for all you solid axle drag racers out there, where did you end up with your spring rates? i am running 130# spring now. The issue i am fighting is when i launch the rear extends slightly, plants the tire, and then and look as if the tire jumps back up in the fender and i start to spin. i have tamed this down some with better clutch control and shock settings but it has never gone away.

Do you think i may be too light on my springs? i ordered a couple spring rubbers just to give this a try.


Car specs:
1990 vette
6.0L ls
6 speed
solid axle with 4 link, DA shocks
Hoosier DR2 16" tires.

 

My rears are 130# also. No problem with launch. 1.36 60'

 

You guys need to figure out your log in name...I was confused as hell for about 10 seconds. 😄

I'd love to see a pic of what that set up looks like.

 

ok, it may just take more laps for me to get it dialed in, i am running 16" tires as opposed to 15 so i wondered if that is causing the issue as well.

if you dont mind me asking where about are you running your anti squat and instant centers?
mine is right about 102% AS and it IC is about 40" forward

 

hahaha yea i think we have confused our self's in the past lol

i don't have any pics of it at the moment but i will try and get some at some point.

 

Have you tried to stiffen up your shocks on compression ?

 

yes, i have them topped out, they are QA1 DA. that's kind of what made me start looking at springs the fact that i topped the Compression setting out on the shock.

with it being a manual i just wonder if i am hitting it so much harder that a stiff spring may be needed.

 

My car uses 110lbs rear springs. Little over 1300lbs rear weight. Typically 1.15 60ft but the car can go 1.11 and has a couple of times on the right track throwing almost everything at it.....

If you're car is separating the rear suspension at the hit and then it's bouncing back up... your shocks do not have enough rebound valving to control the suspension and keep the tire planted... or youhavethereboundadjustmenttoo loose.... Even fully tight is not an uncommon problem with the basic QA1 double adjustables. They are not designed for the kind of forces a hard drag race launch has at the hit. If you have a stick shift transmission this exasperates the problem as the forces applied to the rear suspension are much more violent than a automatic transmission even one with a trans brake.

The full tight compression adjustment is a HUGE mistake.... you are hitting the tire with everything the driveline has.... Its probably crushing the tire initially and then basketballing as thecar mov3s forward.... you need the shock and springs to absorb some of the driveline hit on the launch... Like I explained before stick shift cars are very violent at the hit and stuff you buy off the shelf isn't capable of controlling that.

My recommendation is:

Soften the compression to 11 from full tight.
Tighten the rebound to 3 from full tight.

If it's still not under control... you need to buy better rear shocks.
Will

 

rklessdriver, thank you for the information, i guess i was thinking of it backwards, i was trying to keep the tire planted with a stiff compression setting, i was running 8 on the rebound and about 16 on compression. i have the next race here in two weeks so i will give this a try then. i may need new shocks as well. i think i can stiff the spring up some too if need be until i am able to upgrade shocks.

yea i know i did not do my self any favors staying with the manual and trying to no prep race but man its fun, the best part is when you tree a guy and out 60 ft them and they have a trans break lol.

 

rklessdrive, you hit it on the head as well, the tire does look like a basketball, thats why i was able to find some speed by feathering the clutch at launch. but i knew there was more in the car just not sure where to head.

thank you for the info

 

Where are the coilover located on your Vette in front or behind the rear, Will and vett383? because that makes a difference in spring rate as I'm finding out when I changed my spring mounts around. I had them behind the rear and change them to the front because I wanted the storage\ trunk back
Randy

 

mine are located in front of the axle. the are also located inboard of the frame so im not sure if that's an issue or not.

you are correct, i found a formula based on IC location and position of the shock to calculate out spring rate.

 

ralane5 asked the question I was about to ask. The wheel rate is what needs to be compared, not the spring rate. For a drag launch, you don't care about the wheel rate in roll, just in squat. Therefore, how far inboard the springs are mounting in front/rear view doesn't matter for this use.

You need to measure how much the spring compresses for 1" of wheel travel in compression. That will give you the motion ratio. For example, if the spring compresses 1/2" for 1" of wheel travel, then you have a motion ratio of 0.5. If it compresses 3/4", you have 0.75 motion ration, and so on. Then to get the wheel rate provided by the spring, you need to square the motion ratio and then multiply it by the spring rate. If your spring mount is on a tab on the front side of the axle tube and your four-link is long, the motion ratio might be as big as 0.95. Square that and you get 0.9025, and with a 130lb spring your wheel rate is 117lb/in. If the spring is on a tab at the back of the axle housing, the motion ratio might be 1.05. Square that gives you 1.10, and so the same 130lb springs gives you a wheel rate of 143lb/in. You can see that, due to the squaring function, a seemingly small change in motion ratio makes a significant difference in wheel rate.

 

I have a 300 lbs. coil over spring, and it ride nice and transfer weight good, along with the right trialing arm adjustments.
Randy

 

Than you all for the input, i finally got out to the track as we rained out the past 2 races. i did stiffen the shock up and it did help some but i am still bouncing a little at launch.

MathewMiller,

I need to measure this my springs are in front of the axle so my wheel rate will be less than 130. i got one more race this season, i am going to through a spring rubber in each just as an experiment to see if i can start controlling it. i also though about lowering the anti squat a little as well.

With all that said im still getting off the line better than most even with the 6 speed! lol

 

My coil-overs are ahead of the axle and I'm using a 7" tall 400#/in. spring ... currently averaging 1.35 short times with AFCO double adjustables .

 

i am running 14" 130# springs with a QA1 DA shock. i may just need stiffer springs. i dont have any short times, the track i am racing at does not have timers. instant green heads up stuff.