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  1. #1
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    Am I Just He-Man or is There Something Wrong

    Ok, So there's snow here in AK now and as such, I'm riding in my small chainring a lot. I notice that if I am cranking up hills in the granny, that I get some chain rub and can actually watch what appears to be the rear wheel flex over toward the chain. This only happens when I'm really cranking. I can ride the bigger gears and get no rub until I put a lot of power to it.

    This is on a Pugs with the LM rims. I guess maybe my questions are:
    1) Is it the wheel that is flexing and if so, should I get it in to the shop to have them check the tension on the spokes? They seem even tension to me, but I'm certainly no expert.
    2) Is it the rear triangle flexing that is causing the chain rub? If so, is there any chance that I'm going to damage the frame?
    3) I know there are issues with chain rub on fatties, is that all this is and I should stop being a worry wart and just ride the dang thing into the ground?

    Thanks all!

    By the power invested in me....


  2. #2
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    Quote Originally Posted by blockphi View Post
    Ok, So there's snow here in AK now and as such, I'm riding in my small chainring a lot. I notice that if I am cranking up hills in the granny, that I get some chain rub and can actually watch what appears to be the rear wheel flex over toward the chain. This only happens when I'm really cranking. I can ride the bigger gears and get no rub until I put a lot of power to it.

    This is on a Pugs with the LM rims. I guess maybe my questions are:
    1) Is it the wheel that is flexing and if so, should I get it in to the shop to have them check the tension on the spokes? They seem even tension to me, but I'm certainly no expert.
    2) Is it the rear triangle flexing that is causing the chain rub? If so, is there any chance that I'm going to damage the frame?
    3) I know there are issues with chain rub on fatties, is that all this is and I should stop being a worry wart and just ride the dang thing into the ground?

    Thanks all!

    By the power invested in me....

    Most likely tire flex.
    mtbtires.com
    The trouble with common sense is it is no longer common

  3. #3
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    2) Is it the rear triangle flexing that is causing the chain rub? If so, is there any chance that I'm going to damage the frame?


    I'd say you would have the opposite effect. When the chain has tension from the crank to the freewheel, the two would essentially get pulled together, thus the front part of the back wheel would move to the left.

  4. #4
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    blockphi I have had the same experience on both my pugs and moonlander. I think it is normal but it has also bugged me on the bikes. I have been intending to throw out this question but you did such a good job I guess I don't have to now. I don't think it is the tire. I suspect a combination of wheel and frame flex.
    laotzucycles.blogspot.com

  5. #5
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    Those tires move around quite a bit.....especially under herculean torque!


  6. #6
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    Quote Originally Posted by impalapower View Post
    2) Is it the rear triangle flexing that is causing the chain rub? If so, is there any chance that I'm going to damage the frame?


    I'd say you would have the opposite effect. When the chain has tension from the crank to the freewheel, the two would essentially get pulled together, thus the front part of the back wheel would move to the left.
    My thoughts exactly. I had a similar situation that the OP describes on a "Sneaux bike" I once had, but it wasn't flex from cranking, the dish was a bit off and at one point the trueness of the wheel went bad. The flex wasn't from the forces in the chain, but rather the side to side motion of cranking up hill, exaggerated when the wheel went out of true. It was an old MTB frame that I had a disc brake adaptor in, was never easy to get the rear wheel in that frame, plus if I remember right, I also had to have to old frame re-aligned prior to the re-dish also, so in time order:

    1. Frame was out of alignment in back.
    2. Wheel was originally dished to a slightly out-of aligned frame.
    3. Wheel goes out of true.

    Thus the opposite direction of flex when cranking uphill regardless of chain force.

  7. #7
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    It could be your wheel needs to be a little tighter, but more likely frame flex. The tyre flex will exaggerate the effect.

    Or you're a torque monster
    As little bike as possible, as silent as possible.
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  8. #8
    middle ring single track
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    On a Pugs...

    ...the wheel will pull to the right when the bike is pedaled with effort (as in standing up and "hammering") In the course of adapting my Pugs to belt drive I had an occasion to quantify the forces the chainstays see; I was mostly interested in how much the chainstay compressed but it indeed does move to the right---here's the test set-up:


    My whole write-up is here: DIY Pugsly Belt Drive

    In the Gates Manual :http://www.carbondrivesystems.com/do...ENG_REV041.pdf
    on page 23 there's a good drawing and description of this effect.
    The views and opinions expressed here are those of the author and do not represent any policy of the CA Dept. of Parks & Rec.

