Hi all!

I have a base 1990 coupe that I'd like to upgrade the suspension on. I've already received KYB shocks and Vette2Vette's frame stiffener. Since it's an FE1 suspension car, I currently have a 26mm tubular front sway bar and 24mm solid rear sway bar with the 93.1 n/mm front spring and 39.9 rear spring. I really like the idea of purchasing a 30mm solid front sway bar, but I'm concerned about inducing too much understeer since I'm finding it difficult to find an original 26mm solid rear sway bar. (The 30/26 combination is what the '90 Z51s had, so I'm assuming that would be pretty close to neutral handling in corners). Since I'm thinking of not replacing the rear bar, I noticed on our popular C4 Corvette suspension chart that the '92-'95 Z07s came with a 30mm solid front sway bar, 24mm solid rear sway bar, 90.1 n/mm front spring, and 57.2 n/mm rear spring and realized that this is pretty close to what my car would have if I went ahead and got the 30mm solid front sway bar and purchased a 57.2 n/mm rear spring also. Thus I have two questions, 1) is the difference between 93.1 n/mm and the 90.1 n/mm front springs basically negligible? And 2) does this idea seem like a safe way to keep the general balance and neutrality of the car? If it helps, the car is mostly a road car that is used for legal spirited driving on back roads between the speeds of 30 and 70 mph.

 

It depends on your driving habits or style. You really won't know until you hit highway ramp "hot" it doesn't take long change out. I tend not to use the "chicken" pedal and with z51 springs and a front VBP bar I still had some rear body in roll in the middle of the corner. It did go away when I installed a larger rear bar. I still didn't like it so.installed 84 base rear spring and it seems pretty good on smooth roads absolutely sucks on road patch.

 

1. Yes, I agree those front springs are close enough to probably not make the swap worthwhile.
2. Probably. In general for steady-state oversteer/understeer balance, one end's spring and swaybar rates can be traded off one against the other to maintain balance. The one thing that will be arguably worse will be ride quality, but it probably won't be horrible.

My car had very different spring rates (about 197n/mm front and 96n/mm rear), so it's hard to generalize. However, I liked its balance and would generally shoot for a rear spring rate of about half the front spring's rate (strictly speaking about monoleafs here, not coilovers). Your current combo has a lower rear-to-front spring ratio, and your proposed update would be a little more. Also, I ran a 30mm front bar and had a variety of rear bar diameters. 26mm was too stiff and hurt rear grip too much. Even 24mm was pushing it with that combo, depending on the tires and surface. I usually used 23mm, which was exclusive to the 84 Z51. But the point is: IME a C4 is pretty sensitive to rear bar size changes and it usually isn't good to tie it up with too big a rear bar.

My last piece of advice isn't something you asked about, so I'm just offering it as something to consider. If you haven't opened boxes yet, I would return the KYBs and the V2V frame braces and spend the money on better shocks with single-adjustable rebound. The two "budget" options for that are Koni Sports or Ridetech HQ. I would choose the latter unless you can find a used set of Konis. Shocks are really important (rebound in particular) to handling balance, since they rule the balance while the suspension is moving, such as corner entry and exit or chicanes/slaloms. They also make a much bigger difference in ride quality than springs. The Ridetechs are easy to adjust on the car so you can choose ride quality vs tighter handling for your more "spirited" drives. KYBs are not good shocks, and the V2V braces won't make much useful difference as long as you have the roof on the car.

 

Thank you for your thoughtful answers! They make sense to me, and it's good to hear some confirmation on what I was thinking. Regarding your last paragraph, I got new-parts syndrome and already opened everything lmao. I will keep in mind the Ridetech HQs or Koni Sports for the future. I got a great deal on the KYBs, so if I don't like them, I don't mind ditching them for something better. We'll see what happens. And I got the no-flex kit because I really enjoy driving with the top off and I figured some additional bracing in the frame would be good. Plus I believe it'll help some too with the top on. I'm trying to bolster this car to be the best handling car in can be, even if it only pumps out 250 ponies.

 

What is going on with the suspension and what areas you wish to improve?

 

Aside from the shocks having 75,000 miles on them and the general aging of a 30 year old car, there's nothing "bad" going on, I'd just like to make it a Z51 car and get the suspension tighter. I'd like to eliminate some roll and bounciness.

 

Gotcha… those are basic and reasonable goals, and helps me to pick which direction to ramble :-)

The later spring rates aren’t too bad, and they are in line with earlier/mid Z51 packages, the shocks are another story.

The large solid Z51 front bars will work fine, it’s a subtle change on the balance of the car, and it adds a lot of weight by the way. It will increase the rear traction when cornering, so that can be a benefit depending on how the car handles.

