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Old 06-01-2010, 02:38 PM   #1
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Default Coil Spring Free Length

I am going to buy some 700# front and 450# rear springs for my 95 miata and some coilover sleeves. Does anyone know what free length springs I would need? Thanks!
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Old 06-01-2010, 02:47 PM   #2
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with those rates, depending on the shock either 6"/7" or 5"/6"
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Old 06-01-2010, 03:06 PM   #3
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is there a spring calculator that will help determine the free spring length needed to adjust ride height. iexample: i want a one inch change in ride height. this isnt going to equate to a spring with a one inch longer free length. spring rate will determine a comression factor. sorry about no caps to start sentence, mobile phone makes it a pain.
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Old 06-01-2010, 03:26 PM   #4
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you can use Shaikh's spreadsheet.

But there is roughly 560lbs on each front corner and 530 lbs on the rear corners.

a 700 lb spring will compress only about .8" inches in the front. A 450 lb spring will compress around 1.2". Less if the car is acutally lightened.


I tried 7" 550s in the front and i could barely lower it enough...so i'm assuming 700s need to be a little shorter to have some up/down wiggle room to adjust.
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Old 06-01-2010, 04:42 PM   #5
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Quote:
Originally Posted by Braineack View Post
you can use Shaikh's spreadsheet.

But there is roughly 560lbs on each front corner and 530 lbs on the rear corners.

a 700 lb spring will compress only about .8" inches in the front. A 450 lb spring will compress around 1.2". Less if the car is acutally lightened.


I tried 7" 550s in the front and i could barely lower it enough...so i'm assuming 700s need to be a little shorter to have some up/down wiggle room to adjust.
The load the spring sees will be increased by the motion ratio. For example, if the motion ratio is 1.6, and the corner weight is 560 lbs, a 700 lb/inch spring will deflect:

(560 lbs x 1.6) / 700 lbs/in = 1.28".

Defection at the wheel (from unloaded) for that 700 lb spring will be:

1.28" x 1.6 = 2.05", more or less.
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Old 06-01-2010, 10:29 PM   #6
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Where does a person find the motion ratio? I'm guessing this is probably a manufacturer spec.
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Old 06-01-2010, 10:50 PM   #7
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Shaikh, who has measured it, would tell you it's pretty close to 1.52 for the front, 1.36 for the rear.
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Old 06-02-2010, 12:26 AM   #8
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The quick way is to measure the total length of the lower arm, then divide the length of the shock to the inner bolts of the lower arm by it. You then have to figure out the degree the spring is laying, and add that into the equation....... Or just use the figures Shaikh has already blessed us with.
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Old 06-02-2010, 08:30 AM   #9
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Thanks guys, very useful information. I looked at Shaiks spreadsheet and it is very complete and has tons of info that I can use.
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Old 06-02-2010, 09:16 AM   #10
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Quote:
Originally Posted by Thucydides View Post
Shaikh, who has measured it, would tell you it's pretty close to 1.52 for the front, 1.36 for the rear.
According to his spreadsheets, NA is .66 front and .735 rear, coming out as a multiplier of 1.34 F and 1.265 R
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Old 06-02-2010, 11:22 AM   #11
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According to his spreadsheets, NA is .66 front and .735 rear, coming out as a multiplier of 1.34 F and 1.265 R
I don't think so, Brain.

The mechanical motion ratio is the ratio of the shock mount location to the overall length of the control arm, so whether you use it as a number greater or less than one, depending on which number you choose for the numerator/denominator, either number is the reciprocal of the other. The reciprocal of 0.66 is 1.52; the reciprocal of 0.735 is 1.36.

Shaikh reports both mechanical and bounce motion ratios; I wonder if this is where the difference is, not that I know the difference between the two. I haven't found the ratios you've mentioned; could you refer me to a cell location?

Cool cat fight, btw.

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Old 06-02-2010, 11:26 AM   #12
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look at you busting out words I fought so hard not to learn in high school. probably explains why the numbers never worked out quite right for me...

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Old 06-02-2010, 11:31 AM   #13
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So hypothetically if I wanted to lower my ride height by 1/2" all around, I would need to adjust my coilover perches downward by...
.5" / 1.34 = .37" front
.5" / 1.26 = .40" rear
That is a question. I'm not really going to lower it another 1/2" just yet but I want to make sure I'm understanding the concept correctly.
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Old 06-02-2010, 11:37 AM   #14
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Quote:
Originally Posted by Thucydides View Post
Shaikh reports both mechanical and bounce motion ratios; I wonder if this is where the difference is, not that I know the difference between the two. I haven't found the ratios you've mentioned; could you refer me to a cell location?

Between the tubular sway bar stuff in yellow and springs & sway data.


Spring MR (bounce) | Leverage ratios (mechanical motion ratio)
Front 0.72 | 0.66
Rear 0.88 | 0.735

Quote:
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So hypothetically if I wanted to lower my ride height by 1/2" all around, I would need to adjust my coilover perches downward by...
.5" / 1.34 = .37" front
.5" / 1.26 = .40" rear
That is a question. I'm not really going to lower it another 1/2" just yet but I want to make sure I'm understanding the concept correctly.
This is the way I understood, however, it's never really worked out well for me. and i always have to pretty much just go by guess-and-check method.
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Old 06-02-2010, 11:56 AM   #15
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look at you busting out words I fought so hard not to learn in high school. probably explains why the numbers never worked out quite right for me...

I called my son for those words. He's in colage.
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Old 06-02-2010, 11:56 AM   #16
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Quote:
Originally Posted by ScottFW View Post
So hypothetically if I wanted to lower my ride height by 1/2" all around, I would need to adjust my coilover perches downward by...
.5" / 1.34 = .37" front
.5" / 1.26 = .40" rear
That is a question. I'm not really going to lower it another 1/2" just yet but I want to make sure I'm understanding the concept correctly.
Correct. As with Brain, I've always had better luck with the guess and check. Don't forget to unhook the sway bar or you stand a chance of putting it into preload. I've done that a few times on accident
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Old 06-02-2010, 12:34 PM   #17
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Okay, glad I understand the theory. However, in practice I'll probably just drop all four coilovers approximately the same amount, then take the car to get corner balanced again and the individual corners' ride heights will end up wherever the scales tell them to be.
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Old 06-02-2010, 01:20 PM   #18
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It'd be nice if someone around here had scales. I'm thinking about buying a set then renting them out to get a few of the pennies back.
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Old 06-02-2010, 01:31 PM   #19
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Quote:
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Don't forget to unhook the sway bar or you stand a chance of putting it into preload. I've done that a few times on accident

gimperesting...please elaborate.
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Old 06-02-2010, 02:03 PM   #20
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When setting a car up on the scales, you might have one corner .25-.50 higher or lower then the other side. Unhooking the end links, set the one side up till the bar is parallel with the pavement (or close, this part has little science behind it), then adjust the other end link to where the bolt slips in and out without any force, which gives you zero preload.

I assume you know what preload is?

http://www.longacreracing.com/articles/art.asp?ARTID=30

This is a good read and it explains why I'm able to get away with my 400F/225R setup as a DD/AutoX car
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