Plugging up holes in fork stanchions

[Bhud]

Anyone know about vintage air assist systems?

I'm in the process of putting my Kawasaki GT550 back together. Like many 80s bikes, it has air-assisted forks. These are bog standard, simple 70s-style forks, with a single difference: they rely on air pressure and the forks' spring rate to provide resistance. There's a little hole along each fork stanchion, and a joining assembly between them, and a single Schrader valve that has to be pumped up (min. 11 psi).

I want to remove this air assist system and replace it with new progressive fork springs and spacers. The reasons: 1) it's nearly 40 years old, and while it still holds air and works, who knows when the rubber seals might give; 2) the front suspension works no better than a simple old-school motorcycle fork system; 3) I really want to give progressive springs a try.

I've been in touch with a few makers of prog springs, and they won't do it my way. They suggest replacing the entire fork internals with cartridge emulators, and new damping assembly. Plus use linear rate springs instead of progressive. However, I am pretty sure that I could get away with simply replacing the springs and putting the standard fork oil amount, if I could stop up those air assist holes on the stanchions.

Is this a dumb idea? If not, any thoughts on how I could plug up those holes?

[stinkwheel]

Hard to visualise what you have there but I'd imagine you could tap a thread into the holes and loctite a tapered plug in. What diameter is the hole?

https://www.ebay.co.uk/itm/1-16-2-BSPT-NPT-Male-Carbon-Steel-Countersunk-End-Plug-Hex-Socket-Hydraulic/283524266011?hash=item42035cf81b:g:6PIAAOSwffBdDcum
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[stinkwheel]

Another option to retain the air assist but do away with the complex seal arrangement would be to fit a couple of these in the holes. Schrader valves rarely fail and if they do, they are easily replaced.

https://www.ebay.co.uk/itm/2-GENUINE-SCHRADER-TYRE-VALVE-1-8-CAR-TRUCK-MOTORCYCLE-WHEEL-AIR-TANK-DRAIN-TAP/264530939715?hash=item3d9745cf43:g:I5oAAOSwY3VeeGxj
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Bhud]

Cheers, looks like it might work. The holes are about 3mm in diameter, so if I make them wider I could stuff them with one of those little plugs.
I'm hoping it won't come to that, because there's no turning back from that point. I thought of another solution: covering the holes with the lower fork yoke. To my mind, in theory, that should work, if I turn them so they face inwards and position the stanchions so that the holes face the steel of the yokes. This ought to be enough to stop a pumping-out effect. I'll give it a try with 15W fork oil and long spacers. The springs themselves look like normal springs, but are noticeable softer, with 2 clear coil frequencies. The replacements sold by the likes of Ikon appear to be just like-for-like, with exactly the same length. I think if I beef up the existing springs with a spacer, and cover up the holes, it should sort of work.

[F18]

Will the seals be happy in oil ?

I'd go with your manufacturer, build some designed for the job.
- actually, and especially as they don't leak now, I'd stick with the air; it worked okay.
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[Bhud]

You're right, it worked OK, but it was not a sophisticated system. Picture just an ordinary fork, with a spring and a damper rod and oil. Halfway up each stanchion, there's a 3mm hole. You put an air balancer (just a steel bracket with a cavity and a Schrader valve) in between where these holes are, when the bike is assembled. The balancer has rubber seals above and below, to hold in the air. You inflate it through the Schrader valve, and each fork now has internal air pressure higher than atmospheric pressure.

The only difference from any other basic motorcycle forks is this air assist balancer system, and softer springs. The thinking must have been, air under pressure provides a more progressive resistance than a spring on its own. The standard rear shocks have a similar system. As a result, this bike is incredibly comfortable. However, besides having crumbly old seals that look like they shouldn't be going back on, it's not a "tight" feeling ideal for handling. What I'm looking to achieve is a well-maintained suspension system, a specific amount of static sag with me on the bike (which you can't get with the standard suspension setup), a certain rebound rate suitable for hard braking, minimal dive, etc. The rear shocks have already been replaced (not with Chinese junk) to achieve the handling characteristics I want, and I would like the performance of the front to match.

[kramdra]

Can you get a photo? How does the air balancer part 51044 attach? Is it a split clamp with seals top and bottom?
A strip of rubber sheet blocking the air hole inside the balancer clamp would probably be adequate and reversable.

The fork tubes being quite thin are not ideal to take a thread for a plug.

[Bhud]

[Bhud]

Put back together, with 3-inch steel spacers. Bounced around a lot, with roughly 250ml 15W fork oil in each fork (exact level should be 255ml). No fork oil leaks. So far so good! Sag is in the right ballpark. Will have to take it for a brief run to check no oil is seeping before taking measurements and cutting precise spacers, and dumping the (new) oil and putting in the right amount. But I think I'm there.