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Thread: Chain Rub

  1. #1
    Senior Member koolerb's Avatar
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    Chain Rub

    I recently picked up a super clean 2 X 8 speed Shimano 105 road bike. When on the the small front ring I get chain rub on the big ring when I'm in 7 or 8 on the rear ring. The whole bike looks original except for the seat. When I mentioned it to a friend he said some bikes you have to shift correctly and wont let you use both small rings.

    Do I have a chain alignment issue, or do I need a shifting lesson?

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    S'Cruzer pierce's Avatar
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    don't EVER use both biggest or both smallest gears, the chain is 'crossed over' in those positions. small front plus next to smallest rear is also sketchy. anyways, I'll bet low in front with 7 in back is almost the same gear as high in front with 5 or 6 in the back (or whatever).

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    Chain rub on the inside face of the big chainring when in the little chainring and the smallest one or two cogs is quite common. It's particularly an issue if the chainrings are a 53/39 combination. It can be reduced by fitting a longer bottom bracket axle but that can have it's downside and disrupt the chainline.

    Cure: Avoid the smallest cogs in the small chainring. It called cross chaining and is hard on the chain as well as noisy.

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    coprolite fietsbob's Avatar
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    Shimano designed the cage shape to be fine in anything but those cross chain combinations,
    to avoid the noise avoid those combinations, they are in an Overlap of ratios, anyhow.

    so once in big ring and #6, shift to the small ring and #3, it could be the same ratio.

    [you have to count teeth and do the math to know more clearly.]
    Last edited by fietsbob; 12-27-12 at 07:03 PM.

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    S'Cruzer pierce's Avatar
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    this gear calc is awesome. http://www.machars.net/bikecalc.htm

    put in your wheel size, crankset and cassette, and it gives you the gear inches of all the gears, so you can see your overlap.

    like, my 3x8 hybrid with 700x32 tires is running...

    gear inches
    frontrear 13 14 15 17 19 21 23 26
    48 99.8 92.6 86.5 76.3 68.3 61.8 56.4 49.9
    38 79.0 73.3 68.4 60.4 54.0 48.9 44.6 39.5
    28 58.2 54.0 50.4 44.5 39.8 36.0 32.9 29.1


    so... if I'm in my granny ring (28), I just use the bigger 4-5 max in the back, then go to the middle ring... by the time I get up to middle + 14, I'll step over to big+17. I generally use the big ring with 21-13, right now my strength is such that I'm cruising the flats in the 68-92 gear inch range, mostly around 86 GI.

    my bike came with a stupid 11-34 rear, which I found ridiculous. if I can't push 30 GI up a hill, I should get off and walk (haven't yet).

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    coprolite fietsbob's Avatar
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    NB: the 68.3 on the big ring; 68.4 on the middle ring; 44.6 on the middle ring; 44.5 on the little ring.

    those are the shifting points to make the double shifts,

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    Senior Member koolerb's Avatar
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    I guess all of my other bikes have been very forgiving of my bad habits. I would never be on both big rings at the same time, but both small, yes. I'll shift all the way through the rear without touching the front. Guess I need to start thinking about what I'm doing more carefully.

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    Senior Member koolerb's Avatar
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    Different bike, but to use as an example. I would shift to the big front ring when I get to 4t or 5th on the rear, and then the same on the way back down?

    frontrear 13 15 17 19 21 23 26
    53 107.3 93.0 82.0 73.4 66.4 60.6 53.6
    39 78.9 68.4 60.4 54.0 48.9 44.6 39.5

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    If you're in the big ring, you could use all of the cogs, up to the 53/23, then shift to the 39/16 to get to the next lowest gear ratio, that's nearly the same as the 53/26.

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    As others have said, chain rub on the FD cage is fairly common when riding crossed over (big/big, or sml/sml). To avoid the problem, Campy FDs don't index and the rider can trim the cage according to the angle the chain comes from. Later Shimano added a trip click to their FD lever, moving the cage inboard a bit.

    There are those who say never ride crossed over, but that makes no sense. While riding crossed over isn't as efficient as inline combinations, there are situations where it makes sense. Imagine you're cruising along on the ring when you come to a grade. As it gets steeper you'll shift the rear step by step, and at some point shift the front and continue.

