Planning on doing my 4th AutoX this weekend to try out some minor changes. Here's what I have:

68 convertible
ZZ4 with M21
4.11 gears
15 x 245/60 BFG T/As (I know, they suck. Let me tear these up while I learn)
550# spring up front
360# rear
Bilstien Sports
New CA and sway bar bushings up front
1" sway bar up front, adjustable links
5/8" rear sway bar
Ball joints and steering linkage are all recent.

Here's what I've done since last outing:

Added Borgeson power steering
Lowered the rear with 8" spring bolts (now at stock height)
New rear sway bar links and bushings
New trailing arm bushings
-.75* camber front -1* rear
1/8th inch toe in front and rear.
2.75+ caster
New Master Cylinder
Stainless brake lines up front and braided SS flex lines all around.

In the past oversteer has been a problem. Not even the slightest hint of understeer. Minimal body roll and nose dive. Driving on the road (not being able to push too hard) the front end is now unbelievably tight and responsive, but it still feels like it wants to come loose in the rear.

Here's my dumb question. Would removing weight from the rear help or hurt oversteer? Based on how the car feels my guess is it would help, but logically, it seems more weight = more traction (but more centrifugal force). I could loose the jack, spare tire and run with less than a full tank, losing ~60 or 70 lbs (~2%).

Any recommendations are appreciated. Tire pressure recommendations appreciated as well. It's mostly a weekend cruiser, so I don't want to get too crazy with heavy modifications. I just like the adreneline rush from time to time.

Thanks,

Kevin

 

Weight in the rear helps put the power down out of the corners. I always run with a full tank.

If you want to get rid of the oversteer, disconnect the 5/8" rear bar and watch the oversteer disappear.

 

I say get rid of that rear bar.. cant hurt and you can always put it pack onif your not phappy at the track..

I added a rear bar on mine and did a 720 on my way home from the track...

 

I thought you were supposed to do those in the infield after a Cup win

 

So it's better to run without the rear bar? I would have thought that it increases handling..but it sounds the opposite. What are the pros/cons of running with or without?

 

Disconnect the rear bar.

Get some more negative camber in the front and make the toe in the front 1/16 out. I would think you would want the tires to be around 38 - 40 hot (check after each run) with one or two pounds less in the rear. A bigger front bar would help also.

 

From what you describe, I agree that disconnecting the rear bar will eliminate the oversteer. With that said, I am dead against rear bars that do not have the OEM endlinks, which most aftermarket bars do not have. Being that you are running a 5/8 inch rear bar, I am fairly certain that it is an aftermarket bar with non oem endlinks with endlinks like the front bar. These bars severely limit the IRS on the C3 and since the C3 SB are inherently neutral with 48% Front and 52% rear (my 78 C3), running an aftermarket rear 5/8 inch bar with a 1 inch front bar will cause oversteer. My 78 Gymkhana suspended vette came with a 1 1/8 inch front bar and 7/16 inch rear bar and these cars would understeer from the factory. I currently run the OEM 1 1/8 inch front bar with poly endlink and mounting bushings and a 3/4 inch rear bar with poly mounting bushings with OEM type endlinks. The oem rear bar endlinks allow much greater movement of the rear suspension than aftermarket rear bars while still controlling roll. GM used this type of rear bar for a reason! My car is very neutral with this setup and will only oversteer if severely provoked, gradual and progressive.

What I would suggest to try:

Front 1 1/8 -1 1/4 inch bar with a OEM type rear bar of 9/16 or 3/4 inch rear sway bar.

Get rid of the OEM strut rods and go to competition adjustable struts with heim joint ends which will eliminate strut rod flex, unwanted rubber bushing compression, and undue camber change.

The list of other things you can do is fairly lengthy but I am trying to keep it simple.

 

Just to clarify, as for me, I am referring to daily driving use..

