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  1. #1
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    isis BB and new chainrings hit

    so Ive got an older trex that has the isis BB. the other day I found another chainring and crank set up that I wanted to give a try. the crank arms are 175 ( instead of my current 170) and the biggest chainring is a 52 instead of my current 48. The current chain rings are starting to look a little worn.
    when I went to install the "new" system the chainring slides all the way on and hits the frame not allowing it to turn.
    I really don't know what the issue is. And I cant switch parts form one to another as the "old" chainrings and crank arms attach with 4 pins and the "new" is a 5 pin.
    I thought that maybe I just have to switch to a wider cartridge...when money allows..but would that mess up my chain to rear cassette alignment and how would I determine how much wider to go...if that is something I can do.
    The guy I got the chainring from is a good guy and I don't have to buy it if it does not work. He is not all that knowledgable either and said maybe im trying to put a version 1 on a version 2 ? When I look that up I come up with octalink systems...which I thought were not the same
    BTW this is the second time this year I have tried to put on different chainrings...the first time is when I discovered the difference between 4 bolt and 5 bolt
    going to a 52 count would probabally not make that much difference, but on the flats when I am in top gears...I know I could go faster with little more effort. And down hill I can out pedal what Ive got

    ideas ?

  2. #2
    Senior Member bikeman715's Avatar
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    You need a longer BB . Took out the one you have now and add 5 - 10 MM to it so the chainrings will clear the frame and keep a eye on the chainline . As long as the chain is straight from the middle of the chainrings to the middle cog of the cassette or freewheel (which ever you have ) If it odd number then 3 or 5 cog if it even the between two cogs like 4 & 5 .
    bikeman715

  3. #3
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    thanks, I guess what I can do is reassemble but not tighten things down, then slide the arm out and get my alignment right. then with feeler gauges determine how much I need to slide it out and see about a BB that fits those demensions.
    thanks

  4. #4
    coprolite fietsbob's Avatar
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    maybe a spacer washer under the Right/ fixed Cup ... your LBS should have some ..

    Or a lower Profile crank.. then the crank-arm center comes in, to Meet a shorted BB spindle.

  5. #5
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    Quote Originally Posted by fietsbob View Post
    maybe a spacer washer under the Right/ fixed Cup ... your LBS should have some ..

    Or a lower Profile crank.. then the crank-arm center comes in, to Meet a shorted BB spindle.
    that would be more affordable for now. my local lbs is a pretty small shop. does it need to be a special spacer that matches the profile of the BB. Wonder if I could just use big washers for now, to see if it helps, then find or order the correct spacer

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    Senior Member bikeman715's Avatar
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    nothing special about the spacer you need .
    bikeman715

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    coprolite fietsbob's Avatar
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    freewheels use the same spacer .. as the freewheel thread on the hubs are same as the RH thd Left Cups.
    fixed cup is LH thd.

  8. #8
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    i think the name of the frame part that is hitting your new chainring is called the chainstay. if you look at a number of bikes, especially mountain bikes designed for wider tires, you'll see that some of the driveside chainstays have an intentional dent in them to provide relief for the large or even the small chainring.

    you've exceeded the design limitations for the frame with your large chainring/crank combo.

    fixes include, but are not limited to: wider BB spindle, different driveside crank arm with shallower dish, smaller chainring (not an option, suppose), spacing the BB out to the driveside. pretty much what has already been suggested.

    i suspect the bike is a MTB and the crank is a road crank. i had the same problem when i attempted the same thing a couple of years ago. i hope it didn't take 2-3 hours of screwing around with it to determine the problem, like it did me. i eventually put the old crank back on and called it a day.

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    Quote Originally Posted by niuoka View Post
    I really don't know what the issue is. And I cant switch parts form one to another as the "old" chainrings and crank arms attach with 4 pins and the "new" is a 5 pin.
    You have a hybrid frame made for wide tires and a mountain crank and are trying to use a road crank which doesn't clear.

    but on the flats when I am in top gears...I know I could go faster with little more effort.
    No. The issue is the rider not the bike. You're just not strong enough and need to learn to to pedal faster.

    Your stock 48 x 11 big gear is a 35+ MPH cruising gear (at 100 RPM) at 46+ MPH sprinting gear (130 RPM).

    It's a harder gear than the 52x13 Eddy Mercx used to dominate the spring classics like the 1969 tour de France. That year he sustained a 130km solo breakaway to the finish line after summiting the Tour Malet first and completed the race with the winner's yellow jersey, best climber's polka dot jersey, and sprinter's green jersey.

    Obviously the rest of us not working as professional cyclists aren't as strong as Eddy.

    And down hill I can out pedal what Ive got
    Once you're going 35-40 MPH you'll do better tucking than trying to pedal.

    I've been riding a 50x13 big gear (apart from the experiment with a 14-23 straight block) which is smaller than 48x12 for the last 17 years with 9-10 of them in the Colorado Rockies. It works great.

    ideas ?
    If you really want you might be able to apply a spacer or 118mm road triple ISIS bottom bracket which would get you an additional 5mm of clearance (2.5mm from half the spindle length difference, 2.5mm because it's symmetrical when installed in a road shell that's 68mm wide not 73mm wide like yours).

    ISIS bottom brackets are built so that the crank arms bottom on a flat shoulder. If you use a drive side spacer make sure there's still enough clearance on the left crank for that to happen.
    Last edited by Drew Eckhardt; 01-21-14 at 01:07 PM.

  10. #10
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    Yes of course I could pedal harder and faster. I have a cadence im comfortable with and just thought picking up a few more teeth would increase speed. Pushing a little harder is easier for me then pedaling faster.
    Its hard for me to imagine this old hybrid with racks, panniers, lights etc hitting the speeds you mention

  11. #11
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    The issue is that ISIS spec makes the chainline offset (Distance between the shoulder stop and chainline) of a road crank shorter than the chainline offset of a MTB crank. Ideally you need a 123mm BB but 123mm is not a standard size. There is a 128mm downhill BB but I'm not sure if there exists an ISIS 68x128 BB.
    Last edited by Bezalel; 01-21-14 at 10:13 PM.

  12. #12
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    [SIGPIC][/SIGPIC]
    XXXI

  13. #13
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    Quote Originally Posted by Soil_Sampler View Post


    thanks guys

  14. #14
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    Quote Originally Posted by Bezalel View Post
    The issue is that ISIS spec makes the chainline offset (Distance between the shoulder stop and chainline) of a road crank shorter than the chainline offset of a MTB crank. Ideally you need a 123mm BB but 123mm is not a standard size. There is a 128mm downhill BB but I'm not sure if there exists an ISIS 68x128 BB.

    thanks for the help

  15. #15
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    Quote Originally Posted by niuoka View Post
    Yes of course I could pedal harder and faster. I have a cadence im comfortable with and just thought picking up a few more teeth would increase speed. Pushing a little harder is easier for me then pedaling faster.
    Its hard for me to imagine this old hybrid with racks, panniers, lights etc hitting the speeds you mention
    The fallacy here is that you can maintain the same cadence and effort with a higher gear. Gears do not increase the power transmitted to the pedals. Rather they control over what distance your effort is conveyed. If you go to a higher gear ratio and use the same effort you will expend that effort over a shorter distance, therefore will go no faster.
    There's no such thing as a routine repair.

    Don't tell me what "should" be - either it is, it isn't, or do something about it.

    If you think I'm being blunt take it as a compliment - if I thought you were too weak to handle the truth or a strong opinion I would not bother.

    Please take the time to post clearly so we can answer quickly. All lowercase and multiple typos makes for a hard read. Thanks!

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