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  1. #1
    Junior Member hoov23's Avatar
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    126mm dropout, 73mm BB - advice on converting to fixed gear, bunch of questions

    Got a few questions, if somebody could take a minute to help me out I'd appreciate it!

    BACKGROUND:

    I have this old rigid mountain bike (26", 18sp) that I'd like to make a fixed gear with 700c wheels. Been doing a lot of reading up on the process. Want to avoid buying another frame.

    The bottom bracket is 73mm, and the rear horizontal drop out spacing is 126mm. Also it seems like I have enough clearance for a half-inch wheel radius increase from 26" to 700.

    QUESTIONS:

    I would like to replace my crank with a single-chainring (and some 165mm arms) - will I need to change my bb also, or can I just take off my current crank and put on a new one?

    IF I have to change my bb, Is there a budget one that fits 73mm?

    As for the rear, if I buy a wheelset with flip flop hub, is it likely that it will be possible to space it out to the right size to fit, or are some axles shorter and some longer? Is this a non-issue? I also want to use quick releases, and I have one already on the bike, can I re-use it with a new wheelset?

    Any idea what the dimensions of this bike are gonna do to my chainline? Wondering if the wide bb will cause the sprockets to need to be spaced pretty far from the hub to get a straight line. In this case, maybe a 126mm dropout is advantageous...?


    Thanks y'all, and as all newbs must say, sorry for the dumb questions. Tried my hand at the search for a few minutes, but felt like it wasn't gonna turn up the answers I was looking for, at least not onces I could understand.

  2. #2
    Senior Member Kayce's Avatar
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    Quote Originally Posted by hoov23 View Post
    I would like to replace my crank with a single-chainring (and some 165mm arms) - will I need to change my bb also, or can I just take off my current crank and put on a new one?
    I would suggest your best bet is to use your current crank, and mount the single ring on the inside of the spider first. Then see how your chainline is. After that you may need to get a new BB to get it right. You could do the same with new cranks, but you most likely dont need to buy them.


    IF I have to change my bb, Is there a budget one that fits 73mm?
    I did a quick search, and found Shimano makes a 73 BB in a variety of spindle lengths. Harris has them for $25, but they are usually a bit more expensive, so if you shop around I bet you could find one for $20

    As for the rear, if I buy a wheelset with flip flop hub, is it likely that it will be possible to space it out to the right size to fit, or are some axles shorter and some longer? Is this a non-issue? I also want to use quick releases, and I have one already on the bike, can I re-use it with a new wheelset?
    If youre buying a track hub(you better be) it will come with nuts to strap it down, which is what you want anyway. If your frame is steel you will be able to pull in the stays, and hold it just fine, which is your best option. If its aluminum you probably have to swap out for a wider axle. And defiatly have to play around to get some washers on the adjustment cones.

    Any idea what the dimensions of this bike are gonna do to my chainline? Wondering if the wide bb will cause the sprockets to need to be spaced pretty far from the hub to get a straight line. In this case, maybe a 126mm dropout is advantageous...?
    Surley makes a hub that is specially designed to make conversions out of mountain frames. It runs a different chainline, and has a longer axle. Here it is on harris's site. http://sheldonbrown.com/harris/singlespeed.html#surly

  3. #3
    Junior Member hoov23's Avatar
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    Thanks Kayce for the help!

    Quote Originally Posted by Kayce View Post
    If your frame is steel you will be able to pull in the stays, and hold it just fine
    does this mean that the hubs on most/all wheelsets i'm likely to find (used) are gonna be 120mm specific, but that I can tighten down (bend) my dropouts inwards to fit?

  4. #4
    Senior Member Kayce's Avatar
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    Other than that surley hub, and the odd sized Phil Wood hubs any track hub will be 120mm spaced. I would go for the surley myself. Its probably not worth dropping that much on your mountain conversion, unless youre building yourself a really serious fixed mountain racer or something.

  5. #5
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    Quote Originally Posted by hoov23 View Post
    IF I have to change my bb, Is there a budget one that fits 73mm?
    You can put a 68mm BB into a 73. 73 is an old standard for a wider BB width, but the shell diameter is the same a a 68mm shell.

    I run a 115mm 68mm Token BB in my Surly 1x1 which has a 73mm bb width shell with no issues. The non-drive-side cup just sits further inside the BB shell. My using a 68mm BB you will have a much wider range of BB's to choose from, and it also does not limit you to Shimano (JIS) tapers. You can get Token ISO taper BB's to fit Miche, Campy, Suntour, etc cranks.... you just need to grab a longer spindle length.

    Sorry if this is confusing, but this small fact may open up more options for you.

  6. #6
    Constant tinkerer FastJake's Avatar
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    Quote Originally Posted by hoov23 View Post
    does this mean that the hubs on most/all wheelsets i'm likely to find (used) are gonna be 120mm specific, but that I can tighten down (bend) my dropouts inwards to fit?
    You can bend steel dropouts, but I find it makes things easier to just get the correct length axle spaced out to fit in your frame. Fortunately you already have a correct axle from your original rear wheel. If you get a 120mm wheel (or whatever 1xx mm) you can just switch the axles and then you'll still be able to use quick release.

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