 

Thanks for the tips. The rear sway bar and links are stock as far as I know. The car was originally a BB, judging by the presence of a rear sway bar, the front springs that it came with, and the 4.11 gears (?). Pulling out the caliper and re-measuring carefully this time, the rear sway bar is really 9/16" (I got 39/64" on a quick check and called it 5/8"). The front is really 15/16".

Should I disconnect, or completely remove the rear bar? I'd be inclined to remove it, at least until I get a bigger front bar (?). Can do the strut rods.

Had to delay race day 'til the 22nd.

 

Thanks, I'm going from memory on the alignment setting sheet. I know I've got quite a stack of shims on the upper control arm pins. Enough that I had to shorten my spreader bar to get it in.

Wouldn't the toe-out in front make it a little twitchy on the street. Right now it's very steady on the highway, even on grooved pavement. I like that. The entry and exit on corners feels good. I've got no trouble with the car going just where I want it and zero loss of traction up front. Of course I'm probably not pushing it hard enough for fear of rear end coming loose. I think I understand what the toe out would do for me, but I'm not needing any extra pull on the inside wheel. Thoughts?

 

Just disconnect the bar, and note the change in handling. You could even do it at the event site; make a run or two, then disconnect the bar and see what happens.

And I'd make one change at a time-throw a bigger front bar on there and disconnect the rear bar at the same time, and then you've moved into understeer. See what you get first with one change.

More front negative camber would certainly help, but I would leave the camber and toe where it is unless you start getting serious. I ran 1/8" front toe-in for years before I finally tried 0 toe, and I didn't do that until I stepped up to much stiffer front springs (~900 lb/in) and added bump steer blocks. With your soft front springs and medium front bar putting toe-out in front could make it nervous in corners due to bump steer. If you're just out having fun I wouldn't bother at this point, you said yourself it's mostly a weekend cruiser.

 

Should I disconnect my rear bar for normal daily driving?

 

As 69autoXr states and I agree, try doing one change at a time and disconnect the rear 9/16 inch rear bar especially since your car was originally a BB car. A 15/16 inch front bar with that 9/16 inch rear OEM bar is going to induce oversteer with a SB engine. I am sure that running a 15/16 inch front bar with no rear bar should solve your issue and you can later add a bigger front bar like a 1 1/8 inch or 1.5 inch front bar and reaattach the 9/16 rear OEM bar and you should have more neutral handling. That rear bar is actually a good size and is the OEM bar designed for the C3 IRS. Sounds like a bigger front bar will eventually give you what you want with the rear 9/16 inch bar and more neutral handling. Hope that helps.

 

Thanks guys, this has been incredibly helpful. I think I'll do a couple of runs with the rear bar, then disconnect. The frame bushings on the rear bar seem to be in good shape. I assume that the links will swing down and the bar will just hang there. I don't really see a good place to hang it from.

On the tire pressure issue, I started running around 35, but seems to do better with 30-32. The tires don't seem to be rolling to badly (based on my chalk lines). But the BFG's T/As are really terrible, and with the oversteer I can't push too hard. With reduced oversteer can I increase pressure?

I've had experienced people at the track insist I need to increase/decrease pressure. No help there. With what I have, decrease seems to work, with my best raw times -15, -11, and -7 secsonds off the top time of the day for my first three tries. I'm moving in the right direction, but it hurts to go slower than a (heavily modifed) Chevy Cobalt!

Thanks again,
Kevin

 

Yep, too much rear anti-roll stiffness. Not surprised with your bar/spring combo. If disonnecting the rear bar puts you back into understeer a smaller rear bar may be the easy fix, but they aren't necessarily required to achieve a good setup. Suggest lowering your ride heights now if you're ever going to do it and to pay attention to what phase(s) of cornering aren't working before throwing too much money at chasing setup. FWIW, you won't be the only one who gathers up a fair collection of bits doing so. And, yes, new tires will likely change the equation again. As for pressures, do what works, but if you get serious you need a pyrometer.