    But what if you're near the top of the climb and only need a bit lower gear to crest it. You could shift the front (under load?) and upshift the rear, or you could simply shift the rear one more step, ride it for a short while, then begin the upshifts when you crest. Seriously, which do you think is more efficient?

    If you don't have a trim feature in your left lever, adjust the FD big gear trim to the barest minimum that allows a crisp shift and clears the outside of the chain in high. That will give you the most inboard range.
    FB
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    Senior Member mprelaw's Avatar
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    Quote Originally Posted by koolerb View Post
    I guess all of my other bikes have been very forgiving of my bad habits. I would never be on both big rings at the same time, but both small, yes. I'll shift all the way through the rear without touching the front. Guess I need to start thinking about what I'm doing more carefully.
    I run a compact crank with an 8 speed 12-25 rear cassette. I get less chain rub when I'm using my 34/12 combo, than when I use my 50/25. I never use the 34/12 combo for actual riding, but occasionally I will cycle through all the gear combos just to check everything out. Cross chaining with a 6,7,8 speed chain isn't as big a deal as it is with the thinner links in a 9, 10 or 11 chain. You shouldn't make a habit of it, but I occasionally will be in 50/25 for the last few feet of a moderate hill.

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    Senior Member koolerb's Avatar
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    FB - Both of my road bikes are friction on the down tube for front derailleur shifting. I was talking about rub on the big ring when crossed. And yes, I would agree, if riding a bike where nothing rubs when crossed it doesn't seem to be much of an issue.

  13. #13
    Senior Member oldbobcat's Avatar
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    Quote Originally Posted by pierce View Post
    don't EVER use both biggest or both smallest gears, the chain is 'crossed over' in those positions. .
    Ooh, the cross-chaining police are out tonight.

    Before offering advice I would simply like a clearer explanation of what's happening. Is the chain on the big ring and rubbing against the front derailleur cage or is the chain on the small ring and rubbing against the big ring?

  14. #14
    S'Cruzer pierce's Avatar
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    he said, he was on the small ring and the smaller two cogs, and the chain was rubbing on the back of the big ring.


    a big reason for avoiding small+small is, you have more leverage on the chain in the small ring. if you use the small cog in the back, you're putting a lot more force into each tooth of the cog, so you'll eat up that small cog a lot faster.

    in general, the bigger the cogs, the lower the friction.

    looking at my gear table again...

    frontrear 13 14 15 17 19 21 23 26
    48 99.8 92.6 86.5 76.3 68.3 61.8 56.4 49.9
    38 79.0 73.3 68.4 60.4 54.0 48.9 44.6 39.5
    28 58.2 54.0 50.4 44.5 39.8 36.0 32.9 29.1


    while I could get 54 gear inches with 28:14, I'm *much* better off using 38:19. or I could get 68" at 38:15, but am better off at 48:19.

    Quote Originally Posted by koolerb View Post
    Different bike, but to use as an example. I would shift to the big front ring when I get to 4t or 5th on the rear, and then the same on the way back down?

    frontrear 13 15 17 19 21 23 26
    53 107.3 93.0 82.0 73.4 66.4 60.6 53.6
    39 78.9 68.4 60.4 54.0 48.9 44.6 39.5
    With that combination, going from lowest gear to highest...

    39:26
    39:23
    39:21
    39:19
    39:17 *or* 53:23 (almost same)
    39:15 *or* 53:21 (pretty close)
    53:19
    53:17
    53:15
    53:13

    note the overlap gears with that particular combination are almost the same thing, so in that overlap range, I'd shift up to the big if I expect to keep accelerating, but would stay in the small ring if I know I'm going to be downshifting again soon.

    also note that when you shift the big ring in front, you also need to drop several gears in back to get the next evenly spaced ratio.
    Last edited by pierce; 12-27-12 at 10:18 PM.

  15. #15
    Senior Member oldbobcat's Avatar
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    Quote Originally Posted by pierce View Post
    he said, he was on the small ring and the smaller two cogs, and the chain was rubbing on the back of the big ring.
    Oops, got it. The small-small rub against the inside of the big ring.

    Those are your sacrificial combinations, the result of a wide cassette, short chainstays, and a 14-tooth gap between your chain rings. It does that on my bike with a compact double and 10-speed cassette, too. It was worse on my Giant because the chain stays were shorter. I simply avoid using those combinations. As pierce says, there should be enough overlap that you won't miss them, anyway.

