Is my coilover spring rate too high?

[Captain Obvious]

The FSMs says the following for the 280Z (75 - 78):

 

Front springs for 75-78 two seater  -  1.84 kg/mm or 103.0 lb/in

Front springs for 75-78 2+2  -  2.06 kg/mm or 115.4 lb/in

Rear springs for 75-78 (both 2 seater and 2+2)  -  2.28 kg/mm or 127.7 lb/in

 

So it appears there is a difference between front and rear as the rears are about 25% stiffer.

 

Of course I've never measured any of that for confirmation or accuracy, but that's what the documentation indicates.

[cgsheen1]

Well, you can learn all kinds of things from the FSM - even damping force:  I just found out the 1972  European models had more than twice the damping force than models sold in other countries...

I was actually looking at the 240Z spring rates. They are constant 1970-1973:

Front springs 70-73 - 1.48 kg/mm or 83 in/lb

Rear springs 70-73 - 1.85 kg/mm or 103.6 in/lb

The difference between front and rear is about 20%

In the front, the two springs are nearly identical in specs except for length and load!  The RH spring is 13mm (~1/2") longer (free length) than the LH spring.  Installed they are the same height because the RH spring carries more weight apparently.

 

[AlbatrossCafe]

On 8/14/2018 at 12:45 PM, cgsheen1 said:

Stock Stance USA XR-1 coilovers use 175mm long springs and minimum rate they show is 4K.  They have listed 3K (168 lb.) springs in 200mm length.   If you're interested, talk to Stance USA Customer Service about those springs on your struts and ask if your struts would need to be re-valved to use them.  That is important to know.  The 3K springs would be just over an inch longer than your current springs IIRC.  That means your lower perch would need to drop just over an inch.  At your current ride-height you should have plenty of thread to lower the perch on the strut tube.

(You could move your 4K springs (or strut) to the rear and use the 3K springs on the front.  Stock 280Z spring rates were 103 lb. front and 198  (see below) 128 lb. rear (3K=168 lb. vs. 103 lb. - 4K=224 Lb. vs. 198  128 lb.).  Your springs would be softer than they are now, still stiffer than stock.  edit: the information sited below may change you opinion about the 4K springs...

The 15-step damper adjustment is for damping - it doesn't change the spring rate ("stiffness"), it dampens compression and rebound of the spring.

I picked up the 3k springs from Stance for a very reasonable price. They are indeed 200mm.

Now tell me this... is replacing the spring as easy as:

1. Undoing the coilover adapter from the coilover body to drop the spindle/hub assembly
2. Using spanner to unscrew preload adjustment ring
3. Dropping the spring off
4. Slide the new spring on
5. Using spanner to screw preload adjustment ring back on to 1" (25mm) below the original spring height)
6. Reattach coilover body to spindle/hub assembly at same thread level

Probably a spring compressor would be much faster than trying to use a spanner the whole time but you get the idea.

 

Edited August 30, 2018 by AlbatrossCafe

[Zed Head]

You do know there's a shock absorber inside that tube...  If it's a gas shock it will extend so even if you could get the hub to drop far enough to split the strut it would be a nightmare to handle.

The spring is supposed to come off from the top.  Detach the top of the strut, lower it and swing it out to where you can work from the top.  Lowering the bottom spring perch will allow you to make the strut much shorter so it will swing out easier.

[LeonV]

5 hours ago, AlbatrossCafe said:

I picked up the 3k springs from Stance for a very reasonable price. They are indeed 200mm.

Now tell me this... is replacing the spring as easy as:

1. Undoing the coilover adapter from the coilover body to drop the spindle/hub assembly
2. Using spanner to unscrew preload adjustment ring
3. Dropping the spring off
4. Slide the new spring on
5. Using spanner to screw preload adjustment ring back on to 1" (25mm) below the original spring height)
6. Reattach coilover body to spindle/hub assembly at same thread level

Probably a spring compressor would be much faster than trying to use a spanner the whole time but you get the idea.

Yes, swapping springs that way is the easiest way to do it. You may have to disconnect your ARB endlinks.