Birdman, whether or not you need to disconnect/remove your rear bar really depends on where your balance is with it at the car's limits. If you don't know, I'd suggest disconnecting it until you can safely find out, keeping in mind that what might feel awesome at 7/10's might bite you in the backside at 10/10ths or in the wet (as I've probably said 1K times before).


TSW

 

Even though it's not my thread, I appreciate you addressing my question. I try to keep from cluttering our section by posting relevant questions.

So you suggest starting without the bar connected...flog it, and then decide based on the results. What results am I looking for, in order to justify putting on a rear bar (or going bigger/smaller)?

 

IMO- way to much camber, but only a temp guage will say for sure. "zero" toe front is very desirable. I never liked toe out. Castor should be the max possible. With stock parts about 3.95 is all you can get and have the shims hold.

Get rid of as much weight as you can 1/8th of a tank and remove th whole carrier assembly.

The rear sway or not to sway debate will rage on forever. The truth is that you want minimal body roll to keep as much of both rear tires on the road as possible. To those people who think a vette gets tail happy because a sway bar just don't have big enough sticky tires on the rear and have crappy front to rear weight ratio. I've tried to get 60 rear and 40% front like real race cars

 

From what I remember from my autoX days, stiff sway bar in the front and light or no sway bar in the rear is the hot ticket. Regardless of the tires, a thin rear bar will usually help with overseer. If he uses huge slicks, I'm absolutely sure his times will improve, heck any car will improve with slicks.

On a side note, I use to autoX with a friends drift set-up turbo 240sx. It was extremely low, huge swaybars, wide stretched tires, locking diff, and basically no body roll. I did ok in it, but most importantly, it would kick out into a predictable slide. But, most forum posts I read at the time also suggested using a light rear sway bar to help keep the rear planted.

 

A couple of ideas that are relatively easy and might be useful concerning the setup up you have now before you disconnect the rear bar You may want to change the front sway bar mounting bushings and endlink bushings to poly. Doing so effectively makes the front bar "bigger" since the poly bushings allow much less deflection before the sway bar becomes effective. Also, I just last week put on a speed direct spreader bar in the front of my 78, and even after 28 years of making modifications to the car, I was very impressed of the bar's impact on handling response, steering feel, overall tightness to the frame, and ride. I have a similar setup to you with a couple of differences, one giant one is the tires 255/45/17 ZR's in front and 255/50/17 ZR's in the rear.

For your reference in summary, in Front:

550 springs 1 inch shorter than stock.
Bilstein Heavy Duty shocks with poly bushings
Poly upper and lower control arm bushings in stock a arms
Custom blue printed/rebuillt OEM steering box that now has zero play. Your Borgeson box has a quicker steering ratio but is probably very similar to my OEM box in feel/play (16:1 OEM versus 12.7 :1 I believe for the borgeson).
1 1/8 OEM front bar with poly mounting and endlink bushings
Speed Direct Spreader bar
255/45/17 ZR Summer only ultra high performance tires

In the Rear:

360 composite spring with poly mounting cushions
Bilstein Sports (30% stiffer than the front HD's)
Competition Heavy duty adjustable struts with Heim joints (no bushings)
3/4 Inch OEM type bar with poly mounting bushings
255/50/17 ZR summer only high performance tires.

Next up, the "shark bar" which fits behind the seats and will provide additional frame stiffness to the rear of the car and is an excellent mounting location for a racing type seatbelt harness which would help you for autocrossing.

Hope this helps!

 

Uh-oh more questions:
1. Can I use my infrared temp guage, or is there something better?
2. You seem to disagree with 69autoXr on the rear weight thing. I typically get 7 runs in a day, so it might take a couple of days to sort this out. I'll be out of gas by the end of the day with an 1/8 of a tank!
3. The alignment guy, experienced with C3's, said he maxed out the castor at +2.75. Can't remember what the reason why was, but I asked for +4 based on VBP's spec sheet, which I compromised between sport and AutoX.
4. I realize the limits of my tires. I'll address that later. I'm still inexperienced and not really looking to build a race car.

Thanks,
Kevin