Are there advantages of a narrower chain line?- Mtbr.com
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  1. #1
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    Are there advantages of a narrower chain line?

    Hey Singlespeeders,

    I hope this is not a topic that has already beaten to death, but I just cannot find a convincing answer.
    I wonder whether there are any advantages of the middle chainring position over the outside chainring position when using a triple crank set for a SS conversion. Obviously assuming perfectly matched chain line by properly positioning the cog in the rear.
    The advantage of the outer position is better chain stay clearance and (in my opinion) cleaner looks. The question is whether the position of the rear cog on the outside of the freehub body has any mechanical disadvantage,

    Any ideas that go beyond guesswork?

    Thanks

    dK

  2. #2
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    With regards to the freehub, it all depends on where the bearings are in relation to the cog. Obviously, the freehub bearings may be more likely to last longer if the cog is placed directly above them. Less leverage and all that jazz.

    Off on more of a tangent is the possiblilty that a fame may flex less under power if the chainline is further inboard.
    I see hills.

    I want to climb them.

  3. #3
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    Riders with narrow hips or picky knees seem to have more preference.

  4. #4
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    Quote Originally Posted by OneBadWagon View Post
    Riders with narrow hips or picky knees seem to have more preference.
    Assuming the crank position doesn't change, and only the chainring position does, as the OP suggests, this wouldn't affect hips or knees.

    I highly doubt the frame flex issue would be that noticeable. We are talking about moving the chain roughly 5mm outwards--I just don't think the difference would be noticeable. The change wouldn't affect the amount of flex noticeable to the rider, only the amount of deflection the chain experiences. With a leverage arm of only 42-50mm, I doubt the change would have any real impact on the likelihood of the chain derailing.

    The bearing issue is a possibility, but again I'd be inclined to doubt it's real world relevance. In a laboratory setting it might make some difference, but on the trail, I highly doubt those extra millimetres would affect anything.

    I think there are only two real 'advantages', and they are limited in their importance.

    1) The most likely argument for most people 'round these parts - aesthetics. I think most people would agree that a chainring in the outer position looks better. It won't affect your speed or efficiency, but one of the benefits of an SS bike is that it looks awesome, so you might as well maximise that.

    2) Chainring clearance. This won't affect most MTB riders, but if someone is trying to squeeze the biggest chainring possible onto their frame, in theory the clearance will be much larger, if not essentially unlimited in the outer position. On many frames, the middle ring position will be limited in clearance to about 38 teeth or even less.

    If you are looking to maximise the strength/bearing life of your SS setup, getting an SS-dedicated hub is probably the best bet, as it gives you more even spacing on the spoke flanges, offering a non-dished wheel. Most SS MTB hubs fall somewhere in the 45-52mm chainline range, meaning some use the middle ring position, some use the outer, and some can use either (assuming they are a free-hub).

    Anyways, all that is to say that the chainring position probably doesn't really matter. Pick the hub and cranks you like, and pick the chainring position that makes the two mesh up.

    Done.


  5. #5
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    Quote Originally Posted by Stevob View Post
    Off on more of a tangent is the possiblilty that a fame may flex less under power if the chainline is further inboard.
    To clarify a bit on rereading the thread: frame flex has nothing to do with chainline. It is only affected by crank length and BB spindle length, really. Moving the chainring from middle to outer position won't change the amount of flex.

    The chainline of a chain in the outer position will deflect slightly more given the same amount of frame flex, but given the small difference in deflection (probably less than a couple mm's), and the generally flexible nature of chains, I doubt this would have any real world impact.

    Now, throw a belt drive into the equation and you've got other problems. Belt drives do not like deflection. But even then, the minute difference from middle to outer chainring position is unlikely to cause any real problems.

  6. #6
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    Quote Originally Posted by SSSasky View Post
    Assuming the crank position doesn't change, and only the chainring position does, as the OP suggests, this wouldn't affect hips or knees.
    You're right. I was thinking Q factor and was killing time on my phone. That'll teach me to pay more attention and read the post.

