EPiC Vette

I'm an 89 C4 with base FE1 soft setup, only susp mods being new KYB shocks. Tires are 275 40 17 all the way around. I do 3-4 HPDE a year, but want to corner on rails.

I just put a 32mm on the front, and was expecting bump steer and 'channel-tracking'... got NONE of that! I'm STOKED!!!

Now, for the rear, I was gonna run the 26mm, but it was mentioned to me that if I'm not running 315's at the back, that 24mm was better?

Whattya think?

Slalom4me

My vote is you'll conclude that even with the 26mm rear bar,
you have understeer.

Step up to a 330-350ish lb/in rear spring (ask VB&P to check
for a low testing unit) to better compliment the stiffish OEM
front spring. Step down from 32mm to 30mm if you can find
one, try a 28mm if you can't. Make sure to toe-in at the rear.
Then see how the fr/rr balance feels.

.

EPiC Vette

Some understeer is fine, I'd rather tune for that than OS.


I've been casually looking for a 30mm bar for a while, and haven't run across any here, nor in the aftermarket. I agree, 30/26 is what I was looking for.

larryfs

I run 30/26 with F275/R315's
the car is perfectly balanced.

Sidney004

Eric:
I have got the 32/26 setup with 275's all around and it seems reasonably neutral, it does understeer slightly(such as Thill turns 3,4 and 15, but I think you want that anyways.) What size rear sway bar are you running now? By the way, if you want a 24mm rear sway bar, I happen to have one, just let me know. It was good seeing you at Thill the other day, that was a lot of fun.

EPiC Vette

Sidney, I was 26/22, as this is what came on the car. This last time at the track, I felt like I was pushing the limits of the stock setup, though it is wonderfully neutral feeling. In answer to your question, 22mm.

I don't want to lower the car, and I don't want to sacrifice ride quality, what little is left. Actually, the big frt bar seems to improve all-around driveability.

Thanks for the offer on the 24. I'll keep that in mind.

Slalom4me

EPiC,

Sidney004 has a '94 and larryfs has a '96. You might want to
ask them what suspension packages their cars have, then take
a look at the published spring rates for your '89 FE1 and their cars
when weighing matters here.

I use the following factor to convert to lbs/in
Keep in mind that your year had symetric tire sizes and theirs had
asymetric sizes (except the Z07/Z51's). Their LTx engines (stock)
have a good deal more power than your L98 (stock) one.

If nothing else, consider your remark that you like the perception
of balance the car had with the 26/22 bars. Now at the front you
have gone from a tubular bar to a solid bar and increased dia by
6 mm. And you are deciding between increasing the solid rear
bar by 2 mm or 4 mm.

.

EPiC Vette

Good points, all. But since I'm carrying more mass at the front of the car, wouldn't it make sense that I want to increase the bars proportionate to the mass their trying to control?

In other words, if I'm going bigger in the rear, wouldn't I want to bigger-plus in the front, to keep the proportions equal?

I'm by no means an engineer, nor am I a susp guru. That's why I'm fishing for opinions.

Which I'm thankful for, by the way

Slalom4me

Here is a link to Hib Halverson's suspension chart. It is helpful for
tracking the changes and comparing components.

In some minds, the later Z07 cars marked the pinnacle for C4 handling.
These had 90.1/57.2 N/mm springs, 30/24 mm bars and symmetric
wheels/tires.

The '89 FE1 had a 93.1 N/mm front spring and symmetric wheels/tires.
Adding 32mm/26mm bars would move the '89 in the direction of the Z07
but, without an increase in rear spring rate, the '89 would be prone
to significant understeer. (With the addition of a 330-350ish lb/in
(57.8-61.2 N/mm) rear spring, the '89 FE1 achieves Z07+ specs. )

Increasing the rear bar is a step in the right direction. My vote is
for the 26mm dia to partially counter-balance the rate of the front
spring and the 32mm front bar. I know from experience that the
32/26 combo on an '89 FE1 will still result in understeer.

.

EPiC Vette

Looks like a 26 rr is in my future.

Good to know about the stiffer rr spring, that's an option too.

Thanks!

Slalom4me

If and when you make a move on the rear spring, get the longer
bolts (10") to be able to restore level ride height. The OEM bolts
do not provide sufficient adjustment.

New rubber bushings should be on the agenda at the same time.
These consist of four #10262518 Insulators from GM. VB&P also
offers a version of these under their own p/n#

.

Solofast

If you are going to the Z07 springs without lowering the car you will find that a 26mm rear bar is too much and it will cause jacking and the resultant oversteer and generally weird handling. Without the stiffer springs you can likely get away with the 26mm bar, but you should seriously plan on the 24mm rear bar with the Z07 springs, or you should lower the car a good bit to help get the roll center down.

If you look closely at Hib's chart, they only offered the 26mm rear bar on the non-ZR1's in 90 and 91. To get those cars to handle right in stock trim, you had to increase the front roll stiffness beyond the 30mm solid bar, and sometimes even that didn't get the car right. I am speaking from experience here since I have had both a 90 and a 91, Z51/Z07 cars. If you went to the 315's on the rear, that would also help, but too much rear roll stiffness in these cars can get you some really weird handling...

If you do have a 26mm bar and it results in "edgy" handling you can space the bushing strap clamps off of the frame to soften the preload on the rubber bushings, and that will help soften that bar. We did that on the BSP car and it was an effective means of fine tuning rear bar stiffness, space it about .080 under the two bolts to get you back to where it isn't causing jacking...

Slalom4me

Shrug. There is no debate - Solofast has far more experience with
C4's. But I have the configuration I describe and this is how it drives.

An OEM '89 FE1 with stock ride height that is modified with 32/26mm
Addco bars and a 350# VB&P rear spring exhibits mild understeer while
in steady state 1G turns. It is predictable in transitions. It stops
straight.

The best part of this situation for you is that there is little work or
expense associated with making choices. You already have the front
bar, the rear bars aren't costly or difficult to install. Neither is a rear
spring.

The one consideration to keep in mind is your preparedness to deal
with oversteer if your combination behaves as Solofast describes
rather than as I describe. You have been offered a 24mm, maybe
start there and then move further as you feel comfortable doing so.
I'll be interested to hear how far along the path you proceed.

.

EPiC Vette

I'll keep my eyes out for a rr Z51/Z07 spring. I have lowering bolts.

Looks like this winter I'll be tearing up some empty parking lots tuning the rear.

Solofast

Going to a 32mm front bar is probably the minimum, I was thinking that you had the 30mm front bar until I re-read you initial post. With our BSP setup we had the stiffest stock springs available (84 Z51's up front, and I forget what the backs were, the whole rear suspension was from a 96 and we had an aftermarket spring there) and a 32mm front and 26mm rear bar and then we just had to soften the back by using rubber bushings and opening the clamping on them a bit to take out a little preload and that was a pretty good setup... Sorry for any confusion... I would try the 26 and don't use poly bushings, stay with rubber and you can tune it a bit with that... Just be careful and watch for jacking under braking... Too much rear roll stiffness coupled with the rise in roll center under braking can make the car loose in corner entry. Some folks like that, but it gives me the *******... This is the first thing you will see if you have too much rear roll stiffness. The car will be a bit loose on corner entry, and then as you add power it will squat and will then be stable.... I have seen that characteristic in several BSP C4's, and it can be more than a bit disconcerting...