question about spring rates

[firebob]

I'm looking at replacing my original HD 4 leaf springs from my 69 Firebird with some new ones. The original ;eafs had a spring rate of 126lbs. I can get replacements in either 126 or 140lb spring rates. My question is which would be better? I'm not worried about comfort because it already rides like a wagon. I'd like to get the best spring rate for track use.

Robert

[exwestracer]

Bob, what problem is it that you are trying to fix? Saying you want the car better for track use is like saying you want a chick that is "hotter". Means different things to different people... The original springs are progressive rate (most ANY multi leaf pack is), so the 126lb rating is kind of miseading. If I had to guess, you are probably feeling the car "falling over" at the rear? Maybe worse on one side, like taking left handers? After years of use (abuse?) the springs will take a different "set" from one side to the other. You may not see it visually, but on the scales the corner weights will be way off compared to the ride heights.

If your corner weights are balanced (and holding), I wouldn't chuck those springs "just because" without trying a few other things first. Stiffer springs will reduce traction at the rear, and that will hurt you coming off the corner.

Great shot of my old "home track" by the way!

[Skip Fix]

"If your corner weights are balanced (and holding), I wouldn't chuck those springs "just because" without trying a few other things first."

In my thread on using stock springs but adjusting sway bars/shocks for overall someone else said basically all factory springs are not good for performance just ride. Sounds like you agree also they can be worked around to still handle decently.

[firebob]

The rear of the car runs exactly where I want it to be with the power level I have at the moment(400HP). I can give it everything it has out of a sweeper and it will hang with just a bit of slip. After all these years of adjusting and trying different parts and setups I absolutley love the way everything is working now. I almost hate to mess with it but it's what we do right? The problem is that it's always had a 3/4" lean to the pass side(as measured floor to fender lip). I always assumed it was a weaker spring on that side and figured I would replace them one day. A buddy reminded me the other day that my battery is relocated to that side. I haven't pulled it to see if things evened out but I'm sceptical that it will rise that much from just 20lbs of battery in that corner being removed.
The 140lb multi-leaf springs had a linear rate according to the manufacturer. Sounds like I maybe better off with the 140lb replacements if I they are going to react the same way as the 126lb version. Same money either way so I'm just looking to get the best choice fo my needs.
Thanks for your insights and the help is appreciated.
(here's another pic, tough track for a street car)

[exwestracer]

Seriously, have you checked corner weights?

Car definitely looks good coming off the Corkscrew. There's a lot more inside road to use there, but I've always liked your line better. I swear you make up 10mph going into 9 if you're not so worried about running out of road on the left side...

[firebob]

No, I have not checked the corners for weight. Not quite sure what I would do with that information to tell you the truth.

That was a tough day at the track for me. I only had 1/2 deg camber dialed in and the front tires were rolling over sooo bad on the downhills I kind of had to baby it a little. They were not happy and they were letting me know about it. That next big lefty was the really tough one when I think back. Then just when I was getting comfortable with the track one of my rocker arms gave it up and I was done for the day. Since that day I've upgraded a few things and now she runs pretty well. I run mostly at Sears Point in Sonoma. The car really seems to like that track(me too). Not as challenging but just as much fun. And not as far to get towed home, if needed.

Robert

[Skip Fix]

So whose springs have you been using? If the one has just sagged might could get a spring shop to re arch them the same, most that do that can check the rate also.

Assuming you have a Pontiac motor what oil pan you running, an Accusump also?

Car does look good.

[firebob]

Thanks.
Stock 40+yr old stock springs. After all this time at least the one is weak possibly both and just one worse than the other. In any case she's probably over due for some new ones. A friend has suggested swapping them side for side. Wow, I never would have thought of that. Relatively free and after I drop it back on the ground it should show me for sure if the one is weaker than the other.

Yup,400 Pont motor. Stock baffled oil pan so far but I have a replacement HO Racing copy hanging on the wall the next time the motor comes out. No accusump. So far I just make sure there an extra half to 1qt before I go and I haven't lost pressure yet as far as I can tell. I saw a 3qt one for sale the other day. I was tempted but I'm not sure it was complete.

Robert

[exwestracer]

No, I have not checked the corners for weight. Not quite sure what I would do with that information to tell you the truth.

