Looking for coilover spring advice.

[kaptnick]

Finishing up linking the rear of my xj. Have 14" fox 2.0 coilovers and am trying to get to about 5" of uptravel.

Pretty sure my issue is with the springs I'm using. Unsure of the exact sprung weight of my rig, but its pretty much a stock body 2 door XJ with a full size 37" spare on the back. Figured 500-600 lbs per corner with fuel, tools, etc.. would get me close based on research I've done.

Was recommended to me that I start with a 14" 200lb over a 16" 250lb and that was way too stiff, sat at like 9-10" uptravel. changed to a 12" 150lb over the 14" 200lb and I'm still about 2" too high.
Ride height and uptravel would be perfect if it would come down another 2"

This is also with pretty much zero preload, assuming I'm setting it right, just barely turning the adjuster against the spring when it's fully drooped.

Not trying to spend more money guessing on coils based on this one persons advice who I've been talking to, so was hoping for some more input.

 

[Mac5005]

Can you measure the compressed length of the springs you currently have installed?

I need to know how far you are compressing the current springs installed and their specific rate and length.

This will tell me the actual sprung mass at the shock.

You then reverse the equation to determine the “correct” springs for your desired height at your specific spring mass.

It sounds like you have zero preload correct, the top but should barely touch the spring when the shock is fully extended.

Also make sure the dual rate stop nuts are not touching the dual rate slider when setting the springs up. This will throw off the calculation.

1-2” preload is more desirable than zero preload.

When you get those measurements and spring rate and lengths, I’ll post all the formulas to illustrate the math involved for clarity and understanding.

 

[kyleg336]

Here is a good video that explains how to get corner weights. The only thing I will say is make sure you have the oil and nitrogen charge on them before you do this. If you do it like in the video it will give you a false reading.



I have 14" coilovers now and I like to stay with a 16" spring on the bottom and a 14" on top. If you do a 12" spring you need to add a triple rate spring on top of it. But everybody has a different way of doing them.

 

[kaptnick]

Can you measure the compressed length of the springs you currently have installed?

I need to know how far you are compressing the current springs installed and their specific rate and length.

This will tell me the actual sprung mass at the shock.

You then reverse the equation to determine the “correct” springs for your desired height at your specific spring mass.

It sounds like you have zero preload correct, the top but should barely touch the spring when the shock is fully extended.

Also make sure the dual rate stop nuts are not touching the dual rate slider when setting the springs up. This will throw off the calculation.

1-2” preload is more desirable than zero preload.

When you get those measurements and spring rate and lengths, I’ll post all the formulas to illustrate the math involved for clarity and understanding.

Thanks.

The 12" 150lb on top is currently compressed to 8.25"

The 14" 200lb is compressed to 11.125"

adjusters are not touching the slider

 

[kaptnick]

Here is a good video that explains how to get corner weights. The only thing I will say is make sure you have the oil and nitrogen charge on them before you do this. If you do it like in the video it will give you a false reading.



I have 14" coilovers now and I like to stay with a 16" spring on the bottom and a 14" on top. If you do a 12" spring you need to add a triple rate spring on top of it. But everybody has a different way of doing them.

Watching the WOD video now, surprised I haven't seen their videos before, seems to have some of the best tech I've seen

 

[kaptnick]

Assuming the math for this still works with two springs installed at once, it appears I have a corner weight of 562.5-575lbs depending on which spring I use for the formula. Using the WOD stacked chart it looks like I currently have way too light of a spring installed.

I'm guessing I should redo this with just one spring installed unless there's a formula to do it with both?

 

[kyleg336]

Like I was saying. Make sure you have the oil and nitrogen charge set it whatever PSI fox recommends before u put one spring on it to check your weight. I know in the video he didn't have anything in the shock. But it dose make a difference. believe me I know...

 

[Mac5005]

Thanks.

The 12" 150lb on top is currently compressed to 8.25"

The 14" 200lb is compressed to 11.125"

adjusters are not touching the slider

That would be roughly 570 lbs sprung mass at the shock.

And you want it to sit 2.0” lower than this position?



Do you want to set it up with zero preload?

I would suggest 1” preload minimum and possibly be at 1.5-2” when fully loaded ready to ride.

Ideally if you have a 14” fox coilover, you would use a 16” main spring and a 12 or 14” secondary spring. Some of the length springs also depends on the rate, but just trying to understand what you are shooting for.

