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CM400t build/new member intro

This is a discussion on CM400t build/new member intro within the Project Builds forums, part of the Caferacer.net Forums category; Okay, I'll keep that in mind. The rear shocks are actually in good shape, so for now they stay. The front forks are a little ...

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  1. #11
    Senior Member gearheadE30's Avatar
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    Okay, I'll keep that in mind. The rear shocks are actually in good shape, so for now they stay. The front forks are a little more pressing, as they are a bit leaky...in my eyes, they have so little damping it is a safety hazard. I have new seals for these (came with the bikes) and the shafts aren't pitted at all, so either I will rebuild these with 15w fork oil or something heavier and cut the springs and add spacers (hack, I know, but I can't find stiffer springs for these forks, and they are ridiculously soft. Yes, I will check for coil bind and make sure droop and compression are still correct. I'm not going to lower the bike this way either. I like suspension travel.) to get more stiffness, or upgrade... forks are 33mm. For my off-track use and probable future upgrade, I don't want to spend a ton to run emulators and such, at least not at this point.
    '79 GS750E for sale HERE
    '86 XT350
    '91 BMW 318is Turbo
    1989 Caprice Classic wagon 5.7 TBI, 5 speed manual
    2005.5 KTM 950 Adventure

  2. #12
    Senior Member oldhondacafe's Avatar
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    I don't see how cutting the spring then adding spacers will help? If you want stiffer just add shims. maby 1/2" at a time? The problem is the origional spring has lost some tention from sagging. Adding spacers will take up any slack and add preload. This is a temporary fix, and may or may not help, and you can only add so much till the spring coil binds at full compression. If you could switch to 35mm forks (cb450,cb750, xl250, etc.) You mighe do better. Then if you got, say cb450 forks that are 29" long and looted the springs from a bike with 31" forks you might get much stiffer? It takes some looking and tinkering (and time) but it's cheap if you are patient enough to get the right parts. I don't know specificaly about the cm400, but I have a lot of different cb450-750 35mm forks I Could measure for you if you are interested and want that info. Those bikes are both much nicer than anything I usualy start with, nice score!
    \'78CB750f,\'76 cb500t,\'5-CB/CL450 basketcases,\'68 CL175,\'66 CA77/in a CL72 frame,\'1971 DT1f,1988cbr1000f,1987cbr1000f

  3. #13
    Senior Member oldhondacafe's Avatar
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    I don't see how cutting the spring then adding spacers will help? If you want stiffer just add shims. maby 1/2" at a time? The problem is the origional spring has lost some tention from sagging. Adding spacers will take up any slack and add preload. This is a temporary fix, and may or may not help, and you can only add so much till the spring coil binds at full compression. If you could switch to 35mm forks (cb450,cb750, xl250, etc.) You mighe do better. Then if you got, say cb450 forks that are 29" long and looted the springs from a bike with 31" forks you might get much stiffer? It takes some looking and tinkering (and time) but it's cheap if you are patient enough to get the right parts. I don't know specificaly about the cm400, but I have a lot of different cb450-750 35mm forks I Could measure for you if you are interested and want that info. Those bikes are both much nicer than anything I usualy start with, nice score!
    \'78CB750f,\'76 cb500t,\'5-CB/CL450 basketcases,\'68 CL175,\'66 CA77/in a CL72 frame,\'1971 DT1f,1988cbr1000f,1987cbr1000f

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  5. #14
    Senior Member gearheadE30's Avatar
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    Adding shims won't make a spring stiffer. All that does is change the location of the fork in the travel. If you add enough preload to the point where there is little to no droop, then it will feel a little stiffer, but that is far beyond optimal.

    I'm not sure if the factory springs are progressive or not, but cutting them does actually make them slightly stiffer (note: if the spring gets hot, you will start to anneal the metal, and the spring will lose stiffness. Cut with a hacksaw or a cutoff wheel). A coil spring is basically a torsion bar twisted in a circle. The stiffness of a torsion bar made of a certain metal is determined by its diameter and the length of the bar. If you cut some length off of a coil spring, you are essentially removing some length from the torsion bar, thus making it stiffer. This is why if you take two equal length springs, with the same wire diameter, but one with closely spaced coils and one with coils spaced further apart, the closely-spaced coils will always be softer. If the spring is progressive, cutting the springs has an even greater difference because you can cut specifically on the softer end of the spring, and the endmost coils are often 'dead', at least in cars. This will stiffen the spring and shorten it with no coil bind issues. Hard to explain this without pics... The trick has been around in the hotrod/muscle car crowd for a long time. Not to say it's the best way, but it does work.

    I know about the coil bind issue-I'll be checking for that and if my plan is even workable once I get it all apart. I know the spring has lost tension, but I'm just working with what I've got, hah.

    I'm going to do some more research and see if there are any triple clamp options that would fit easily on my frame that would allow 35mm forks, and then I may be emailing you. Again, though-longer springs from other forks will raise the front end without a rate change, unless static compression is significantly greater. What might work, though, would be springs from, say, a CB900 or something that are likely stiffer from the factory due to the extra weight. Like you said, lots of time and tinkering haha.

    Thanks for the compliment; I wasn't expecting the bikes to be anywhere near this good, or even really running well, when I went to look at them. Needless to say, I was pleasantly surprised!


