spring rates from corner weights and motion ratios

ok...

My FF has a very old set of Sachs shocks with an unknown spring rate. I have taken a look and cannot easily determine what the existing spring rates are. There are no visible markings that I can find.

so, given the known corner weights and the known motion ratios, how can I choose a spring rate to establish a new baseline so I can start to tune the suspension?

What ever spring rates I have, are very soft and just not suitable for what I do (autocross) I am planning on purchasing a new set of shocks, but I need to calculate a baseline setup.

what is the accepted practice? From the searches I have done it appears to be a matter of personal preference, but I need a baseline before I can figure out my personal preference.

What kind of car do you have? Saying it's a formula ford doesn't say much... Determining the rate of a set of springs is easy. Put them on a scale, put a weight on them until they compress a set amount (half-inch at least, a full inch is better) and see what the weight is.

Different generations of car tend to like different corner spring rate to corner weight ratios. Note that I am referring to true installed spring rate, so you need to know the motion ratio's to determine the actual springs spring rate. Stiffer is usually never bad but it can induce some uncomfortable handling characteristics in older cars. Cars in the late 1960's would probably like corner rates around 35% to 50 % of corner weight, cars in the 1970's would start with 50% of corner weight, more modern cars went towards 100% of corner weight. Cars on treaded Dunlop type tires could be a little softer, cars on slicks could be a little stiffer. Stiffer cars can be harder to drive, softer cars will seem to have more progressive grip. Note that the original equipment springs for older cars tended to develop corner rates around 15% to 25% of corner weight - take a look at F1 pictures from the 1960's and you'll see high ride heights and tremendous wheel travel...

Brian

Brian,

I have a swift fb91, its one of the euroswift models. The car was designed to run on the Avon treaded tires which have a larger diameter than the slicks we run. I have very little setup information to go on, and I have so far "borrowed" setup knowledge from a swift DB1-DB3 to get started. What information I have seems to indicate that I have a typical UK factory shock setup for rain and the treaded Avons. I am neither driving in the rain, nor using the Avons. So, yeah, I need to figure this out.

Based upon what you have said, I think I might try for 70% of the corner weight, and work my way up from there. I need to work on the ride heights, and basically start from scratch (almost) and see what happens.

Did you look on the flat surface of the spring for number's? Sometimes they are engraved.

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Originally Posted by BURKY

Did you look on the flat surface of the spring for number's? Sometimes they are engraved.

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The flat surfaces have some surface corrosion which is making it difficult to read the spring rate.

I, had the same problem one time. cleaned the flat's, then took a crayon across the surface and it was exposed.

Get out a caliper or micrometer.

Spring rate in LBS/IN =

(11000000 * d^4) / ( 8 *N * D^3)

Where d is the diameter of the spring wire/rod

N is the number of active coils. (start counting where the spring is no longer in contact with the perch)

D is the mean diamer of the spring (outside Diameter minus wire diameter)

That will at least let you know what you have now, at least close enough.

For an estimate of the spring rate see "bluecoilspring.com/rate". This will get you in the ball park.

If you know the rate of the springs, the motion ratio of the spring to the wheel, and the weight on that wheel, you can calculate the ratio of weight to spring rate for that corner. The range will be some where between 1 and 2. Where you want to be is your decision. Best do some testing.

An other way to think about this ratio is that when you set the car on the ground, you will move the wheel between 1 and 1/2 inches between just touching the spring perches to fully loaded. One is kind of soft and 1/2 is stiff.

For example

Diameter of wire : .313"

Outside diameter of spring : 2"

Number of effective coils: 7 3/4

d=.313
D=1.687 (2-.313)
N=7.75

11,000,000 * .313^4 = 105577.2

8 * 7.75 * 1.687^3 = 297.7

105577.2 / 297.7 = 354.6# spring

Quote:

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Originally Posted by S Lathrop

For an estimate of the spring rate see "bluecoilspring.com/rate". This will get you in the ball park.

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Only difference between Steve's link and the formula I quoted was the Torsional Modulus of Elasticity for the steel. I used 11,000,000; Steve's link uses 11,250,000. I believe it varies between 10,000,000 and 12,000,000 depending upon the steel utilized.

Quote:

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Originally Posted by Daryl DeArman

For example

Diameter of wire : .313"

Outside diameter of spring : 2"

Number of effective coils: 7 3/4

d=.313
D=1.687 (2-.313)
N=7.75

11,000,000 * .313^4 = 105577.2

8 * 7.75 * 1.687^3 = 297.7

105577.2 / 297.7 = 354.6# spring

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Nice! that is just what I needed

Quote:

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Originally Posted by chrisw52

Based upon what you have said, I think I might try for 70% of the corner weight, and work my way up from there. I need to work on the ride heights, and basically start from scratch (almost) and see what happens.

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Chris,

From what I've picked up over the years from personal experience and from talking with the fast CM guys it appears that your 70% range is a very good starting point for the front and maybe somewhat lower for the rear. This assumes your shocks can handle those rates.

Dick
CM 85

Quote:

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Originally Posted by Dick R.

Chris,

From what I've picked up over the years from personal experience and from talking with the fast CM guys it appears that your 70% range is a very good starting point for the front and maybe somewhat lower for the rear. This assumes your shocks can handle those rates.

Dick
CM 85

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I don't care if the shocks can handle higher spring rates. I am bottoming out on course. On the course we ran this past sunday, we had a right hand sweeper that had a bump on corner entry at the end of the braking zone. As I hit that corner I bottomed out hard on the front end enough to lose traction on both fronts, causing me to lose a lot of time.

I can compensate any crappy shock handling with air pressures to some degree by mimicing Formula 1 and use the tire sidewall for a little extra compression dampening. At least that is the theory :)

One fairly legitimate way to arrive at spring rates is to keep the car off the ground when you start out at your proper ride height. Another good option is to run droop limited and put pre-load in the front. I can see that working in an auto-cross car that needs to transition very quickly. All part of the fun!

Brian

Chris
Before you start buying springs, call me. I have a box of springs you can borrow before you buy- 2.25" X 8" that I think will work for you.