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Lowering forks. BMW K75S cafe build.

This is a discussion on Lowering forks. BMW K75S cafe build. within the Project Builds forums, part of the Caferacer.net Forums category; Originally Posted by Panchoplanet What about cutting down the new progressive springs, say a 1/2 inch. That may be what I'm looking for, just have ...

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  1. #21
    Senior Member hillsy's Avatar
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    Quote Originally Posted by Panchoplanet View Post
    What about cutting down the new progressive springs, say a 1/2 inch.
    That may be what I'm looking for, just have to figure out what dust seals to order so I can delete the accordion gaiters.

    Pp.
    Your new progressive springs will come with a big long spacer. The springs will be shorter than the stock ones, so you just cut down the spacer. The spacer is just PVC so it's easy.

  2. #22
    Senior Member Farmer_John's Avatar
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    Quote Originally Posted by brad black View Post
    that's the point.
    I was following along.

    In a simple damper rod fork, there's only one way to change damping. Lighter or heavier oil. It's always going to be a compromise and never optimal.

    But I believe the progressive springs to not be the direction I'd travel. Cut a coil or two at a time off the stock springs, get some heavy duty PVC such as this

    Name:  threadedriser12.jpg
Views: 77
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    at 2.75 for a foot, it's a cheap way to set your static sag.

    Then play with it until you're happy with it.

  3. #23
    Senior Member brad black's Avatar
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    Quote Originally Posted by Panchoplanet View Post
    What about cutting down the new progressive springs, say a 1/2 inch.
    Pp.
    you can move forward with baseless guessing if you like, they celebrate that sort of brainless shit over on dotheton.

    spring rate is calculated from a fairly simple formula based on the wire diameter (d), the mean diameter of the coils (D), the number of coils (n) and young's modulus (G), an engineering constant express in Mega pascals.

    in kg/mm, rate = (G * 100 * d * d * d * d) / (8 * n * D * D * D)

    the k75s specs i found say it has 41mm forks (probably showa from memory) and ime showa 41mm springs tend to have wire 4.5mm diameter and coils 31.65mm mean diameter, and G is 76.9 so that leaves you needing about 13 coils for a 0.9kg/mm spring, which given the 225kg wet weight i'd say is the minimum you'd want.

    problem with that is the space between each coil times the number of coils gives you the compressed length (coil bind), and you might not have enough for the travel. my list shows 12mm b/w coils for ducati springs - 12 x 13 is 156mm so maybe if you fit some internal spacers to cut the travel back to 100 - 110mm or so.

    and even at that rate, it'll probably need quite a bit of preload, which you have to add to your coil bind calc. the st series ducati i fit 0.95kg/mm springs to usually, and even with that you need 25mm preload, whereas most times 15mm preload is nice.

    if you find it bottoming out with the lesser travel you add oil to the forks to increase the air spring effect. 10mm change in oil height at a time is a good start.

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  5. #24
    Senior Member kenessex's Avatar
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    I have had pretty good results with Progressive Suspension springs in racebikes for the past 30 years or so, by setting the preload via PCV spacers and using the right weight and volume of oil. That being said, I believe that the trend to the proper weight straight wound springs and the proper weight and volume of oil with emulators will give a better result. That is what I have in my SV650 and it is good. That is also what I would suggest for the K if the cut down stock springs with an internal travel limiter and the correct preload and oil doesn't cut it.
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  6. #25
    Junior Member Panchoplanet's Avatar
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    Quote Originally Posted by Farmer_John View Post
    Quote Originally Posted by brad black View Post
    that's the point.
    I was following along.

    In a simple damper rod fork, there's only one way to change damping. Lighter or heavier oil. It's always going to be a compromise and never optimal.

    But I believe the progressive springs to not be the direction I'd travel. Cut a coil or two at a time off the stock springs, get some heavy duty PVC such as this

    [iurl="http://www.caferacer.net/forum/attachment.php?attachmentid=58353&d=1492394709"]Lowering forks. BMW K75S cafe build.-threadedriser12.jpg[/iurl]

    at 2.75 for a foot, it's a cheap way to set your static sag.

    Then play with it until you're happy with it.
    Are you suggesting taking out the stock spacers and adding longer ones with the pvc pipe?

    Yes I'm new at this, but all suggestion and advice will be greatly appreciated.

    Did find a seal / wiper kit on ebay for the K75 , $14 and it's on order.
    Still sourcing other parts and components including a Koso speedo that's going to sit under the Venon headlight fairing.

    Pp.

  7. #26
    Senior Member hillsy's Avatar
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    OK - all talk about spring rates and damping aside, here is how you "lower" a fork in a standard damper rod style fork with a top spacer:



    If you don't have a top spacer, then you add one to the damper rod and cut the top spring to suit.

    Of course, this reduces travel and has the potential to bottom out unless you change the springs to suit.

  8. #27
    Junior Member Panchoplanet's Avatar
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    Quote Originally Posted by hillsy View Post
    OK - all talk about spring rates and damping aside, here is how you "lower" a fork in a standard damper rod style fork with a top spacer:



    If you don't have a top spacer, then you add one to the damper rod and cut the top spring to suit.

    Of course, this reduces travel and has the potential to bottom out unless you change the springs to suit.
    Thank you for that simple explanation, will explore the notion while resealing forks, I don't want to compromise handling and ride quality, I'll settle for a half inch reduction in length just to keep it safe.

    Need to focus on other aspects of this build like electrics, exhaust, seat design and cleaning up cosmetics in order for this to come together.

    Pp.

  9. #28
    Senior Member brad black's Avatar
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    the problem with all of this is you're working on the assumption that anything you do will be appropriate. and that's just wrong. the whole space it x then cut x off the springs or x off the preload tube is not good. reduce your travel by 2 inches with the same or marginally increased spring rate? what happens when you hit a bump that would have used the last 2" of travel? i just don't get the idea behind that.

    you need to measure your loaded sag is. then ride it around for a while and see how much travel you use. then you pull a fork out and compress it and find out how much load you have at the sag setting and the maximum travel you use. it's not that hard to work out - i use a press and a set of electronic scales. then you have end points of how much you need your forks to support and you can also test the air spring effect your forks have and then you decide what spring rate you need and what oil height you need to use to hit the targets. then you'll have a reduced travel fork sprung to work on your bike.

    suspension isn't a black art. at this level - damper rod forks and springs - it's pretty basic science.

  10. #29
    Junior Member Panchoplanet's Avatar
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    Oops, ordered seals thinking showa forks, wrong ?
    I have Brembo forks, it says Brembo on back, (yes I'm a brick newbie) . Now I'm having no luck finding dust seals for these bremdos, no luck with Google or ebay.
    I did the 35mm film wipe on oil seal 2 days ago and no leak so far, good thing I didn't take it apart.

    I have too many items on list to get involved with forking mods at present like lowering!,so just concentrating on getting bike together and running. Before you know it winter will be back and I'll have plenty of time to tinker.

    All comments and responses much appreciated, and if anyone can enlighten me on these Brembo fork dust seals that would help me a ton. ?

    Pp.

  11. #30
    Senior Member steveo's Avatar
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