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Howdy
I'm looking to replace the swaybars on my base 85 that I mostly use to autocross. I put in a set of Z51 Bilstein shocks and lowered the rear about an inch ( it sits about level) . Im running Nitto 555 street tires 275/40/17 in front and 315/35/17 rear. I would like to reduce the understeer
Any advice would be appreciated!
Last edited by spro2395; 11-30-2014 at 06:14 PM.
Reason: spell check
I installed a set of sway bars from Vette Brakes last season; front is 32mm solid and the rear is 26mm solid. I wasn't too sure about the size of the rear bar but it seems to be working fine. The bars came with poly bushings.
My '87 also has Banski rear tow rod ends and camber rods.
Alignment is a big part of getting the car to handle. Also, have you played around with lower rear tire pressure?
I installed a set of sway bars from Vette Brakes last season; front is 32mm solid and the rear is 26mm solid. I wasn't too sure about the size of the rear bar but it seems to be working fine. The bars came with poly bushings.
My '87 also has Banski rear tow rod ends and camber rods.
Alignment is a big part of getting the car to handle. Also, have you played around with lower rear tire pressure?
I have a lot of rear camber over 2.5 degrees) all came in from lowering the rear. In the front its got about 1.5.
It feels the best to me with 39 psi in the tires. The car seems to have a large amount of body roll too along with the lack of front grip.
I have a lot of rear camber over 2.5 degrees) all came in from lowering the rear. In the front its got about 1.5.
It feels the best to me with 39 psi in the tires. The car seems to have a large amount of body roll too along with the lack of front grip.
First of all you really need a set of Z51 springs. That car will roll like a pig on the base suspension. Look at Hib Halverson's C4 suspension sheet for the spring and sway bar combinations that were available on the stock cars. ( http://www.netmotive.net/articles/hib/c4/sustunch.pdf). More spring than a Z51/Z07 will be hard to stomach on the street the 85 Z51 setup is a good choice for autocross duty.
You don't want anywhere near that much negative camber in the rear of a C4. If you have really sticky tires you'd want -1.5 degrees (assuming you get the proper springs under the car). Nobody runs the base springs and the car rolls a lot more with them, but 2.5 in the back is way past what is necessary.
If you do the Z51 springs then go with the 85 Z51 bars. With the stiffer springs you don't want to go more than a 24mm rear bar or the car will jack in the back. These cars are sensitive to too much rear roll stiffness and the handling deteriorates pretty fast of you go over stiff in the rear.
Because the rear roll stiffness is limited by the rear roll center geometry these cars really like to have a square setup. Putting bigger tires on the back usually results in a lo of push that won't go away. If you try to stiffen the rear bar to take advantage of the bigger rear tires the back end will jack up like an old VW.
I don't know where folks get the idea that bigger rear tires are needed on these cars, they don't have so much power that you need more tire on the back and with more rear tire than at the front they push like a pig.
If you stay with the base springs you could put some more rear bar (like the 26 mm rear bar that came on the ZR1 and the 1991 Z07) and that may help your pushing problem, but the car is still going to roll like a pig with the base springs. That may help balance it, but if you want to get the car to work you need to start with the right springs and bars and get a square setup with the same wheels and offset front and rear. If you go with bigger springs the biggest bar you should go with is a 24 mm rear bar, for the above mentioned reasons.
As an aside we were surprised when we were testing C4's in that the 1/4 inch of offset allowed by the SCCA when added to one end or the other really upset the car if we didn't do it at both ends. That is, if we ran the extra offset on the front the car was really loose, and if we put it on the back the car pushed really badly.
Sorry if there aren't any easy fixes, but that's just the way it is.
I run Hoosier A6's in 275/40-17 on all corners and with the new bars and new Z51 Bilsteins (like yours) I don't feel a lot of body roll. Springs are stock base but the car came with the Z52 package.
For tire pressure, I start with 32 front and 24-26 rear. I try to keep those pressures at that point but on hot days it's hard to do. I also have the rear down 1" and have new poly bushings on the spring. I would suggest trying something like 5 psi less in the front tires. Even with using my Nitto NT05's for some events, I never go over 32-34 psi up front.
With the early C4 suspension, it can be trying to get lots of negative camber but I do have about -2.5 degrees in front and -1.75 rear. Front toe is 1/8" out and in back it's 1/16" in. The other thing I have in front is a camber brace; that helps a bunch to tie everything together.
I run Hoosier A6's in 275/40-17 on all corners and with the new bars and new Z51 Bilsteins (like yours) I don't feel a lot of body roll. Springs are stock base but the car came with the Z52 package.
