Need Help! Changing Rear Chassis Springs

[Rick Parker]

I am in the middle of changing the rear springs on my car. I am out of my element in this part of the car consequently I have put this off for quite a while, but today was the day. The shocks ar Alden coil overs with only the rebound being adjustable. The current springs are 10 inch rated at 240#. The replacements I have are 8 inch rated at 300#. Is there something I am overlooking by replacing the 10" with an 8" as long as there is enough thread adjustment on the body of the shock absorber. The reason for the change is that sometimes the rear suspension bottoms out on some bumps while driving, this is only with me (a svelt 210#) in the car. Aside from going on a diet.??????...........am I on the right track by putting on stiffer springs (?) Or should I be looking at the shocks? The existing ones are in good working order. Is the the additional 60# too much of an initial change (?)

Chassis Guru's!!!

With the frame rails on jack stands, I raised the center of the differential to the point that the chassis just began to lift off of the Jack stands and I measured the compressed length of the 10" spring in an attempt to determine an installed length of the shock (at ride height) so I would have a gauge to keep the ride height the same, short of getting under the car and measuring the actual ride height. The compressed spring measured 7.5" long at the point it lifted off. I neglected to measure the installed length of the shock itself (out of my element!!!) Is there a formula to calculate the comparative length that the 8" long spring would to be with the same load so I could install it at a precompressed free length and be assured that it will maintain the same ride height? I don't want to have to climb under the car several times in an attempt to set the ride height.
The image below is what the shocks & springs looked like before the body was installed.

Is this clear as mud???

Thanks guys

Rick

[RICK LAKE]

Rick Parker Rick 2 thoughts First just add a 1 inch spacer to the lighter springs in the car all ready and stay with them. The rear needs to stay flat on the road surface and not bounce around going over dips and uneven spots. If you go with the heavy springs either get the longer ones or again add the spacers to the spring. I run the stock ERA spring that came with the Jag rear. The car rides great on the street but bottoms out on the track. I added 1" spacers and stayed with the same springs. I am 240 lbs, also going on a diet The car doesn't bottom out and the ride is still nice. When I drag race, the car squats and leaves nice, left for wheel just about off the ground, good wieght transfer for this racing. If you don't mind that floating ride, go with larger sway bars on the car front and rear that are adjustable. Take out any rubber bushing and go to hard plastic or aluminium ones. They will need more maintainance but tighter ride. Have you had the car wieghted and balanced? This is a good place to start. Alot of guys don't do this before wheel alignment is done. They measure the frame in 4 corners and when even number comes out they are done. My car was 82lbs right to left front with me in it. and 46 lbs rear. Reset the car with the balance correct and the car handles alot better. I am in the process of getting bigger sway bars for my car that are adjustable from ERA. Hope this helps Rick Lake

[Rick Parker]

Thanks Rick

With the shock fully extended, a 10" spring "just" fits with the threaded collar being able to be fitted. A spacer wouldn't fit or change the rate or load capacity but would just allow a different ride height adjustment.
I only have a front sway bar, it is a solid round bar that is splined on each end, it is carried in a metal tube with delrin bushings on each end. The tunability is created by using different diameter bars.

I hunted around on the internet last night for engineering sights with formulas for coil springs. All I was I was able to determine (laymans view) was that the 8" spring will compress "less" with the same load (static weight), still unable to detrmine installed height.

So let me state it another way:
How can I determine how much a given spring will compress (from free length) with a given load???

Thanks
Rick

[John A. Simpson]

I'm plagiarizing Herb Adams book 'Chassis Engineering' here but to get you started if your springs are rated at 300 lbs-in. then when you compress the spring one inch it will support 300 lbs. If you compress another inch it will support another 300 lbs. for a total of 600 lbs. Your spring compressed three inches can support 900 lbs. Your coil-overs are close enough to vertical it shouldn't be a big factor. If your know your sprung weight you can get close. If your sprung weight, basically what you're picking up with your jack when you were taking the weight off yours springs but not lifting the differential etc., you can figure it out. If your sprung weight is 1200 lbs. you'd need 1200 lbs. of spring load to support the car at rest. Each spring will be compressed two inches to support your hypothetical 1200 lbs. So I'm guessing IF (big if) in this example if you put the car two inches above your desired ride height and had the coil-overs collars snugged up to the spring, you would be close. Do you know the weight of the rear half of your car? I'll bet you can figure it out from watching what your current springs do. For example, if your 240 lbs-in. springs compress 2.5 inches (to support 1/2 of our theoretical 1200 lbs. )the 300 lbs-in. springs would compress 2 inches. I think if you measure and watch your springs you can get pretty close, but I have to admit this is all arm chair theory on my part, I have NO experience. Good Luck young Skywalker. If nothing else if I'm completely out of the ballpark maybe someone who really knows will chime in. I sounded pretty good though didn't I.