Typical suspension design advice is to calculate wheel rates so that it accommodates the corner weights of the car and the frequency/dynamics desired… and then look at different sway bars to aid in for proper cornering balance. The first part takes a lot to figure out, and GM has done some of the math and also revised over the years as they respond to deficiencies and feedback. Not sure where you’re ride height is, what you plan to do for shocks, but one simple way I know to raise spring rates is to install 2” tall “soft” bump stops on the shocks (such as AFCO foam type, not the rubber donut type, the car at stock height is right at 2” static just sitting there on its own weight)… for all four corners this will add progressive spring rate with little effect on comfort and general driving. The shocks ideally would need to have the high speed rebound adjustment dialed in for the higher rebound velocities created by the bump stops, but most non-advanced shocks have too much high speed damping so it should be fine. Please DOO install a good set of Bilsteins or QA1’s at least. If the rear is too harsh and towards oversteer with this setup, you can experiment with a lighter or no sway bar even.

A long time ago, performance suspensions had the strategy of high spring rates (high frequency of wheel rate to be more precise) and small sway bars… the trend subsequently went to lower frequency springs and higher rate sway bars. Then for the last 5-10 years the best race car setups had lower frequency springs, smaller or no swaybars, and progressive bump stops. Some of that was developed for stock car racing with aero at first, but has evolved for much other forms of racing. For the Corvette specifically, no too many have done it, and the stock bump stop in the rear (more of a sudden bump limiter) was detrimental to good handling. This would need to be removed or at least made a lot weaker (such as carved into a skinnier triangle from the fat cube shape). Anyway, just some ideas To think about.

 

Good shocks will do a LOT to give you good ride quality and less bounciness.

I've had both the Koni Sports and Ridetech HQ shocks. Both are very good. The Ridetech's are better. Monotube, made by Fox, easier adjustments, wider range of adjustment, lighter to boot.

 

You never mentioned your tires! Make sure they're up to par or any of the great advice you have received will be useless! Good luck with your quest👍

 

I added the 30mm front bar and NYU rear spring on my '91 with stock front FHA 93.1N/mm and 24mm rear bar. No regret. I used Bilstein shocks for 88-90 Z51.

FYI...the 26mm rear bar on '90 Z51 is a misprint on the chart (depending on which chart you look at), it was 24mm. 26mm rear bar was '91 Z07 only (275 rear tire cars...not including ZR-1 or Grand Sport in that statement).

 

Oof, a lot to think about! I will need to digest it and conduct some more research to understand this, particularly how these bump stops are used and installed. Thank you for the insight!

 

Oh awesome, I'm glad to hear someone has already done what I was thinking of doing! Also thank you for noting that misprint. If the '90 Z51 had the rear 24mm, do you know if the R9G models are the same? (I would assume so, right?) Also, how did you know the chart is wrong? Is there a list of corrections anywhere?

 

 

Great pics. Also very clean, nice job. I will note that with the KYB shocks I'm currently planning on installing, there is no place to add a bump stop like that. If I ever do decide to go with Toni's or Ridetech's, I'm assuming I could then use the bump stops?

 

The sway bar typo has been pointed out on this forum in the past…it’s the only typo that I recall but I mainly focus on the ‘88-95 years. Search for posts by member Solofast…he did EXTENSIVE C4 autocross development back in the day and directly points out that the 26mm rear bar was a one year only mistake by GM 🤣. Also, it’s easy to measure on an original ‘90 Z51. I believe R9G was identical, but don’t quote me on that.

 

Looks like Wilwood brakes on there, do you also have the 4 piston rears??

I put the 6 piston fronts on not long ago, exceptional.

 

Chasethecorvetteman -- Yes Sir, it's the 4 piston rear, same caliper basically with only 4 pistons and evenly sized pucks. Running a B pad up front and the street pad in rear until I can get a brake bias adjuster installed. Converted to 1985 spec manual brakes recently (new master/lines etc). Note that the Wilwood kit has some very heavy cast iron hats, so I've got some aluminum hats the same offset (5 lb weight savings per side), and modified the hand brake / parking brake setup to be pass through hydraulic with a Speedway parking brake set button (haven't installed that yet). I don't recommend their rear brake setup for this reason for the 1984-1987 setups anyway, didn't look at the 1988+ kits.

 

My front kit came with aluminium hats, and from all i can see the rear kit does too.

If i knew how well they were going to fit and work i'd have ordered the rear kit at the same time, but wasn't certain how much space i'd have under the Bridgestone wheels and the rear kit needs a little bit more space for the park brake caliper unit, but knowing what i know now there is no doubt they will fit just fine.