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Help Honda CB125S USA spec - I need better fork springs.

Discussion in 'Honda 250cc Singles' started by grcamna5, Oct 3, 2021.

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  1. DannoXYZ

    DannoXYZ Well-Known Member

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    Solve equation for ending higher rate, that's all that matters when you're close to bottoming. Because softer end coil-binds and is removed from operation to yield the higher rate. You'll be doing same thing pre-emptively by cutting off softer coils anyway.

    Progressive spring went out of fashion decades ago. Because they cause too many compromises with damping. Fork will be overly damped and harsh at initial travel because of low spring-rate. Then on big bumps when they're close to bottoming, damping will be too soft and allow fork to rebound to quickly. Ends up with pogo-stick for fork.
     
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    DannoXYZ, Mar 14, 2022
    #41
  2. grcamna5

    grcamna5 Active Member

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    Danno,when you say "softer coils",are those the close-wound coils on one end of the springs which I'll be cutting off ?
    I like giving myself a 'straight rate' set of springs.
    Do you know how much I would need(not too good with math)to cut off to arrive at .50kg straight spring rate ?
     
    grcamna5, Mar 14, 2022
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  3. DannoXYZ

    DannoXYZ Well-Known Member

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    Yes, softer end of spring is where coils are wound closer together.

    Ok, so let's plug into equations:

    R0 = 0.34kg/mm
    L0 = 457mm
    Lf = final length
    Rf = 0.50kg/mm

    Rf = R0(L0/Lf)
    0.50 = 0.34*(457/Lf)
    Lf = 0.34*(457)/0.50
    Lf = 311mm final-length
    457-311 = 146mm to chop off

    Now spring will still be progressive, but just not with as wide range as before. For more square end, cut off only 140mm or so. Then grind end so it's square to arrive at 146mm.

    Another thing to consider is preload, unknown at moment. So measure how much of original spring sticks out of fork above where end-cap is. If end-cap bottom sits 5mm below top of fork, with spring sticking out 5mm, then you've got 10mm of preload. You might want to add tiny bit more pre-load to 15mm for higher initial height to give you little more suspension travel. In which case, use 146mm+5 = 151mm spacer to replace 146mm of spring you cut off.
     
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    DannoXYZ, Mar 15, 2022
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  4. grcamna5

    grcamna5 Active Member

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    I'm going for it;thanks Danno. :thumb_ups:
     
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    grcamna5, Mar 15, 2022
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  5. grcamna5

    grcamna5 Active Member

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    I just today had them cut and the cut end shaped/tapered by a metal worker with a smal(3"?)one-hand air powered cut-off wheel;this worker did it 'to my spec' ! The next thing is to measure my preload and install :thumb_ups:
     
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    grcamna5, Mar 17, 2022
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  6. grcamna5

    grcamna5 Active Member

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    Danno,I also just found a pair of "adult rate" fork springs manufactured here in OK. USA by a company called Performance Spring,inc. This set of springs are the correct O.D. dimension and the length is 16 1/4" long and the spring rate is 32 lbs/in. They are sold to replace the stock fork springs on a 2001' Yamaha TTR90 mini-dirtbike.
    I'm considering purchasing them as another option for fork springs and want to see if you know how "32 lbs/in." would compare to .50kgs ? Thanks,Bill ;)
     
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    grcamna5, Mar 17, 2022
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  7. DannoXYZ

    DannoXYZ Well-Known Member

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    Let's do maths, we can convert by canceling out units:

    lb/in * kg/lb = kg/in * in/mm = kg/mm

    32 lb/in * 1kg/2.2lb * 1in/25.4mm = 0.57 kg/mm
     
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    DannoXYZ, Mar 18, 2022
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  8. DannoXYZ

    DannoXYZ Well-Known Member

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    Woohoo, progress!!!

    I hope you measured preload before cutting. Now that they're cut, how do you know how much preload there was originally???
     
    DannoXYZ, Mar 18, 2022
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  9. grcamna5

    grcamna5 Active Member

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    I didn't check but vaguely remember the springs were just down a few threads inside.

    Thanks for that info on those 32lb/in.
    Those springs would still require me to add 1 3/4" spacers to bring them up to 18".. and that may cause them to be too stiff for my purpose.I'll see how these ones that I just had cut feel considering brake dive etc. being at .50kg
     
    grcamna5, Mar 18, 2022
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  10. grcamna5

    grcamna5 Active Member

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    Excellent explanation,thanks Danno :thumb_ups:
     
    grcamna5, Mar 19, 2022
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  11. grcamna5

    grcamna5 Active Member

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    I have cut my stock fork springs and installed them with the long PVC spacer,plus I added about 20cc more 15wt. fork oil.
    I like how it's tightened-up my front end ! :D Stiffened it up just right;could use a bit more rebound.
    I will probably switch over to 20wt. fork oil and possibly add more of it to raise the level inside the forks..
    The stock spec oil was 10wt and 85cc.
     
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    grcamna5, Apr 9, 2022
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  12. DannoXYZ

    DannoXYZ Well-Known Member

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    hey good job!

    Don't make too many changes at once. Higher oil-level will kinda countreact heavier oil. Start with changing to heavier weight 20w oil 1st of same level.
     
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    DannoXYZ, Apr 10, 2022
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  13. grcamna5

    grcamna5 Active Member

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    Ok,thanks.
     
    grcamna5, Apr 10, 2022
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