The Fox Shock Perch Adjustment Mod

[BigJ]

Great post Mark. Thanks!

 

[BigJ]

By the way, what are your thoughts on an increase of 3-5degrees of approach angle (assuming that proves to be true)? Worth adjusting the perch?

 

[frogslinger]

I have to disagree with one technical detail of MarkT's post...

"Some people will say by increasing preload the springs gets "stiffer" and ride quality is reduced. This would defy the laws of physics and is not true at all! "

If you add preload to a prgressive spring it dos get stiffer... not relavent probably but hey...

"This puts the piston in the shock in a different position which will affect the "internal bypass" tuning."

Very true, and it might be a good thing... a couple of reviews have said that the shocks are great except for the first couple inches of travel which is too stiff...

Nice writeup

 

[BigJ]

I have to disagree with one technical detail of MarkT's post...

"Some people will say by increasing preload the springs gets "stiffer" and ride quality is reduced. This would defy the laws of physics and is not true at all! "

If you add preload to a prgressive spring it dos get stiffer... not relavent probably but hey...

But... if you move the perch up, and allow the top perch to float (as in you don't decrease the "at rest" spring height) won't the spring find the same equilibrium it found before? As in, won't it return to the same height it was at before the perch adjustment?

If so, then the spring is under the same compression before or after, no?

 

[frogslinger]

That is true assuming two things;

all forces are linear... which for a raptor they are not...
the upper perch is floating... which I am pretty sure it is not...

From the pics I have access to of a raptor suspension all that this mod does is move one mounting point for the spring... when assembly is out of the truck he spring will be under slightly less load... in the truck it will be under the same load, will therefore be the same length and as the bottom of the spring is now mounted higher, the whole truck sits higher...

An analogy would be if you hold a book (your truck) on your head...

if you are on the bottom step of your staircase (lower perch) then the book is going to be lower than if you are on the top step (upper perch). This is regardless of the fact that the book has not moved relative to your feet... (same spring length and preload).

As far as i can tel his mod does not change preload at all, once the shock and spring is installed all the load simply moves from the shocks internal stops to gravity... total length of the spring is determined by total load on the spring... the load on the spring is all from the internal stops on the shock whilst the assembly is outside the truck... once the trucks weight is in play, the shocks are no longer topped out and that load is gone... all load is supported by the spring and it will compress the same amount...

or to put it another way... unless the preload on the spring from the shocks internal stops is greater than the 1500 lbs weight exerted by the truck, then the distance between the upper and lower perch will remain the same...

 

[pirate air]

I know I would like to try this mod. It will cost me nothing but my time because I can perform the work and re-align it myself. If I don't like it, no harm done. IIRC the only thing you have to mod is the Fox sticker on the shock housing. It gets in the way when you slide the perch up.

If the 6.2 springs are different then the 5.4, the parts catalog should tell us. I doubt they are though.

 

[MarkT]

I have to disagree with one technical detail of MarkT's post...

"Some people will say by increasing preload the springs gets "stiffer" and ride quality is reduced. This would defy the laws of physics and is not true at all! "

If you add preload to a prgressive spring it dos get stiffer... not relavent probably but hey...

Actually J has it right frog... it doesn't matter one bit if the spring is "progressive".

Also, your stair analogy is a good one... the distance between the upper perch and the lower perch will not change. When you raise the lower perch, the upper perch (along with the truck!) moves up. Which is exactly why it doesn't matter if the spring is progressive or not.

Maybe the misunderstanding is in regards to the definition of "preload"?

Preload is the amount the spring is compressed with the suspension at full droop.

 

[bstoner59]

it's like on my motorcycle...I tighten the spting to increase preload and eliminate sag. By moving the perch up a notch you are in essence tightening up the spring thus increasing the preload. The ride may get stiffer but that is due to the position of the shock in the stroke...since they are position sensitive. I am basing this all off my limited experience with my motocross suspension setups so I may be off...if so please let me know so I can be enlightened.

 

[frogslinger]

Actually J has it right frog... it doesn't matter one bit if the spring is "progressive".

Also, your stair analogy is a good one... the distance between the upper perch and the lower perch will not change. When you raise the lower perch, the upper perch (along with the truck!) moves up. Which is exactly why it doesn't matter if the spring is progressive or not.

Maybe the misunderstanding is in regards to the definition of "preload"?

Preload is the amount the spring is compressed with the suspension at full droop.

A progressive spring is one that changes its stiffness with compression...

"In progressive springs each coil is spaced differently and have a variable spring rate. When free, it is easy to compress progressive springs for first centimeters. As you apply more forces, coil on a progressive spring come closer. After a certain point, coil at the top 1/4 of progressive springs begin to touch each other and finally become inactive or dead, and that makes the spring stiffer. Apply more forces to a progressive spring then it becomes stiffer because as the number of active coils in a spring decreases, the spring rate increases. So, progressive springs may both be sensitive to very small bumps on the road, while giving the stiffness you need during hard braking and turning."

The whole point of a progressive spring is that it is like stacking a bunch of very short springs on top of each other... as the load (dynamic or static) is increased the softer springs bottom out, leaving the stiffer springs "unbottomed" and free to move... but as they are stiffer they move less...

Motion is not directly proportional to load.

to use an example...

if you have a spring 1 foot long uncompressed that when compressed is 6 inches long, and where half the spring take 500lbs to compress an inch and the other takes 2000 lbs to compress an inch...
if you put a 2000lb load on it the top half will try to compress 4 inches... down to two... it will not be able to and will compress to 3 inches... the bottom half will compress 1 inch so your spring will now have compressed a total of 4 inches with the 2k lbs...

add another 2k lbs and the spring will only compress 1 more inch
add another 2k lbs and the spring will only compress 1 more inch
add another 2k lbs and nothing happens your spring is bottomed out.

the effective overall stiffness when measured with 2k lbs was 1 inch per 500 lbs
the effective overall stiffness when measured with 4k lbs was 1 inch per 800 lbs
the effective overall stiffness when measured with 6k lbs was 1 inch per 1000 lbs
the effective overall stiffness when measured with 8k lbs was 1 inch per 1333 lbs

if you have progresive springs and you add weight they get stiffer...

You are 100% right on the defenition of preload... I may have bent the defention a little...

J is indeed right... except that the upper perch is fixed... he is still right in his conclusion... the spring will force the perch down to where it was before... causing the shock to elongate and the truck to sit higher... I was not debating that...

 

[frogslinger]

I had this same debate with an engineer buddy of mine... I showed him essentially this mathematically...

still did not believe me until we did it experimentally...