jderson

OK fellow competitors, please, please bare your secrets to an old Z car autocrosser. Last year, all year on hard turns my inside rear left the ground, which appeared to be excessive diagonal weight transfer. It occurs at corner entry especially under heavy braking and continues in mid corner and corner exit. Do not feel any wild push or loosness. Most damaging time loss is on exit where the inside rear takes it's time to get back on the ground. Using Quaife; Tryed many different spring front to rear combos in range of 300 to 450 lbs. Tryed changing roll axis. Tryed bars. Tryed rebound settings. At this point I do not know what I'm to do. This is really sad! I'm not kidding. I'd just love to even have the two inside wheels leave the ground simultainiously.
Signed,
Desparate John

zlalomz

Are you sure it's not a VW Rabbit your driving? I have never seen a Datsun Z do that. I assume you have asked Tom Holt since your car is on his website in the competitors section. Vic Sias at www.siastuning.com and Andy Craig at www.dandos.com are 2 past national champs that could help. Also Jay Morris at www.ground-control had a champ ITS car in past years and has helped with suspension problems before.
I love the look of your car and would love to see more pics and info about it. Steve

tholt29

Hey John!
First, what are you running now for springs and bars? Also, I assume you've confirmed everything is moving free, no bushings or shocks binding? Are your shocks valved for your spring rates?
The only experience I have had that is similar occurs in Topeka on the rough concrete. About mid turn on some of the bumpier sections I was getting up on two wheels. It's the only time I've ever felt like the rear was free wheeling. It did it with the old soft suspension, it does it with the current stiff suspension. I didn't notice it so much under braking... mostly bumpy fast right handers. (Don't blame the ample mass of the driver, the car is balanced with my butt in the seat!)
Let us know your setup and maybe we can figure something out.

Tom

FPguy

We've had the same problem with our Quaife. With only 6 autocross events on the new (very expensive) Quaife we pulled it out and sent it back. They checked it and said it was like new - no problem. We had a Nissan clutch style diff for years and did not have the problem (same car set up). They suggested we loosen the rear of the car (spring or bar) - it requires 35 lbs of preload to work properly. I'm shocked that we're unloading the inside tire that much.

The wheelspin only happens in fast sweepers (none during tight hard corners) - so here's our plan...
1) We're installing a wing/spoiler.
2) We're gonna soften or remove the rear swaybar.
3) We're gonna change the rear toe to compensate for no rear bar.

We currently fun 300 lbs springs in front and 350 in the rear - with custom Blisteins all around. We don't plan to change those at this time.

We're rebuilding the engine this winter (lots more power) - so the problem will be MUCH worse without some kind of fix.

I've heard of others with a Quaife having the same problem. Guess it requires a change in the setup. Let's stay in touch - we might be able to help each other find the right one.

Steve

Spudz

Hey John,
Like the others said if you can give us the exact or close settings then we might be able to help a bit more. What are your corner weights and ?

Like Steve said Quaifes tend to react differently to changes then the clutch type LSD's. Up here in the wet country some of the big guns over in Seattle were commenting on the way it breaks away on the rain ( low break away torque numbers ) they had a few incidents due to it .

If you gather the information up for us I will fire off an email with it to Erik from EMI Racing he is a suspension guru and knows the Z well.

Take Care,
Mike

FPguy

Here's our corner weigthts - without driver - with about 8 gallons of fuel.

Left Front - 500 Right Front - 520

Left Rear - 620 Right Rear - 480

Total Front - 1040

Total Rear - 900

As you can see the car is fairly light. Corner weights aren't exactly what we want - but not bad out of the box - we have not tried to adjust them yet.
It's on the "to do" list.

Steve

tube80z

John,

This may or may not help. First make sure that you have enough rebound travel in the back, which I'm pretty sure you've already ruled out. Also make sure that you don't have the rear role center too high and this is a jacking effect. Have someone take a picture when this happens and look at the outside. It it's extended then you may have the rear RC too high. I got bit by this once.

My trick is a droop limiter. I limit the front droop to keep the car from doing this and to get better traction out of the corners with my really loose LS. I normally set the front droop to about 3/4 of an inch. You may need to adjust the number a little but you should find this is an interesting tuning tool.

I tried zero drooping the front like a formular car and can't say that I ever really got that to work. I may play with that more this year. Adding preload made the car feel very tight.

My car is fully caged, advance design shocks, and 500 fronts, 450 rears. Front sway bar is 7/8 and the rear is 3/8.

