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  1. #1
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    For Any Physics People Out There... Derailleur Return Spring Rate

    Or anyone that can help, really.

    I got some SRAM rear derailleurs (late model X.9 and X.0) with the main body / return spring in the parallelogram broken. I tried reshaping the X.0 spring but the Ti was brittle and kept snapping. No clue what happened to the other one.

    I can get a new spring in there, but there are no replacements available. No official replacements, that is.

    Amazon seems to have everything under their Industrial and Scientific category, including the right kinds of springs. They're available elsewhere as well I'm sure. But what kind is that?

    I can measure the overall length of the spring from another derailleur I have. But can anyone approximate a spring rate in lbs/in or N/m? Or, can anyone get a digital caliper and measure the diameter of the spring wire? Any specific material I should get (stainless vs. not)?

    Thanks!

    Edit: Using a ballpoint pen as reference, it appears that the spring wire is around .045-.055" (1.2-1.4mm) in diameter and measures around 1.25" long fully compressed.
    Last edited by pinkston32; 12-02-2011 at 06:24 PM.

  2. #2
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    I measured by XX and it is 1.57 mm diameter wire.

    Try ordering a couple different spring rates from McMaster Carr or MSC.

    I didn't take my derail apart to measure the rate, and my physics degree isn't helping at the moment, but I'd start with 10lb/in. Maybe buy a few different rates and save on shipping costs?

    Good luck.

  3. #3
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    Thanks for your help!

    Your estimate of 10lb/in is about right really. With the broken Ti spring (even though only around 2/3 of it is left), I measure 15lb/in. I attached one end to my bike stand and loaded the other end by attaching bricks to it, and it took 15 lbs to stretch the thing 1". I'll just look for something from 10-15 lbs/in.

  4. #4
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    Nice work on measuring the spring rate. I thought about suggesting this but wasn't sure how savvy you might be. Good to know smart people are on this forum.

    Given you measured 15lbs, I wouldn't go any less, it would likely cause worse shifting. Too much spring rate and you will have a really strong thumb!

    Good luck.

  5. #5
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    I know what you mean; one can't be sure what to expect from people these days, especially on the interweb.

    I've got an XTR shifter, so I can live with a higher spring rate. I'm running 10spd Dynasys with a SRAM rd and it works fine.

  6. #6
    namagomi
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    I've often toyed with increasing the spring rate so the rear mech resists freezing and clogging with mud... pinkston have you considered increasing it a bit?

  7. #7
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    I was thinking about it, but since I only have a very rough estimate of what the actual spring rate is, I'm not sure how much I would want to bump it. I'll definitely skew to the higher end of the range, and will see if I can pick up multiple springs with different rates.

    I generally ride in dry and dusty conditions, and even in winter my rd gets muddy but not clogged. Really my derailleur shifts to a higher gear plenty fast I think.

    How did your spring rate experiments go?

  8. #8
    namagomi
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    Quote Originally Posted by pinkston32 View Post
    I was thinking about it, but since I only have a very rough estimate of what the actual spring rate is, I'm not sure how much I would want to bump it. I'll definitely skew to the higher end of the range, and will see if I can pick up multiple springs with different rates.

    I generally ride in dry and dusty conditions, and even in winter my rd gets muddy but not clogged. Really my derailleur shifts to a higher gear plenty fast I think.

    How did your spring rate experiments go?

    I've just adjust my old shimanos so that the pulley spring is stronger by changing the hole in which the arm sits, but never the parallelogram spring. So I only toyed with the idea in my head. For instance, does shimano's SAINT line have a higher spring rate? What are the wear effects on the gearing in the shifter pod and the rivets on the derailleur cage with a stronger spring.

  9. #9
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    Ah I see. Was there a noticeable difference? I'll probably try the same with my derailleur.

    Apparently Saints do have a higher spring rate. "Shimano Saint M810 Rear Derailleurs feature an integrated skid plate, increased spring tension,and a rigid structure that resists twisting and wear." Saints look pretty cool to me.

    Not sure about the wear, but it shouldn't be that much more.

  10. #10
    namagomi
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    Quote Originally Posted by pinkston32 View Post
    Ah I see. Was there a noticeable difference? I'll probably try the same with my derailleur.

