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Estimate springs for use on coil-overs

mrblaine

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This is a way, it is not "the" only way.
You need to know the sprung corner weight (CW). I'll use our current coil over (C/O) project as the example since it is fresh.
We bought springs in 200 lb/in x 14" long. We installed them on the shocks with a spacer to hold the rig at our design ride height (DRH) After they were installed, we checked several things to ensure we are getting a reasonably accurate amount of compression on the springs. Shake the rig by rocking side to side with the front tires chocked to help remove any suspension bind and try to get the rig to settle to what it is designed at. Trans in neutral so the rear tires can roll to relieve bind, cargo that is going to be carried in the rig where it goes. Or basically "weigh" the rig how it will be driven most of the time.
That gave us 4.125" of compression in the front, 3.0" of compression in the rear. The math is basic, compression x rate = sprung weight.
Front = 825
Rear = 600

The sprung weight is used with the compression of the shock length from fully extended to get your target spring rate. In our case we are using 14" travel C/Os so here is how that breaks down.
Front DRH is 6" of shaft showing so 8" of shaft is in the body. How far the shaft moved into the body is how far the spring stack compressed which is why we use that number.
Our tuners like 1" of preload (PL) in the front and 2" in the rear. A discussion with your tuner should be had to get what they like. Preload is the amount you compress the spring stack from fully extended when installed on the shock.
That puts us at 9" or 8" + 1" of PL = Total.
The math is dividing the CW by the amount the shaft moved into the shock + PL at DRH.
825 divided by 9 = 91.6 lbs Target Rate.
Once you have the TR, then the guessing game starts. Our tuner doesn't want the upper and lower spring to be more than 100 lbs apart in rate. It is easy to see what springs are out there by going to some place like Poly Performance and Kartek or even Eibach to check the offerings.

The math for 2 springs on a C/O to get the combined rate is product over the sum. Or, multiply the two rates and then divide that by them added together.
After that, then you start with a few selections to see which way it needs to go to get you the rate you are after.

Target Rate = 91.6

200/250 or 50,000 divided by 450 =111.1 which is higher than our TR. We'll start dropping them down with numbers based on what we are able to get in a spring.
175/250 or 43750 divided by 425 = 102.9 so still a bit high.
150/250 or 37,500 divided by 400 = 93.75 which is very close to our target spring rate. It needs to be understood that the gas charge in the reservoir will add some small amount of lift to the spring stack so we shoot for a target rate slightly under what the math gives us for the original target rate.
150/225 or 33,750 divided by 375 = 90 which would work for us but I'd like just a bit less plus I couldn't readily find a 225 lbs/in x 16" lower spring.
150/200 or 30,000 divided by 350 = 85.7 so that's very close to what I think we can use and in fact what we did use.

The above is just to show that you can move the numbers in and out of the formula based on commonly available springs to understand the process easier which is why I called it "guessing" because you are plugging in numbers to see how that moves the rates around to get to what you are after.

Of note, a 14" C/O typically runs a 16" lower spring and a 14" upper. 12" runs a 14" lower and a 12" upper. Our experience is with Fox and King shocks and this works on those. I don't know if others are the same, they should be but you should verify.

To get your preload correct, once the spring stack is installed on the shock, hold it vertical and screw the lower nut down until it just touches the top spring, then screw the lock nut down on that until it touches. Then hold the lock nut and start screwing the main nut down to compress the stack, measure between the two until you reach your preload then screw the lock nut down and lock it in.

The above is pretty accurate. When we got the stacks on the shocks, suspension settled a bit, the rear came in at 7.25" of shaft showing at ride height with a target of 7", the front came in at 6.125 with a target of 6.0. We'll start driving it and get the shocks loosened up, get everything moving and used to moving to settle in and then we'll dial it back in to hit our target numbers. I showed how to arrive at a target rate for the front, the process is the same for the rear.

Disclaimer- I need to go back over this more intently to check for errors, if you see any, point them out.
 

AMS417

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Good write up. Part of the reason I’m trying ORI. I’ve watched buddies struggle with tuning coil overs. I might struggle as well, but I figure I can tune a lot easier with gas pressure. Will see if it pays off.
 
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mrblaine

mrblaine

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Good write up. Part of the reason I’m trying ORI. I’ve watched buddies struggle with tuning coil overs. I might struggle as well, but I figure I can tune a lot easier with gas pressure. Will see if it pays off.

It shouldn't be a struggle. The struggle only happens when they make the same mistake the rest of the suspension flat earthers make and try to tune ride quality with springs which winds up violating every rule about how springs actually work that exists. There is no mystery to coil overs. They are a tunable shock, they hold the rig up at the DRH with springs.

