I definately think the larger rear bar will help. The car seems to need it to rotate at this low power level. I run an RB rear bar on my car and it is very predictable. It does not snap loose ever, and is very close to neutral. It is free off power on entry, and depending on the corner there is a very slight push on exit.

Actually the ST 86-88 rear bar is slightly thicker than the RB and I was thinking of putting one of those on my car. The ST 89-91 bar is the same size as the RB.

My setup sounds similar to yours. I run 8k front and 6k rear springs, which is very close to the rates you are running.

In the rear I have 1/16 toe in and -1.3 rear camber. I need a bit more rear camber, and will probably go to -1.75 degrees.

This is on 235/40/17 NT05 tires all around.

 

yeah we ran the stock bars, or nothing in the rear. in ITS the car is setup so that you can be WOT or close to it at turn in. since you're limited to the AMOUNT of power, you have to maximise the full throttle time

 

For a track car I can definitely see not running a rear bar. You guys don't have to turn much and you want the car stable in transitions. I don't so much.

 

I definately agree with LargeOrangeFont on the subject of the Rear bar.

It makes the car more predictable and repsonsive for an autocross setup. I drive my car every once in a while on the street and sometimes I scare myself cornering at higher speeds with the big RSB. You get the feeling you're going to kick *** entering a turn or in my case an on-ramp, then it just feels like it's going to whip around. My times at AutoX improved a few seconds after I put mine on. If I was ever to track the car I'd probably unbolt it though.

You're more than welcome to try out my car someday. Perhaps this Saturday, at the Woodbridge VA meet?

 

good build, and great find. but I thought you had a red one with 20b? or is it someone else.

 

Nice Build on your GTUs
I am picking up another black 90 GTUs this weekend that was a club racer for the past 10 years. I have some plans on racing it also possibly in STS?

 

Well I threw the ARP studs on the front hubs last night in anticipation of putting some spacers on the front wheels. I've got 35mm offset wheels all around (7.5" wide) and get some rub at full steering lock. Widening the front track is also a way to tune the balance of the car towards more oversteer. I bought 10mm and 15mm spacers so I should be able to test them out next weekend.

Bad news is I noticed that the plastic tank (upper) on the radiator is starting to crack. I had been smelling a very faint coolant smell and now I see the leak. I was going to do some testing this weekend but it's not going to happen now. Just ordered a replacement rad and it should be here next weekend.

 

I'd try tire pressure before the spacers. Tire pressure affects rate. The spacers will add scrub and increase steering loads. I can't see either as a good thing. You might try changing the links to the rear bar. Closer to the actual bar will decrease the leverage and increase roll stiffness.

GD

 

Changing tire pressure will not have any effect on the tires rubbing on the inner fenders, though.

Tire pressure effects the spring rate of the tire but there is a "happy zone" of tire pressure with regard to grip and I will only be able to find that happy zone through testing. Changing the stagger of tire pressure front to rear tends to affect the car more transitionally than in steady-state and is a very minor effect around the 2-4psi sweet spot.

With how the endlinks attach to the '89-'91 rear bar (don't know how the earlier ones mount) there is no way to move the endlinks to a shorter position on the lever arm.

 

We used to do the track width change thing with Karts because you really don't have springs to fiddle with. You had frame stiffeners to change the rate front to rear. It works, but we had more effect with tire pressure as the spring rate is affected by anything in the system, from the upper strut/shock mount to the asphalt. This worked , of course after we had the chassis rate dialed in. Nascar teams do it often and they go in 1/2 lb increments.
I have a 90 GTu and the rear bar where the link attaches is like a "blade" about .250" thick and about 3.0" long. The production mount point is near the rear most end. I drilled two more hioles further forward, toward the actual bar. This effectively stiffens the rate, as it lessens the leverage. Of course only to a point where the rate is peaked.

GD

 

On a kart changing the percent spring rate change due to tire pressure is huge (soft tires and flexy chassis). Not so much with a car.

From what I saw looking under the car when I re-did the suspension it didn't look to me like drilling more holes on the rear bar would give much of a rate change for the bar. I guess I have to get under there and take some actual measurements and run a calculation now that you've pointed it out. Thanks!

John

 

Looks like re-drilling the blade of the bar may give a 10-20% increase in the stiffness of the rear bar. It's worth a shot, so I did it. I just put two holes as far inside on the blade as I could without getting into the curve of the blade. About an inch and a half inside the original holes. If it isn't noticable I'll pick up an aftermarket bar to try.

 

Well, first event in the car was today. My co-driver and I battled with the rest of the STS field in the Washington DC region. Even with no development time on the suspension and running a stock tune on the engine, we managed to run times good enough to take the STS win. Greg ended with a clean 54.7 and my best was a dirty 54.9. Our competition was mostly in the 56's but there was a nationally competitive driver in a CRX at a 55.0 and a Miata at 55.2. Greg's time was good for 12th in index, in a field of probably 220 cars. Not bad at all.

I should be able to get a better tune on the car this week. As for the suspension, it worked better with a little more rear bar . It's a monster in slaloms and its only real weakness is slow corners - it's very slow out of low RPM digs.

Also had some problems with locking the rear brakes and stalling the motor. That's something I'll have to get used to.

Very happy with the results so far, looking forward to finding more speed as we go on.

 

Congratulations! You need to put some vids up for us to see. Great to hear that your setup is performing.

 

We don't have our video set up in the car yet . Shortly!

Forgot to mention that we threw the car on the scales. It was 2,560lbs with all of the gear in the car but no driver. The gear was probably about 20lbs of extra weight. Additionally my seats are each 10lbs above the min weight (going to be replaced soon) and we had almost a full tank of gas (about 4 gallons shy). Based on this I think the car should be about a hundred pounds lighter when it's maxed out for the rules and running low fuel.

That's 600lbs heavier than the best CRXs in the class and 400lbs up on the Miatas. Not great, but almost exactly where I expected it to be.

 

 

Sweet!

 

Car looks awesome John!

 

yeah that turned out really nice!

 

Started the tuning after work yesterday.

From 3,000 RPM to 4500, the AFR ramped from 11.0 to 12.5. From 4500 to redline it began a slow decline from 12.5 at 4500 RPM to nearly 9.0 at 8,000 RPM. No wonder it felt sluggish on course.

After a quick tune with the RTek, I got the AFRs to about 12.5 across the board except from 6500 to redline, where they were still in the 11's. More tuning after work today and I'm getting consistent 13-13.4 across the rev range at full throttle. Need to work on the part-throttle tuning now, but it is making a bunch more power now! Way more fun.

I also need to work on the front end. I actually think the front shocks are too short, and there is almost zero droop travel. Not a great thing, especially for the street ride. I'd rather not get new shocks, but I'm going to talk to ground control to see if they can do anything.

Need to get the sucker on a dyno, too.

 

Not sure why the first pic went away, and with this retarded forum setup you can't edit an old post.

 

^ Wheelspin or blowing out a lot of smoke b/c of how rich it was running?

That's a great pic btw. I see what you were saying about the lack of droop travel.

 

The smoke is the rear brakes locking up.

 

Damn, his driver side front tire is not contacting the pavement. Too low of a spring rate?

I bet he got a good snap steer when that tire made contact again.

 

Subterranean roll center combined with a high roll rate and almost no droop travel. Nothing that can be done about the first two (struts suck). The third one I'm going to correct.

You don't even notice it. That tire isn't doing much of the cornering work when it's touching the ground anyway.