(UPDATED: I’ve corrected this how-to so it follows the steps Chris wrote. Same info as before but steps are easy now thanks to Chris. -JohnnyMc)
This how-to was submitted by Chris Haertel, aka chktm on rcgroups. Replacing the rear shock on the VMX450 & MM450 should be up at the top of your mods list. The stock shock just doesnt cut it for handling the heavy gyro in the rear wheel. Doing this mod is like sending your bike to Mitch Payton at Pro Circuit. 😉
The shock you purchase goes by different names, either the HotBodies 16mm Big Bore front shock 97mm, or the HPI Vorza 16mm big bore (front) shock. As long as the size is measured from eye-to-eye about 97mm-100mm your good on length. You can find them on ebay for about $30 a pair. Piston used is the 6 hole 1.2mm hole size. You may need to purchase a piston set for about $10, which I found at amainhobbies.com.
Step 1. Pic 1
Drain oil and carefully dismantle both shocks removing shafts and pistons but leaving the seals and bushes in place. Next place a washer over the std shaft before pushing on the big bore 1.2 piston. Screw on the nut to retain the piston. Use a soft cloth to prevent damage to the shaft when holding it with pliers.
Step 2. Pic 2
Put a drop of oil onto the shaft or into the seals before pushing it through the shock. Clean the thread of any oil once its through. I use a bit of carby cleaner on a cloth. Make up a solid bump stop of about 5 or 6 mm. The spacer in the pic is a servo horn cut off. Anything solid plastic similar would do. In another shock I use two old pistons drilled out. Make sure its a nice wide bump stop that’s pretty robust.
Step 3. Pic 3
I fill the shock with 70wt oil and bleed the air then screw the cap on. I put a drop of super glue into the thread hole in the shock end and let it dry before screwing on the end until there is almost 2 threads showing. The measurement from end of eye to eye is 106.5mm. ( Edit… I’ve since reduced that to 105mm ) You could also make up a alloy shock end to make the setup absolutely bulletproof.
Step 4. Pic 4
86mm is the measurement from end of eye to eye with the shock fully compressed. For bitumen use I’d recommend reducing travel more and stiffening oil more. Also maybe try 80wt on grass for less wallow.
Step 6. Pic 6
Make up some kind of upper bush to suit the hole in the cap. These are Hong Nor bits with the outer cut and shut about 1mm to fit the ball tightly. (This is John, I purchased some of the Vorza shock stand off upper mounts and cut off the part that fits inside and it works too.)
Make sure to set the droop screw gap to about 3.5mm. Just a tip when servicing the shock make sure the diaphragm is not compressed when installing the cap otherwise the shock action will not be correct. I don’t bother using any sort of dust protection as it generally last five minutes for me before being to torn up. Change the oil every so often and the seals and maybe shaft when it starts leaking. I also move the top mount back towards the rear two holes to stiffen up the shock setting.
Here is 2 videos of Chris’s Atomik MM450 that show how good the suspension handles the jumps and landings. The rear wheel settles down amazingly well.