  9. #9
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    My necro does the same thing.
    Thought it was tire flex.

    interesting.....

  10. #10
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    The flex isn't necessarily a bad thing unless you want to fit a belt drive. It's all part of the feel of the bike.
    As little bike as possible, as silent as possible.
    Latitude: 5736' Highlands, Scotland

  11. #11
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    My Moonlander does it. Drop the small cog, throw a spacer in behind the big one. Adjust limit screws accordingly. Problem solved. Not much call for the 12 tooth on a MTB except for the rarest occasion.
    Cheers-
    What the EFF is "All MOUNTAIN"???

  12. #12
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    Sorry Steve- auto log-in here at the bike shop from your last visit.

    Chad
    What the EFF is "All MOUNTAIN"???

  13. #13
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    Thanks for all the responses. I guess I just find it a bit disconcerting coming from an aluminum bike that had no flex at all and prior to that destroying the rear triangle of an older steel mid-range Mongoose by cranking too hard through some mud. I think I'll try the spacer trick and just keep riding it. Nice thing about purchasing at REI, if I destroy it, I can always return it

  14. #14
    middle ring single track
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    Quote Originally Posted by blockphi View Post
    Thanks for all the responses. I guess I just find it a bit disconcerting coming from an aluminum bike that had no flex at all and prior to that destroying the rear triangle of an older steel mid-range Mongoose by cranking too hard through some mud. I think I'll try the spacer trick and just keep riding it. Nice thing about purchasing at REI, if I destroy it, I can always return it
    I wouldn't lose any sleep over the frame flex; try Googling "pugsly frame failure" and you won't find anything. If things don't flex some they tend to break; plus the ride "feel" is softer which can be a good thing.

    Aluminum frames need to be stiffer because of that metal's inferior fatigue properties.

    I think a Pugs frame will fail from rusting before it fails from flexing!

    (BTW; how heavy are you?)
    The views and opinions expressed here are those of the author and do not represent any policy of the CA Dept. of Parks & Rec.

  15. #15
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    I hover around 268 most days.

  16. #16
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    Quote Originally Posted by Rightcoaster View Post
    My necro does the same thing.
    Thought it was tire flex.

    interesting.....
    that damn assyemtrical chainstay is the weakspot on the pugs. becuase of this flex, inability to dispearse load equally, it will crack at the right hand chainstay where it meets the seat tube. on my second frame in two months

  17. #17
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    Quote Originally Posted by pliebenberg View Post
    I wouldn't lose any sleep over the frame flex; try Googling "pugsly frame failure" and you won't find anything. If things don't flex some they tend to break; plus the ride "feel" is softer which can be a good thing.

    Aluminum frames need to be stiffer because of that metal's inferior fatigue properties.

    I think a Pugs frame will fail from rusting before it fails from flexing!

    (BTW; how heavy are you?)

  18. #18
    will rant for food
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    Quote Originally Posted by captbuck View Post
    Yes, Pug frames "can" be broken.

    A Pug frame crack I saw similar to that one was warrantied.

    The other Pug I've heard of broken locally, the rider has a local rep of breaking "everything".

    Safe frame as far as I'm concerned.
    Latitude: 44.93 N

  19. #19
    will rant for food
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    Also, have you guys ever done an off the bike shimmy test?

    Seriously. Take your bike by the handlebars, no rider. Shake the thing hard left to right. Watch what the rear tire does during as well as a moment after the shaking. WooOOoooOOooo wacky shack!

    I guarantee you mine does it worse than yours because I built a flexy frame. But, they all do it. The wheels are heavy.

    After that discovery, I wondered - do all / a lot of bikes do this? Yes, a lot of bikes do this.

    You just noticed because of the very voluminous tire. I wouldn't sweat it.
    Latitude: 44.93 N

  20. #20
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    I stand corrected...

    Quote Originally Posted by captbuck View Post
    ...and how much do you weigh?

    (don't you mean "seatstay"?)
    The views and opinions expressed here are those of the author and do not represent any policy of the CA Dept. of Parks & Rec.

  21. #21
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    Quote Originally Posted by pliebenberg View Post
    ...and how much do you weigh?

    (don't you mean "seatstay"?)
    190

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