lateral forces. So if I was going to recommend a bushing change, I'd recommend that one. Does anybody have pictures of the individual links and bushings off the car?

I refuse to beleive that this is a unique problem only to the RX8. This must be an issue that other cars have and has been confirmed and fixed. Where do we find this info guys?

Idea.....let me get back to you. HINT: I believe the MX-5 has the same rear suspension as the RX8.

Eric, any idea what ride heights the koni challenge MX-5's are using?

Eric, any idea what ride heights the koni challenge MX-5's are using?


Same. Minimum of 3.5" to the lowest part of the car. We found this was the front of the nose or depending on how much rake you had in the car our exhaust just under the diff. The lower you run the car in the rear the lower in height you decrease your rear roll center and the less rear wheel spin you'll get under power on the inside wheel during lower gear turns. Remember, we run a 5.12 ring and pin so 2nd gear (which you almost never use unless something has checked up pretty bad in front of you and traffic goes to a near stop)....so 2nd gear can be a spin master.

I talked to my good friend Tom Long about this. Tom is VERY, VERY fast and experienced. Tom offered that the MX-5 has the same tendancy and that ANY throttle input in these types of corners goes to understeer. ANY. This would echo my experience with the car and with looking at the TPS data.

But I still want to figure out how to change the suspension mounting points so I can add throttle earlier and corner exit at 1, 2 or 3 more mph.

Same. Minimum of 3.5" to the lowest part of the car. We found this was the front of the nose or depending on how much rake you had in the car our exhaust just under the diff. The lower you run the car in the rear the lower in height you decrease your rear roll center and the less rear wheel spin you'll get under power on the inside wheel during lower gear turns. Remember, we run a 5.12 ring and pin so 2nd gear (which you almost never use unless something has checked up pretty bad in front of you and traffic goes to a near stop)....so 2nd gear can be a spin master.
That's my next question -- how much rake did you typically run in your RX-8's? What about the MX-5 guys?

Here is how I understand roll center as it related to suspension geometry. Its not entirely my words since most of it is coming from a text book but I paraphrased most of it to fit neatly in this thread.

As cornering force is applied to the Center of Gravity (CG) it causes the car to rotate around the Roll Center (RC). The greater distance between te CG and the RC, the more leverage the CG will have against the RC and thus with the same cornering force cause the car to rotate even more around the RC. If the CG and RC have no distance between them then the car will not rotate at all around the RC. In most cars, the RC is below the CG and thus causing the chassis to roll away from the cornering force. If the roll center was above the CG then the Chassis would actually roll towards the corner force or in other words lean into the corner like a motorcycle.

The position of the roll center as already explained is controled by the position of the upper arm on our cars. If we can move the position of the lower A-arms then we could also use those as a way of adjusting the roll center aswell. The closer the upper arm becomes to the lower arm, the higher the roll center becomes thus causing less roll (think back to the comparison of Roll Center in a motorcycle).
When raising the upper arm thus moving it away from the lower arm, the roll center becomes lower thus creating more roll.

Hope this helps people's understanding of roll center. Any questions or disagreements I'd be glad to continue contributing my knowledge to this very constructive thread! I'm going to make a few diagrams once I get some time.

Steve

That's my next question -- how much rake did you typically run in your RX-8's? What about the MX-5 guys?


just got back. great question! ;)

beers :beer:

Or also how much scrub inducing wheel offset are you running as well?