You want the springs to not fall out of the bucket at full extension. You need them to hold the rig at the design ride height. In our case you want some determined amount of preload and we want the rates in a stack within 100 lbs of each other. Very easy, very basic and with a tiny bit of guidance, not even remotely mysterious.
 

gasiorv

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Question - If you have spacers holding the rig at DRH during your initial setup with the 200 lb/in springs. Would the spacers not carry some of the load once it compresses the springs and makes contact?
 
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mrblaine

mrblaine

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Question - If you have spacers holding the rig at DRH during your initial setup with the 200 lb/in springs. Would the spacers not carry some of the load once it compresses the springs and makes contact?

They are the same as a spring cup just moved up or down. We could crank the top adjusters down if we had enough thread on the body and felt like spinning them to get them all the way down. We don't so a spacer makes up the difference. We do that to keep an approximation of the weight of the links, driveshafts and what not at roughly the same angle so their portion of the sprung weight is transferred to the chassis.

Given that the method shown above nailed the target ride height within fractions of where we wanted, then that shows why the spacer is just that, a spacer.

The reason that we don't use two springs and deal with the combined rate is that increases the chances that we might wind up with a spring that is slightly out of spec on its weight rating.
 
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gasiorv

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Thanks. I was incorrectly assuming that the spacers were a hard stop installed between the Chassis and the axle to hold it at that height. If I am now understanding correctly, you are only installing one (1) spring per corner and the spacer is making up for the distance of the missing upper spring, this way the entire weight is going through one spring per corner.
 
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mrblaine

mrblaine

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Thanks. I was incorrectly assuming that the spacers were a hard stop installed between the Chassis and the axle to hold it at that height. If I am now understanding correctly, you are only installing one (1) spring per corner and the spacer is making up for the distance of the missing upper spring, this way the entire weight is going through one spring per corner.

Correct.
 
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AMS417

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It shouldn't be a struggle. The struggle only happens when they make the same mistake the rest of the suspension flat earthers make and try to tune ride quality with springs which winds up violating every rule about how springs actually work that exists. There is no mystery to coil overs. They are a tunable shock, they hold the rig up at the DRH with springs.

You want the springs to not fall out of the bucket at full extension. You need them to hold the rig at the design ride height. In our case you want some determined amount of preload and we want the rates in a stack within 100 lbs of each other. Very easy, very basic and with a tiny bit of guidance, not even remotely mysterious.

I think that is the issue here. Most guys are throwing springs at it until its close. Not accounting for gas charge etc. Part of the reason I went ORI was ease of tuning, part was packaging. Still plan to incorporate a rear sway bar. Existing anti rock if I can make it work with the stretch, or a TK.
 
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mrblaine

mrblaine

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I think that is the issue here. Most guys are throwing springs at it until its close. Not accounting for gas charge etc. Part of the reason I went ORI was ease of tuning, part was packaging. Still plan to incorporate a rear sway bar. Existing anti rock if I can make it work with the stretch, or a TK.

All that does for you is put a big pile of springs over in the corner you don't really need to own. The math is simple and basic. Folks just don't want to do it. I know they don't because I see post after post from folks asking what everyone else's rates are and not once do they ask how to figure it out or discuss the shock tune. Same shit as always.
 
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mrblaine

mrblaine

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Thanks. I was incorrectly assuming that the spacers were a hard stop installed between the Chassis and the axle to hold it at that height. If I am now understanding correctly, you are only installing one (1) spring per corner and the spacer is making up for the distance of the missing upper spring, this way the entire weight is going through one spring per corner.

To clarify, if you just ran one spring without the spacer, or tried to run a very high rate spring to keep the rig higher, then the softer spring would let the rig sit very low and that may skew the sprung weight readings. The high rate spring has to be measured very accurately and the first few fractions of an inch of compression are not as accurate as a few inches.
 

Apparition

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To clarify, if you just ran one spring without the spacer, or tried to run a very high rate spring to keep the rig higher, then the softer spring would let the rig sit very low and that may skew the sprung weight readings. The high rate spring has to be measured very accurately and the first few fractions of an inch of compression are not as accurate as a few inches.

What are you using for a spacer?

