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.
.
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.

Looking at you springs in the photo I had the same bushing on my old springs. When I tighten up the rear shackle it made it so there was no movement at all. It road like crap obviously never looked into it any more until I decided to replace them. I was in shock (punny) when I loosed the shackle and the read dropped 4 inches.

Also a stiff shock like KYB with a stiff spring is not a winning combo. I ran KYBs too. Great for a soft stock spring though.

I now run the Hotchkis springs in the back with their new Foxshocks. I like the ride in the back now. I don't like the riide in the front. Still working on that. The leaf spring bushing are coned shaped and can articulate correctly now.

What ever you do I would consider a spring/shock combo. Suspension parts are not cheap and ride quality is subjective.

DSE, Hotchkis, RideTech are all good names.

I don't understand how or why your shackles wouldn't allow your rear end to seat properly, or allow the bushing to not work, what did you find for the cause? I installed new urethane bushings when I redid everything so I know they're good. The shakle bolts are shoulder-bolts and only tighten to a certain torque. I will check out the shakles, I know when I first installed everything I didn't tighten the shakles down until the car was sitting on it's own weight.

Sent from my SAMSUNG-SM-J327A using Tapatalk

Waiting on reply as to why your rear shackles didn't seat?

Maybe he tightened them without weight on the suspension, causing it to bind

Sport - can you advise the distance in inches from rocker panel to ground (location immediately to front of rear tire and rear of front tire.

I'll have to get that for you as soon as I can get my car off the jack stands. I'm parking my snow blower behind it right not, and so far we haven't been catching a break on the snow here in Pa. I will get that to you as soon as I can.

I will see if I can describe it better.

Your leaf bushing look flat on the end like mine did. They also look like they don't have a metal sleeve in them. Again like mine did.

So when I tightened up the shackles good and tight, I was squeezing the shackle into the bushing and the bushing into the frame or leaf, making it so the leaf couldn't extends or retract. The shackle needs to be able to be able to swing a little bit as the leaf flattens and curves. Mine could not budge.

The Hotchkiss leaf's I replaced them with, had a cone shape bushing with a metal sleeve. They can be tightened up without pinching on the bushing and the shackle will still articulate.

Those ones in the picture are the old bushings before removal. When the shoulder bolts are used you don't use the metal sleeves. They tighten up to the threads allowed by the shoulder bolt (preset tightness) and no further. I don't understand what your saying about being flattened on the ends, they have a larger end than the center which allows them to seat into position and no further? Shoulder bolts are cleaned and greased before assembly.

rohrt, ????

It's 10" from the ground to the rocker.

Just to clarify, that's 10" from the ground to the rocker with 15" wheels and tires.

Thank you!

Sorry guys, I forgot to follow up with this post. Here goes;

I removed the shock extenders and re-installed the shocks in the correct installation order, but it really hasn't made a lot of difference. Re-checked the leaf spring shackle seating, all good there. I run 32# of tire pressure all around, is that an issue? I remember that these cars originally called for that much, but I saw where someone said to run 29-30 in the rears. I don't want to lower the rear anymore than it so I'm at an impasse! I'm not sure different shocks would make a difference?
I did just think, since I changed the original tire height from 14" to 15" would that affect the stock shock specs? If it would then maybe I need to look at a different shock? It still seems like a leaf spring issue but I'm really not sure anymore.

Very unlikely to be the shocks. Try a different rear spring. If the new ones are more to your liking, the current springs should be easy to sell.

Just make sure you have the right length shocks. They should be neutral when the car is on its suspension. Too long (especially gas) can lift the light rear of these cars.

Factory tire pressure was 24-28 but that was for belted tires. Nothing wrong with 30-35 for radials.

I had a similar problem on a 67 Acadain (Nova). But adding to the stiffness was horrible creaking sounds.
It had bushings and shouldered bolts like yours.
Like rohrt said, the shackles would pinch the bushing ends enough to bind them.
To test the theory, I loosened the nuts and spread the shackles with a pry bar. A quick trip confirmed the problem.
I then ground the bushings ends down a bit and made them cone shaped. When it put it together, I added a washer on the bolt that made the shoulder longer.
It worked.

Great suggestions guy, thanks. I'll give these a try, but it's Winter again so everything will be on the back burner till Spring.

I'll check the shock length, and the rear shackles for binding. Tire pressure is at 32 lbs. all around so it sounds like that's good. If all else fails then I'll have to try a different set of leaf springs.

