Nitrogen Bump Stops 

   
 
 

Every rock buggy needs some sort of bump stop to keep metal to metal contact from occurring. In the past the typical bump stop material was either rubber or polyurethane.  These worked well for keeping parts away from each other under the chassis but when called on to dissipate energy when the rig bottomed out their weakness was exposed; a rubber or polyurethane bump stop will release stored energy on rebound making your primary shock work harder.

Nitrogen bump stops (or air bumps as they are commonly referred to) are the next step. They are basically a short stroke air shock and offer the advantage of tunable velocity sensitive valving.  Air bumps will dissipate energy both on compression and rebound thanks to their valving and you can adjust them via valving, nitrogen pressure or oil volume. These miniature airshocks have just recently become more popular with the rockcrawling crowd who are looking for a way to make their rigs handle better at speed.  If you have no desire to drive fast off-road, rubber bump stops are a better option and much more economical.

I purchased a set of Fox Racing 2.0 bump stops with 4" of stroke and installed them on my buggy late in 2006. My suspension was setup with 4-5" of uptravel up front and about 6" of uptravel in back which meant up front the air bumps were almost always in play and acting like a secondary suspension. My first impression was they did work good at high speeds, much better than the old rubber bump stops, however at low to mid speeds they were a little stiff and you could physically feel every time they impacted the axle which made for a rougher ride than I would have liked.  Surprisingly when crawling you could not even tell they were there showing the velocity sensitive valving was indeed working.

To make the air bumps less noticeable at medium speeds I first tried lower nitrogen levels.  The initial charge was 200 PSI so I tried dropping the pressure in 50 PSI increments.  I can't really say I could tell much of a difference in ride quality at the different pressures. The next thing I tried was adjusting the oil volume  with the goal of making the spring rate of the airshock a little more progressive. Again I couldn't really say it made the ride any better at medium speeds.

After sitting back and re-evaluating the situation I decided to try shortening the stroke on the air bumps.  My theory is I really only want them to come into play at the last second to absorb any really big impacts.  So I need to make the suspension have as much uptravel as possible before the air bumps are engaged but still need the air bump to be the limiting factor on my suspension to avoid contact between the axle and frame.

While in Moab I asked Dave from Polyperformance (where I bought my air bumps) about shortening the stroke of the air bump using an internal spacer and he thought it would work.  The next step was to figure out how the air bump came apart.  The only thing I could find on the internet was some disassembly instructions for a Fox airshock.  These turned out to work fine with the air bump so here is a pictorial walk thru of the disassembly.

Step 1.  Holding the shock vertical so the schrader is up, discharge the nitrogen (don't breathe the discharge) and drain the oil. Clean the shock and then place it in a vice.

Step 2. Remove the bearing cap by tapping it out with a hammer using a flat blade screwdriver or punch (don't try prying it).  I inserted the screw driver in the slot and angled it as far back as possible, a few taps started the cap moving.

The cap pulled back exposes a bearing which is retained by a snap ring.

Step 3.  To expose the snap ring, press down on the bearing assembly (you may have to tap the screwdriver gently with a hammer).

I used a small screw driver to fish the snap ring out, just be careful not to scratch the cylinder walls.

Step 4.  Pull the shaft/bearing assembly out by rocking it back and forth while pulling.  I found I had to work the shaft like a slide hammer to pop the bearing out the last quarter inch.  I was expecting a negative spring but instead found a small spacer made from 1.50 x .120 DOM tubing between the valve stack and bearing.  To shorten the stroke on the shock I simply had to replace the tubing spacer with a longer spacer.  For reference my stock spacer measured approximately 1.41" long. the new longer spacer I cut is shown in the second picture. 

Step 5. To remove the stock spacer, place the bearing assembly in a vice, remove the top nut.

Step 6. Carefully remove the valve stack then the old spacer. Remember, these parts need to go back on in the same order.

Step 7.  Slide your new spacer on the shaft and then re-assemble. I'd suggest putting new oil in before re-installing the bearing assembly in the housing.  It's much easier than trying to feed oil thru the schrader valve opening. I refilled my air bump with 5w shock fork oil to the factory capacity: 125cc's.  Once I slid the bearing assembly back in I found I had to bleed off pressure via the schrader valve to get the bearing seated below the retaining groove so I could install the snap ring.  Once that was in I compressed the  bump so the snubber was resting against the bearing cap.  I then hit the snubber with a hammer which knocked the bearing cap back into place.

   

Here's a shot showing the new 2.5" stroke vs. the old 4" stroke. It'll be a bit before I can thoroughly evaluate the change but I will update this page with the results. I am pretty confident the shorter stroke will work much better since the air bump will be acting more like a bump stop and less like a secondary suspension.

Update - 5-16-07

I have logged a few runs with the shorter stroke and I am happy to say I can no longer tell the air bumps are there. With the old 4" of stroke you could hear and physically feel when the bumps contacted the axle, this got annoying on the easy stuff. Despite the shorter stroke there still appears to be enough stroke to dissipate the energy when the rig bottoms out.  I hit a few bumps pretty hard last weekend and the air bumps did their job. For reference I am runni gthe bumps at 150pi.