  7. #7
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    Quote Originally Posted by SSSasky View Post
    To clarify a bit on rereading the thread: frame flex has nothing to do with chainline. It is only affected by crank length and BB spindle length, really. Moving the chainring from middle to outer position won't change the amount of flex.
    My El Mariachi frame flexes visibly towards the drive side under power. It happens whether it is the left or right foot applying the torque. This indicates that the moment of force increases as the chain moves away from the centre of the bike. If chainline had no impact on flex, then the rear triangle wouldn't move to the side, regardless of how much force was applied. So yes, BB spindle length has an impact with regards to where the chainline sits (and therefore the leverage point), but crank length does nothing other than to increase/decrease the force applied at that point (yes I know that has a resulting effect on flex but it's not the point being discussed).

    The chainline of a chain in the outer position will deflect slightly more given the same amount of frame flex, but given the small difference in deflection (probably less than a couple mm's), and the generally flexible nature of chains, I doubt this would have any real world impact.
    The chainline will deflect linearly with the amount of flex. Inner or outer is irrelevant when flex is the same. I do agree on the lack of real world impact though.

    Happy trails.
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  8. #8
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    Quote Originally Posted by Stevob View Post
    My El Mariachi frame flexes visibly towards the drive side under power. It happens whether it is the left or right foot applying the torque. This indicates that the moment of force increases as the chain moves away from the centre of the bike. If chainline had no impact on flex, then the rear triangle wouldn't move to the side, regardless of how much force was applied. So yes, BB spindle length has an impact with regards to where the chainline sits (and therefore the leverage point),...
    I think you're right in theory and that the effect is minimal in reality. I moved my chainring to the outer position to get clearance with an oval ring and was thinking about the effect. If the chain / drive force went along the centre line of the bike there may be a little twist but shouldn't be any side flex. Move it say 1m to the side and there will be a lot of leverage and side flex under chain load. Moving the ring from middle to outer is about 8mm, so probably makes very little difference - maybe as much / little difference as a change from a 18 to an 19 or 20T cog, or a 5mm longer crank arm etc. Ideally I'd move it back in, but I'm not worried about my bearings or any frame flex as it is. The Hope SS hub I use has a bearing right at the end of the freehub body as well as a needle roller at the other end, so I'm even less worried.

  9. #9
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    Thanks everybody for the replies. It confirms so far what I already suspected: the mechanical impact of the chain line position might be minimal. But it was obviously one of my first world problems that needed to be addressed desperately.


  10. #10
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    Quote Originally Posted by Stevob View Post
    My El Mariachi frame flexes visibly towards the drive side under power. It happens whether it is the left or right foot applying the torque. This indicates that the moment of force increases as the chain moves away from the centre of the bike. If chainline had no impact on flex, then the rear triangle wouldn't move to the side, regardless of how much force was applied. So yes, BB spindle length has an impact with regards to where the chainline sits (and therefore the leverage point), but crank length does nothing other than to increase/decrease the force applied at that point (yes I know that has a resulting effect on flex but it's not the point being discussed).
    I think there may be a bit of a confusion here, possibly on both of our parts. First of all, we aren't talking about changing spindle length at all -- simply moving the chainring from the inner/middle to the outer position. So the leverage being applied by the cranks/rider doesn't change at all.

    Secondly, I think I may not understand what you are saying about your bike. If I've read your explanation correctly, when you apply force out the right crank, the BB actually shifts to the right side, against the force you apply. Assuming you are tilting your bike left and right in an out-of-the-saddle climb/effort, this would mean that as you push down with the right crank, you lean the bike to your left (normal so far). However, on your Mariachi, instead of the BB bending closer to the ground as it would on most people's bikes under the weight and force of their drive side crank, your BB actually lifts away from the ground, pushing you up, regardless of your weight and force.

    Is that correct? Yet when you apply pressure from your left crank (and lean the bike to the right), the BB bends towards the ground? If that's the case, I don't understand the physics at play. I have never seen a bike BB lift the rider further from the ground under force, but I may also be missing something / have misread you.

    Anyways, I've probably misunderstood, and that's okay. Bourbon has that effect.

    At the end of the day, I think we can all agree that the real world effects of narrower or wider chainline probably won't really affect your bike. In which case it must be urgently addressed with new, expensive, shiny parts.

    Ride on, friends!


  11. #11
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    wider chainlines, and wider q factors are a response to cassettes and more gears, not the rider's anatomy.

    unless you're 'big in the hips', the perfect ss fixed gear bike for most riders would have a narrow q factor and a reduced chainline, as was the case before multiple gears began to drive the geometry of frames.

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