Emodspitfire.....What to try next ?

So,

I have a persistent understeer condition on the autocross car. Have tried multiple stuff: 1. swapped spool diff for LSD 2. New slicks 3. Baselilned and reset the cornerweights. (Not too bad prior to adjustment). Checked tire temps at last event. All temps within 5% across the individual tires.

Weight distribution is 53/47 F/R.

My first thought is to lower the front spring rates and add a small amount of negative camber at the front. (Keep the existing RH).

The guy that I borrow scales from suggested raising the rear spring rates.

Current rates are 450 at the front. I have a set of 300 pound springs. (Wheel rate is approx 300) Current rates are 350 at the rear. I have a set of 500 pound springs. (Wheel rate is approx. 280)

What say the GRM setup wizards?

Thanks,

Rog

On most cars I wouldn't ask this question, but how's the Ackerman? Spitfires have a stupid stock steering geometry which leads to 'reverse Ackerman', that's where the outside tire turns more than the inside. It leads to excessive slip angles on the outside front (i.e. the most heavily loaded tire) and also tries to force the front of the car up. I fixed that on my car by moving the steering rack back. No, not easy, but it had a pretty strong effect.

When does it understeer: corner entry, corner exit, steady-state, slaloms???

Hey,

RE: the ackerman: I lowered the rack to allow a lower motor install. I swapped the steering arms to maintain the existing tie rod angles. (Approx). Car ran many years with decent balance.

RE: Pete: Predominant push is on low to mid speed corner entry.

Thanks guys.

Rog

f it ran like that with decent balance then I'd say it's time to dig in the ol' memory bank and see if you can recall when this problem started. Try to remember what was changed at about that time.

Something which may have an effect is driver confidence; if the car was being driven conservatively as the driver learned its limits then the understeer may not have surfaced. As the driver starts pushing the car harder, the problem begins to manifest itself, although it was really there all along.

Low to mid speed corner entry makes me think that's under acceleration, so the idea to go with a limited slip was a good one. It may be too aggressive; is there enough friction modifier in the oil? If the plates can't slip against each other, that's effectively a spool.

Also, about raising the rear spring rate: it's my contention that the way to tune your suspension is to attack the problem end, not the other end. Stiffening the rear spring rate adds oversteer which is not really the same thing as lessening understeer, i.e. for instance I'd soften the front before stiffening the rear.

This 'shortcut' chart may help jog your memory:

I would try more negative camber in the front first.

In reply to jstein77:

Even with the even temps across the tire face he's seeing?

I ask in the "I want to know" sense, not the "you sure about that, buster?" sense...

My (probably simplistic, mostly-books) understanding is that the even temps suggest he's using the tire about as efficiently as possible. I only got a couple of (frequently cold) events in with my camber-plated E30 and tire temp probe before I got rear-ended and the experimentation thus suspended, so I don't have much empirical data of my own...

I'd leave the camber alone too.

Have you set up a circle to "simulate" the speed and situation, and then checked tire temps? I'd wonder that the tire temps are partially getting confounded with good braking data when you do post run checks.

Also, have you tried a little toe out? That may help on initial turn in, but not really the mid turn part- is it like Crumudgeon asks that it's under power?

If it's under power, I would next ask what LSD you are running? And can you tune it out some more.

Car has +3/8" toeout.

Diff is a 1.5 Kaaz

With the corner entry understeer, does the car not point in and get worse or points in and then washes out or points in and then darts?

Does not point in...I have to wait on it or go VERY slow.

I would try raising the front tire pressure first because that is easiest, then adjust the camber towards negative and then soften the front suspension. Do you have adjustable shocks?

Where are the front and rear roll centers?

emodspitfire wrote: Car has +3/8" toeout. Diff is a 1.5 Kaaz

Say WHAT??? The thing must be nearly uncontrollable. I ran a MAX of 1/8" toeout on the Abomination and even that made it twitchy as hell in a straight line at high speed. Zero toe in the rear or it was really strange feeling.

You say you have to 'wait' on the car or go really slow. When you 'wait', does it slide, then grab? If so the infamous Spitfire reverse Ackerman could very well have something to do with it.

