67 Firebird Mono Leaf Spring Question
 

For the sake of originality, I have mono leaf springs and factory radius rods on my 67 Firebird 400. When going over bumps, it feels like the rear is bottoming out on the bump stops. I have replaced all the spring bushings, pads, and have new Koni Classic adjustable shocks.
I am considering replacing the springs as they may be original but when I read of how to tell if the springs need replacing, the only thing I see as indicators is if the springs sag and the car isn't sitting level. They don't sag and the car sits as it should.
I don't really want to spend $400 for new ones if I don't have to.
Any thoughts would be welcome.

There's about 7" of travel in the rear end of a first gen firebird. It's possible what you are experiencing is not that the suspension is bottoming out, but that the leaf springs are hyper extending as they arc past center and reverse.

This could be an indication that the springs have lost some of their spring rate and are no longer able to cope with the weight of the car in high loading conditions like bumps, or if there is extra weight in the back.

If the leaf springs are original and the car has spent it's life in the northeast where I see you're from, it's also very possible that they aren't one homogenous piece anymore. When I upgraded the rear end in my 69 Firebird I was shocked to see that both the driver and passenger side springs were cracked in the middle. The spring perch and lower shock plates were keeping the springs together in the car. This is a worst case scenario and would be a big safety hazard.

I personally would recommend changing the springs based on your testimony alone, but if you are keen to diagnose first, get the car up on jack stands, support the load of the axle housing and drop the lower shock plates and at least inspect the springs condition where the axle mounts to them.

Thank you for the response. When I replaced all the bushings and pads about 8-9 years ago, I had the springs powder coated and they had no cracks then but I will recheck them to be sure.

I agree with what Jason is saying. Also, I've seen it where as springs get tired and sag a bit, people will sometimes replace the rear shackles with taller ones trying to get the rear squat corrected. This can make some odd stuff happen back there too. As the rear goes up and down, the springs get longer to the rear. The longer shackles makes all the movement worse.

Have you adjusted your Koni shocks for ride improvement, they are an excellent choice.

Tim john---

Yes. When I first installed them, I adjusted for maximum "stiffness". Thinking that setting may be playing a role in the issue, a couple of weeks ago, I readjusted to a lower setting. No change.

Powder coating leaf springs can be done, but it has to be well controlled and to the lower end of typical settings to avoid affecting the spring properties; as the coating temperature is very close to the annealing temperature. If they were exposed to temperature >350F for a period it could have softened them affecting spring rate.

I agree. I cracked the right side main leaf twice and the left side once even with the five leaf bundle on my '70. A lesson learned on the potential damage from installing a B&M Transpak Shift Kit. They always seemed to crack below freezing temperatures in Ohio winters which made for a chilly repair too.

400 degrees F isn't nearly hot enough to change the temper of any spring steel. All the leaves in my '59 GMC were powder coated twelve years ago- the springs have not changed at all, and the coating is still like new.

I had a banging sound from the rear when driving a newly acquired 67 400, turns out both leafs were broke at the front eye bushing. Break was not easily visible from below as the break was hidden near the top of the eye. Installed new monos and all was good.