The new spring is an inch longer so the perch will indeed be an inch lower but, assuming 600lb corner weight, the car will drop approximately by another inch. Therefore, the damper body will need to move up to maintain ride height.

Another effect of the softer spring is that you will be taking away an inch of bump travel. This may or may not be ok, depending on where your damper is currently. This can be adjusted via spring preload, e.g. one inch of spring preload will raise the car an inch and gain one inch of bump travel (assuming you haven't run out of travel).

Edited August 30, 2018 by LeonV

[Zed Head]

Take some pictures and report how it goes.  Maybe I'll learn something.  Should be fun.  Good luck.

[AlbatrossCafe]

15 hours ago, Zed Head said:

Take some pictures and report how it goes.  Maybe I'll learn something.  Should be fun.  Good luck.

Will do. The reason I suggested doing it the way I did is because I was trying to avoid having to re-do my whole alignment from unbolting everything from the camber plate at the top.

On these coilovers the height adjustment is independent of the pre-load adjustment. So I will do my best to get the shock in the middle of it's travel range then match the height to match what I had before the swap.

At any rate, springs aren't showing till Tuesday so I'll have to do it next weekend. We'll see how it goes.

[cgsheen1]

22 hours ago, AlbatrossCafe said:

I picked up the 3k springs from Stance for a very reasonable price. They are indeed 200mm.

Now tell me this... is replacing the spring as easy as:

1. Undoing the coilover adapter from the coilover body to drop the spindle/hub assembly
2. Using spanner to unscrew preload adjustment ring
3. Dropping the spring off
4. Slide the new spring on
5. Using spanner to screw preload adjustment ring back on to 1" (25mm) below the original spring height)
6. Reattach coilover body to spindle/hub assembly at same thread level

Probably a spring compressor would be much faster than trying to use a spanner the whole time but you get the idea.

 

1. You DON'T need a spring compressor!

     a. There isn't a strut cartridge insert - you don't need to worry about anything shooting out or springing apart.  The spring is NOT compressed between it's two perches!

2. Take the strut cartridge out however you can and/or want to.  Personally, I'd just pull the whole thing out and work with it off the car.

    a. So, yes - loosen the nut at the top of the threaded adapter and unscrew the strut cartridge from the adapter.

    b. If you can work with it after it's loose at the bottom, unscrew all of the nuts under the spring and take the spring out.  I'm going to refer to the two nuts locked together just under the spring as the LOWER PERCH (or lower spring perch)

    c. put the new spring up there and install the two nuts that make up the lower perch.  When you're tightening the topmost nut, INSTALL IT SNUG TO THE SPRING so the spring doesn't move, BUT DON'T tighten it much beyond that.  (If you tighten it more than snug, you're adding PRELOAD to the spring - the spring doesn't require ANY preload (unless you're corner balancing the car - then, your suspension expert will add preload if necessary to get the balance he wants...))

        A. If you pull the whole strut cartridge off the car the same thing will be true.  Personally, I'd remove the entire cartridge, remove the top nut which holds the pillow block, spacer, and upper spring perch, and remove the spring from the top.  Then you'll lower the lower spring perch more than an inch or so, install the spring and replace everything you took off the top in the reverse order.   You should have slack between the spring perches, so this is the point that you snug up the lower perch and it's lock nut.   Re-install the strut cartridge.

Before I remove a strut cartridge I usually measure the distance between the bottom lock nut of the lower spring perch and the locknut on top of the threaded adapter to make sure I've re-installed the strut to the same height.  You can't do it this way because you're changing the height of the lower perch.  I guess subtracting the additional spring length will get you close though.

*** PLEASE GUYS!!!  Don't make comments about a specific strut cartridge unless you know how it actually works! ***

[cgsheen1]

18 hours ago, LeonV said:

Yes, swapping springs that way is the easiest way to do it. You may have to disconnect your ARB endlinks.

The new spring is an inch longer so the perch will indeed be an inch lower but, assuming 600lb corner weight, the car will drop approximately by another inch. Therefore, the damper body will need to move up to maintain ride height.

