Leaf spring and rear shocks?
 

Here's my conundrum. I have a 69 coupe with multi-leaf springs (5) and KYB gas-adjust shocks on the rear. The rear end is a GM 10-bolt posi with a 8.5 diff. I'm also running 15" wheels with 245/60-15 tires. My car handles great, corners like it's on rails, but she rides like a buckboard! When I put the new shocks on at the end of my restoration I had to use 2" shock extenders and I believe when I hit a good bump that they're maxing out the shocks extension. I like the stance of the car, but I would like to soften the ride, any suggestions?
I did look through past postings, but didn't come up with any clear answers. I'm willing to change leaf springs if need be (my current ones are heavy duty-660 capacity-125 rate-5 leaves), go to longer shocks, or both, but I absolutely don't want to lose the stance. The car was originally a mono-leaf 350 and was changed to a multi-leaf 400 (yes I changed the spring shackles, and it has new bushings everywhere).

Maybe pull a leaf out of the pack?
500 lbs in the trunk would do it too!

Front suspension looks very high. Check to see if the snubbers on the upper control arm are hitting the frame as it sits. With the car sitting so high, it will ride terrible. My 69 TA had the wrong springs in it and I know exactly what you're talking about. It also sat high and would extend out over every bump.

I run DSE springs front and rear along with DSE valved/tuned Koni shocks in the front with DSE tuned JRI shockes in the back. DSE coils in the front w/ DSE leafs in the rear. Car is a 69 Firebird convertible. Very pleased with stance and handling.
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Tim john---

Will do.

I have no problem with the front end, just the rear being too stiff. I also don't think having to add shock extenders is the correct fix either. I believe switching to 15" wheels, and with the multi's, there should be an alternative shock replacement available that has the correct throw so it doesn't top out going over bumps, but allows for more shock absorption.

As long as your suspension has pretty equal travel on both jounce and rebound your rear shocks should be ok. Saying that, you shouldn't have to add extenders unless they are the wrong shock.

I'm going to remove the shocks, remove the extenders from them, and re-install the shocks to see what happens. I read the correct way to install them is to:
1. Have the frame/body supported on Jackstands and let the rear end hang unsupported.
2. Install the shocks in the upper shock mounts.
3. Once the shocks are attached fully extend them, they should be 1-2" from the shock mounting plate on the rear end. If they aren't you've got the wrong shocks.
4. Using a Jack, raise the rear end up until you're able to finish attaching the shocks to the mounting plates.

After I do this I'll see where I'm at and how it feels. Thanks for the input.

I found interesting you mention the car originally had mono leafs, that was never offered on 69's. typically the issue with a hard ride on the back of first gen cars (apart from the obvious like too high spring rates or too stiff shocks) is lack of suspension travel and/or bottoming out.

Ironically softer leaf springs and shocks often worsen the ride because they allow the rear to bottom too easily.

Sorry north, I misspoke on the mono leaf spring, it had multi's. Someone somewhere along the line swapped them out with new(er) multi's (21-339) and had a different rear end put in it. I'll figure this out eventually.
I have a friend who has the exact same problem. He has a coupe and a convertible, the convertible rides great and the coupe rides like mine, and they both have (what appears to be) identical multi-leaf springs???

Here's a pic of the current multi's I have.
Product Specs for Leaf Springs 21-339
SRI Number:21-339
OEM Number:337230
Width:2-1/2 Inches
Length (A): 25 Inches
Length (B):31 Inches
Arc (C):7-1/4 Inches
Number of Leaves:5
Pack Thickness (D):1-3/8 Inches
Capacity:660 lbs.
Front Bushing:Included / RB-95
Rear Bushing:None - Berlin 1-5/8" Eye
Location:Rear
Spring Construction:2/291 3/262
Note:1-5/8" eye uses (2) HB-1000 bushings; Order separately
MPN: 21-339

I had a similar problem with my 68. Have heavy duty leafs and the car was very rough riding. I changed shocks to a single adjustable qa1 setup. Putting them on a low setting made a significant difference.

This is a timely thread for me. One of my winter projects is to replace the rear leaf springs on my '68 400. Currently have Bilstein gas shocks all around. I'd like to get that old school muscle car look where the back ends sits a little higher than the front. I don't mind a harsh ride as long as the car handles well. Trying to decide on what rear springs to order and if I should go 1" or more higher. Once the rear is done I'll change out the front coils to complete the project. Thanks

That car looks like it sits right, I wouldn't touch it!

I'd just drop the front 1" instead (watch your header clearance) rather than raise the back.

Thanks. Car is a numbers matching. According to PHS it has a 400 HO engine. I'm planning on removing the current exhaust system that has long tube headers and replace it with the correct stock exhaust manifolds to tailpipes for 1968.

Car does bottom out so still going to replace the rear leaf and front coil springs as part of winter makeover.

Just my addition really late in the conversation. When I was 16 YO (1969) I bought a 67 326 HO 4 speed Firebird, with the mono leaf, and the quasi lift bars rear suspension. I worked at the same new car dealer where my father worked and sometimes we rode to work together. The first time he rode in my car within the first mile, he told me that my car "Rides like a buckboard wagon", funny that back 51 years ago his impression was the same as one of the first posts in the thread.........:)

Back to the problem, are you sure the shocks haven't bottomed out with 2 inch extenders on them? From the ride height shown in the pictures, you shouldn't need any shock extenders if the shocks are the correct ones for the car. Extending them 2 inches may make the shock piston bottom out in the cylinder making the suspension solid upon mild compression. Just my thoughts, but I'd remove the extenders and see where your at, won't cost you anything but a little time. It may not do anything, but I'd give it a try.

I hope you find your problem, but I know from owning one of these cars ride quality isn't one of their redeeming points. If you want a ride improvement, an air ride conversion in the rear would help immensely, but that type of suspension isn't for everyone, although I find that after converting my K3500 dually to air ride in the rear, my old ailing back really appreciates it.....LOL It no longer "rides like a truck".......;) The air ride is infinitely adjustable and you can choose the firmness as well as height in a simple adjustment. I'm seriously thinking of fabricating an air ride system for my 2005 GTO just to make it a better cruiser on long highway trips.

:focus:

Thanks. I'm going to remove the extenders sometime soon and see where I'm at then.

No headers, just factory Long Branch manifolds. I don't know if something was done with the front coils or not, how many coils were on the stock ones? I like the front height now, I'm not sure how dropping it would affect it?

Removed the shock extenders and got everything bolted back up correctly, I just have to wait till there's no snow on the ground now to see how it feels. Thanks.