Introduction: Improve Your Bicycle Fork Suspension

In this Instructable I will show you how to turn the regular coil suspension on a mountain bike into a shock absorber working on compressed air. With this Instructable you can save lots of money because the bicycle forks working on compressed air are usually quite expensive compared to the "normal" ones.

Step 1: Preparations

First I will remind you of that I take no responsibility of whatever you decide to do. Any warranties will probably be voided if you disassemble your fork suspension.  But I think that the chances of anything failing are very small since this modification is easy to perform.

This Instructable will NOT cause your bike to stop working with the old suspension system consisting of coils.

The parts you will need for this Instructable are:
-A bicycle fork that has a COIL suspension and rubber seals between the two legs
-A Schrader valve, I got mine from an inner tube for a bicycle tyre
-Epoxy glue and spatula
-An air pump
-Some cable ties (might not be needed)
-A hex key
-Some tools for bending (Leatherman, knife, flat screw driver...)

Step 2: Getting Started

Since I've already done this on my bicycle I don't have precise pictures of every step but I'll still explain as good as I can.

The first thing to do is to separate the two parts of the fork from eachother. This is easily done by removing the two bolts at the bottom of the fork. This is also the step where any warranty might be voided. If you haven't done this before then prepare for an extremely dirty part.

clean everything inside the fork, sand or mud will cause the air to leak

Step 3: Adding the Valve

Now you have to remove the cap that is not used for any adjusting of the suspension. In my case it was the cap on the right side. Remove it using your bending tool, I found out that the knife works pretty good.

Then drill a hole through the cap for the valve extracted from the inner tube.

Mix your epoxy (read instructions manual) and glue the valve in place from both inside and outside of the cap. Depending on the epoxy, let it dry for anything between a few hours and one day. I couldn't wait for more than one hour but it worked out fine anyway.

When the valve is in place, glue the whole cap back in place. Remember that surfaces have to be clean and dry for the epoxy to work.

Step 4: Cleaning the Seals

 while waiting for the glue to dry you want to take a look at the rubber seal. You might have to buy a new one, but my seal was in surprisingly good shape so I sticked to it.

The seal has to be as clean as possible as well as the rest of the leg.
Clean the seal using a detergent that dissolves fat 

With this done we can move to the next step

Step 5: Reassembly

This is where we put it all back together. Slide the legs together, apply some lubricant on the seals to make it easier. Tighten the bottom screws, the one on the right leg has to be very tight to keep the air inside.
Start pumping air into the leg, if you're lucky there will be no leaks.
leaks are revealed when adding soapy water on the spots that you think could leak 

If a leak however appears it is probably at the seal, to fix it you can:
-Tighten the seal using a cable tie
-Swap seals, only one of the legs have to be airtight. While swapping seals proceed with caution, remove the seals slowly without damaging them. I swapped rubber seals because the steel spring broke on one of the seals, and cable ties didn't work too good for me.
-Cut a ring of plastic foam and put it right under the seal, seals bought from the store also include this foam ring
-Buy a pack of new seals with the same size and install them

Leaks are also possible at the bottom bolt so tighten that really hard or try to make a rubber seal for the bolt out of the inner tube used earlier  
When the suspension is airtight you are ready to test it.

Step 6: Final Thoughts

I'm not an expert in mechanics so this modification is not foolproof, i'm not sure how long the air stays inside the leg or if it could damage the system (I'm almost positive it won't)
I'm quite sure that without new seals you will have to pump in more air quite often.
But if it doesn't work at all, then no harm done, the coil suspension is always there for you, as it was not damaged during this Instructable. 

So this is how you turn your lousy coil suspension into a better air suspension.
Thanks for reading this Instructable, which by the way was my first one.
If you have any questions please ask them in the comments and I try to answer them.