Another effect of the softer spring is that you will be taking away an inch of bump travel. This may or may not be ok, depending on where your damper is currently. This can be adjusted via spring preload, e.g. one inch of spring preload will raise the car an inch and gain one inch of bump travel (assuming you haven't run out of travel).

Yes, you may need to disconnect the sway bar end links.

NONE of the rest of this is true for Stance-USA strut cartridge!  Please strike out the lower two comments.

edit: In regard to ride height:  The softer springs will compress slightly more when loaded than the springs he's using now - not an inch I don't think - but you're right that the strut cartridge will need adjustment in the threaded adapter to get his desired ride height after the spring replacement.

Edited August 31, 2018 by cgsheen1

[cgsheen1]

21 hours ago, Zed Head said:

You do know there's a shock absorber inside that tube...  If it's a gas shock it will extend so even if you could get the hub to drop far enough to split the strut it would be a nightmare to handle.

The spring is supposed to come off from the top.  Detach the top of the strut, lower it and swing it out to where you can work from the top.  Lowering the bottom spring perch will allow you to make the strut much shorter so it will swing out easier.

You're correct in that the damper rod coming out the top of the strut cartridge is always fully extended when the strut is unloaded.  You really can't do anything to "shorten it up" when you're removing the strut cartridge.

The spring can come off either way, but I prefer to remove it from the top.  The thread pitch on the cartridge makes unscrewing the lock nuts and lower spring perch annoying and time consuming...

[AlbatrossCafe]

1 hour ago, cgsheen1 said:

 The thread pitch on the cartridge makes unscrewing the lock nuts and lower spring perch annoying and time consuming...

lol you got that right. My forearms are gonna be tired for sure. I'll try my way first and see how frustrated I get. Thanks for the help. I always get confusing advice on my coilovers since Stance operates a bit differently than most.

Edited August 31, 2018 by AlbatrossCafe

[LeonV]

 

1 hour ago, cgsheen1 said:

*** PLEASE GUYS!!!  Don't make comments about a specific strut cartridge unless you know how it actually works! ***

 

1 hour ago, cgsheen1 said:

Yes, you may need to disconnect the sway bar end links.

NONE of the rest of this is true for Stance-USA strut cartridge!  Please strike out the lower two comments.

edit: In regard to ride height:  The softer springs will compress slightly more when loaded than the springs he's using now - not an inch I don't think - but you're right that the strut cartridge will need adjustment in the threaded adapter to get his desired ride height after the spring replacement.

Unless Stance-USA struts have managed to defy the laws of physics, everything I've said is true.

Here's how it actually works, with some math to prove it:

Assume a corner weight of 600lb. Old springs were 224lb/in (4k), new springs are 168 lb/in (3k).

600/224 = 2.7 inches of spring compression

600/168 = 3.6 inches of spring compression

Therefore, the car will sit about an inch lower (accounting for motion ratio) AND the damper rod will lose 0.9" of jounce travel due to the spring compressing more.

There are two solutions to getting ride height back. The first is to simply thread the damper body up by an inch. This works to get ride height back but you're still missing 0.9" of bump travel. Assuming damper travel was set correctly in the first place, you'll want that jounce travel back. Therefore, I suggest getting your travel back to where it was by preloading the spring by 0.9" (using a spring compressor makes this easy). This accomplishes both getting your ride height AND damper travel back.

I've never corner balanced a car that has separate height and preload adjustments with spring preload. That's best done via damper body adjustment because preload changes where the damper sits at nominal height, as you can see from this exercise.

In summary, preload adjustment sets damper position and damper body adjustment takes care of ride height and corner balance. It doesn't matter whether the metal bits were supplied by Stance, or BC, or Ohlins. It all works the same, according to the laws of physics.

1 hour ago, cgsheen1 said:

The spring can come off either way, but I prefer to remove it from the top.  The thread pitch on the cartridge makes unscrewing the lock nuts and lower spring perch annoying and time consuming...

Removing from the bottom is not difficult and probably faster and easier than through the top, a friend of mine did it this way to not mess with the top nut and assorted hardware.