John

[RICK LAKE]

Rick Parker I have the same book John talked about and he is right. You need a spring compressor with the spacer to get on. You can start with a 1/2" spacer and see how that works. You have adjustable sway bar, this makes the car handle. The spring holds the wieght. I went with a soft spring and tighen the sway bar for cornering. As I said you don't want the ride of a tractor with your car, you need the tires to have full contact patch to the street or track. Bigger, stronger, larger, or heavier don't always work better. Car only wieghts 2500 lbs. Rick Lake

[Richard Hudgins]

Rick,

10 inch spring 240 per inch rate, compressed 2.5 inches = 600 lbs preload.


8 inch spring 300 per inch rate, compressed 2.0 inches = 600 lbs preload.

But the next inch is:

10 inch, 240 lb. 3.5 inch compressed = 840 rate.

8 inch, 300 lb. 3 inch compressed = 900 rate.

Therefore, depending on your motion ratio, 300 might not be enough to stop the bottoming out.

[RICK LAKE]

Richard Hudgins Rick I was under the thought that the more angle the shock and spring are at, the less support and dampening they do. This is why you use the canterlever system in the front end of your car. The straight up and down shock does the most accurate work. I have seen the F1 car with the shocks laying flat on there sides. I figure the the oil control discs in them are different for this control or a remote res. is there to help. Are your cars setup soft? light springs with good adjustments on the sway bars? Wieght is balanced, with some brake bias. You want the tire to stay flat on the track for the most contact patch. You want the rear of the car to squat when accerating and put more pressure on the rear tires so they don't spin or loose grip. A heavy spring may cause the tire to load and unload, you loose traction and maybe the car spins out. Rick Lake

[Richard Hudgins]

Rick,

You pretty much answered everything yourself.

It seems that you have a good bit of knowledge about chassis dynamics.

Now, for my little bit of opinion.

(And it is opinion only, as I am sure that there are many folks on this forum way more knowledgeable than I and more current in the art as well.)

I like low wheel/spring rates.

I like high motion ratios. The dampers need to traverse as much as possible to work well.

I dislike angle in the spring/damper unit as this reduces motion. (See coilover angle)

The valveing design in F1 dampers (Or any damper that lays down) is the same as a upright damper. You will notice that all of these dampers have a remote reservoir. This is for rebound control and for gas volume. They run some serious pressures in those damping systems. Well over 200 bar.

The above answer is a bit simplistic, but dampers are very complex and they hold the answers to making a chassis work.

No matter if the chassis is as stiff as the proverbial wedding cock or as limp as a noodle, proper dampers can make virtually any chassis work well.

Even a "REAL" Cobra can be made to work reasonably well on a road course.

[Rick Parker]

Repeat: Out of my element, Out of my element:

I called the shock manufacturer (Aldan) today while I was looking at their website and thumbing through an old catalog of theirs. It became appearant that the shock I had was not in the catalog. I asked if it had been discontinued, but the response was that it had not, but was not a very common item (13" fully extended, eye to eye with 3" of travel) actually quite short. I purchased these several years ago but they are still in quite good shape.
Today I recalled that when I first got them they came with 300# springs and I felt they were too harsh at the time. So I had removed the springs (?) or sent the both of the shocks back to Aldan who at that time exchanged them for the 240 (or 250??) that I actually now have, but acording to the fellow I spoke with today they should only have an 8" or possible a later designed 9" spring not the 10" that I actually have. Somehow the 10" springs were sent, or installed and I have had them that way since day 1. I didn't realize until today the incorrect springs were on the shocks. This development will probably not solve my bottoming out issue but sure helps me understand why the spring seats were difficult to remove as there was tension on them the entire time I was unscrewing them from the threaded body of the shock. We'll see.............Out of my element.........
Stay tuned...................

Rick