back coil over shocks

fishman · Post by **fishman** » Wed Jul 04, 2007 1:04 am

my car weight is 2450 with me in it and i am guessing the back end weight is no more then 1100 lbs as the car is front end heavy...anyways i measured the thickness of the rear coil springs and they work out to be 150 lbs spring. and they r 10 inch long springs that r preloaded to 9 1/2 inchs....everybody i talked about this figures that i need a 85 or 95 lbs spring....looking in the catolgue i can only get 100 lbs spring r the smallest made......so my question is if i put in 100 ibs spring should i preload the to what and do u think that these 100 lbs spring would be the right way for my application..........thanks

Ed-vancedEngines · Post by **Ed-vancedEngines** » Wed Jul 04, 2007 1:19 am

What is your spring's ID and length?

That will depend on what is available.

I can't tell you waht you would need but I can tell you that what you have is very stiff. Too stiff.

How are you guessing at your weights? Can you weigh each end on something? Truck scales or something? If you can't then eigh the entire car and read my suspension article and you will be able to know what percentage is on the front and the rear.

http://forums.racingjunk.com/viewtopic.php?t=2363

In the past I have gotten springs as light as 75# but it all depends on what your spring size is.

The diameter of the coils is but one measurement for spring rate. Also you need free length and amount of coil windings in that length. AND I do not remember the formulas to convert thise to a spring rate.

Ed

BillyShope · Post by **BillyShope** » Wed Jul 04, 2007 6:15 am

spring rate = (1.45E6 d^4) / [n(D-d)^3]

where "d" is the wire diameter, "D" is the O.D. of the coil, and "n" is the number of active coils.

(No, I didn't remember it either, Ed, but my college textbook was just a few feet away.)

fishman · Post by **fishman** » Wed Jul 04, 2007 8:27 am

The springs r still on the car so i cannot measure the id of the spring but i can tell you that they are 10 inchs long with 10 coils and the thickness of the spring is .430 inch........i hope this helps.....the new ones will be here tommorow and i will be able to get a id of the old spring

BillyShope · Post by **BillyShope** » Wed Jul 04, 2007 9:33 am

fishman wrote:...i cannot measure the id of the spring....

The equation uses the OUTER diameter of the coil.

Oops! I see you're responding to Ed's post. Sorry.

Still, I'd like to know the O.D. so we can use the equation from my old textbook.

fishman · Post by **fishman** » Wed Jul 04, 2007 7:17 pm

rough measurement looks like id is 2.35 inch give or take a little hard to measure on the car.........thanks

Ed-vancedEngines · Post by **Ed-vancedEngines** » Wed Jul 04, 2007 8:00 pm

If you can't get what you need, I think that I have several different rate 10 inch springs upstairs in a box. If I remember right they were 95# 110# 120# and 140#. Now I am trying to remebr this form maybe 20 years back so I may be a little wrong. I do know they are 10 inch long though. Come to think of it, I believe I also have some that size on a set of Armstrong shocks I also have up there. I would have to check on the ID though. I know they are 10 long. They may not be small enough in the ID for you though. Let me know and I will dig them out and measure them.

Ed

BillyShope · Post by **BillyShope** » Wed Jul 04, 2007 8:08 pm

The equation says 231 pounds per inch (3.21 inch O.D.). I hope you can trace them back to the supplier's stated rate.

fishman · Post by **fishman** » Thu Jul 05, 2007 2:03 am

I took one off and took it to a local speed shop and the shocks r aladin shocks and he matched them up to a set he has in another store, they r 100 pound springs he told me that for that shock that will be the lightest spring they make so i oredered them and they r suppose to be here tommorow.....he figured me old ones were at least 150 pound springs.....way to much for my car what i have been told by others anyways, i hope they r right....some guys told me 85 pound and other told me 95 pound springs

fishman · Post by **fishman** » Thu Jul 05, 2007 7:00 pm

3.4 od and 2.5 id is what it measures

Ed-vancedEngines · Post by **Ed-vancedEngines** » Thu Jul 05, 2007 9:11 pm

By the way;
In an earlier post yoy mentuoned about pre-laoding the springs. Don't unless your car needs it. If the springs were just a little wweak you could make up some of that in adjustments but let the car tell you what springs it wants and if you need to make adjustments or not.

