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KZ750 project

This is a discussion on KZ750 project within the Project Builds forums, part of the Caferacer.net Forums category; Originally Posted by brad black you can probably cut the fork springs down quite a bit to make them a decent rate, then preload them ...

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  1. #21
    Member Oldjeep's Avatar
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    Quote Originally Posted by brad black View Post
    you can probably cut the fork springs down quite a bit to make them a decent rate, then preload them up again. that'd have to help.
    I think for starters I will just get them back to proper stock and see what they feel like then. I'm assuming that they contain whatever was put in there in 1981. Not to mention that looking at the manuals provided there is an air pressure spec for the forks - no idea if there is any air in there Will check that tonight.

  2. #22
    jcw
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    it's interesting many of the japanese bikes at the time had two versions of the essentially the same bike. they set their cruiser version with trailing forks then corrected the trail with smaller offset triples. the standard with regular fork bottoms and larger offset triples. both bikes ended up riding similarly.

  3. #23
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    Pulled the forks off tonight and got one side cleaned and new seals put in. Need to pick up an 8mm impact hex tomorrow, the other side doesn't want to come out. The oil was nasty sludge.
    All in all pretty simple and no special tools required yet. I will need to pick up a fork pump to get the air pressure set right.

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  5. #24
    Senior Member brad black's Avatar
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    cut those springs in half they'd probably be about right.

  6. #25
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    Quote Originally Posted by brad black View Post
    cut those springs in half they'd probably be about right.
    Talk to me like I'm dumb. How does cutting the spring in half make the suspension more firm? Isn't that going to drop the suspension a bunch? Or are you suggesting cutting the progressive part of the spring off and then using a larger spacer on the top of the spring to preserve the ride height? I guess I would think that the first step would be to play with the air pressure - manual says stock is something like 8.5psi with a max of 35psi

    Worth noting that this bike is driven on MN roads, so some non jarring suspension travel is a good thing.

  7. #26
    Senior Member TrialsRider's Avatar
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    Did you replace the slide bushings? That's the part that actually wears out.
    More like 4 or 5 psi in your forks should be good if they will even hold it, otherwise play with your spring rates.
    woodsman likes this.

  8. #27
    Senior Member brad black's Avatar
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    spring rate is directly related to number of coils. cut it in half, twice the rate. by cut it in half i mean coil wise, not length wise on that spring shown in your pictures because the gap between the coils varies. you can calculate the rate of the spring pretty easily.

    you also need to be sure that the total sum of gaps between coils exceeds your fork travel and preload added together, otherwise it's going to coil bind. if you need 150mm travel i'd want at least 200mm potential travel sort of thing. 6" vs 8" if you're so inclined.

    then you make a spacer of some sort to give it the preload you want. usually 15mm or so, but that can depend on what sort of bike it is.

    i don't deal with this kind of skinny old shit much, but when i do i see springs that look like that. what i would do it work out the total travel (easy when it's out), then ride it around and work out how much travel you're using. i wouldn't use any air, that's just going to confuse things and it's not something you can quantify in rate / load terms.

    i dislike progressive springs for a couple of reasons. the first is that generally they start off way too soft, so all you get is a shitload of sag and excessive low impact dive. then they often get too hard at the end, especially when combined with the air spring effect. because they're old damper rod forks with no low compression damping that just makes them worse dive wise. get the initial rate good, and you don't really need a progressive. or a gentle progressive if you must, as long as the initial isn't too soft. it'll certainly help control dive under brakes, while making the from more stable in general.

    you can make a linear spring fork progressive with oil height variations. measure the height of the oil from the top of the compressed leg. don't add a given volume and call it done. the measurement is an absolute.

    if you want to get a feel for the air spring variation oil height gives, fill one of them with oil set to 100, 150 and 200mm variations then assemble it and bounce up and down on it. you'll feel how far less you can push it down at 100mm compared to 200mm.

    those forks are long, so the given oil volume might give a height of 300mm or lower, dunno. you sort of make it up as you go along. if you cut off that top section of the spring, for instance, and replace it with a thin wall steel preload tube, you've already decreased the material inside the fork, so you would go up in oil height to get the same air spring effect. if that makes sense.

  9. #28
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    Quote Originally Posted by TrialsRider View Post
    Did you replace the slide bushings? That's the part that actually wears out.
    More like 4 or 5 psi in your forks should be good if they will even hold it, otherwise play with your spring rates.
    No, only got seals and dust covers. I assume the bushings you are talking about were the ones on the inner rod that is bolted to the bottom of the fork that look like automatic transmission seals? If so then they seemed to still be good. Bike has about 20k on it.

  10. #29
    Senior Member brad black's Avatar
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    second photo, the split ring at the very lh end and the other steel ring on the slider under the seal washer. does it really have teflon bushes from 1981? man marzocchi were behind the times. even m1r don't have them.

  11. #30
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    Quote Originally Posted by brad black View Post
    second photo, the split ring at the very lh end and the other steel ring on the slider under the seal washer. does it really have teflon bushes from 1981? man marzocchi were behind the times. even m1r don't have them.
    Ok. The 2 parts you are describing are steel or something like that. The seal I was talking about is not in the pictures. It is on the short inner rod that holds the other spring. That seal is a phenolic type material like shaft seals used in automatic car transmissions.

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