Spring Rates for 2nd Gen Firebird???

[blackknight]

I have a 78 Bird With alot of work done and I am looking for some advice on some fine tunning the suspension from you guys.

I have 275 rubber in front and 315 in rear, lowered body mounts, all poly graphite bushings, 150-180 progressive rear leafs with 1 1/2 drop with QA1 12 way adj shocks, Baer track kit all around, QA1 coil over(250-350 progressive spring) with 2 in drop with 12 way adj shock. I have subframes and g braces and sway bar braces. stock 1 inch sway bar up front and 1 1/4 racing rear sway bar with adjustable stiffness collars. the car is driven on the street but does see some track time.


I did the brakes and wheels this summer. now that i have been driving it the front feels a little soft and I have under-steer. the front always plows. The back is glued between the 315's, solid leafs and the racing sway bar.

Are my front springs too soft, should i go stiffer???
Any other input would be appreciated.

thanks

Lucas

[silver69camaro]

I don't know what your motion ratio is, but a 250-300 rate at the wheel should work well. A 250/350 seems very, very soft.

[79T/Aman]

with what you have there 600-700 front springs would work well take our GT or Comp front springs will fit the bill

[blackknight]

I dont know what my motion ratio is. I actually have never heard of that term before.

But I know the shocks were sold for my car with the 250-350 progressive springs. It has been a few years since I bought them, but recently I was figuring that I need the spring rate around 600. So it looks like I am going to be talking to QA1 and getting a stiffer spring for the coil over up front. I think they only make up to a 550 spring though.

The other question is.....Are the spring rates different between a standard springs and the tapered springs in a coil over??? Does 600lbs in a standard spring mean the same as 600lbs in a tapered spring??

I appreciate your input guys.

[Twentyover]

Motion ratio is a term used to describe the ratio of distance traveled by the springs as compared to the wheel. If a wheel travel of 4" results in a spring travel of 2", the motion ratio is 2:1. These also cascades into a term known as wheel rate. Wheel rate is the spring rate measured at the wheel, and is the square of the motion ratio divided into the spring rate.

For instance, if the spring were located 1/3 of the way between the inner pivot and the wheel on the lower A arm, the motion ration would be 3:1, and the wheel rate would be spring rate/9.

Spring rate is spring rate, it's the force required to compress a spring a given disance. Dosn't mtter if the spring is tapered coil, coil, or leaf.


To meaure motion ratio, I'd pull the wheel, tire, and spring. jack up the hub so it's approximately an inch or two below ride height. Measure the length of the shock (assuming it rides in the center of the spring). Jack the wheel up a couple i inches and meaure shock length again. Divide the change in shock length with the change in hieght of the hub.

Can also be done grapically if you know the dimensions of your parts and pivot points.



OK, on to your situation. You say the car pushing and you want to improve this by raising front spring rate. While I think going up in spring rate ius a good idea, I think it will make the car push more. I'm thinking you need more rear roll stiffnes, either by raising the rear spring rate or adding more rear bar. I think the 315's are causing the rear to grip a bunch more than the fronts

[Norm Peterson]

I'm wondering if we have all of the information. Heavy understeer with that much rear spring and bar compared to what's up front seems a little odd unless there simply isn't much front grip to work with.

Mechanical things like alignment specs, what the tires are (rears = DR's?), rear diff/locker/spool, etc.

Under what conditions does the plowing occur? What are you doing with the throttle, brakes, and steering, and how rapidly are you doing it?


Norm

[David Pozzi]

Talk to Global West about stiffer front springs. I think they have stiffer options for the QA1 shocks. The springs you have in front now are probably allowing the front to bottom out. Take a look at your bumpstops, I'll bet they show a LOT of use. You need a larger front bar too.

[BMR Tech]

I definitely agree that the car needs stiffer front springs and a larger front sway bar to balance the car out. I am surprised that a 350 lb. progressive front spring works at all for a handling application and feel sorry for your bump stops :injured:. As shown in the chart below, even OE springs ranged from just under 300 lb/in to just over 400 lb/in. These are all the OE rates from '70-'81:
https://static1.pt-content.com/images/noimg.gif
Until a few years ago, QA1 only offered two spring rates for the Pro Coil kits (what they called small block and big block springs). They were progressive rate and mainly designed for street rods, not really for road handling. They have gotten a little better since then and offer more choices now. All of their springs are now linear wound and they have 4 rates available - 250, 350, 450, and 550 lb. springs. In my opinion, the 550 lb. springs just barely cut it for what you are wanting to do. They would be a huge improvement but I prefer a 600-650lb linear spring unless you are building a dedicated track car. Also a 1.25" solid or 1-3/8" hollow front sway bar should help tame your understeer woes. Your rear sway bar seems a little large. I am not aware of a rear bar that big designed for a second gen F-Body, is it something custom?

[406 Q-ship]

Back when I played with 2nd Gens at Guldstrands we used 650 lb fronts with the WS6 rear leafs (150 lb if memory still serves) a 1 1/4 Trans Am front bar and no rear sway bar. With a good set of shocks (Bilsteins is what we used) Guldstrand won a suspension shootout back in the 1980's and it works bitchin on the street.

[Skip Fix]

I think Guldstrand told me my TA front springs from them were closer to 740. For my driving and stock TA rear springs I liked a 5/8 rear bar with them.

[blackknight]

Thats for the breakdown on the motion ratio, I had never heard of that.

heres the rest of the info on the car....(the cars a work in progress one job at a time).....