    If you insist, though, you can cheat your chain line a bit by inserting a 1mm shim between the right bottom bracket cup and the frame, and readjusting the front derailleur. This is a cheat, though, and it will pretty much preclude use of big-big combinations. And any thicker than 1mm will cause serious chainline problems on the other side, so don't go there.

  16. #16
    S'Cruzer pierce's Avatar
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    my last hybrid, came with a cheap steel triple, I replaced with with a 50-34 compact double cheap FSA crank but continued to use a 7 speed rear and chain. I was using friction thumb shifters (nice old Suntour alloy ones from the mid 80s), so I first changed cassettes to one that put the big ring in the MIDDLE of the chainline, and then I put spacers on the small ring so it was farther over, more like old school 2x7 spacing instead of 2x10-speed narrow chain spacing. I used the 34 ring strictly as a granny gear, and otherwise treated the bike as a 7-speed on the 50T. in the 34T I could only use the lowest 3-4 cogs but thats all you want in granny gear anyways. it actually worked surprisingly well. my low gear was a 34:32, it worked out like this....

    frontrear 14 16 18 21 24 28 32
    50 98.2 85.9 76.4 65.5 57.3 49.1 43.0
    34 66.8 58.4 51.9 44.5 38.9 33.4 29.2

  17. #17
    Senior Member oldbobcat's Avatar
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    Short chainstays, fat cassette blocks, and wide gearing are the bane of easy shifting. And every ten years or so somebody designs something to make it easier, and then we go pushing the limits again.

    When I started racing in '73 our setup was a 52-49t or 48t double with a 14-24 or 14-22t 5-speed in the back. With 120mm rear axles. And probably 17-inch chain stays. No chain rub there. The main problem was getting the chain to drop down a 3- or 4-tooth gap when you were half-way up a hill and realized you needed the small ring.

  18. #18
    S'Cruzer pierce's Avatar
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    standard setup in the early 70s I remember was a 52-42T front, and a 13-17 5 speed 'straight block' if you were racing entirely on the flats, and a 13-24 or so if you had serious hills. some racers ran 53T fronts for a little more top end. regina deloro freewheels on campy nuevo record hubs, with sedis chains, yeah, bebé!

  19. #19
    Senior Member oldbobcat's Avatar
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    You guys were more up to date than we were. Gitane Tours de France and Peugeot PX-10s came with 52-40 Stronglights, and Raleigh Pros and Internationals came with 52-45 NRs, and we all replaced the small rings with a 49 or 48. Normandy hubs on the Peugeots, Tipo on the Gitanes, and Record on the Raleighs. Sedis chains were my favorite because they seemed a bit stiffer and more durable than Regina, which was popular, too. When Sun Tour started making good freewheels we were happy to start using them.

    By '75 we were on the 52-42 or 53-42 overlap train, with 6-speed coming around, the exact size of the freewheel depending on the rider and the route. But when we started, somebody, I don't remember who, was getting us all into half-step.

    Point being that chain rub (and chain slack, too) was never a problem on a setup like this.
    '

  20. #20
    Senior Member koolerb's Avatar
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    Thanks for the feedback everyone. So sounds like no on thinks there's anything wrong with the bike. I'll adjust my shifting habits to fit.

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    I think you should be able to run your chain on the smaller chainring and next to smallest cassette cog without chainrub on the big ring. A 2mm bottom bracket spacer would probably allow you to do that. A BB spacer is inexpensive and easy to install. But first You should check to see that the bottom bracket was installed with enough torque. A slightly loose BB can cause your problem.

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    a lot of the advice here reminds me of the old Henny Youngman joke:

    guy says to his doctor, "It hurts when i do this". doctor says, "don't do that".

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    S'Cruzer pierce's Avatar
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    Quote Originally Posted by Al1943 View Post
    I think you should be able to run your chain on the smaller chainring and next to smallest cassette cog without chainrub on the big ring. A 2mm bottom bracket spacer would probably allow you to do that. A BB spacer is inexpensive and easy to install. But first You should check to see that the bottom bracket was installed with enough torque. A slightly loose BB can cause your problem.

    or run the big ring, and a larger cog, which is likely the same ratio anyways, and easier on the whole drive train.

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