For tire pressure, I start with 32 front and 24-26 rear. I try to keep those pressures at that point but on hot days it's hard to do. I also have the rear down 1" and have new poly bushings on the spring. I would suggest trying something like 5 psi less in the front tires. Even with using my Nitto NT05's for some events, I never go over 32-34 psi up front.
With the early C4 suspension, it can be trying to get lots of negative camber but I do have about -2.5 degrees in front and -1.75 rear. Front toe is 1/8" out and in back it's 1/16" in. The other thing I have in front is a camber brace; that helps a bunch to tie everything together.
What wheels are you running ?
I have 9.5 wide in front and 11 rear. I've been looking for same size both to try!
First of all you really need a set of Z51 springs. That car will roll like a pig on the base suspension. Look at Hib Halverson's C4 suspension sheet for the spring and sway bar combinations that were available on the stock cars. ( http://www.netmotive.net/articles/hib/c4/sustunch.pdf). More spring than a Z51/Z07 will be hard to stomach on the street the 85 Z51 setup is a good choice for autocross duty.
You don't want anywhere near that much negative camber in the rear of a C4. If you have really sticky tires you'd want -1.5 degrees (assuming you get the proper springs under the car). Nobody runs the base springs and the car rolls a lot more with them, but 2.5 in the back is way past what is necessary.
If you do the Z51 springs then go with the 85 Z51 bars. With the stiffer springs you don't want to go more than a 24mm rear bar or the car will jack in the back. These cars are sensitive to too much rear roll stiffness and the handling deteriorates pretty fast of you go over stiff in the rear.
Because the rear roll stiffness is limited by the rear roll center geometry these cars really like to have a square setup. Putting bigger tires on the back usually results in a lo of push that won't go away. If you try to stiffen the rear bar to take advantage of the bigger rear tires the back end will jack up like an old VW.
I don't know where folks get the idea that bigger rear tires are needed on these cars, they don't have so much power that you need more tire on the back and with more rear tire than at the front they push like a pig.
If you stay with the base springs you could put some more rear bar (like the 26 mm rear bar that came on the ZR1 and the 1991 Z07) and that may help your pushing problem, but the car is still going to roll like a pig with the base springs. That may help balance it, but if you want to get the car to work you need to start with the right springs and bars and get a square setup with the same wheels and offset front and rear. If you go with bigger springs the biggest bar you should go with is a 24 mm rear bar, for the above mentioned reasons.
As an aside we were surprised when we were testing C4's in that the 1/4 inch of offset allowed by the SCCA when added to one end or the other really upset the car if we didn't do it at both ends. That is, if we ran the extra offset on the front the car was really loose, and if we put it on the back the car pushed really badly.
Sorry if there aren't any easy fixes, but that's just the way it is.
Did you ONLY lower the rear? If so, while the fender gaps may look even the car is leaning back. This will take weight off the nose and make it understeer as well. These cars like about 1" of rake (rear higher than the front), but measure the height at the frame right behind the front tire, and in front of the rear tire. Too much variance in fender height.
I too believe that a square setup works best on these cars. I have used anything from 275s square up to 335s square. For Autocross stiff springs and softer bars work well, but for the track I like softer springs and big bars. On my race car I use the '88-'91 Z51 front and rear springs. Like Solofast mentioned, if I use the '84 Z51 rear spring its just too stiff in the rear. I also have aftermarket 32mm front and 26mm rear solid sway bars. It is very very neutral with square tires in both autocross and road racing.
For camber I like to be around -1.25° to -1.5° on the rear as too much will make the car loose on exit and harder to put power down. Try to get as much camber as you can in the front. When I lowered my car I was able to get -2.5° in the front with no shims and that really helped any mid corner push the car had. A bit of toe out in the front helps with turn in as well.
What wheels are you running ?
I have 9.5 wide in front and 11 rear. I've been looking for same size both to try!
I run a square setup with 17x9.5" AFS ZR-1 style wheels using the correct 38mm offset and 275/40-17 Hoosier A6's. I had thought about trying the 17x11 wheels and 315's but I think I would wind up having to re-invent everything. Also, the 315/30-17 tires are getting hard to come by at least in terms of good sticky tires.
Originally Posted by spro2395
Thanks for all the info!
I measured at the frame
Front 6 3/8"
Rear. 6 1/4
so it is lower in the rear.
I had bought new bolts locally and installed new poly bushings on the rear spring. I marked the new bolts at a point 1" different than the OEM bolts using the hole for the cotter pin as a reference point. The car sits level. but your measurements aren't that bad.
