There are several reasons you may want to run air bumps, and to us the biggest reason is that they can eliminate the bone jarring ride of a polyurethane or rubber stop when you’re blistering through the desert or dirt road on the way to your favorite trailhead. Rather than run a super stiff shock, you can add air bumps which are nothing more than a small air shock, and come into play at a pre-determined amount of up-travel. When we built The Wendy Project we used Fox 4″ bumps, but with only 5″ of up-travel the ride was a bit harsh at moderate speeds. They were transparent at low and high speeds, so we shortened the stroke from 4″ to 2″ rather than send them back to Fox for re-valving.
After you release the nitrogen pressure, you can drain the oil through the shraeder valve, then pop the bearing cap off with a punch and a small hammer.
Once the bearing cap is out of the way, the bearing/seal assembly is exposed.
Gently push the bearing/seal just far enough into the housing to expose a snap ring that needs to be removed.
The groove in the housing for the snap ring is tapered to the inside making removal of the snap ring a breeze. Using a small pick, push it into the housing a little, then fish it out.
This is what the guts of an air bump looks like, and that sleeve is the spacer we're after.
We secured the shaft in a vise and removed the valve nut, then removed the valve/shim/washer pack. The shaft is hollow, so do NOT tighten the vise more than just enough to hold it or you could damage it
If you didn't sneeze when you removed the valve pack scattering the shims all over the floor, you should be ready to remove the spacer.
If you've never seen a valve assembly from an air bump, you can now scratch it off your bucket list.
On the left is the 4130 spacer we removed, and will replace it with the one we made on the right. We used 1.5" .120 wall HREW.
The guts are ready to reinstall into the housing with our new spacer. We torqued the retainer nut to 30 ft. lbs.
We cut the new spacer to be 3 3/8" long, and we were able to achieve the 2" bump travel we were after.
The oil volume spec for a 2.0 bump stop is 125cc. You can get a measuring container at any motorcycle shop, so don’t guess at what 125cc is. We used 5w Torco fork oil that we also picked up at the motorcycle shop to refill our bumps.
The final step is a nitrogen charge. You can play with pressures to fine tune the force of the shock. The minimum pressure recommended is 50psi and a maximum of 200psi. We set ours at 100psi, and we’ll adjust as necesssary.
We made a run through the alley behind our shop at 20 MPH, and reducing the stroke got us off the bumps and gave us the nice smooth ride we were after.
For information on the full line of Fox Racing Shox products go to www.foxracingshox.com
Great article! I used it to rebuild my bumps and shorten them from 4″ to 2″ in the back and 3″ to 2″ in the front!