 

[kaptnick]

That would be roughly 570 lbs sprung mass at the shock.

And you want it to sit 2.0” lower than this position?



Do you want to set it up with zero preload?

I would suggest 1” preload minimum and possibly be at 1.5-2” when fully loaded ready to ride.

Ideally if you have a 14” fox coilover, you would use a 16” main spring and a 12 or 14” secondary spring. Some of the length springs also depends on the rate, but just trying to understand what you are shooting for.

based on what a couple other rigs I've seen do I was originally planning on about 1" of preload and going from there depending on where it sat loaded down.

I've read a lot of conflicting talk on spring lengths, but I originally planned on doing a 14" over a 16"

And yes I am wanting to sit about 2" lower than I currently am with the 12" 150lb over the 14" 200lb

 

[Mac5005]

based on what a couple other rigs I've seen do I was originally planning on about 1" of preload and going from there depending on where it sat loaded down.

I've read a lot of conflicting talk on spring lengths, but I originally planned on doing a 14" over a 16"

And yes I am wanting to sit about 2" lower than I currently am with the 12" 150lb over the 14" 200lb

I’ll respond in the am when I’m fresh and caffeinated

 

[Mac5005]

For 1” preload, 5” uptravel I suggest going with a 14” 110lbs spring over a 125lbs 16” primary spring. This would be a light spring for your application but about the lightest you can run without having severe spring bowing issues.

Next choice would be 100/150, a little heavier spring for a better step up rate but not far off you avg. the loss in preload is negligible once you load everything up.

Your combined rate needs to be around 57 lbs.

Pics below should help explain.

I don’t like going below 100lbs springs for bowing issues.

I like at least 50 lbs split between spring rates for a good step up when it hits the dual rate stop, to fight body roll and to help get back to ride height on repetitive bumps.

The rates listed in the chart at the bottom are compiled from commonly available rates.

Here is an excel file I use for most stuff. Yours I just did the math by hand.

Big thanks again to Scott @olejeeps for hosting it.

http://metalwerxdesign.com/files/springcalculatorv2.xlsx

 

[kaptnick]

For 1” preload, 5” uptravel I suggest going with a 14” 110lbs spring over a 125lbs 16” primary spring. This would be a light spring for your application but about the lightest you can run without having severe spring bowing issues.

Next choice would be 100/150, a little heavier spring for a better step up rate but not far off you avg. the loss in preload is negligible once you load everything up.

Your combined rate needs to be around 57 lbs.

Pics below should help explain.

I don’t like going below 100lbs springs for bowing issues.

I like at least 50 lbs split between spring rates for a good step up when it hits the dual rate stop, to fight body roll and to help get back to ride height on repetitive bumps.

The rates listed in the chart at the bottom are compiled from commonly available rates.

Here is an excel file I use for most stuff. Yours I just did the math by hand.

Big thanks again to Scott @olejeeps for hosting it.

http://metalwerxdesign.com/files/springcalculatorv2.xlsx

Thanks for all this and saving me from more guessing haha.

Will try the 110 over 125 first and see how it feels.

Also have a friend who has like 95lb springs in his setup and we see a lot of bowing issues with them, so I'll pass this on to him as well

 

[Mac5005]

Thanks for all this and saving me from more guessing haha.

Will try the 110 over 125 first and see how it feels.

Also have a friend who has like 95lb springs in his setup and we see a lot of bowing issues with them, so I'll pass this on to him as well

Stiffer rate or larger diameter helps with the Bowing issue.

I think it was all German Motorsports that makes adapters to put 3.0” springs on 2.0” shocks that also help with Bowing.

The best fix for this is getting the shocks onto the links in a trailing arm setup.

Lets you get up in rates where bowing isn’t an issue. Also helps a ton on valving as it takes out a bunch of harshness and gets you a better shim stack. That’s the biggest flaw with light weight, light sprung weight, and relatively heavy unsprung weight.

It’s a challenge to spring and valve sometimes.

 

[paulevans76]

This is likely ignorance talking, but why are coilover "combined" rates (i.e., x rate over x rate = z rate) typically so much lower than a single "stand alone" coil spring rate? I ran a 160 lb/in coil spring on the front of my rig and it was like a marshmallow, but when I went to a heavier rate, it felt much better. Then I see these 150 over 200 setups which make an "combined" 85 lb/in and I'm thinking my rig would lean and dive like crazy. Not to mention the frequency difference with the rear (leaves in my case). I know weight plays a factor and mine is probably close to 5k lb all dressed, but still, an XJ with a cage would be close. Am I just thinking of the combined rate in the wrong way?