    EDIT: look http://en.wikipedia.org/wiki/Spring_%28device%29, under 'theory', and there is a good equation for spring stiffness. spring force has negative association to number of active coils.
    '79 GS750E for sale HERE
    '86 XT350
    '91 BMW 318is Turbo
    1989 Caprice Classic wagon 5.7 TBI, 5 speed manual
    2005.5 KTM 950 Adventure

  6. #15
    Senior Member gearheadE30's Avatar
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    Adding shims won't make a spring stiffer. All that does is change the location of the fork in the travel. If you add enough preload to the point where there is little to no droop, then it will feel a little stiffer, but that is far beyond optimal.

    I'm not sure if the factory springs are progressive or not, but cutting them does actually make them slightly stiffer (note: if the spring gets hot, you will start to anneal the metal, and the spring will lose stiffness. Cut with a hacksaw or a cutoff wheel). A coil spring is basically a torsion bar twisted in a circle. The stiffness of a torsion bar made of a certain metal is determined by its diameter and the length of the bar. If you cut some length off of a coil spring, you are essentially removing some length from the torsion bar, thus making it stiffer. This is why if you take two equal length springs, with the same wire diameter, but one with closely spaced coils and one with coils spaced further apart, the closely-spaced coils will always be softer. If the spring is progressive, cutting the springs has an even greater difference because you can cut specifically on the softer end of the spring, and the endmost coils are often 'dead', at least in cars. This will stiffen the spring and shorten it with no coil bind issues. Hard to explain this without pics... The trick has been around in the hotrod/muscle car crowd for a long time. Not to say it's the best way, but it does work.

    I know about the coil bind issue-I'll be checking for that and if my plan is even workable once I get it all apart. I know the spring has lost tension, but I'm just working with what I've got, hah.

    I'm going to do some more research and see if there are any triple clamp options that would fit easily on my frame that would allow 35mm forks, and then I may be emailing you. Again, though-longer springs from other forks will raise the front end without a rate change, unless static compression is significantly greater. What might work, though, would be springs from, say, a CB900 or something that are likely stiffer from the factory due to the extra weight. Like you said, lots of time and tinkering haha.

    Thanks for the compliment; I wasn't expecting the bikes to be anywhere near this good, or even really running well, when I went to look at them. Needless to say, I was pleasantly surprised!


    EDIT: look http://en.wikipedia.org/wiki/Spring_%28device%29, under 'theory', and there is a good equation for spring stiffness. spring force has negative association to number of active coils.
    '79 GS750E for sale HERE
    '86 XT350
    '91 BMW 318is Turbo
    1989 Caprice Classic wagon 5.7 TBI, 5 speed manual
    2005.5 KTM 950 Adventure

  7. #16
    Senior Member oldhondacafe's Avatar
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    The stock springs are well known for looseing tention over the years wether they are progressive or not You'll get something from shimming them, possibly close to what you'll get if you can determine what end of the spring to cut off. The deal is, if it dosent work, you can take the shim back out. Can you weld the spring back together if it dosen't work when you cut it? I'd personaly try this first, if I'm wrong I'll have to drink two or three extra beers to get over my shame. If you're wrong you'll be buying parts. Just saying...
    \'78CB750f,\'76 cb500t,\'5-CB/CL450 basketcases,\'68 CL175,\'66 CA77/in a CL72 frame,\'1971 DT1f,1988cbr1000f,1987cbr1000f

  8. #17
    Senior Member oldhondacafe's Avatar
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    The stock springs are well known for looseing tention over the years wether they are progressive or not You'll get something from shimming them, possibly close to what you'll get if you can determine what end of the spring to cut off. The deal is, if it dosent work, you can take the shim back out. Can you weld the spring back together if it dosen't work when you cut it? I'd personaly try this first, if I'm wrong I'll have to drink two or three extra beers to get over my shame. If you're wrong you'll be buying parts. Just saying...
    \'78CB750f,\'76 cb500t,\'5-CB/CL450 basketcases,\'68 CL175,\'66 CA77/in a CL72 frame,\'1971 DT1f,1988cbr1000f,1987cbr1000f

  9. #18
    Senior Member gearheadE30's Avatar
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    True, shims are reversible. Once I get it all apart I'm going to check the rest length (I know it isn't tension but its as close as I can get in my garage) and see how close it is. I'll have to see what it looks like and then go from there on shimming/cutting. The ride height and sag are about right though right now, and the last thing I want to do is raise the front end more with shims. But yeah, you're probably right. I may just try shims as first and see how that goes. I can always take it apart again later.

    And no, you can't weld a spring. The heat will cause it to lose tension, thus making it not much of a spring anymore.
    '79 GS750E for sale HERE
    '86 XT350
    '91 BMW 318is Turbo
    1989 Caprice Classic wagon 5.7 TBI, 5 speed manual
    2005.5 KTM 950 Adventure

  10. #19
    Senior Member gearheadE30's Avatar
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    True, shims are reversible. Once I get it all apart I'm going to check the rest length (I know it isn't tension but its as close as I can get in my garage) and see how close it is. I'll have to see what it looks like and then go from there on shimming/cutting. The ride height and sag are about right though right now, and the last thing I want to do is raise the front end more with shims. But yeah, you're probably right. I may just try shims as first and see how that goes. I can always take it apart again later.

    And no, you can't weld a spring. The heat will cause it to lose tension, thus making it not much of a spring anymore.
    '79 GS750E for sale HERE
    '86 XT350
    '91 BMW 318is Turbo
    1989 Caprice Classic wagon 5.7 TBI, 5 speed manual
    2005.5 KTM 950 Adventure

  11. #20
    Senior Member Geeto67's Avatar
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    dude.....bio....fill it out
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

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