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.

Comments

author
skaterqwertyuiop made it!(author)2010-07-15

Nice instructable and idea. I like the idea, but I wouldn't trust my life to it, Sorry :( Anyway, how has this worked for you so far?

author
somerandomguy made it!(author)2010-08-29

It has worked quite fine actually. My rubber seals are a bit too old, which means that the pressure is equalized in a few days, But the suspension is a lot better since my upgrade. With new seals I believe that there would be no problems at all.

author
faiyaz+khan made it!(author)2016-10-21

place an o ring under the seal
and check for leaks on the top cap.
if it doesn't work then buy a new seal and then put an o ring under it .

author
skaterqwertyuiop made it!(author)2010-08-29

I might actually try this with one of my spare bikes. But I'm getting the Rock Shox Tora shock soon, so I won't bother doing it to my Haro.

author
faiyaz+khan made it!(author)2016-10-21

I have made it and successfuly Passed the off-road test.

Don't worry about seals if you place an o ring under the seal and lock the seal with super glue the air won't be able to get out. broken seals must be changed. and if u add a soft spring of a normal fork the air will get support and you will fell the difference .

temp_1361252326.jpg
author
crazypj made it!(author)2015-09-17

If anyone is bottoming out fork on a regular basis, the springs are too soft and adding air caps would be a good way to assist the springs rather than doing a full 'air conversion. I worked on high pressure hydraulic systems from around 1990 until 1998, (JCB, fork lift's, etc) Special high pressure seals are relatively easily available but you will have to order them. 'O' rings will work for the relatively low pressures but if making an air piston it would be better to use double acting 'U' seals or double up standard 'U' seals 'back to back

author
crazypj made it!(author)2015-09-16

Good Instructable but there are a few things you should be aware of. Majority of dust seals are only rated for about 15psi (I forget where I read that?)

High pressure seals are available but will probably need modification to top of fork slider

You will get increased 'stiction' due to seal grabbing fork tube when air pressure applies force, generally making forks inactive over small bumps. If seals are not physically located into tops of sliders there is a real good chance the increase air pressure on 'full bump' will eject them from slider (pressure can easily exceed 150+PSI) My main source of information for all this was the proliferation of air suspension on motorcycles in the 1980's and the amount of problems I had to deal with. Pressures will be lower on bicycles because the entire machine is at least 200lbs lighter but maximum pressures will be similar as there is less volume in smaller diameter forks

author
somerandomguy made it!(author)2015-09-16

Thank you!

As for the pressure ratings, I recently tested the maximum pressure that the seal would withstand after being glued with PU sealant. It turned out to be about 12 bar (170 psi), which of course is a lot more than necessary due to the 'stiction' you mentioned, which occurs at about 5 bar. When doing the math, 5 bars of pressure over a cylinder with a radius of approx. 12 mm gives about 25 kg (55 lbs) of "force" in the direction of the cylinder.

When the fork is compressed, the total volume of the gas chamber is reduced by a maximum of one third, meaning that the pressure is increased to 7.5 bar (p1*V1*T1^-1=p2*V2*T2^-1) which is far from the critical 12 bar.

Interesting to hear the motorcyclists point of view, a lot is different, such as the volume change during compression, but the maximum pressure seems to be the same.

author
alopez80 made it!(author)2014-05-09

still not a very safe idea, specially for me since I ride my bike a way my fork's always getting a bottom. I do not oppose coil to air conversion but I would recommend a better way. In this case, you will not entrust your air suspension on the bushings and mud seals as air seals. We would not be including the fork's casting as "air tank"
1. improvise a new AIR PISTON. This would be placed on the top of the rod which pushes the spring to compression. This could be done by joining two circle plates (by glue or something !but leaving some 1mm space!) and placing an o-ring between them (this makes the seal)

2. pouring some ml of oil inside (to really seal the air in) -about 10ml

3. molding a new top cap. we can use steel epoxy and mold it inside the threaded part of the stanchions; leaving a hole that would exactly fit with the valve. use epoxy to seal the valve.

This way, if successful, would give you 100% confidence with your improvised air suspension and ALSO, you can go back to your coil spring if you wanted because we would not be wrecking or gluing any stock parts permanently.

I've already did this with my RST Dirt. It works fine considering that I do dirt jumps. It has very high preload pressure. (60psi)

Happy experimenting, guys! :)))

author
TEDDY256 made it!(author)2012-10-27

awesome project finally got everything i needed to do it and i did it on a old manitou comp 80 i was a little bit nicer then your fork but i kinda did that same thing and so the actual seals popped off after a few pumps so i put some gorilla glue on them hopefully they will hold to the lowers better

author
TEDDY256 made it!(author)2012-10-27

ok well this is where i have the issue it actaul seal shoots well pops off i just used some gorilla glue on it so... hopefully it will dry over night

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