 

[adamk]

Look at the photos that @Mac5005 posted above. The equation for determining spring rate of springs in series is there.

When comparing a single spring, like that of a stock cherokee, to a coilover, the coilover spring(s) is quite longer. A longer spring to keep the jeep sitting at the same height as a shorter spring means that the longer spring must have a lower spring rate (lbs per in).

 

[Mac5005]

This is likely ignorance talking, but why are coilover "combined" rates (i.e., x rate over x rate = z rate) typically so much lower than a single "stand alone" coil spring rate? I ran a 160 lb/in coil spring on the front of my rig and it was like a marshmallow, but when I went to a heavier rate, it felt much better. Then I see these 150 over 200 setups which make an "combined" 85 lb/in and I'm thinking my rig would lean and dive like crazy. Not to mention the frequency difference with the rear (leaves in my case). I know weight plays a factor and mine is probably close to 5k lb all dressed, but still, an XJ with a cage would be close. Am I just thinking of the combined rate in the wrong way?

You want preload to force the axle to droop as fast as possible, then use rebound dampening to slow it down.

It also comes down to the effective travel range you are using.

Too stiff and it’s difficult to get the force of a bump to compress the springs. Too soft and it won’t hold up the load correctly.

For the best ideal setup, You want the lightest springs possible to hold up the load.

You also want 1-3” of preload (depending on setup) on the springs so that you still have decent spring pressure at full droop.

The other helpful hint I’ve shared all ready is the step up in rate when the slider hit the dual rate stop nuts. This transfers from the combined rate to primary rate.

Useful for big g outs and controlling body roll. It is also for good push to get the axle back off the bump stops after a big hit to get to droop in preparation for the next event. Think whoops, or whoop de doos, or repetitive water bars.

Regardless of the advertised spring rate, you need the correct spring for your load and setup.

If you take any rig with the shocks mounted at 1:1 motion ratio, ie on the axle, you would theoretically need 2x the spring rate if you moved the shocks to a trailing arm setup with a 2:1 motion ratio.

Then anyone looking at the setup would say “my gosh, it has 250/500 springs, how can it possibly weigh 3500 lbs when my JK unlimited as 175/250 springs and weighs 5500 lbs”

My point to all that is don’t get wrapped up in the numbers bc all that matters is that you get the correct springs for your unique setup.

 

[Scooter402]

This is likely ignorance talking, but why are coilover "combined" rates (i.e., x rate over x rate = z rate) typically so much lower than a single "stand alone" coil spring rate? I ran a 160 lb/in coil spring on the front of my rig and it was like a marshmallow, but when I went to a heavier rate, it felt much better. Then I see these 150 over 200 setups which make an "combined" 85 lb/in and I'm thinking my rig would lean and dive like crazy. Not to mention the frequency difference with the rear (leaves in my case). I know weight plays a factor and mine is probably close to 5k lb all dressed, but still, an XJ with a cage would be close. Am I just thinking of the combined rate in the wrong way?

To answer your question of why the combined rates are lower...and expanding on the math from @Mac5005 … In short - science!

The long answer is as follows...

Think back to Hooke's law from Physics class: F=-kx where k is the spring rate and x is the displacement of the spring by the applied force F. Now with two springs connected in series, (as in coilover shocks) you have one "force" on each corner of the rig, applied by the weight of the vehicle. So both springs have the same force exerted on them. i.e. F=F1=F2 where F1 and F2 are the forces on each spring respectively. Applying this to Hooke's law:

F=-k1x1=-k2x2 (1)

We're looking for the force for the combined displacement of the springs, such as F=k_eq(x1+x2). (2) where k_eq is the combined spring rate.

Substituting equation (1) into equation (2), you'll see that -k2x2=-k_eq((k2/k1)x2 + x2)

Simplifying some... k2=k_eq((k2/k1)+1)

Solving that for k_eq (and skipping the boring algebra ) k_eq=(k1k2)/(k1+k2)

The derivation above is analogous to resistors in parallel, or capacitors in series for those electronics nuts out there. I think of it as capacitors in series, since both springs and capacitors store energy.


Phew, I feel better now that I let some nerd out today.

