air shocks

[grader]

if the top fork caps were drilled and fitted with schrader valves and even plummed together, can you run air pressure in the forks to firm them up? if so, how much? 5, 10, 15 psi? anyone done this on a gsf?

[crackin]

You probably could, but at the risk of blowing your fork seals. I wouldn't put more than a couple of psi in them. It's worth a try.

[grader]

i know that other bikes both dirt and street use air pressure just wondering if the fork seals are different. should hold some air to firm up the travel, just never read of anyone doing it on the gs.

[ShowBizWolf]

I know I saw a thread that talked a bit about this once... I've been searching but can't seem to find it. 

[barry905]

Thinking about the design of the forks, there appear to be three components that go to make up the performance characteristics: the oil that must pass through orifices in the piston head, the physical spring and the compression of the air in the cylinder. If you raise the initial pressure in the fork that will stiffen the air component of the spring curve, but I have no idea to what extent. Given that by doing so you would also have to ensure that, at a minimum, the top fork caps were properly sealed. I suspect that you can get the same results for more easily by changing the physical spring, either in length or spring rate.

But try it by all means.

Barry

[gregjet]

First , your forks are already partially air forks. You put the caps on at 1 atm , when you compress them to full travel the pressure rises. Ignoring the spring volume ( for simplicity the real value is higher because of the ration/volume change) , with a 4" to fork compressed fluid height, your air volume goes from 9" to 4" , so 1 atm to 2.25atm . As the air and fluid heats up during use that pressure will increase. Your seals will handle a lot more pressure if they are in good nick, than most people think.
The biggest problem with oiur type of forks and using air pressure to assist ther spring is the lip seal design starts to create a lot of drag against the fork leg ( called Stiction by some). The makes the damping/spring curve do funny things.
I used air cap valves a lot in the 70's and early eighties when fork springs weren't as easily available. Nowdays with these type of forks more practical to get the CORRECT spring rate...cause you can.
A set of GVE will make a huge difference as well.

[mr72]

Excellent points by gregjet.

Of course the amount of pressure the fork seals will take will be revealed when you exceed it Anwyay, no point in getting in a more technical discussion.

Point I wanted to bring up is that adding air above atmospheric pressure you also add preload. And the air spring is very progressive, so it will change the characteristics a lot.

And that extra air pressure also pushes on the oil directly and may lead to cavitation. IDK what pressures this matters for motorcycle fork oil. Mountain bike air shocks isolate the damper oil from the air chamber by mechanical means, often with the air spring in one leg of the fork and the damper in the other.

On a mountain bike, which I am most familiar and air shocks/forks are common (I currently have an air-spring fork and rear shock on my current MTB), most air forks have a soft spring in addition to the air to reduce preload and stiction, making the fork more supple and improving small bump performance a lot. The only reason mountain bikers generally choose air springs is because they are lighter than coil springs, especially in the rear shock. But coil springs outperform air springs 100% of the time.

I do think if you kept the air pressure pretty low it might give a meaningful fine tuning of bottoming resistance vs. small bump performance near the end of the travel (like while braking), but given the limits of pressure the fork seals may sustain, it might not be nearly enough to make a difference.

[gregjet]

As mr72 pointed out, the spring rate becomes particularly non linear , which I actually like ( other do not) , BUT ,    the damping is linear, so it makes it difficult to control the extention and compression evenly. Another reason it isn't a better way to to get your optimium suspension without accompanying damping matching.
Originally mtb didn't separate the air and oil, that came later ( thankfully). The oil would oxidize quite quickly ( months). You never forget the smell of oxidized fork oil , especially if it gets all over you from an air valve release....

[mr72]

I would also point out that you could also bleed air out of the fork so you wind up with less than atmospheric pressure with the fork topped out. Ordinarily when we assemble the for caps it's done with the front end suspended so that the fork is completely extended but when you put the weight of the bike back on the fork it sags 1-2" into the travel and this compresses that air column, so you might choose to use a valve to bleed off air with the bike at sag-height. This would make the bottoming performance less progressive, or more compliant. Or at least it would allow you to set the air pressure at the bike's static ride height, rather than just taking what you get. Adjusting preload or rear shock position, handlebar position, fork legs position in the yoke, etc. can change the sag on the front which has the effect of changing the air pressure, maybe you want to compensate?

Who's tried adding a valve to the GS's top caps?

[sledge]

There is another option.....a proven one!

http://gstwins.com/gsboard/index.php?topic=40097.0


Part no 5135101D60

[grader]

thanks for all the info, dont plan to do this mod as it really is the springs that are the weak link. will probably pick up a set in the future.

[ShowBizWolf]

Those caps are the ones it took me a while to track down. Thanks to Suzi Q, I was able to get them.
https://www.parts-sale.jp/SUZUKI%2051351-01D60?search=51351-01D60

And the O-Rings that go with them:
https://www.motosport.com/oem-parts/part-number/51117-38260