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Old March 18th, 2014, 09:24 PM   #1
stuftmunky2k
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Default spring rates for coilovers

whats the best way to figure out spring rates for 2'' dual rate coilovers without have the exact weight of the rig. tube buggy with yota drivetrain on 38's the front height is about where i want it but is way stiff when flexed. the rear needs to come down about 2''s but flexes pretty good. i know the rear springs are 150 over 200. im just so damn confused on how to figure out what i need to change. the springs are 14'' over 16'' when weight of rig is down the front upper spring compresses to 10 1/4'' over all length of upper spring with 8 1/2'' of shaft showing the rear is 150 14'' spring over 200 16'' spring. rear spring compresses to 11 3/4'' over all length of upper spring with 9 3/4'' of shaft showing. any help would be much appreciated the front springs i cant find any marks on the springs to tell what they are
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Old March 18th, 2014, 11:29 PM   #2
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The springs rates are measured in lb/in. So a 150lb spring will compress 1" when 150lb are placed on it, 2" when 300lb are placed on it, etc. Based on the amount of spring compression you should be able to figure out an approximate sprung weight. Obviously it's a combination of the compression of both springs on each corner. Take that info and use it in the spring rate calculators on the FOA or SwayAWay websites. The FOA one is easier, SwayAWay gives a lot more detail.

For the front, if you know you want it softer you could step down 1 or both spring rates and use the adjustment collar to set the ride height back where you want it. If the ride in normal driving is good, but it only gets too stiff when under severe flex, then I suspect your combined spring rate is fine, but when the slider hits the stop collar and transitions to the secondary rate(step-up rate), it becomes too stiff. So you may just want a softer lower spring. Have you tried adjusting the stop collar so it transitions to the secondary rate later in the travel?

For the rear if it needs to come down 2" but the adjustment collar is already backed all the way off, then a shorter coil is an option. The thing to be careful about there is that you still have pressure on both spring seats with the shock at full extension. If not, you could add a triple rate kit.....that's pretty much all those are used for....but it takes up additional height as well so you may need to account for that with spring lengths too.
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Old March 19th, 2014, 10:21 AM   #3
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Makes little more sence. Are you saying there should be some preload on the springs? As its now the front adjuster is turned down roughly 3" and the rear is backed all the way off. Should i back them all the way off then measure from there see where it has to come down? Thanks for the help
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Old March 19th, 2014, 01:41 PM   #4
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The adjusters are normally used to set right heigh via spring preload, but the secondary thing you want to watch for is that at full extension, the springs are still making contact with the upper and lower spring seats and slider. It doesn't have to be any huge amount of pressure, but you don't want the springs coming out of their seats...this can cause anything from springs gouging the shock body, breaking the slider, depending on the shock the lower spring pad could even fall out entirely. Usually people run 2 springs the same length as the shock's travel. 14" travel gets 2 14" springs, 16" travel gets 2 16" springs, etc.
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Old March 19th, 2014, 01:52 PM   #5
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Alrite that makes sence. Im try and do the math again when i get home see what i can come up with.
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Old March 19th, 2014, 02:23 PM   #6
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As bbaXJ mentioned, you should be able to find the sprung corner weight based on how much your springs are compressed. One way is to use the online calcs.

I find it easiest to put a single spring on each coilover at each corner with no preload (at full shock extension with the top adjuster just touching the spring). Then install shock and put the weight on the rig and see how much each spring compresses. For instance, if you just install your 200# bottom spring and it compresses the spring 4" then you know the sprung weight of that corner is 800#.

From there you can do some figuring to pick a spring rate.

For instance, let's say you've 800# on the corner with a 14" coilover and you want 5" uptravel and 2" spring pre-load, that means you need to compress 11" with 800#. (shock travel - remaining uptravel + preload = 14-5+2 = 11")

Take the 800# and divide by 11 and get 72#/inch spring rate. Then you gotta run through the spring rate calc. What I do is pick a rate for the bottom, and then play with the top rate to see if I can get it close to where I want it.

I can never remember the formula, but I do remember that in spring terms x/x = x * 1/2. (the full formula is (upper*lower)/(upper+lower))

To start, 150/150 is 75# combine spring weight, which gets us close to target of 72#. Adjust the upper down 25# to 125/150 and the combined rate is 68, kinda low. Bump the lower up by 25# and we have 125/175 and we are at 73#.

Or if you want to keep the 200# lower, you could move to 125# upper for 77# and just back off the adjuster some.

Our experience to to set the spring dual rate adjuster to hit 2-3" before bump to help prevent from blowing through to the bumps.

One of the better discussions on spring choice. Wayne (ZukIzzy) is one of the top shock tuners in ultra4 and is now the official tuner at Fox shocks.

http://www.pirate4x4.com/forum/gener...ch-thread.html
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Last edited by JohnnyJ; March 19th, 2014 at 05:10 PM. Reason: small edits for clarity
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Old March 19th, 2014, 03:26 PM   #7
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I agreee with everything you said, except in the last sentence of the second paragraph it should be "sprung weight", not "unsprung weight". To clarify for the OP, sprung weight is what's being held up by the springs. Unsprung weight is what's underneath them(axles, wheels, tires, tie rod, and in most cases, half the weight of the control arms, shocks, draglink, driveshafts, etc).
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Old March 19th, 2014, 03:51 PM   #8
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14" travel shocks or 16" travel shocks or X" travel shocks?

Assuming vertical shocks, you have a 338lb/corner rear weight to compress a 150/200 combo upper 150lb 14" spring 2.25" to your measured 11.75".


If you really want to know you're going to have to take the rear springs where you do know the rates and put them on the front to get a weight/corner from the compressed values. Using 1 spring makes it even easier. Using your 16" 200lb spring from the rear, you measure and subtract the compressed height from 16" and multiply by 200lbs to get your front corner weight. Once you know your front corner rate, you can use the free vs compressed measurements and that front corner weight to figure out the rate of your mystery springs. Once you know all your rates, then you can look at what to swap where to get your desired ride height with preload....ect. It sounds hard but really isn't that difficult.
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Old March 19th, 2014, 04:36 PM   #9
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Quote:
Originally Posted by bbaXJ View Post
I agreee with everything you said, except in the last sentence of the second paragraph it should be "sprung weight", not "unsprung weight". To clarify for the OP, sprung weight is what's being held up by the springs. Unsprung weight is what's underneath them(axles, wheels, tires, tie rod, and in most cases, half the weight of the control arms, shocks, draglink, driveshafts, etc).
Fixed it. Sorry, should have reread one more time before hitting the post button.
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Old March 19th, 2014, 05:35 PM   #10
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so found out the front springs are 175 over 250 the rears are 150 over 200 so if i did my math right the front weighs roughly 625 at the corner. the rear is at roughly 375ibs per corner used the calculator came up with 131 over 198 for the front. so imma switch the rear springs i have to the front. now the rear came out to 78 over 118. so to get the rear close came up with 90 over 125 then should be able to get ride height close on the adjusters. make sence to anyone else? was just thinking way to much into it
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