Spring Rates

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Geoff
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Spring Rates

Postby Geoff » Sun Mar 11, 2007 12:35 am

So here is that spring rate spreadhseet:
Image

The first five kinds on top are ones that I measured. Notice how the calculated front spring rate for the SPG springs is within 15% of what Luke measured (310 lb/in). Whats interesting is that Luke's measurements are slightly different and provided drastically different calculated rates. Take that into consideration when looking at the spring rates in the bottom part of the chart. These were taken by someone else and I think they are a compilation from other sources. The wire diameter is only in hundreths, not thousandths, the number of coils is more estimated (6.5 vs 6.6 for the SPG springs on the bottom vs. the ones on the top of the spreadsheet) and look at how different the results are between the SPG springs on top vs. the ones on the bottom. It shows that measurments need to be taken very carefully. Also the data below didn't tell the condition of the spring ends, which is VERY important because it determines the number of active coils. I modified the original spreadsheet I had received with the data in the lower half of the spreadsheet becuase the number of active coils were purely guessed at. I assumed that all springs had squared ends on both sides, which may not be the case, so the spring rates may be totally off from actual...

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Postby Jordan » Sun Mar 11, 2007 5:44 pm

What is the difference between plain and squared ends?

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Postby Luke » Sun Mar 11, 2007 5:51 pm

Geoff, i think you have the mean diameter wrong for the SPG springs 5.513 should be the OD for the front and 4.5 the OD for the rear, not the mean. I think thats one reason we are coming up with different rates.

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Postby Hans » Sun Mar 11, 2007 6:43 pm

The data for stock springs differs from my Saab manual. I'll scan the pages and get them to ya...

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Postby Geoff » Mon Mar 12, 2007 9:57 am

Hmm.. I may have entered that coil diameter wrong, not sure how I did that. I should re-measure the springs sometime

Assuming I have the OD in there for those springs instead of the mean diameter the spring rates for SPG springs would be
Front: 388 lb/in
Rear: 524 lb/in
with the diameter adjusted

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Postby Geoff » Mon Mar 12, 2007 9:59 am

jordan wrote:What is the difference between plain and squared ends?


Plain ends are ones that continue at the same pitch as the coil ends. Squared or "closed" ends are ends where the coil pitch decreases to 0 at the end of the spring.
Pictures here:
http://www.engrasp.com/doc/etb/mod/stat1/spring/spring_help.html
The end conditions of the spring effect the spring rate

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Postby Luke » Wed Mar 21, 2007 7:48 am

I had to pick a few things up at the race shop last night so I brought a rear spring to measure. It was a stock 900 turbo rear spring. I measured the average rate over its travel at about 390 lbs. I say average because I saw quite a bit of change of rate throughout the travel, it started out slightly stiffer at around 440 lbs/in. I measured the rear spring perches and it appears that standard 3"ID racing spring should just squeeze in there so if we want to play around with rates, that might be an option.

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Postby Geoff » Wed Mar 21, 2007 10:31 am

interesting as the calculated value is 398 lb/in. That's only a 2% difference.

Somewhere I read that when you measure the rate it should be in the middle of the deflection range. I think they said to measure the average between 30% and 70% of travel.

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Postby Luke » Tue Apr 03, 2007 8:55 am

Ok so I did get around to installing new rear shocks and springs before RNY and I can say that i am very happy with the results. I took out the rear SPG's and put stock turbo springs in. I also made some adapters out of stainless on the lathe to mount 7" travel bilstien race shocks to the rear. They are valved 120/220 and combined the ~400 lb turbo springs the car feel superb on gravel, much less bouncy than before. It just floats over the rough stuff.

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Image

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Postby Geoff » Tue Apr 03, 2007 12:24 pm

neat! How did you adapt the bottoms? You might want to be cautious with the adapters made from stainless as its much softer than the hardened steel that shock tops are made from. They could break or bend much more easily.

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Postby Jordan » Tue Apr 03, 2007 12:28 pm

So is that the second set that you ordered?

Speaking of springs...you guys mentioned I should just cut my rear springs.. should I cut the 99 ones or find some 900 turbo springs? Any big difference in spring rates?

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Postby Luke » Tue Apr 03, 2007 12:54 pm

The bottoms pretty much bolt through the normal shock mount but with some spacers on both sides of the rod end. I don't think the stainless will be a problem. Most of the force that it sees is in tension on extension of shock, unless the axle get ripped sideways or something. The bushing sits on a thick washer that rests on the shoulder of the adapter.

Cut some rear turbo springs and you have a rate similar to SPG's but with a slightly lower ride height.

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Postby Jordan » Tue Apr 03, 2007 1:26 pm

Ok , next time I'm down there I need to go junk-yarding to grab some springs and look for a 91+ trans. Heck maybe I'll just drive the 99 down.

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Postby Hans » Thu Apr 12, 2007 1:16 pm

Here's a pdf with the spring rates for c900s, from Saab's tech specs manual:

http://www.bluesedan.com/c900springs.pdf

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Re: Spring Rates

Postby Crazyswede » Tue Nov 11, 2014 2:48 pm

http://www.hypercoils.com/spring-calculator

Using the above calculator with a guessed corner weight of 600# for the rear I came of with 300lb/in rate spring. Looking at the above table I can only assume this is a min requirement....was based on 18" from rear pivot to center of axle and 10.5" from pivot to center of spring with 3" of shock ride height. I am going to order some 3" ID springs on Thursday and will probably take a guess at 550# rate to start with.
I am the 73%


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