#1  
Old 03-26-2024, 09:45 AM
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Default '67 Firebird 400 Bottoming Out

I have been trying to resolve a ride problem with my '67 with no success.

When driving over 30mph, anytime I go over a dip or bump in the road, the car seems like it bottoms out with a loud bang.


I keep the bird original with mono-leaf springs, both radius rods, Koni Classic adjustable shocks.


All the suspension bushings have been replaced in the past couple of years. I went with the Konis two years ago. Last year I replaced the rear springs with new mono-leafs from Eaton, thinking that the ones I had may have been original and lost some of their effectiveness. I can't say there was much, if any, improvement.


I can't believe these Birds acted like this when new. Anyone else have this issue?

  #2  
Old 03-26-2024, 10:48 AM
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Can you give us an idea where you think it's bottoming out? In the rear, or in the front?

If you haven't yet, you can adjust the koni's stiffer, which may help.

Especially in the rear, the first gen firebird has a lot of compression travel. I'd get under the car and start looking for witness marks on things like exhaust tubes, shock mounts, inner wheel wells etc. It's possible the suspension is not bottoming out, but instead it's hitting something that is out of place.

What wheel and tire package are you running? Does the car have headers with collectors that hang below the subframe rails?

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Old 03-26-2024, 01:10 PM
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My 68 was doing it as well, ended up installing the StreetGrip kit (for nova) which came with dual rate new front springs (don't remember rate), 170lb rear composite springs and fox shocks. Lowered it 1+ inches but it doesn't bottom out anymore. Not sure if it was the springs or shocks that fixed it.

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Old 03-26-2024, 09:31 PM
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Quote:
Originally Posted by JLMounce View Post
It's possible the suspension is not bottoming out, but instead it's hitting something that is out of place.
That's what I was thinking as well.
Look really closely at the exhaust especially.

You might also try setting your shocks on full soft (or removing them if that's not enough), and just get the car bouncing up and down by pushing on it while parked, to see if you (or a friend) can determine what's hitting.

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Old Yesterday, 12:24 PM
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Hi Everyone. Thanks for the suggestions


Some more detail.


As far as the possibility of exhaust, wheel well area, and other parts hitting, that is the first think I checked while up on the lift. I don't see any signs of rubbing, hitting, etc.
I don't have headers, but LB manifolds. I run 15x7 front and 15x8 rear Rally 2s with 235/60/15s all around.


I also experimented with the Koni settings. I set to full firm when I installed them, then adjusted to medium later. No difference. I may try the soft setting.


However, one area that I didn't cover was the front coil springs. They may be original and worn, so my next job is to replace them. Maybe the front end is lifting too much?
That's a job I don't look forward to but in the next couple of month, I will replace them.

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Old Yesterday, 03:25 PM
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Front coils aren't terrible in these cars, I'd much rather replace them than the leaf springs.

Based on your summary I don't necessarily see anything that is really out of whack. Do you have heavy stereo equipment or anything else in the rear that would change the dynamics back there? Is the car currently equipped with a lot of sound deadoning, or interior that would add weight over factory? Basically anything that would have added weight to the car that its suspension package wasn't designed around. I'm not purposely being rude, but what about the drive and passenger? People were generally much thinner in the 60's. If the occupants are carrying more weight, it can have an effect. My apologies for being direct, that can be a sensitive subject.

While the suspension does work in a choreographed manner, front springs being too soft wouldn't really effect the back end bottoming out to any great extent.

All that said, I have long told anyone who asks me that the first generation birds are woefully under sprung. Especially in the front. The back isn't as big of an issue typically because it's got a mile of suspension travel. Out of curiosity, is the bump stop still in place on the rear frame rails?

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Old Yesterday, 05:19 PM
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No heavy added equipment nor "overloaded" driver or passenger. Lol! Bump stops are all there.

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Old Yesterday, 06:00 PM
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Quote:
Originally Posted by 1stgenbird View Post
No heavy added equipment nor "overloaded" driver or passenger. Lol! Bump stops are all there.
Funny I know, but I've seen it. Cars bottoming out or vehicle performance isn't what the owner thinks it should be. Meanwhile there's 600 lbs between the husband and wife sitting in the car.

Anyhow, onward.

I ran your wheel and tire setup for the rear into a calculator. I assumed based on your statements a 100% factory original suspension and axle length as a result. I used the 6Jx14et8 wheel that should be on the car from the factory with the optional wide tire, that equates to a current 205/75/14. Wheel 1 would be the factory fitment, wheel 2 would be your current fitment. I'm assuming you have something like a Coker rally II wheel that has a standard 0 offset, 4.5" backspace.

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What jumps out to me is the change in tire location towards the outside wheel lip. Your tire sits almost an inch and a half further out than the factory wheel would have. On a 67-68 car with the rounded fenders, you can get away with poking the tire outside of the sheet metal, or even directly under the wheel lip, but you do run the risk of slamming the sheet metal onto the tire itself.

As a 69 owner, we have to be much more careful and sensitive to this situation.

Is there a chance that your car is coming down on top of the tire? Look for witness marks on the underside of the wheel lip and possible marring/scraping on top of the tire tread.

If that's the situation, either pulling the wheel inward towards the suspension, or sourcing a wheel with a proper backspace would resolve the issue. You could also go to a stiffer spring and shock setup which would not allow the car to travel as much in bump.

If the bumpstops are in place, bottoming out the car should not result in a loud bang. You should feel the car stop moving, but noise should actually be minimal. The bump stops in the rear are several inches thick if in good condition.

It may not be the tires, but if there's a loud bang, you're definitely hitting something.

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Old Yesterday, 06:59 PM
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600 lbs between driver and passenger? Need to get a bigger car than a Firebird!


Definitely not any type of interference with the tire/rim/wheelwell. I was running 225/60/15s with 15x7s all around until recently and still same issue.

I did have 275/50/15s on the 15x8s at the rear and the wheelwell lip did rub and grooved the tire so I took those off.

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Old Yesterday, 08:50 PM
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Quote:
Originally Posted by 1stgenbird View Post

However, one area that I didn't cover was the front coil springs. They may be original and worn, so my next job is to replace them. Maybe the front end is lifting too much?
That's a job I don't look forward to but in the next couple of month, I will replace them.
Springs do not lose "rate" with time. They only lose load capability. So if your car is riding really low, that might be a problem. But if the ride height is near stock, don't waste your time changing springs - they aren't the problem.

Have you looked at all the jounce bumpers to make sure they are present and the rubber is compliant?

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Old Today, 06:37 AM
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