Example;
If you have a total of 6 inch shock travel and the car is sitting with all weight on the springs and the springs are adjusted all the way down, Good would be for 4 to 3 1/2 inches of the shock ram to be inside the shock housing. I am using 3 1/2 and 4 inches as minimums and as maximums. 4 inches in would still leave you with 2 inches of compresion travel for weight settling with wheels up or for ride control going down the track. 3 1/2 inches in would still give you a marginal amount of suspension travel for ride and weight transref witout bottoming out.

If you have 6 inch of travel and 3 inches are in and 3 inches are out, to me that means that spring is a little too stiff. If you have 5 inches in, that still could be ok because you have spring adjusters to make the difference.

With a drag car you need to have appx 3/4 of your total travel available for rear suspension action on the launch and at least 1 1/2 of travel available for weight transfered and for ride.

Someone else may disagree but this is how I do it. This is what I mean by letting the car tell you what spring it wants. The rear springs hold up the weight of the car. That is all they do. Rear springs are not supposed to be absorbing torque or do anything except to hold up the rear of the car.

Ed

fishman · Post by **fishman** » Thu Jul 05, 2007 10:25 pm

thanks for your reply ed and all of the rest of you...the reason i preloaded the shocks was because i thought maybe i didn't have enogh clearance between the car and the fenderwall...i probalby have 2 inch clearance..so i figured that i can try it this way and if it doesn't rub i will loosen them up some more....i will find out tommorow as the rocky mountain nationals start tommorow

af2 · Post by **af2** » Fri Jul 06, 2007 1:17 am

Ed-vancedEngines wrote:By the way;
In an earlier post yoy mentuoned about pre-laoding the springs. Don't unless your car needs it. If the springs were just a little wweak you could make up some of that in adjustments but let the car tell you what springs it wants and if you need to make adjustments or not.

Example;
If you have a total of 6 inch shock travel and the car is sitting with all weight on the springs and the springs are adjusted all the way down, Good would be for 4 to 3 1/2 inches of the shock ram to be inside the shock housing. I am using 3 1/2 and 4 inches as minimums and as maximums. 4 inches in would still leave you with 2 inches of compresion travel for weight settling with wheels up or for ride control going down the track. 3 1/2 inches in would still give you a marginal amount of suspension travel for ride and weight transref witout bottoming out.

If you have 6 inch of travel and 3 inches are in and 3 inches are out, to me that means that spring is a little too stiff. If you have 5 inches in, that still could be ok because you have spring adjusters to make the difference.

With a drag car you need to have appx 3/4 of your total travel available for rear suspension action on the launch and at least 1 1/2 of travel available for weight transfered and for ride.

Someone else may disagree but this is how I do it. This is what I mean by letting the car tell you what spring it wants. The rear springs hold up the weight of the car. That is all they do. Rear springs are not supposed to be absorbing torque or do anything except to hold up the rear of the car.

Ed

Excellent!

fishman · Post by **fishman** » Mon Jul 09, 2007 11:47 pm

I took the car to a chassy guy and scaled the car.
the car is 55 % to the front and 45% to the back, he also found that the front springs are to short they have been cut and the drivers side is even shorter...he figures that the front end of the car needs to come up 2 inchs in the front...he also found that the back shocks r hooped and he recommends putting 150 pound springs on because the shocks r mounted in front of the rear end and not at the back...he told me that after these changes the car will probably need wheelie bars and my et should be close to 10.1 to 10 flat from my best 10.29

Ed-vancedEngines · Post by **Ed-vancedEngines** » Tue Jul 10, 2007 12:08 am

The shock placement of where it is mounted and the mounting angle do have quite a lot to do with what rate springs you need. Still the words I wrote will apply about how to know if you need a stronger or weaker spring rate in the rear. I do agree that a front monted shock will be needing a stiffer spring than a rear mounted shock with all else equal.

You are correct that the cut down front springs will only hurt you in drag racing, but you also should ne be trying to rasie the front of the car just because of changing springs. A variable rate sft spring rate spring is what I suggest. You do want the compressed coils to be loaded when the car is setting at reast and to assist on getting the front up more when you are launching if you need the additional weight transfer.

Perhaps Mr. Billy will come on herer and offer spring suggestions for the front. MY only suggestion is to see if you can get a lower rate spring for the smallest engine for your car in the variable rate style from Moog. Perhaps Moroso has some.

Ed