Rear end is stock 8.5 10 bolt with 3.42 open diff., stock 305 chevy with cam, carb, manifold..about 300 horses. Stock control arms with the polygraphite bushings. Alignment specs are -0.7 camber, +1.3 caster, +0.19 is total toe. Tires are Sumitomo HTRZ(all directional) DO I HAVE TO MUCH TOE?????

the rear bar is a old Rancho bar from the 80's i picked it up from a guy who use to have a trans am race car. Rancho doesnt make car stuff anymore, only trucks.

the car seems to only plow when your turning on a bumpy road or if you have hard braking(heavy load) in a turn...it feels like the car is bouncing on the bump stop and that would explain why I get the front end to push. Usually plows when im turning from 35mph and up after a decrease in speed, usually from like 45-65mph. Slow 90 degree(like residential streets) hammered in second gear are no prob..no plowing.

the bump stop is a polyurethane stop, so it looks brand new, but u can see how it has hit the frame alot. ALSO JUST A QUESTION? At static ride height...I have about 1 1/2 inch clearance between my frame and my lower control arm bump stop, the QA1 lowered the car about 2inches. Is that enough clearance...what do you guys have.

Yea I have to get a larger front bar, and I am going to get a stiffer front spring, I have to do a little shopping around and call QA1 and talk to Global West and see what is available I might have to go back to a traditional 650lb spring with a QA1 12 way shock and ditch the coil over, which suckS cauz it was expensive.

That should fix my prob and if it doesn't I might try taking off my rear bar and see how it rides.

thanks

[Norm Peterson]

If they're standard 2.50" coilover springs, there are a number of sources for different rates going beyond 1000 lb/in in various free lengths. 2.25" OD c/o's, maybe not as heavy.

Hypercoil is one source (Google it), Ground Control is another (note that GC is sometimes a bit slow, and that I know of one vendor who has stopped carrying their stuff for that reason), but maybe you can get Eibach c/o springs directly and avoid GC entirely.


Norm

[79T/Aman]

that rear bar is a bit too much unless you realy stiffen the front, i have one of those but don't use it and I run 800# front springs with an adjustable front bar.

[Ripper]

Where did you find that 1,25" rear bar?

I ran 1050lbs/in up front and Hotchkiss rear springs together with 1" rear and 1,25" front bar on my -74 Firebird. The car pitched too much, mostly because the drop on the rear suspension wasn't limited.

The rear springs felt slightly too soft during acceleration out of corners and had slightly understeer on R-rated Nitto NT01-tires. That was mostly because I couldn't get enough camber and caster (more 1,5ºcamber gave less than 3º caster. 2-3º camber would be nice...).

I'm currently building new suspension, but if I kept the stock I would have upgraded the rear springs to stiffer versions.

[79T/Aman]

ripper, the old Rancho rear bar was 1" adjustable, we have 7/8" adjustable and a 3/4" adjustable

[BMR Tech]

If they're standard 2.50" coilover springs, there are a number of sources for different rates going beyond 1000 lb/in in various free lengths. 2.25" OD c/o's, maybe not as heavy.

Hypercoil is one source (Google it), Ground Control is another (note that GC is sometimes a bit slow, and that I know of one vendor who has stopped carrying their stuff for that reason), but maybe you can get Eibach c/o springs directly and avoid GC entirely.


Norm

Norm, I was thinking the same thing but then remembered that the QA1 Pro coil is a special tapered spring QA1 makes that only works for this application. It is not a true coil-over that captures the spring at each end. The top is a 5" diameter spring that sits in the OE spring pocket and the bottom tapers to 2.5" for the adjustable shock collar.

Blacknight, I think you could probably keep your coil-over shock, remove the adjustable spring collars and run a standard spring on the A-arm. QA1 Stocker Star shocks are not really any different than the QA1 coil-over shocks you already have except they don't have a threaded body. This would save you the expense of buying a whole new adjustable shock.:idea:

[blackknight]

Brett, thank you.

I am either going to do what you suggested by buying a normal spring and pull the collars off the coil over, or I just got the Heidts new catalog and they have coil overs which i believe are the same size as QA1(with the tapered spring 4.5 to 2.5), I have to get the exact specs from Qa1 and compare to Heidts, but heidts offers up to a 700 pound spring. so that would work too. and they are cheap 100 bucks for a chrome pair.

thanks for all you help, I should be able to fix these few little probs in the early spring. I just cave to call some people and then order.

[79T/Aman]

2nd gen use a 5.5 OD spring , a 5" or 4.5" will not fit the top frame pocket

[BMR Tech]

2nd gen use a 5.5 OD spring , a 5" or 4.5" will not fit the top frame pocket

That's right, 1st gens are the ones that use the 5" spring, 2nd gens and practically every other GM car use 5.5". I stand corrected!

[blackknight]

ok i talked to QA1, and they have some old stock springs. they have a 600lb and a 800lb that will work with my coilover. What do you recommend.....i think the 800 would prob do better with the wider rear wheels. it should balance out the handling but will the 800 pound spring beat up my aluminum wheels to much on the street??????

i have got the feeling that the 600 is a little soft and the 800 might be fine or a little to much for the street.

any input is appreciated.

thanks

[79T/Aman]

the 600# will match your rear springs better

[Procharmo]

As you have a 90% street 10% track car and are concerned about street manners/ride wouldn't medium to firm front springs with a stiff anti roll bar be better? You will preserve some ride quality yet reduce body roll. You can then further increase front end grip with softer compound tyres and additional negative camber!!!( Toyo R888's).