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Air in forks?

I think practically, that adjusting the air cap (by changing the oil level) is not really all that critical for our "plush" front suspensions. But I found another series of posts from about 4 years ago by Tom Cutter who suggests that it is something to be considered. He's big into suspensions (he races and sells a specific brand of suspensions and springs that can be customized) so I figure he knows what he's talking about. Tom made reference to this chart by Wibers:

http://www.docstoc.com/docs/107690689/Gabelfedern-_Fork-springs_-WILBERS-PROMOTO

One section is for BMW and the one column to the right that has a cloud over it is the air cap over the oil. Probably a lot more going on in this chart, but it does appear that the amount of oil could be fine tuned for each rider. But in general, the value listed in Haynes or the manuals will cover a wide range of riders and situations.
 
An easy way to visualize the effect of oil LEVEL in the fork, is to start by visualizing a fork with the springs removed (minimum mechanical spring effect) and NO air (maximum oil level=full, maximum pneumatic spring pneumatic spring rate) . It will not compress.

As you lower the oil level, you create a compressible air pocket. The larger the air pocket, the farther the fork must travel to bring the air pressure up to a given pressure. This air pressure resists fork compression just like the spring resists fork compression, except that as the air space gets smaller the spring rate gets higher ( the additional force required to compress the fork the NEXT inch is higher than the force required to compress the fork the first inch) this is known as a rising rate spring effect.

In a fully assembled fork, with springs and an air space, the two spring rates are additive. The smaller the air space, the quicker the pneumatic spring rate becomes significant. The larger the air space, the further the forks will compress before any significant pneumatic spring effect happens. With a very large air space, pneumatic spring rate could almost be negligible.



:dance:dance:dance
 
Ride report

I did a ride test (OK, twist my arm to get me out on the thing...) and I am very pleased with 235cc of 7.5 fork oil. It rides very nicely...not as harsh as before the change..either the PO had more fluid or thicker fluid, but I like the new ride.

Thanks to all..and a good spirited discussion too

S
 
An easy way to visualize the effect of oil LEVEL in the fork, is to start by visualizing a fork with the springs removed (minimum mechanical spring effect) and NO air (maximum oil level=full, maximum pneumatic spring pneumatic spring rate) . It will not compress.

As you lower the oil level, you create a compressible air pocket. The larger the air pocket, the farther the fork must travel to bring the air pressure up to a given pressure. This air pressure resists fork compression just like the spring resists fork compression, except that as the air space gets smaller the spring rate gets higher ( the additional force required to compress the fork the NEXT inch is higher than the force required to compress the fork the first inch) this is known as a rising rate spring effect.

In a fully assembled fork, with springs and an air space, the two spring rates are additive. The smaller the air space, the quicker the pneumatic spring rate becomes significant. The larger the air space, the further the forks will compress before any significant pneumatic spring effect happens. With a very large air space, pneumatic spring rate could almost be negligible.



:dance:dance:dance

I think your explanation is spot on. Very well done and accurate.

I would suspect that the amount of air space as designated by BMW's recommendations is probably in the non-significant category. My hesitation (which is contrary to my usual nature!) is to think that BMW didn't engineer these forks to be significantly sprung by air. However, I was reminded by Kurt, who was correct, that my usual position on these types of matters is to give new ideas a try (whether engineered by BMW or not!) and see what works!

I just have a "gut" feeling that that it has been tried and probably in theory works, but they found out that the results weren't satisfactory. Hence the lack of on-going conversation about the concept.

As a matter of fact, I have been working on my forks lately, and may experiment myself. My problem is I want to do it scientifically, but don't have that kind of time.

For me, and probably most casual riders (not racers), the front forks are adequate as long as they are in good condition with good springs. After all, they weren't great shocks/forks when they were new, but they were adequate for most riders.

I really enjoy this back and forth conversations as it really forces (me) to think using other's ideas and perspectives. Just this one thread has taught me a lot! Helps me understand physics principles on a variety of subjects (pneumatic, hydraulic, spring tensions, etc).
 
I did a ride test (OK, twist my arm to get me out on the thing...) and I am very pleased with 235cc of 7.5 fork oil. It rides very nicely...not as harsh as before the change..either the PO had more fluid or thicker fluid, but I like the new ride.

Thanks to all..and a good spirited discussion too

S

I may get shot for this, but here is what I just did:

After doing a minor rebuild this past weekend, I went with 250cc in each fork of 5w BelRay fork oil. Took the bike out yesterday afternoon, and I was happy with the performance on the roads and interstate (certanly better than before). Though, I am not a fussy rider. My main concern is a smoothly running engine. A plush ride isn't that much on my radar - that's for those guys who have a ton of money who can afford those big bikes that are more like an auto on 2 wheels - a big boat. That's why I usually like running both my front and back shocks/springs in stiff mode. It makes me feel like a "tough" guy!!!

Don't you just love these old airheads?
 
There is a slew of 70s bike models that had "air assist" forks. One of them the Honda CBX, first had it on the 1980 model. As the forks are almost identical to the 79 model, many modify their 79s to "air assist" by using 80 parts (fork caps and valves) I am not sure, whether the additional air pressure puts extra load on the fork seals and makes them leak easier. But when I park my KZ1300 with "air assist" forks for the winter, it will leak after just sitting for while. It does not leak while I ride it all summer. When I release the air pressure before storage, it will not leak either.
 
Since the pneumatic effect is only significant when the forks are significantly compressed, many manufactures of traditional style forks would use the pneumatic effect to help prevent fork bottoming on forks fitted with relatively soft springs. That way the could get a plush ride through most of the forks travel and have the pneumatic effect stiffen the forks as they approach being bottomed out.

Progressive springs and dual rate springs (a softer main spring with a short stiffer spring in each fork tube such as in the early K bike sport forks) kind of do a similar thing but can allow for more flexible design of suspension qualities.



:dance:dance:dance
 
Since the pneumatic effect is only significant when the forks are significantly compressed, many manufactures of traditional style forks would use the pneumatic effect to help prevent fork bottoming on forks fitted with relatively soft springs. That way the could get a plush ride through most of the forks travel and have the pneumatic effect stiffen the forks as they approach being bottomed out.

Progressive springs and dual rate springs (a softer main spring with a short stiffer spring in each fork tube such as in the early K bike sport forks) kind of do a similar thing but can allow for more flexible design of suspension qualities.



:dance:dance:dance

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
 
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

By adding air pressure to the forks, you are increasing the initial pressure in the air pocket to above atmospheric. Sort of like preloading a spring. That will cause the pneumatic function to be stronger and at an earlier ( less fork compression) point.



:dance:dance:dance
 
have you actually done this? Seen it done?
I ask, because I suspect that the only thing one would "feel" is a fork fluid shower upon the first compression action.

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.
 
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!!
 
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
 
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.
 
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!
 
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.
 
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. :laugh
 
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. :laugh

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.
 
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).

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/209...o4&network=g&gclid=CJD-yOX0iroCFcd_QgodaB8AqA

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


:dance:dance:dance
 
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.
 
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!
 
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