What this number represent? and Advice for oversteer

[Guest vizcounty]

Hello,

 

I was working on my car today and I decided to check the spring rates on my GC coilovers because I feel that I might have a low rate springs and my rear of the car keep sliding away.

Anyway, for the

 

FRONTS:

17/98

1200/250/0350

 

REAR

22/97

1200/250/0300

 

What does the number represents? I know that that 250 is the spring rates, which one is diameter and lenght? Also what 17/98 for the FRONTS and 22//97 REAR means?

 

If I do have the same spring rates for both front and rear, I felt that the fronts are stiffer because when I kinda give pressure to the front, it's stiff however it's kinda spongy in the rear.

 

I also check my sway bar for fronts and rear and I got F=27mm and R=19mm. What can I do to correct the oversteer? The rear of the car tend to go away; once I was doing like 40 in a corner and when i slightly pressed the brake, the car started to slide so I release the brake.

 

I got this car not long ago and the previos owner does not seemed to know alot about the car as well because he had it for a month or so before he sold it to me.

 

Thank you for help, appreciated,

 

Tony

[TimZ]

Hello' date='

 

I was working on my car today and I decided to check the spring rates on my GC coilovers because I feel that I might have a low rate springs and my rear of the car keep sliding away.

Anyway, for the

 

FRONTS:

17/98

1200/250/0350

 

REAR

22/97

1200/250/0300

 

What does the number represents? I know that that 250 is the spring rates, which one is diameter and lenght? Also what 17/98 for the FRONTS and 22//97 REAR means?

 

If I do have the same spring rates for both front and rear, I felt that the fronts are stiffer because when I kinda give pressure to the front, it's stiff however it's kinda spongy in the rear.

 

I also check my sway bar for fronts and rear and I got F=27mm and R=19mm. What can I do to correct the oversteer? The rear of the car tend to go away; once I was doing like 40 in a corner and when i slightly pressed the brake, the car started to slide so I release the brake.

[/quote']

 

not sure what the 17/98 and 22/97 numbers mean, but the other numbers are:

 

length/spring inside diameter/rate

 

So your springs are 12.00 inches long, 2.50 inches id and 350 lb/in front and 300 lb/in rear. This confirms your suspicion that the front is stiffer. However, I'm having trouble imagining a 300 lb/in spring feeling spongy under any circumstances on a Z. Hopefully you were just speaking comparitively? Those are fairly high spring rates for the street, unless you have very smooth roads...

 

All else equal, I would think that this spring combo would tend to make the car (a 'normal' Z car, anyway) understeer, not oversteer. However, there are many other things that could be contributing here. Your sway bar sizes don't sound too unusual so I'm not going to go there first. What shocks do you have in the car - are they the same type front and rear? Are they adjustable? How about tires (size, brand, inflation pressures)? Any other non-stock suspension pieces we should know about (sectioned struts, camber plates, any other adjustable pieces)?

 

Also, you only mentioned it oversteering when you applied the brakes - what brakes do you have on the car - are they stock or modified? This could also be a front/rear brake bias issue.

[TimZ]

Your sway bar sizes don't sound too unusual so I'm not going to go there first.

 

I take that back - there is one thing you should check here. The combo looks reasonable, but those are much stiffer bars than stock. One common thing that happens with bars this thick is that the mounting points get torn apart from the added stress. This is most prevalent in the front. Usually the spot welds in the frame rails literally tear apart. If this has happened, then the effectiveness of your front bar is greatly diminished, and this would result in oversteer with that stiff of a rear bar.

 

So go take a good long look at the frame rails where the front bar attaches. It wouldn't hurt to check the rear, too.

[Forrest]

And if they're tearing, consider reinforcing them with a strut cage.

[TimZ]

And if they're tearing, consider reinforcing them with a strut cage.

 

I agree that the mounting points need reinforcement, but I'm not familiar with a 'strut cage'. What exactly does this look like?

[74_5.0L_Z]

I would do the following:

 

Put the 350# spring in the rear, and the 300# in the front.

Remove the rear sway bar.

 

Rear sway bars on the Z make them oversteer horribly, and unload the inside rear tire during corner exit.

[JMortensen]

I disagree about rear sway bars in general, but I agree that you probably don't need huge sway bars at either end seeing as how you've got such stiff springs. I'd switch the springs first, then see how it handles. Taking the rear bar off is going to increase the tendency to understeer, if you just do that alone. You may want to consider taking the rear bar off and getting a smaller front bar, maybe even a stock front bar with some poly bushings.

 

Jon

[Guest vizcounty]

Hello,

 

Wow, thank you for all the replies, I didn't expect to get this many replies, but please keep them coming , sorry for the late update, I was working on my car from 8am-4pm today, trying to change the bushings.