When I ran the Mazdaspeed limited slip (which was basically a spool) in the Jensenator it just never grabbed in the front because the rears were locked together and due to the big contact patch (22/10x13 Hoosier slicks) there was no tire 'slip' to act like a half ass differential so the front tires just slid, even with no throttle. When I went back to a standard RX7 limited slip, letting off the gas let it bite in the front. It's still twitchy as hell because of the sorta hard compound tires (R45's), overall grip is sorely lacking.

Brotus: Don't have the roll center numbers on this computer, Basically, the front RC is stock TR. The rear RC is about 2" higher than the front. The rear suspension is custom SLA.

Curmudgeon: I have been running 1/4 to 3/8" positive Toe out for a long time. Car has never been twitchy, it just pushes....(Only more push recently)

On corner entry, it typically pushes wide, then starts to rotate. (not really a "grab", more gradual)

I'm gonna recruit another driver to get a second opinion.

Thanks to all,

Rog

Good luck. Please report back on your findings and ultimate solution.

Twice you've mentioned that it was fine, and then changed, becoming an understeering plowing pig.

It's a mechanical device, it won't simply change how it operates. Something has happened that has created this new handling characteristic.

Curmudgeon pointed you this way, and you avoided it.

So again, what changed just prior to this problem arising? New tires? New alignment? Driving school? New brakes? Etc.

I'd round out that 3/8" toe to 1/2"

emodspitfire wrote: Car has +3/8" toeout. Diff is a 1.5 Kaaz

That is a lot of toe out. Could tire scrub be some of the reason for the understeer ?

I would adjust the toe out as well but it seems like that if toe out was the big problem it should point in.

I keep thinking Ackerman. Here's why:

If both front wheels turn the same amount, the inside tire will slip more because it's trying to cover a smaller distance/tighter radius than the outside. This comes back to the driver as a steering wheel that 'fights' in a turn and with street tires a lot of squealing. Typically Ackerman is set up so the inside tire turns more than the outside to correct this (to an extent). On a Spitfire, the steering geometry is all wack because the steering rack is so far ahead of the tie rod ends, meaning the outside tire turns more than the inside, which is why I call it reverse Ackerman.

That's not typically a big deal with street tires because they don't grip that well. Now, put some wide sticky race rubber on it and try to turn under power... The outside front tire gets most of the weight in a given turning situation. The reverse Ackerman causes the outside tire to have a lot of slip angle.

Remember the circle of friction; a tire can generate X grip in any direction, or combination of directions. but once it's exceeded you are slip sliding away. So what happens is that part of the grip available from that outside front is being just thrown away because of the excessive slip angle. That means it's much more prone to understeer.

It will also cause the steering to start 'fighting' the driver in a low speed tight turn situation. That's because the traction input from the tire is trying to force the tire to turn even more sharply and this causes the steering knuckle to try to 'shove' the rack itself, which comes to the driver as 'it turns okay when I start in but as I add steering input I suddenly have to start steering the other direction or fighting the wheel because it suddenly started trying to turn on its own'.

This also causes the front of the car to try to rise in a turn, which brings its own set of problems to the table.

This became very noticeable on the Abomination as I got used to driving it and pushed it ever harder. A couple of my co drivers said the same thing. In the quest to go ever faster, I wound up moving the steering rack back so that the centerline of the rack was 3/4" behind the center of a line connecting the center of both outer tie rods. I also narrowed the rack at the same time, this was to correct bump steer. It made a HUGE difference in the way the car drove, it was much more predictable at the limits.

This can also be done by going with a double Pitman arm steering setup (parallelogram steering) but talk about reinventing the wheel... I went through that in my head and drew it out on paper, it was going to be muy complicated so I decided to narrow/move the rack instead.

Toyman has the Abomination now, he can get pics/measurements for you.

Folks,

Thanks to all for the inputs.

I ended up dropping the front spring rates from 300 to 200 (Spring rate at the wheel). Ride ht ended up about .12" lower which added a small amount of static negative camber.

Recruited one of the local hotshoes, (Thanks Kevin W!) to do a test drive after the tires were warm. He said the car felt good, and maybe needed more front anti-roll bar. Offered to co-drive in the future. (the answer is yes)

I went out and finished my runs. The car turned in well, was wagging the tail in some of the faster transitions. Tire temps are OK.

Didn't get close to Kevin's time......

So the car is pretty good....now we gotta train the driver (Grin)

Thanks again,

Rog