 

[paulevans76]

OK I digested a good bit of that. I get that for X distance (shock travel) you need Y rate to hold it at Z position. As well as trailing arm/cantilever ratios. But:

For the best ideal setup, You want the lightest springs possible to hold up the load.

I guess "the lightest possible spring" would in context of the performance you are trying to achieve, right? Like, it may hold up the load at 6" of uptravel, but that might be so soft that every corner you feel excessive body roll and every time you brake it's a nosedive. But the other things you mention - preload, bottoming on the dual rate stops and moving to the primary rate - that is the sort of stuff that will help with G's and roll and higher speed stuff. And I guess the link/suspension geometry plays a huge role as well.

To answer your question of why the combined rates are lower...and expanding on the math from @Mac5005 … In short - science!

The long answer is as follows...

Phew, I feel better now that I let some nerd out today.

Thanks - I understand the equation and how the combined rate "works", my head-scratch was more in regards to how guys are running setups with a combined rate of 85lb/in when my single coil setup with 160lb/in felt super squishy until I stepped up to 230lb/in springs - and it probably has more to do with COG, suspension design, etc

Good stuff guys, appreciate the lesson.

 

[Mac5005]

OK I digested a good bit of that. I get that for X distance (shock travel) you need Y rate to hold it at Z position. As well as trailing arm/cantilever ratios. But:



I guess "the lightest possible spring" would in context of the performance you are trying to achieve, right? Like, it may hold up the load at 6" of uptravel, but that might be so soft that every corner you feel excessive body roll and every time you brake it's a nosedive. But the other things you mention - preload, bottoming on the dual rate stops and moving to the primary rate - that is the sort of stuff that will help with G's and roll and higher speed stuff. And I guess the link/suspension geometry plays a huge role as well.



Thanks - I understand the equation and how the combined rate "works", my head-scratch was more in regards to how guys are running setups with a combined rate of 85lb/in when my single coil setup with 160lb/in felt super squishy until I stepped up to 230lb/in springs - and it probably has more to do with COG, suspension design, etc

Good stuff guys, appreciate the lesson.

You want the lightest spring possible with 1-3” preload to achieve your desired ride height somewhat regardless of usage,

So that you can use the dampening for your benefit.

For most of our rigs with shocks mounted to the axle, the sprung mass is not much more than the unsprung mass. This makes it difficult to get the axle to droop bc the chassis above each axle and each axle has nearly the same inertia while moving.

It’s a battle to keep the chassis movement minimal while allowing the axle to move freely. To help force the axle away from the chassis, as fast as possible, hopefully faster than just free fall speed, you need preload.

Preload is getting the axle to full droop, dual rate stops aids that.

Rebound dampening slows that.

Light springs allows you to use the compression damping to actually do its job.

Think of it like this, would you rather use a long lever and fulcrum to jack up your vehicle to take the tires off, or would you rather use a hydraulic floor jack? Most would use the hydraulic jack.

Why use the springs to stop a bump event when a much better way is to use the hydraulic force of the compression dampening.

You can only get the dampening so light in the shock, while it’s easy to add more. This is why I want the axle to move as freely as possible via the springs used, and then control that with the shock dampening.

That’s the same reasons you tune the springs and preload first, get that to near 90% and then tune the shocks.


As far as suspension geometry it has some to do with it, but not usually enough to restrict movement. Typically with good joints in the links, even poor geometry wont inhibit free movement of the suspension bc of the heims rotating, and rotating on the bolts. Edit: there are of course anomalies with this.

Yes the geometry affects the dive/lift/squat/ pitch/roll of the vehicle, and having the wrong springs can band aid a problem. The same is true with valving.

The lightest springs to hold the vehicle at correct ride height with 1-3” preload is a good catch all for the majority of rigs. Sure there are anomalies but that is a great starting point for 90% of the stuff out there.

If it’s completely nose diving or body rolling in a turn, it’s going to be far worse in any bump event. The force is far greater in a bump event, than braking, accelerating, or turning.

If it is that bad for “normal” driving, it’s going to be exponentially worse in an actual event where the suspension is used. Just thing about the forces.


As far as hierarchy it starts with design geometry, building constraints, spring selection, then shock tuning.

For me, in my opinion, that is why it’s crucial to get the suspension geometry as good as possible from the start. Both with anti’s front and rear, and roll centers left to right.

Then build it as good as possible, get the springs as close as possible, and then the valving as good as possible. Put all the variables in your favor from the beginning. But that’s an argument to use a suspension calculator vs not, and that’s not for this thread.