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Thread: Air in forks?

  1. #31
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    Quote Originally Posted by DennisDarrow View Post
    Back in "The Day", it was an experimental practice and even written up in "The News" as a technical item (Don't ask for a footnoted reference).
    My references say the issue about "Forks, air" was March 1979, page 23. My membership doesn't go back that far!!
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  2. #32
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    Quote Originally Posted by melville View Post
    I had caps that wouldn't seal with the currently supplied crush (that don't seem to) washers. I replaced them with Viton o-rings and found quite a difference. Not so much a shower as a modest sheen emerging from the cap area after a ride on a rough road.
    Drilling the hole in the cap and inserting a tire plug/air stem, was done on the old /5 and /6 and perhaps later that had a flat dome type huge cap on top of the fork. One, uses/used a pin wrench in the tool kit that was combined with a spanner to loosen and take off the cap. Not being readily familiar with the /7 and later, I really do NOT know what the top of the fork leg is like.........God bless.......Dennis

  3. #33
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    Quote Originally Posted by DennisDarrow View Post
    Back in "The Day", it was an experimental practice and even written up in "The News" as a technical item (Don't ask for a footnoted reference). Folks would drill out the top cap and insert a valve stem. Then put in a few pounds of air.........It did exactly as stated above; BUT......there were seal problems and the practice died a lingering death over perhaps a season or two. This is when folks were installing the Reynolds bottoming springs, PVC spacers, and progressive rate springs were just beginning to be developed. It is interesting to note that "hybrid" experimentation and evolution became the standard of today...........God bless......Dennis
    I sort of suspected that it had been tried, but probably didn't work out too well - at least in the long run.

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    Quote Originally Posted by 20774 View Post
    My references say the issue about "Forks, air" was March 1979, page 23. My membership doesn't go back that far!!
    I probably read that article! That was around the time when I was a member for a few years until I sold my two BMW's because I wanted the kids to grow up with a dad and also because we were strapped for money. Bikes had to go, and I now really regret it!

  5. #35
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    Quote Originally Posted by DennisDarrow View Post
    Drilling the hole in the cap and inserting a tire plug/air stem, was done on the old /5 and /6 and perhaps later that had a flat dome type huge cap on top of the fork. One, uses/used a pin wrench in the tool kit that was combined with a spanner to loosen and take off the cap. Not being readily familiar with the /7 and later, I really do NOT know what the top of the fork leg is like.........God bless.......Dennis
    On the /7 one could still get the top caps machined out, plugged, with a center hole sized for the tire stems, but personally I think it would be an exercise in futility. Pre-loading the springs would be more easily accomplished, I think, bu using spacers atop the springs. Of course it wouldn't do much for variable rate - that would have to be accomplished by buying springs for that purpose, I think.

  6. #36
    Day Dreaming ... happy wanderer's Avatar
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    Early 80's I owned 75/5 that someone had installed the "air" mod on the forks. 1/4" threaded brass inlets in each fork cap connected to a brass T that went to a tire valve mounted on a hole made in the top fork brace. Adding no more than 3 lbs pressure to this system made a big difference in the ride. The T provided balanced pressure between the two forks. Taigon tubing was used and that got loose and started to lead over time which was the Achilles heel of the whole idea. I got tired of it and put stock caps (no holes!) back on and removed it.

    I think implemented more robustly though the idea did work. I don't recall if it caused the fork seals to leak or not. That old bike had a lot of leaks.
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
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    Quote Originally Posted by happy wanderer View Post
    Early 80's I owned 75/5 that someone had installed the "air" mod on the forks. 1/4" threaded brass inlets in each fork cap connected to a brass T that went to a tire valve mounted on a hole made in the top fork brace. Adding no more than 3 lbs pressure to this system made a big difference in the ride. The T provided balanced pressure between the two forks. Taigon tubing was used and that got loose and started to lead over time which was the Achilles heel of the whole idea. I got tired of it and put stock caps (no holes!) back on and removed it.

    I think implemented more robustly though the idea did work. I don't recall if it caused the fork seals to leak or not. That old bike had a lot of leaks.
    Hmmmm? 3 Lbs. of air pressure doesn't seem to be enough to do anything.

    I don't even know what gage would be able to measure such a low amount of air pressure.

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    Quote Originally Posted by jimmylee View Post
    Hmmmm? 3 Lbs. of air pressure doesn't seem to be enough to do anything.
    That is a 20% increase in the initial charge. Remember when the forks are first sealed (filler cap tightened) they have atmospheric pressure (14.7psi).

    Quote Originally Posted by jimmylee View Post
    I don't even know what gage would be able to measure such a low amount of air pressure.
    Probably any normal 0-15psi guage would work fine. http://autoplicity.com/products/2092...Fcd_QgodaB8AqA

    Obviously you wouldn't use your standard 0-100 or 0-60 psi tire guage.


