|Jul 20, 2012, 04:25 PM|
MM450 and VMX Fork Clinic
Please post your questions and solutions about fork tuning and suspension.
The VMX forks are the same as the MM450 however they can be improved further with a little bit of effort.
I have a tune up guide I call Blue Printing so you can follow it and get the same results as many others and myself.
The object of blue printing any mechanical device is to get it as close to matched as possible for optimal performance. It can be anything but typically this term comes from building engines for cars, etc.
We are not modifying the forks we are simply tuning them and making adjustments to the existing parts making them work better as intended.
I'll post the steps here soon...with photos....
|Jul 20, 2012, 04:31 PM|
United Kingdom, Beverley
Joined May 2012
|Jul 20, 2012, 05:37 PM|
Getting Started - part 1
Take the upper spring perch and fine sand paper like 600 grit and hone the piston cylinder. Knock down the high spots and test fit it back in the fork tube to check the fit and smoothness. Your looking for a nice smooth feel and not a whole lot of play. Too much play and you have gone too far.
Do the same for the lower piston. Hone its outside edge and check its fit in the fork tube.
Next take the lower bushings, 3 of them, and make a dowl out of the sand paper and hone the inside of the bushings. Place them back on the fork shaft and see how they slide and hone as needed. All 3 per side so 6 parts in all. Your just looking for better smoother and easier movement, bind free, but dont over do it.
Next check the assembly stack for compression when you assemble all the cleaned parts back together for a check fit test. Lube the parts slightly. If you tighten the lower cap fully and the fork shaft binds there is too much compression on the o-rings. Take out the plain flat bushings and hone them flat on 600-1000 grit sand paper on glass (or hard flat surface) and try to do it evenly just a little bit at a time. Its better if you can measure them first and then sand and measure one at a time. Your looking to decrease the compression of the o-rings but maintain the seal so dont go too far on the sanding.
The stepped bushing that keys into the lower cap can also be sanded but only on the flat side not the step side.
Check fit them again but keep the parts clean and lubed to see whats going on. You should be able to tighten the lower cap fully and have the fork shaft operate smoothly. If not then adjust the bushings again and repeat until the fork slides with little effort. You also want to keep the fork seals working so watch for leaks and do not damage the o-rings. A minor amount of weeping is permitted from the fork seals.
Once you have done this you will see a huge difference and the forks will respond quickly to the terrain and tuning them will be more accurate and easier too.
|Jul 20, 2012, 05:50 PM|
For you machinists out there... with a 4 jaw chuck....
You can chuck the fork tubes up and zero your dial out on the inside diameter where the fork seals and bushings reside.
Inside you will see a step where the top bushing seats. If you can reach with a boring bar type cutting tool and take a small amount off that face to increase the depth of the step in relation to the outer edge this will decrease the o-ring compression.
In addition, you can also increase the ID of the tube by a minor amount to further decrease the o-ring pressure.
Both methods will work and the amount of material to remove is very minor but very accurate.
|Jul 20, 2012, 06:34 PM|
|Jul 20, 2012, 06:49 PM|
|Jul 20, 2012, 11:05 PM|
Australia, QLD, Burua
Joined Jul 2012
This is just my two cents worth.
Front forks. Strip the forks out of the clamps and drain the oil. Remove the lower caps and pull the fork tubes and seals out. At this point I remove the fork pistons, seals and bushes and polish the shafts using a polishing wheel not sandpaper. They come up with a mirror finish. They give a lot less sticktion meaning a smoother faster ( better ) action, thats why i use fairly heavy oil. Next clean the holes for the piston screws and the screws and assemble back the seals etc. Use plenty of locktite on the piston screws and make sure they are firmly tightened. These screws coming loose are the main reason for fork seal premature failure as the shaft slips out and the sharp edge catches and tears the seals. While my fork tubes are clean i wrap 3 or 4 layers of plumbers thread tape around the top and bottom threads. This I find allows you to leave the bottom collar slightly loose for a few runs while the seals bed in. Tighten as forks free up more. The top threads benifit from tape by not leaking if the o ring doesnt seal properly and it keeps the thread from being worn out. I fill my new forks with 40wt oil , pushing the shaft up and down while filling to disperse the air. I fill to the top of the fork chamber with the shaft fully compressed. With old and worn ones I use 60wt. Once filled check each fork for smooth and free action, tighten or loosen cap as needed. Next assemble the forks into the clamps tightening only the top two screws. Next with the front wheel in place push the front end through its full travel and release several times. There should be no sticking. Tighten the lower triple. clamp screws lightly and check again for binding. Nip up don't over tighten. Next I tighten only one axle clamp screw. I don't blueprint my forks as I find the above method works well without resorting to sandpaper. The forks are usually bedded in after a few runs I find. Oh and with the sticky forks after a big crash ( rarely happens they are just tweaked in the clamps ) this is what I do with the lower screws on the triple clamps and even one axle clamp screw loose push the forks through their range several times, they should feel smooth with no sticking. I then nip up the lower triple clamp bolts and try again. If the forks start to bind I usually just back the lower bolts of 1/4 turn. I always leave one lower axle clamp loose as well. This gives me forks that work well, are ultra smooth, last a long time between seal changes and never stick . I change the oil every 5 to 10 hrs use depending on running conditions. About every forty hours or when the forks start to leak or develop excessive slop I change the pistons, seals, and bushings and if the shafts look good re polish and reassemble. Try 60 wt oil at this point. I find it best almost always once the forks have some time on them.
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