Air Shocks

[Redcbr007]

air shocks dont work, I already tried them... fox makes great products but the air shocks are not made to withstand the weight of the xj. Go with the fox c/o.

-Redx

 

[Matt]

air shocks dont work, I already tried them... fox makes great products but the air shocks are not made to withstand the weight of the xj. Go with the fox c/o.

-Redx

The XJ is a pretty darn light vehicle in the grand scheme of things...

Since alot of desert racing guys use them I find that difficult to believe (most switched to c/o due to heat issues not weight cap.) - what nitrogen PSI and oil level were you using? - and what was the stroke length of the shock???

Some specs on your rig like corner weights and net weight would clarify things as well...

Since the XJ and ZJ (oem to aftermarket) coils are some where in the 280 - 350lbs per inch range and the air shocks are capable of rpoducing spring rates of up to 500lbs per inch - something does not add up! I smell a tuning issue

Matt

 

[Neil]

Don't forget that it's real easy to get 300 + pounds of gear in your rig. I know that I have been known to exceed 600 pounds of gear and the family (Lost coast). My thought is that as you get too close to the maximum weight, you may become unstable. :roll: Coil springs have a progressive rate. The more you push on them the harder they push back. As far as I know (and I may be wrong) the "air" shocks are a constant rate. Providing the same ammount of resistance throughout it's travel. Great for a buggy, not so good for an XJ.

 

[REDXJ4FUN]

Don't forget that it's real easy to get 300 + pounds of gear in your rig. I know that I have been known to exceed 600 pounds of gear and the family (Lost coast). My thought is that as you get too close to the maximum weight, you may become unstable. :roll: Coil springs have a progressive rate. The more you push on them the harder they push back. As far as I know (and I may be wrong) the "air" shocks are a constant rate. Providing the same ammount of resistance throughout it's travel. Great for a buggy, not so good for an XJ.

actually you can adjust the fox units buy the amount of oil you put in to controll the spring rate.

 

[Neil]

actually you can adjust the fox units buy the amount of oil you put in to controll the spring rate.

I know you can adjust the rate. What I am saying is that the rate stays constant, it's not progressive like a spring. It would be a real PITA to adjust the rate every time you load and unload the vehicle. Buggies are usually the same weight all the time (limited gear) XJ's usually get overloaded until they get to camp. Also let's not forget weight transfer. As you climb the weight will transfer to the rear causing the rear "air springs" to become overloaded.

 

[Redcbr007]

No, its not a tuning issue, especially since I live about 5 min away from fox themselves who valved and tuned it for me...call them if you think it was tuned wrong. It was a 80/80 valving, about 400 on the psi and almost 300cc of oil, it held the xj up ALMOST to the ride height, but they said it would gurnade after it heat up, so I went coil-overs.

.... I smell a tuning issue

Matt

 

[Matt]

No, its not a tuning issue, especially since I live about 5 min away from fox themselves who valved and tuned it for me...call them if you think it was tuned wrong. It was a 80/80 valving, about 400 on the psi and almost 300cc of oil, it held the xj up ALMOST to the ride height, but they said it would gurnade after it heat up, so I went coil-overs.

Good info on the shock settings, but what does your rig weigh and what were you shooting for as ride height?

I never said I thought anyone was wrong! I just think there is something else going on besides "they don't work on an XJ"

The heat issue would if anytihng cause fade - I doubt you could truely rupture one of these shocks due to thermal expansion of nitrogen gas - if so why would Fox list 500 psi as their max pressure - also what is the plan for these - desert racing, prerunner, rockcrawler, some of everything...



Neil: - the spring rate is controlled by the gas - the oil level sets how fast the rate begins to become progressive - since you have a closed chamber with a fixed amount of gas there is no way the rate be anything but progressive...

If you look at the volume/ pressure curve you will see the ride height pressure is just that - the PSI at ride height, in any other position the volume has changed and the PSI reflects this - so your initial charging pressure is not the pressure in the shock at ride height (when the PSI changes the force transmitted to the shaft from the piston reflects this change...) (PV=NRT)

This is the same priciple their (fox) bump stops use to function - as you make a set amount of charged gas go into a smaller space you need to impart a larger and larger force to compress each additional inch - it is not a linear relationship...

A coil spring or a leaf spring acts the same way - in a coil spring (XJ type) the smaller windings at the top allot for a softer initial rate and then the main coil transitions into the primary rate - as the windings begin to touch each other the rate begins to increase dramatically... With a leaf spring you are transitioning different leaves that all represent distinct spring rates - as they get added together the total spring rate begins to get higher...

Matt

You can see on this chart from POR by Billavista and Dave from Polyperformance - that the spring rate increases dramatically as the chamber volume shrinks...

http://www.pirate4x4.com/tech/billavista/PR-Airshox/index.html