Cary

jderson

Thankyou all for your responses.
I reply with these questions and comments: Cary: I do have travel on rear rebound; What is a droop limiter and how does it work?; And your comment of the rear roll center being too high could be one of the causes of inside rear wheel lift. Need more input on that like Carroll Smith says if the front RC is too low-or rear is too high, the result is the roll axis is too for out of parallel with mass centroid axis leading to non-linear generation of chassis roll and lateral load transfer and therefore in your suggestion of the rear RC being too high causes too much load transfer at the front, thus understeer and three wheeled motoring on corner exit. ???? (Which wheel does Smith mean here that leaves the ground?)
Mike, I ran the original Gleason-Torsen rear end and the Quaife feels the same.
Dear Tom(Ample ***), Springs:tried combos of 400/400; 350/400; 300/350; Bars: 25mm frt; 22mm rear. There is no binding. Shocks: Koni single adj. (It appears my problem occurs after initial shock action and shocks are set on full soft rebound. On the last race of the seaon I was gonna try it without the rear bar and I blew the clutch on the first run...so.
Steve-FPGuy, You said something about your rear toe change. From what to what? I was running 3/16-1/8 toe in. You say your wheelspin happens in sweepers. Probably after the shocks have done their thing. Also, what is your rear/front bar settings?. Removal of your rear bar sounds possible, but what about introduction of push? BTW, your corner numbers don't add up to your total. And also BTW, I know a roadracer who runs 500frt and 300rear springs. I was offered to drive it (autocross) and then his car broke
Anyhow I appreciate your inputs, all of you.
John

FPguy

Hi John,

>You said something about your rear toe change. From what to what?

We've been running total toe-in at .34 (.17 on each side). Without that much toe-in the car was very loose in the rear end. Not sure how much we'll change it - most likely a little at a time. We'll monitor tire temps to make final adjustments.

>I was running 3/16-1/8 toe in. You say your wheelspin happens in >sweepers. Probably after the shocks have done their thing.

Yes - it happens at the exit of the turn (fast sweepers often in third gear) leading into a long straight. Very frustrating because tremendous time is lost.
Third gear happens quickly for us since we're running a 4.6 rear end.

>Removal of your rear bar sounds possible, but what about introduction of push?

Actually our first step will be to use a wing on the car. If that's not enough - reducing the toe-in should take care of the any push. We have adjustable lower control arms front and rear so we have tremendous adjustability.

>BTW, your corner numbers don't add up to your total.

You're right. The corners were done with the driver - front to rear was done without the driver. It's interesting to note the wheel spin happened both in left and right turns. So even with the lard *** driver in the car and the steady state LR corner weight at 620 - we still had wheel spin.

Steve

tube80z

A droop limiter is really nothing more than a mechanical limiter for suspension droop. In my case I have a cable that runs from the top of the strut tower to the swaybar pickup. I have this currently set to only allow 3/4 of an inch of droop/rebound. This helps to limit weight transfer on the inside rear tire when turning.

The mass centroid axis in Smith's books is really not correct. I hate to say that but It's not really anything that's close to reality. For all intents and purposes the car works as if it as a single point (CG) that forces work through. How you connect to this point and lever it are called couples and moments. It sounds very complicated but if you can get someone to explain it in less engineering terms it makes sense.

A high roll center generally leads to high jacking forces. Why this can be a problem is that at some point the forces will actually lift tires and in extremes will lever the entire car up. That might be what the problem is.

If you have a large incline in the roll axis you may end three wheeling a lot. In this case the droop limiter can be a good cratch to help.

Well, that's about all I know, which isn't much.

Cary

Spudz

John and Cary,
My freind is using this kind of strap on their 325is ITS car. I was going to use one on my Z , because in the back I had cut factory springs and at full drop my springs were not in contact with my perches

They use a cable like Cary suggested , so it had some play in it to keep the spring from wrapping up and snapping .

Mike

Hmmm... maybe Cary would be interested in welding a cage in my car for me

jburge01

Maybe it's as simple as stiffening up the front?

Your car sounds like mine was with the open diff - I was going through all the sweepers with the throttle on-off-on-off-on until it would finally get enough weight on the inside to grab. But it would stick good enough to get up on two wheels on blacktop at Gulf Greyhound Park.

My setup is 300/300 lb springs and illuminas on 3, 1" front, 7/8" rear sway bars. I thought I really needed to stiffen up the front, and so did a good driver who tried my car, but after I put an R190 limited slip in things really changed to the point where I'll probably leave it the way it is (for a while anyway).

Don't know how much power your making, probably a lot more than me . I haven't done anything to the internals on my L24 yet, so I don't know if it is an apples to apples.

jderson

I thankyou all for your inputs. I learned somethings from them:
1. Spring play(loosness) on rebound will not necessarily affect the swaybar action, but visualizing not the wheelset movements, but the car body movements, the car body is free to experince extra travel in elevation on the inside corner. Not a good thing.
2. Suggestions given of a cable inbibitor are a fix and probably an effective one from what I can see. A multimillion $ race team would do it right in the first place by getting the upper and lower limits of shock travel in the right place.
3. Roll axis is a contributing factor, but to what degee, I don't know.
John A.

jderson

I thankyou all for your inputs. I learned somethings from them:
1. Spring play(loosness) on rebound will not necessarily affect the swaybar action, but visualizing not the wheelset movements, but the car body movements, the car body is free to experince extra travel in elevation on the inside corner. Not a good thing.
2. Suggestions given of a cable inbibitor are a fix and probably an effective one from what I can see. A multimillion $ race team would do it right in the first place by getting the upper and lower limits of shock travel in the right place.
3. Roll axis is a contributing factor, but to what degee, I don't know.
John A.