You didn't say what organization you are running with, but if you are going to do lots of suspension mods, I would certainly recommend finding a camber brace and installing it.
[QUOTE=c4cruiser;1588385626]I run a square setup with 17x9.5" AFS ZR-1 style wheels using the correct 38mm offset and 275/40-17 Hoosier A6's. I had thought about trying the 17x11 wheels and 315's but I think I would wind up having to re-invent everything. Also, the 315/30-17 tires are getting hard to come by at least in terms of good sticky tires.
I had bought new bolts locally and installed new poly bushings on the rear spring. I marked the new bolts at a point 1" different than the OEM bolts using the hole for the cotter pin as a reference point. The car sits level. but your measurements aren't that bad.
You didn't say what organization you are running with, but if you are going to do lots of suspension mods, I would certainly recommend finding a camber brace and installing it.[/QUOTE
I race with the Florida Corvette. Racing Club in NCCC class 2E.
I did the same thing but I only did the rear. I never thought I would throw the balance off. That's why I asked for advice! ]
It seems that I remember something about using late C4 front springs in a early C4. Does anybody have clarification on if / how? Good to see a little love for the C4 track rats.
Howdy
I'm looking to replace the swaybars on my base 85 that I mostly use to autocross. I put in a set of Z51 Bilstein shocks and lowered the rear about an inch ( it sits about level) . Im running Nitto 555 street tires 275/40/17 in front and 315/35/17 rear. I would like to reduce the understeer
Any advice would be appreciated!
Just so you know, you have a goldmine of info from these guys, lucky dude, wish I had this info when I had my LT4 about 10 years ago.
I never tried the square setup, I had the full 96 Grand Sport rims, springs, sways, but if these guys say it works then do it. Get the ride height up in the back and drop the tire pressures. C4's had DOZENS of spring combo's over the years.
Just so you know, you have a goldmine of info from these guys, lucky dude, wish I had this info when I had my LT4 about 10 years ago.
I never tried the square setup, I had the full 96 Grand Sport rims, springs, sways, but if these guys say it works then do it. Get the ride height up in the back and drop the tire pressures. C4's had DOZENS of spring combo's over the years.
Yes the info is outstanding! I would like to lower the front an inch but I'm trying to decide if it's above my skill level. One thing I forgot to mention is that we race on a very old concrete runway that has Lot's of pretty big chunks of it missing in places and is quite rough. Would that play into the bar selection?
..........Here's an article I wrote that covers a lot of the sway bar issues.
........
Richard Newton
Richard, I have read that article a few times and there is one point I don't understand: I do get that the inside spring loading by the sway bar assists in adding spring rate to the outside wheel, and I understand that the sway bar has the tendency to 'pick up' the inside wheel by loading that spring thus taming body roll. But, I don't see anyway possible for the sway bar to "push the inside wheel back down." Can you explain this for me please?
It helps if you think of the sway bar as a spring. A sway bar is really nothing more than a torsion bar if you think about it. Now think about twisting this torsion bar.
The arms on a sway bar are really nothing more than handles. They allow you to twist the bar. If you make the handles bigger (longer) it's easier to twist the bar.
If you increase the diameter of this torsion bar it's harder to twist.
You then have 2 variables. The diameter of the bar and the length of the handles. You can play with those two variables all day. It's still about twisting the bar.
It helps if you think of the sway bar as a spring. A sway bar is really nothing more than a torsion bar if you think about it. Now think about twisting this torsion bar.
The arms on a sway bar are really nothing more than handles. They allow you to twist the bar. If you make the handles bigger (longer) it's easier to twist the bar.
If you increase the diameter of this torsion bar it's harder to twist.
You then have 2 variables. The diameter of the bar and the length of the handles. You can play with those two variables all day. It's still about twisting the bar.
Richard Newton
Well, I get that, as far as that goes, but when the outside tire compresses the spring, the lower a-arm rises which leverages the inside a-arm upward, also, by way of the sway bar. This is going in the opposite direction of pushing the inside tire back down. The only dynamics going on here, as far as I understand it, is that the spring on the inside suspension is added to the spring force of the outside tire and, also, since the inside wheel is being lifted instead of being pushed back down, the body has less tendency to roll. If this is not the case, please explain. What I get from your article is that the inside wheel is being pushed down. I don't think so.
11-30-2014, 05:23 PM
C4 Swaybar Recommendation
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If you can afford new parts, the Corvette Challenge springs are the same rate as the '88-'91 Z51. I just found used springs.