At least I have some things to check over now, I'll get back to you in the Spring. Thanks again, and Merry Christmas!

I'm going to add my two cents in support of the potential problem as outlined by others. I seriously doubt this problem has anything to do with the shocks or tire pressure.

As others have mentioned, your car sits a bit high both front and back, yet it looks relatively level. That tells me your back end has an issue with the springs and or shackles as they are installed. It's absolutely true that if your springs are too strong, your car "may" sit high and could potentially have a harsh ride. That said, there are two mistakes people make that cause both the front and back to sit high. Both have to do with bolting everything together before the vehicle has weight on its wheels. Again, per the suggestions above, it is very likely that the rear shackles were tighted with the weight of the body off the wheels. It certainly can then pinch the rubber bushings in the rear when the shackles / bushings are in the wrong rotational positon. What's happening is the bushings twist a bit as the springs flex during your ride. The problem when this mistake is made is that the rotational neutral position of the bushings / shackles is now with the body up high. This causes nasty problems in the ride and will cause the bushings to wear prematurely.

A very similar problem happens when the front control arm bushings are tightened before the cars weight is on the wheels. The front end sits high.
Of course, the front end issue could also be from using stiffer springs.

Anyway, if the issue is tightening things up before weight is on the wheels, the rear end issue can be fixed by proper installation of the rear bushings, shackles and springs, but your rear end will then sit lower when all is correct. Lowering the front will require properly setting the front control arm bushings if those are the issue. If it's high because of the front springs being too stiff, then change them.

You probably already know this, but don't take for granted the potential DANGER in doing any of this. The car / body / rearend and other things can suddenly drop when doing this if you don't support it properly. Springs by their nature are dangerous, especially the front.

On a different note, if your problems are none of the above, then I ask, is your car really bottoming out on the shocks, or are your rear tires hitting the inner wheel housing? I've seen it where people run larger tires (not referring to rim size) or have a different wheel offset and they bottom out on the inner wheel housing. I don't think that's the issue here since your car sits high, but didn't want to leave that out as a possible problem.

BTW just installed all new springs in my 68 convert. Sits good, bounces good, but haven't driven it yet.

Old Joe

What brand leaf springs did you end up going with? Mine are tired, stock '69 and I've got it apart, so.....

On the rear I used Eaton Springs and bought them from NPD. Eaton makes several leaf styles for our cars. I used part number C-7503-10. While they are new springs, I bought them years ago, so not sure if that part number is still valid.

I know it's been a few years since I addressed this issue but it's still there, and I finally made my way back to it. A couple things I did do was to get rid of the shock extensions and remove one short leaf in the springs, it didn't make much difference at all. I also changed the KYB Gas adjust shocks to the stock version, again, no difference. I did notice while I had the shocks removed to switch them, I pushed down on the rear of the car and still had very little travel, this would seem to point directly at the leaf springs, what do you all think? I'm thinking I need to swap the set that's on there out for an original, less stiff, set, any suggestions?

I just found the picture I took before I cleaned and painted the springs that were on the car when I bought it, seems like they may be made for a truck, or a heavy duty/performance application (660 lb. load capacity/5-leaf) and wrong for my car.

Here's the number off the leaf spring, S&G21-339-42, and here's the info I found matching that number:
Product Specs for Leaf Springs 21-339
Length (A): 25 Inches
Length (B):31 Inches
Width:2-1/2 Inches
Location:Rear
Number of Leaves:5
SRI Number:21-339
OEM Number:337230
Arc (C):7-1/4 Inches
Pack Thickness (D):1-3/8 Inches
Capacity:660 lbs.
Front Bushing:Included / RB-95
Rear Bushing:None - Berlin 1-5/8" Eye
Spring Construction:2/291 3/262
Note:1-5/8" eye uses (2) HB-1000 bushings; Order separately
MPN: 21-339

660 lbs must be the load capacity not the spring rate.

A little google search
Stock leafs are 125 lbs/inch, Hotchkiss are 150-180 Variable Rate. 68 Z28 rear "BR" springs have a 135 lb rate, and a 690 lb load.

Loosen your leaf shackles completely and see if you are able to get some movement in the rear.

Yes it is.

Put the new 4-leaf leaf springs on and it solved the problem. It now rides like it's should, the car absorbs the shock, not me.
Like a friend of mine said, who had the same issue, I could've run over a dime and told you if it was heads or tails, lol.

Sport - what spring brand and part number did you buy?

Part #3932719. I got them from a local spring shop (Penn Public in Altoona, Pa), not sure of the brand.