That was a tough day at the track for me. I only had 1/2 deg camber dialed in and the front tires were rolling over sooo bad on the downhills I kind of had to baby it a little. They were not happy and they were letting me know about it. That next big lefty was the really tough one when I think back. Then just when I was getting comfortable with the track one of my rocker arms gave it up and I was done for the day. Since that day I've upgraded a few things and now she runs pretty well. I run mostly at Sears Point in Sonoma. The car really seems to like that track(me too). Not as challenging but just as much fun. And not as far to get towed home, if needed.

Robert

Roll it on a set of scales and check the L/R split in the rear (with you in the car). The left rear (high side) should be reading more weight if the RR spring is "sagging". Check to see if the spring is bent near the front eye. Years of hard wheel hop launches will permanently deform the leaves.

If you do go to a stiffer spring, it needs to have less static arch; or your rear ride height will go up. It sounds like you're pretty happy with it the way it is.

SCALE IT BEFORE YOU CHANGE ANYTHING. If nothing else, you will have a ride height and corner weight baseline that you know you're comfortable with.

[Merlin]

There are several things to check. Are there any binding in the shackles/bushings? Stiction between the leafs? Are all the bolt torque proper? Are the shock in good shape. Is the engine/tranny aligned with the pinion angle?
You can have a stiffer spring with great shock and still have a great ride. Softer springs with crappy shock can make for a horrible ride. So, if you have a good shock set-up that is valved for the spring rate, then you will be happy with the ride.

Cliff
369 CP Chevelle

[firebob]

Thanks for the replies.
I had the whole car weighed a few years ago(lots of changes since then) and it was 3740 with me in the seat and half a tank. I wonder how heavy of a scale I need to weigh each corner alone?
Shocks are new gas, non adjustable. Not the best, but ok. I looked at spring rates of newer Fbody cars and they are all over the place but it looks like it wouldn't hurt me to go with the 140lb rated springs if I decide to go with some new. Not sure what pinoion angle would have to do with ride height being different side for side?
The wheel hop thing has me wondering...this car always had quite a bit of wheel hop for the majority of it's life. It wasn't drag raced much but did go through a couple of teenager type drivers so not babied much either. It lived most of the time with a one legger so that right side spring would've taken the most beating.
A few more things to consider.

Robert

[Skip Fix]

FWIW both my second gen TAs one spring ,driver's, is 3/4 taller than the other when removed(free no weight off the car). 79 Camaro both the same. I know the custom drag springs I had made the gut often put some extra bias in the passenger side, likethe coil spring guys putting more air in an air bag on that side to compensate.

Get a Canton pan they have really good baffles, better than my Milodon.Hard braking my HO pan would get the lifters ticking.

[exwestracer]

Bob, typical race car scales go up to 1500lb per corner. Ask around the local race shops, somebody has to have a set and they will probably charge you $50 to set them up and scale the car. There is a dirt track in Petaluma, probably a lot of teams in that area. One of the shops at Sears Point can do it for you, but expect to pay double for them to mess with it.

[Bryce]

Bob,

If you find your self in San Diego, contact me and we can scale your car.

[firebob]

Ok thanks. I'll bet that if I ask around the next track day there will be somebody there with all the toys that can do it for me. Maybe I'll run down a CHP with a set of those portable scales and try and make a deal with him.

Robert

[David Pozzi]

Roger Kraus Racing Tires in Milpitas, has scales.
Disconnect one end link of the rear sway bar if you have one, then place a jack dead-center of the front crossmember and jack it up until the front wheels are off the ground. Then measure the rear for height differences. This removes the front springs & bars from influencing ride height, leaving only the rear to worry about. You can also jack up the rear and measure up front. If level without bars connected, then shim the bars at the end links to level it out.

F body leaf springs have a slight off-center to the front rubber bushings. They usually won't swap over left to right without moving the bushing over.

Most aftermarket high performance Tuner leaf springs are around 175 lb/in.
David

[Skip Fix]

So David for a street/autocross car what spring bushings do you like?

Poly kind of binds but is more solid than rubber. Front spherical on the street gets alot of debri to wear it out. GW Dela luma don't allow any roll