Kawasaki GPX600R original rear shock: model number relevance

[Bru]

Hi BCF. Long time lurker, etc.

I have been working on a GPX600R-C1 (89 registered), and have a question about the rear suspension unit. This is because the bike has quite a pronounced weave coming up to 80 mph. Local MOT tester said that the rear suspension has too much sag, compared to the front, so the bike basically squats at rest. Alignment check was a 'pass'.

I have rebuilt the front end completely; while it can't hurt to re-check, let's just say that the basics are done (and did not draw the same type of comment). Tyres are quite new BT45's; pressure nominal. I also drained and refilled the original rear damper unit, which is quite simple to do, but probably opens a can of worms in terms of questions of oil viscosity and quantity (which I settled on after some browsing). No air added, as the hose is cracked.

The question about the rear - original - suspension is that I have discovered on eBay that there appear to be about three different model numbers for the original KYB rear suspension: 1308, 1440, 1367. This is by checking images, and the fact that my spare unit has a model number which does not match the 'original'.

So long story short: is there any correlation between KYB model number and bike model, eg. spring rate to suit the bike? I know that several different bike models have this type of rear suspension.

I could 'just' buy a new after-market unit, but with starting prices of ~ £300, I hope that some knowledge would lead to a far cheaper solution. I have written to Kawasaki, KYB UK and KYB Europe, but only KYB UK responded to pass me on to KYB Europe. Hopefully someone here understands the question (if posed correctly) well enough to give me a steer.

Thanks.

[Fizzer Thou]

Are all of the shocks relevant to your bike visually the same?

Are the spring rates the same for all?

If you looked for an aftermarket shock from say Nitron,Hagon,Ohlins or Penske,is there only the one shock from each specified for the GPX600R?
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[Bru]

Hi. Yes, the shocks all look the same.

I don't know about the spring rate, which is why I am trying to find out how relevant the model number is. I can only imagine it relates to spring rate - I should really measure the free length of the two units I have, but that requires removing one from the bike...

But otherwise, I would expect that most of the big names will have a single, standard offering, maybe with opportunities for customisation to suit the rider (like Nitron does, but at a serious cost).

[I like the standard unit, because the spring is enclosed within the plunger, and the plunger (chromed rod) is itself covered by a gaiter. As it is unpressurised (although like many 80's bikes, this can take a shot of air), replacing the damping oil is easy. Too practical, but in this case, maybe too far gone!]