jakers

Problem: When driving hard the car feels like it is pushing in corners, Is the correct term "oversteer"? Car is pushing through the steering response and I have to ease off the accelerator to get a response. I prefer the car balanced in the corners or slightly lose in the rear.

Car
94 coupe with modified n/a engine and estimated hp around 400+/-. (Balance & Blue Printed, head work, roller rockers, headers, 3 " exhaust, no cats, and tuned)

Other important items
R&D Camber brace and Harness Bar both installed.

Current suspension
Base Coupe with no Z51

Suspension Upgrades
32mm solid front sway bar
Retained original rear sway bar. A 24 mm, I think.
Changed shocks to KYB basic shock
Stock springs.

Should I change out the rear sway bar to a bigger diameter to get a more balanced car?

Aardwolf

The road race section might have more on this. The term for push would be understeer. And there is a lot of it! Oversteer is the rear end sliding out.

There are a number of things you can do.

30MM front and 26MM rear sways would be more balanced. You would want to increase the rear or decrease the front, to help with understeer.

A larger front tire would help if you are running two different sizes. I have 285 on all four. Tweaking the tire pressure front and rear may help as would alignment.

If you just want it loose in the rear could can try some rear toe in. I road race mine and at high speeds I would not want oversteer. It can be useful for autox though to slide the rear around in corners.

JrRifleCoach

When driving hard the car feels like it is pushing in corners, Is the correct term "oversteer"?

Yes

Car is pushing through the steering response and I have to ease of the get a response. I prefer the car balanced in the corners or slightly lose in the rear.

Balanced is best, You'll learn to control the rear with the throttle

Suspension Upgrades:
32mm solid front sway bar
Retained original rear sway bar. A 24 mm, I think.
Changed shocks to KYB basic shock

Should I change out the rear sway bar to a bigger diameter to get a more balanced car?

Yes A 26 or 28 rear bar would be better suited with the 32

Slalom4me

What you describe is known as understeer everywhere outside of
NASCAR-land.

'94 CPE FE1 73.2/39.9 N/mm springs with 26T/24S bars, 255/285 on 8.5/9.5 wheels.

My vote is that the aftermarket 32mm front bar in conjunction with the
OEM springs, rear bar and asymmetric rubber is the principal cause of
the understeer.

The largest rear bar I know of is the 26mm. My vote is it will help, but
not enough for your current combination.

Increasing rear spring rate will reduce understeer. 39.9 N/mm = 228 lbs/in
(I use N/mm * 5.710 = lbs/in to convert) A Z07 or Z51 rear is 57.2 N/mm
or equivalent to 327 lbs/in. On request VB&P can provide a rear spring
in the vicinity of 350 lbs/in. Don't select their higher rate rear springs
they are too stiff for your purposes.

A smaller front bar will help - the OEM 30mm is hard to find, people say
they have purchased aftermarket ones following their being discontinued
by GM, but every time I followed up with the vendors they indicated
they had no 30mm. ZBRA claimed VB&P substituted a 30mm for him
but VB&P has told others including me repeatedly that they do not have
the 30mm.

There is an aftermarket 28mm front bar. With the existing 255/285 tires
and a 26mm rear bar, this could restore neutral-to-oversteer characteristics.

Edit: correct rear sway bar typo - 26T/24T changed to 26T/24S

.

Aardwolf

Guldstrand is listing tubular bars in stock. You would have to call to select fitment though as the bars can be made in different ways so you cannot select what you need based on size.

Slalom4me

Guldstrand offers

88-96
31.75mm (1.250") Solid - stiffer and heavier than 31.75 Tubular
31.75mm (1.250") Tubular - less stiff & lighter than 31.75 Solid
28.58mm (1.125") Solid

85-87
31.75mm (1.250") Tubular - stiffest
28.58mm (1.125") Solid

The OEM Z51/Z07 bar is
30.00mm (1.181") Solid

I'm citing dimensions to tenths only for conversion purposes, not
to imply that much precision in manufacturing tolerances.

The point is that Guldstrand is not offering something between
32mm and 28mm. A used 30mm bar would lie somewhere between
Guldstrands, VB&P and other vendors 1-1/8 - 1-1/4" bars.

(Edited to correct statement about relative stiffness of tubular and solid bars.)

.

jakers

The tires are 275 front and 315 rears. They are Sumitomos, they were on the car when I bought it. I understand the relationship between good rubber and handling but new tires are not in the budget until 2008. The front bar was purchased from another Forum member an is actually a 30MM solid bar. Supposedly it was OEM... It is heavy as hell.

The OEM Z51/Z07 bar is
30.00mm (1.181") Solid


???? I would have to re-measure it but I remember the approximate 1.181 number.

Highest and best use for this car (As of today) is high performance street with some autoX in the near future.

Help me understand the how tire pressure can impact oversteer and understeer??