Are there rules of thumb for the length of that spacer?
 

psrivats

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@mrblaine this is very clear. The formula for combined spring rate is just like capacitors connected in series, and if you think about it, that's exactly how the two CO springs connected in series in the system function in terms of how they support the weight individually if the rates are different. The differential equations governing the systems have the same form, and the math works out exactly the same. There is a broader methodology that is used for both understanding and problem solving using mechanical / electrical analogues.

https://en.wikipedia.org/wiki/Impedance_analogy

The methodology you described is exactly what I would use if I were designing a circuit. I entirely understand the tuner wanting top and bottom spring rates not to deviate from each other too much. However the preload requirements are still somewhat a mystery to me. I guess this more a question for your tuner, but if you know .. I would like to understand. Why the 1" vs 2" difference between front and rear? To account for the weight differences? What happens when you use say 0.75" and 1.5", or the other side 1.5" and 3"? It's seems that this will have an effect on the eventual shock tune, correct?
 
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mrblaine

mrblaine

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mrblaine

mrblaine

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However the preload requirements are still somewhat a mystery to me. I guess this more a question for your tuner, but if you know .. I would like to understand. Why the 1" vs 2" difference between front and rear? To account for the weight differences? What happens when you use say 0.75" and 1.5", or the other side 1.5" and 3"? It's seems that this will have an effect on the eventual shock tune, correct?
No idea why they request that amount which is why I suggested a conversation with your tuner if you want to start down this road.

We wind up with small variances in preload dialing in the ride height after the springs settle in, the shocks get broken in, etc. We don't retune the shock for that plus with the DSC reservoirs we use, any minor tuning issues that would arise are quickly dialed back out.

Bear in mind this is all for the recreational side of things and if one wants to deep dive into it for the race side, that will all have to be done with help from your tuner. This is just A way to get the pile of springs over in the corner to be much smaller. It isn't wholly exact, but very close to being able to get the average person very much in the ballpark with the least amount of effort.
 
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NashvilleTJ

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No idea why they request that amount which is why I suggested a conversation with your tuner if you want to start down this road.

We wind up with small variances in preload dialing in the ride height after the springs settle in, the shocks get broken, etc. We don't retune the shock for that plus with the DSC reservoirs we use, any minor tuning issues that would arise are quickly dialed back out.

Bear in mind this is all for the recreational side of things and if one wants to deep dive into it for the race side, that will all have to be done with help from your tuner. This is just A way to get the pile of springs over in the corner to be much smaller. It isn't wholly exact, but very close to being able to get the average person very much in the ballpark with the least amount of effort.

As a paid member of "the pile of springs over in the corner" crowd, thanks for taking the time to write this up, Blaine. Great information.
 
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mrblaine

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Mr. Blaine:

I left you a voicemail suggesting an alternative method (calibrated springs) a few weeks back when you were trying to put together load cells to measure your sprung corner weights. There's a lot more to that, which is why I called and left a message. It's very difficult to manufacture coil springs to tight tolerances on the spring rate. I learned this in my AMA Superbike days. I was responsible for spring design for the vehicle, and I worked with Renton Coil Spring (a Boeing supplier in the Seattle area) to make the springs I designed out of 6-4 titanium to save weight.

We ended up measuring the actual spring rate on every spring, and the actual rates were significantly different. We kept a range of spring rates in order to tune the chassis for the rider and for the race track conditions. I don't remember the actual tolerances we observed on the spring rates, but they were so far off that they didn't fit into the range they were supposed to (for instance a spring made for the 350 lb/in range may have actually measured to 300 lb/in, and a 300 lb/in spring measured 350 lb/in). We measured and labelled each spring.

We discovered this when trying to do what you're currently trying to do for TJ. We were calculating installed height for a spring of a different rate based on the measured installed height of another spring with a different rate, assuming that the rates were correct. We never got it right, so we measured the spring rates and figured out the problem. When measuring the rates, we also discovered that they are not perfectly linear, either. This variance was not a bad as the variance in average spring rates (measured from 25% compressed to 75% compressed), but it did affect things.

I appreciate your efforts to further the TJ knowledge base by sharing your "experiments" with us all - so, thanks again!

My reason for the weight gauge was not for accuracy, it was for convenience. The fact that I did this by compressing the springs, measuring that compression and then using that for a calculated weight to determine my spring rate which turned out to be within fractions of where I wanted it tells us this method is accurate enough for what we do.
 

jfetters

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Mrblaine, Thank you for taking the time to post this info; I'll be using it to sort out the springs on my LJ . So far this info is the most straight forward and succinct I have come across, and I appreciate seeing/learning how this is figured out. The tips and insight you provide, like accounting for the small amount of lift from the reservoir, are invaluable.

Thanks again for this and all the information you share with this forum.
 

B00mb00m

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Great info thanks! I need to re-set my ride height after spare tire mount was added.