    Apparently Saints do have a higher spring rate. "Shimano Saint M810 Rear Derailleurs feature an integrated skid plate, increased spring tension,and a rigid structure that resists twisting and wear." Saints look pretty cool to me.

    Not sure about the wear, but it shouldn't be that much more.
    Marginal improvement, I think originally it was designed to prevent dropped chains. I am not sure. Moving to a friction shifter helped my situation, but ice still poses and issue at certain temperature combinations.

  11. #11
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    Interesting. I ride in whatever weather is here, but in GA, it doesn't really get that cold. The worst weather related issue I've had on my bike was when my pedals were getting jammed with ice (riding in the snow). Haven't had any other issues. I'll see if I notice a different and let everyone know how my spring experiments turn out.

  12. #12
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    I like this thread. It has me all learnin' and stuff. Subscribed.
    Instead of rims, put sundials on your wheels and run for the hills...

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  13. #13
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    This is something completely new, and I don't know of anyone else (online, and I search the internet pretty well!) that has done this sorta thing. This could be a breakthrough. This could be a whole new frontier of derailleur tuning!

    Maybe not. But still cool anyway. But some interesting stuff can be done. I'd imagine if I can get the spring between the opposite pivots in the parallelogram I could make a "rapid rise" type derailleur.

    As to my progress, I've done something.

    Why trying to measure the spring rate of the broken Ti spring, I did manage to rather cleanly get the two ends bent like they should be, with the two loops at the end to hook around the pivot. So I managed to stuff it back in there, and it works! I can't be sure that the plier-bent spring ends won't unbend somewhere along the way, but for now I'll try it out.

    No worries, I will still continue with finding a new derailleur spring (since I also have an X.9 with the same problem ).

    Unrelated, I also managed to make an improvised short cage for the X.0 which had a broken carbon outer plate and bent alloy inner plate. I just cut them both and drilled new holes for the pulleys. Seems pretty good.

    As for more background of how I managed to end up with this clusterf***ed derailleurs... I bought a lot of 3 broken ones off the bay for $30, an X.0 and 2 X.9. Only one X.9 had a main spring. Missing screws, cage parts, broken cage, etc. but that one X.9 is fully functional. Hey, at least that one is worth at least $30 itself.

  14. #14
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    Quote Originally Posted by pinkston32 View Post
    Thanks for your help!

    Your estimate of 10lb/in is about right really. With the broken Ti spring (even though only around 2/3 of it is left), I measure 15lb/in. I attached one end to my bike stand and loaded the other end by attaching bricks to it, and it took 15 lbs to stretch the thing 1". I'll just look for something from 10-15 lbs/in.
    If 2/3 of the spring gave 15lb/in, then a whole spring should give 10lb/in I reckon. So I would err on the lower side of your range to match the factory spring.

  15. #15
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    Exactly what I was thinking. But at the moment with 2/3 of the spring stuck in there anywhere it is fine. The tension is just barely noticeably increased, but that's in relation to X.9 derailleurs that may have different tension to begin with. I think I'll try one on the lower end (12-13ish lb/in) and one slightly higher (15-17ish lb/in).

  16. #16
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    Quote Originally Posted by pinkston32 View Post
    Exactly what I was thinking. But at the moment with 2/3 of the spring stuck in there anywhere it is fine. The tension is just barely noticeably increased, but that's in relation to X.9 derailleurs that may have different tension to begin with. I think I'll try one on the lower end (12-13ish lb/in) and one slightly higher (15-17ish lb/in).
    Very cool! I'm interested in how this works out--please publish your findings if you get a chance. It's a funny thing: those who know to seek the advice of a physicist are often pretty decent ones themselves (not a physicist myself).

  17. #17
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    Yeah it's funny, since we just got through springs and forces in AP Physics, not that F=-kx helps me at all... Here I am trying to find k, even though it's supposed to be given in the problem!

    Now that I think of it, I really shoulda taken a pic of the spring. I'll get some pics sooner or later, with the whole thing.

  18. #18
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    Pictures of the product!

    Derailleur with modded cage (and interesting dropout... )




    This end of the spring had to be looped in...


    But the other end I was able to remove the pivot to put the spring in.

  19. #19
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    You have all the right ingredients - carbon fiber, titanium, and zipties!

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