Spring Wt Help

Suspension Tuning, Troubleshooting, Design and Discussion

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allryz70
Posts: 3
Joined: Wed Jun 09, 2010 9:30 am

Spring Wt Help

#1 Post by allryz70 » Wed Jun 09, 2010 9:50 am

I am looking for some advise on choosing the proper spring rate for car (think that might be an issue). It is a 68 Firebird with square tube chassis. Steel body with fiberglass front, doors, and trunk lid. Car is cert to 7.50 but being a rookie old grandpa (The Farmer) though we should start a little slower. Car has a pretty much a street/strip 455 motor in it. Nothing special, probably just over 600HP. Nothing that should over power the chassis (having inconsitant 60' times thus the reason for asking for help). Car runs 9.80 at 136mph and 60' have been from 1.30 to 1.42 depending on the day and how the car wants to react. Car is deadly if I could get the 60' time to repeat instead of running a 1.34 then running a 1.37 the following round.
The car weights either 2481# w/o the driver or 2689# w/ the driver. W/O the driver the car is about 50/50% weight distrubution. With the driver is about 49.3% front and 50.7% rear.
The car is currently equipped with Strange Shock (4.077" travel, 11.32" compressed length, 15.4" ext. lenght) with Hypercoil 12" lenght at 110#/inch (it had 125# springs on it). Up front I have VariShock (2.8" travel, 8.55" compressed length, 11.35 ext. lenght) and VariShock spring that is 7" long at 450#/inch spring.
The spring rate I calculated used the formula of:
(Total weight (front or rear) - Unsprung weight / 2) / Shock travel + 1". Please note I haven't calculated any leverage effects (I have been to Morgon's classes if you reconized the the formula). I can tell you the rear shocks are behind the housing with a 4 link, and the front suspension is a double A-Arm car (Alston ChassisWorks) and the shocks are at some angle.
The front weight is 1250# w/o driver and 1326# w/ the driver and there is around 101# of unsprung weight. The rear weight is 1231 w/o driver and 1363# w/ driver and there is around 307# of unsprung weight.
So what I calculated I need a spring rate of:
Rear w/o driver = 91#/inch; est. around 83#/inch with leverage effect
Rear w/ driver = 104#/inch; est. around 95#/inch with leverage effect
Front w/o driver = 151 #/inch; est. around 219#/inch with leverage effect
Front w/ driver = 161 #/inch; est. around 233#/inch with leverage effect.
I am battling a inconsistant 60' time and I think I have a way to stiff front spring and I not gettting the weight tranfers like it should. From what my crew is telling me (haven't had a chance to video the launch yet) the car hooks and about 1/2-3/4 turns out the rear tire seem to unload and have a slight tire slip. It seem like any shock adjustement is not working (rear shocks were just rebuilt and dyno). I am stating to wonder if the rear end is in control back there since the back end is so slight.

Any advice would be greatly apperciated. If you have any specific question please let me know and I will anwser them the best I can.

Sincerely,
Ryan Wiersema.
Arnie Beswick Racing

bert
Posts: 41
Joined: Sun Feb 22, 2009 2:10 pm
Location: Sacramento CA

Re: Spring Wt Help

#2 Post by bert » Wed Jun 09, 2010 10:12 am

Got good tires? :?:
[youtube]pWMP4iSIAhk[youtube]

allryz70
Posts: 3
Joined: Wed Jun 09, 2010 9:30 am

Re: Spring Wt Help

#3 Post by allryz70 » Wed Jun 09, 2010 10:52 am

I orginally had Hoosier 13.5x31.5-15 with a 3.89 gear in it then we went to Hoosier 14.5x32-15. Still had 60' going all over the place with the tires being brand new. Is it possible to have to large of a tire?

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John_Heard
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Re: Spring Wt Help

#4 Post by John_Heard » Thu Jun 10, 2010 7:37 am

Should be able to hook in the middle of a ice storm with that big of a tire, ET and weight!

Yeah the front springs do sound stiff, a 450 is what I have (I think?) on the front of my 3400 lb car.

Before doing anything, good video of the car launching to study how it reacts would be a very good thing.

Also if you have data logger info, that would be very helpful.

allryz70
Posts: 3
Joined: Wed Jun 09, 2010 9:30 am

Re: Spring Wt Help

#5 Post by allryz70 » Thu Jun 10, 2010 10:13 am

LOL. That was what I was thinking.
At this time I don't have a video camera but I will work on trying to get a video of the launch. No data recorder either.

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