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resr spring shim rate

4K views 20 replies 8 participants last post by  dlbehrns 
#1 ·
I know ive seen this infor before here but hoping someone can tell me before I spend the weekend looking for it. What is the shim ratio on the rear springs- or how much shim gives how much lift when adjusting ride height?

thanks
 
#3 ·
Well, it is approximately 1:1. It might be more like 1.1 to one - that is, adding a .50" shim would raise the car by .55". The spring on a 105 is mounted on the trailing arm close to the axle tube, but not right over the axle (as it is on a 750/101).

You might measure the length of the trailing arm, bushing to bushing, and measure how far the center of the spring is from the front bushing. The quotient of those two lengths would be the ratio.
 
#4 ·
Jay,
I believe you are correct. My spider has stacked Autodelta shims both front and rear, with an old set of Autodelta competition springs. What I did was essentially trial and error in back, after I got the front negative camber set as I wanted. My front A frames are only shimmed for alignment, and I have just enough front wheel arch clearance for the street Bridgestone Potenza, RE 950, P185/65 R 15's. These were close in rolling diameter to the Goodyear Bluestreaks used when it raced. Once I got the front set as close to -0- (-.1 degree), I checked my sill level, and shaved 1 mm from my thin rear shim. That did it. The car sits level, and my track set, front negative camber is largely gone, for street use. It was all trial and error, but fortunately not too bad. My Jack Beck built 2L LS rear axle (replacing the 5:13 GTA locked unit) runs .2 degrees negative camber on each side. Jack did a great job on this, some years ago.
I guess my formula was "by-guess-and-by gosh". Sorry I cant help more.
 
#6 ·
On Gordon's 101 series car, the rear should be exactly 1:1, because the spring sits on top of the axle tube. Without measuring, I would guess the 105 series cars are more like 1.2:1. For the fronts, the ratio should be pretty near 2.6-2.8:1. On the front, you can easily see the changes by putting spacers between the spring pan and the lower a-arm. A 3/8" spacer should give you about a 1" drop in ride height.

Erik
 
#8 · (Edited)
For the fronts, the ratio should be pretty near 2.6-2.8:1. On the front, you can easily see the changes by putting spacers between the spring pan and the lower a-arm. A 3/8" spacer should give you about a 1" drop in ride height.
Erik:

Since the spring intersects the lower suspension arms at an angle that is less than 90 degrees, shortening the spring by 3/8" does not have the same effect as adding 3/8" shims between the pan and the arms. Shims will have a greater effect than would spring shortening, for a given dimension of shortening/shimming.

For spring shortening, I have found the ratio to be close to 2:1. By "ratio", I mean the difference in ride height / change in the COMPRESSED length of the spring. Admittedly, if you measure the length of the arms, and calculated the ratio based on the location of the centerline of the spring perch you will get a number above 2:1. But, experimentally, it seems to work out to 2:1. My guess is that the difference is because a) the front spring is not normal to the pan, and 2) the spring's base is not attached to the pan with a joint that can rotate.
 
#10 ·
It can get even worse, guys. A friend is an engineer for a company that makes springs. (small ones!) With many designs, as coils are removed, the effective spring compression rate changes. This implies that any shortening of any type spring will alter is compression geometry as compared to the full coil version, or one of the same height OA, with a different compression rate.
I am NOT an engineer, but have studied this with both automotive valve springs, and springs built for our military in various weapon systems.
For example, consider shimming a valve spring, as compared to using either a stiffer and shorter spring, as compared to shortening a spring.
As I do considerable work with Alfa oil pumps, it's interesting to me to note the number of pumps from trashed engines that come in with shimmed relief valve springs. These springs are in constant motion (hopefully!) when the engine is running. They are designed to take a set, and shorten with use as they become tired. They require replacement rather than shimming, as the shimming alters the designed in compression rate. A shimmed spring, has a different compression rate than the same length spring without compression. A shortened spring of the same compression rate, is a different animal entirely. Try it with a valve spring at different heights of compression and you will see what I'm talking about here.
In Alfas, as far as I know, the suspension springs are designed to be shimmed for height and compression rate, not cut. Now, that said, this is JUST MY OPINION based on my experience. I am sure that a spring engineer can cut a spring and get a useful and proper result, if he has the facilities to test his result and knows exactly what he is trying to achieve.
 
#11 ·
Now, that said, this is JUST MY OPINION based on my experience. I am sure that a spring engineer can cut a spring and get a useful and proper result, if he has the facilities to test his result and knows exactly what he is trying to achieve.
Agreed, Gordon. As an aside, you can pretty accurately predict the affect of cutting coil springs using a spring rate calculator. There are a lot of them available on-line. They require wire diameter, overall length and number of coils as inputs. I've tested the formulas on springs of known rate and they work pretty well.

As for oil pumps, I'm like you Gordon. I don't understand why you would shim one. All it does increase COLD oil pressure. Once everything is warmed up, the relief valve should be closed all the time anyway. It won't raise your oil pressure when the engine is hot!

Erik
 
#12 ·
It's actually worse than that Erik. The pumps in good shape can provide necessary volume and thus pressure pretty well up into the 8,000 RPM range. At lower speeds where volume requirements are less (less effective area of spinning bearings) pressure may increase beyond that required. All this assumes a fresh pump and engine. In that case, the relief valve will be in some degree of constant motion relative to the port in the pump body. Notice how the erosion of the check valve itself over time is linear (up and down). This valve is considerably harder than the aluminum pump body. The aluminum does wear, but minimally, as compared to the oil wash erosion on the side of the valve itself by the port. It should be moving up and down some, as it constantly adjusts pressure relative to the engines volume demand. Pretty neat! Alter that spring though, and you are looking for trouble! As the pump and engine wear with use, the volume demand is higher and pressure lower. Eventually the pump cannot maintain pressure at high engine speed and the valve does remain closed. Time for a pump or engine rebuild, or both. Meanwhile, back on the farm, the shortening and weakening spring is trying to offer more warnings that all is not as it should be. I find all this quite interesting. Thanks for your comment Erik!
 
#13 · (Edited)
reviving an old thread: When I pulled the rear end apart for the rebuild there was one rear shim on the top of the driver's side rear spring that appeared to be made of some sort of plastic. It was broken in three places and, like an idiot I threw the pieces away. Now, that side is about 3/4" lower than the passenger side. I can't imagine a shim about 1/8" thick could cause 3/4" difference with all else being equal. With that said, anyone know where I can find one?
 
#14 ·
If I remember correctly the rear spring shim sits inside the spring cap or, between the cap and the top of the spring, no? If someone can tell me the outside diameter of a regular 105 rear spring and the inside diameter of the spring cap hole I could have some fabbed up. Anyone? Please and thank you. DB
 
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