 

Anyway, the car has

- slot mags 14"x6 wheels

- kumho in fronts and bf goodrich on the rears with 195/65/14

- stock front disc and stock rear drums, and I recently repaired the wheel cylinder on the rear

- No camber plates, No Front and Rear strut bars

- Shocks is Kyb's(the manager at autozone who had 7 Z's confirmed this to me) and I usually run 40psi in the rears and 35 fronts

 

I am rather suspicious about the spring rates because it doesnt behave like they are suppose to do. I autocrossed the car before and the rear like to slide away esp under quick slalom corners on moderate speed(35-45). I met another guy who drove a prepared a 240Z with R#275 and F#225 spring rates, custom front and rear strut bars(welded), aftermarket front sways and no rear sways and I sat in his car, his car is MUCH, MUCH stiffer than mine.

 

When we discussed about my problem, we started to give pressure on the front and rear and compare them between my car and his car. His car is stiff on both F and R. Mine is stiff on front but kinda spongy on the Rear.

When I say give pressure, what i mean is: simply put hands on the top of the fender and started to push down.

 

I will try to exchange the springs from F to R since Im doing the bushings now and I will see the results.

 

Thank you again for the replies, I really appreciated,

 

Tony

[jt1]

The rear of the car tend to go away; once I was doing like 40 in a corner and when i slightly pressed the brake, the car started to slide so I release the brake.

 

Sounds like a brake bias problem to me. How does the car feel without any brake application?

 

John

[TimZ]

Anyway' date=' the car has

- slot mags 14"x6 wheels

- kumho in fronts and bf goodrich on the rears with 195/65/14

- stock front disc and stock rear drums, and I recently repaired the wheel cylinder on the rear

- No camber plates, No Front and Rear strut bars

- Shocks is Kyb's(the manager at autozone who had 7 Z's confirmed this to me) and I usually run 40psi in the rears and 35 fronts

 

[/quote']

 

Okay, what model Kumho and BF Goodrich tires? It's very possible that the front tires are just stickier than the rears.

 

Also, do you know which kyb shocks you are using?

[blueovalz]

jt1 brings up a good point here. The suspension should be characterized without the brakes as far as balance is concerned. My understanding is that applying brakes unloads the rear of the car anyway. I would instead base the balance of the car on a skidpad or series of S's.

[JMortensen]

Here's a rundown of what I think you've got going on:

 

KYB doesn't make a strut that can handle the stiff springs you have, not that I am aware of anyway.

It sounds like your rear struts are blown.

You've got too much sway bar both front and rear for those springs IMO.

Mismatched tires front to back.

No strut bars.

Front sway bar mounts may not be reinforced.

Possible brake bias problem.

 

Fixes:

 

Option 1: Keep the front strut cartridges if they aren't blown, replace the rear struts, get softer springs probably like 225F/250R or somewhere in that range. Keep the sway bars that you have. Strut tower braces would be a good idea. Reinforce the front frame rail where the sway bar bolts on. Get the same tires front and back, check brake bias. Possibly reduce rear tire pressure.

 

Option 2: Get Koni sport shocks front and back. Remove rear bar. Replace front bar with smaller aftermarket or possibly stock bar. Install strut tower braces front and back. Check the frame rail for damage, fix if necessary. Get the same tires front and back, check brake bias. Possibly reduce rear tire pressure.

 

Maybe JohnC will see this post and throw his ideas in on this one. He would be a better source for suggestions than just about anyone on this board...

 

Jon

[260DET]

Potentially a big topic. On the sway bars, I have replaced both front and rear stock bushes and retaining brackets with heavier/wider items and strengthened the stock rear bar mounts as well. After a lot of experimentation my front bar is now 22mm dia, the rear 14mm dia which may be a bit light. Track test day coming up so we'll see.

 

My firm conclusion is however that the stock bar bushes are too narrow and too thin in section and consequently allow too much (poly) bush flex and distortion under load which does not allow the bars to work properly.

 

As an example of how complicated suspension setup can be, the rear end of the car was breaking away prematurely. What to do? Got the Koni rear shocks revalved to increase bump significantly and with no other changes rear grip improved dramatically. But its a matter of attending to considering and attending to every suspension component because they all effect handling. Get the basics right and work up from there.

[johnc]

You've got way too much spring in the car! On my race 240Z I run 275 front and 300 rear srpings with a 23mm front and a 17mm rear anti-roll bar.

 

For a street/autox car reduce your spring rates to 200 front 225 rear, keep the anti-roll bars that you have, buy a set of Tokico Illumina shocks, and get 4 of the same tires all the way around.

 

Once you've done the above, then start figuring out the car's handling. Right now, unless you have a 10 point welded in roll cage, your chassis is moving and causing your suspension to do odd things.

[260DET]

Its interesting John that our springs and bars are pretty much the same and the differences are likely due to slightly different total weights and front/rear weight distribution.

 

And mine were arrived at independantly after a process of trial and error. So naturally I support your opinion although the view of a couple of experienced racers is that my setup is not bad at all for ordinary road use as well.