Aardwolf

As far as I know you cannot compare the diameter of the bars to how stiff they are because of tube wall thickness in the tubular bar and swing arm length. Not every bar is made the same. That is why I suggested to call them for application, since I do not know what bar they are selling.

Slalom4me

Oops, Slalom4me is incorrect.

A hollow shaft will be stiffer than a solid shaft with equal section
modulus. But for the hollow shaft to have equal section modulus, it
must be larger in diameter.

A hollow shaft of equal diameter will be less stiff than a solid shaft.

Edited post above to correct.

.

jakers

Thanks for the feed back folks. I am still listening...

Slalom4me

The wrong tire pressure reduces grip, the right pressure increases grip.

Less grip at the front leads to understeer. Less grip at the rear leads
to oversteer.

.

Corvette Kid

As a quick fix you could increase your front negative camber. Just going to 1/2 to 3/4 degree negative will make a big difference. If you drive on a lot of curvy roads as I do, you could go as much as 1 degree negative with no or minimal change in inner tire wear.

RonRed89

The timing on your post couldn't be more perfect for me. I am trying to get the same as you, and after searching forever on a 30mm bar, it's proving to be nearly impossible to find. VB&P is now recommending a 32mm tubular bar with a 24mm Solid Z51 rear bar (which i was a little surprised about). My car is a 89 base car with Koni single adjustables. Trying to avoid the same understeer scenario that you're describing here. Any thoughts from the more knowledgeable folks in this thread?

Ron

CFI-EFI

Like the NASCAR announcers say about hitting the wall. If the front end hits first the car was under steering (tends to go in a straight line). If the rear end hits first (it came around on you) the car was over steering. Just for yuks, remove a sway bar link in the front of the car and go for a ride. I'll bet you will have a much easier time getting the rear end to step out (over steer) on you. As has been said, more rear bar and/or less front, will help the balance.

RACE ON!!!

PBnf

If you don't steer enough and you're going in too straight a line then you've understeered the car. If you find the car turned in too much and the back end wants to come around, then you've oversteered the car. I find that an easy way to remember.

By the way, that book on 101 mods for a 1984-1996 Corvette has a section on sway bars.

Corvette Kid

As just another dumb-assed old country boy, I find "push" and "loose" just so much easier to remember.

LouisvilleLT4

Most of the work of balancing the car is done by YOU. If you want some oversteer as you corner, make sure you're at a high RPM (downshift if necessary), and right when you're ready for some oversteer tendency, lift off the throttle hard.

The reason this works is your RPMs will rapidly fall, as will the amount of torque applied to the road, pitching the car's weight forward which lifts some weight off the rear tires. Instant oversteer.

Even if you have to brake to slow down for the corner, try to brake independently of this action; that is, start braking before you release the throttle, waiting to lift off until you're ready for the tail end to get loose. Doing this will give you control over your understeer-oversteer balance at all times.

CFI-EFI

Same ole, same ole. Just hang on tight when it gets pushy-loose. Boogity! Boogity! Boogity!

RACE ON!!!

Slalom4me

My vote is that the 32T/24S combo would not be at all satisfying
on an otherwise stock '89.

'89 CPE FE1 93.1/39.9 N/mm springs with 26T/22S bars, 275/275 on 9.5/9.5 wheels.

The 93.1 N/mm front rate on the FE1 is the second highest rate ever
used on non-ZR-1 C4's. GM dialed back the Z07 rate from 115.5 to
90.1 in '92 but continued to use the 57.2 rear rate the Z51/Z07's
had used since '85. So, if you located a Z51/Z07 rear or an aftermarket
replacement with a rate of 330-350 lbs/in for your '89, this simple,
inexpensive swap would get rid of the deep kneebends the rear does
now over any sizable dip and make a big stride toward more neutral
characteristics in steady-state cornering.

My vote is that even with the stiffer rear spring, the 32mm front bar is
too big. I feel that without chassis stiffeners like the R-D Camber-Brace,
Cross-Brace and Cross-Frame, the additional stiffness of the 32mm is
given up to chassis flex. With the additional stiffeners, a 350# rear
spring and a 26mm rear bar, the 32mm returns understeer in steady-state
for me. More camber, different bushings, wider track, bigger tires
would all help increase grip at the front and reduce U/S.

For an otherwise stock '89, my vote is that if a 30mm is available, a
30mm/26mm combo with a 330-350 rear spring is interesting. In the
absence of a 30mm, then a 28mm/24mm combo would be worth a try,
IMO. especially on less-than-smooth circuits.

You'll need the longer rear bolts for the spring. Don't use the stock bolts
with nylock nuts positioned below the cotter pin hole, the bolt will
eventually break at the c/p hole.
.

teufelhund

Just go with a 28MM solid rear bar and your worries are over. That and double check your tire pressures all around that that will more than likely fix your problems. Also any time you do ANYTHING to the suspension you need to have the alignment adjusted so you might as well get that done while you're at it.