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  9. #39
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    Quote Originally Posted by jimmylee View Post
    Hmmmm? 3 Lbs. of air pressure doesn't seem to be enough to do anything.
    Doesn't sound that small to me...what's the pressure in a basketball or football. What about the compressability of that? As it was stated earlier, the air works as a addition and/or progressive spring, adding to the existing spring under larger compressions. And isn't it the psi across the area of the inside of the shock? Seems like every bit might help at some point.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
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    Quote Originally Posted by 20774 View Post
    Doesn't sound that small to me...what's the pressure in a basketball or football. What about the compressability of that? As it was stated earlier, the air works as a addition and/or progressive spring, adding to the existing spring under larger compressions. And isn't it the psi across the area of the inside of the shock? Seems like every bit might help at some point.
    A soccer ball is properly set for college play to 12 lbs. psi. My son is an ACC licensed referee, and he just told me.

    Footballs are 13 lbs.

    3 lbs seems a small amount to me for the application, but is right if that is what the person did.

    I am also surprised that they make a gage that is that intricate that will properly show just 3 degrees.

    Learn something new everyday!

  11. #41
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    Quote Originally Posted by jimmylee View Post

    3 lbs seems a small amount to me for the application,
    Boyle's Law: when the volume of a closed container is cut in half, the pressure of the gas inside doubles

    On a fork with 8" of air column ,and no precharge, the air inside the fork is at atmospheric pressure (14.7psi absolute) when the fork is fully extended. If the fork has 8" of air at the top, the air pressure would be 29.4psi at half way through its travel (4"). At 6" of compression (3/4 of the way compressed), the pressure would be 58.8psi absolute). Acting on a 40mm fork end (about 2 1/2 square inches), the force would be 147 pounds minus the 2 1/2 sq in x 14.7 psi atmospheric pressure acting in the opposite direction (35.75) or 111 pounds PER FORK! The air in the two forks would be exerting 222 lbs of force trying to resist compression at 3/4 travel.

    Now add 3 psi preload to the calculations. 17.7psi at full extension. 35.4 psi at 4". 70.8 psi at 6". Times 2 1/2 sq in. = 177 lbs. minus 35.75lbs = 141.25 lbs x2 = 282.5 lbs vs 222 lbs with no precharge.

    60 lbs of force difference to the front of the bike from ONLY 3psi!!!!





    Quote Originally Posted by jimmylee View Post
    I am also surprised that they make a gage that is that intricate that will properly show just 3 degrees
    First, I know you meant to say precise instead of intricate and psi instead of degrees.

    Precise low pressure tire gauges are EXTREMELY important for many forms of racing. A typical Top Fuel dragster will run about 6psi rear tire pressure. Swamp buggies, sand rails, and monster trucks all run very low tire pressure.

    There are also innumerable industrial needs for low pressure air gauges.


    Last edited by 98lee; 10-10-2013 at 04:18 AM.
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  12. #42
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    CC Products Air Forks Kit

    Some of you may be interested is seeing one of those air fork kits that were around back in the 80's, this is a CC Products unit. They did indeed work, experimented a lot with them back then. The gauge was mounted on the handlebar so it could be easily monitored........................George

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    Another Air Fork Kit

    Here is another air fork kit, think it is for R65's....................George
    Attached Images Attached Images

  14. #44
    Day Dreaming ... happy wanderer's Avatar
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    Wow! Now those are way fancier than the home jigged rig on that old R75/5 I had with air forks. While not willing to argue the reward/benefit or science side of the mod I can tell you from riding that old bike that the difference a few pounds of air in the tubes made to the ride was like night and day. The front end felt "stiffer" and better suspension when hitting potholes. Also less diving when grabbing a handful of front brakes (such as they were). I don't recall any issues with leaking seals but due to poor implementation it did leak air slowly.

    There was no science applied by me though. Never measured a thing other than the fork oil itself. I would just stop by a gas station and pop it with the air hose very very briefly and that was that. You could feel the difference immediately.
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
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  15. #45
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    Quote Originally Posted by 98lee View Post
    Boyle's Law: when the volume of a closed container is cut in half, the pressure of the gas inside doubles


    First, I know you meant to say precise instead of intricate and psi instead of degrees.

    Precise low pressure tire gauges are EXTREMELY important for many forms of racing. A typical Top Fuel dragster will run about 6psi rear tire pressure. Swamp buggies, sand rails, and monster trucks all run very low tire pressure.

    There are also innumerable industrial needs for low pressure air gauges.

    I did mean to use the word "increment" rather than "intricate" but even that needed a restructured sentence. My meaning was that for normal bike riders, it would be very difficult to hold such a small amount of air pressure, and get it precise at all times. Incrementally, one could say that if 3 Lbs. would be deemed too hard, why not 2.5 lbs.? Why not 2.2? You see, to be able to set those increments would be very impractical, and a gage that would do so, would be very expensive, even if these forks could be trusted to maintain such an exacting pressure. The reason is that there is only a small amount of volume of air in there. It doesn't take much to "fill up" that space with more air and pressure would increase exponentially. The solution, would be to "plumb" some sort of tank to increase the volume, thus making it easier to maintain one's favorite pressure - whatever that may be. These forks simply weren't made for that sort of use.

    I would suspect that is why this concept never really caught on. It was deemed impractical for the average rider. A novelty, yes, practical, no!

    I would bet that those who make their living using these small pressure settings (as mentioned dragsters, etc.) don't use a $5.95 gage.


    Using air pressure is, however, an interesting concept.

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