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  1. #1
    Senior Member
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    Need help planning a crank upgrade..

    Hi All,

    I have a 13 year old Trek 800 mountain bike that i would like to upgrade. It currently has a 3-ring crank with the chain rings welded together. It also has a 6-speed freewheel.

    For the first phase of my upgrade, I would like to switch out the old crankset with a new crankset. Originally, I wanted to replace my inner chain ring with a smaller diameter ring to give me a lower climbing gear, but since the chain rings are welded, I need to replace the whole unit.

    I've talked to the folks at a LBS and I've also consulted my Bicycling Magazine's Bicycle Mechanics (circa 1990) book on replacing a crank set. I have some questions as I plan my upgrade and am hoping to get some help from this forum.

    1. Bike Nashbar sells an upgrade crank set that "mounts to the standard square taper bottom bracket." Does this mean that I can mount this crank onto my existing bottom bracket? How can I tell if this or another crank will fit my existing bracket? Here's a link to the Nashbar crankset,

    http://www.nashbar.com/profile.cfm?c...20ATB%20Cranks

    2. I've been looking into some other cranksets that would require that I switch out the bottom bracket (Shimano cranksets). In my Bicycle Mechanics book it says that removing the fixed cup of the bottom bracket is difficult to do and usually requires a special tool to remove it. Is replacing a bottom bracket (and not just disassembling it for cleaning and lubrication) easy to do or will I need to buy some bottom bracket-specific and expensive tool to unfasten the fixed cup of the bracket? I realize that I'll need tools to remove the crank arms and the lock ring, but the book that I have suggests having a bike store remove and replace the fixed cup. Is this still true today or can I do the whole job myself (without spending too much money on special purpose tools)?

    Thanks in advance

  2. #2
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    Most bottom brackets are the standard square tapered type. What you need to determine is whether your crankset and bottom bracket are low profile or not. If it is newer than 10 or 12 years old, odds are it is a low profile crankset. These can be identified by a curve in the crank from where it attaches away from the frame width wise. ( hope that makes sense ) If your bottom bracket is the normal square type, and is not creaking or feeling grindy when you turn it, then you are probably fine not removing it. If it is creaking (check your seatpost if you hear creaking first) or feels grindy when you turn it, replace it. The cups can be a pain in the but to remove if you do not have the specific tool, however, a flat head screwdriver and a hammer have been known to work. Spray ithe thread or outside of the cup with some PB blaster first. Take the bracket to a shop to make sure you get the new one in the right size. All of this is moot of course if you cannot get the old crank off. Again a special tool is required, or, a deft hand with a hammer. DON'T hit the stuff hard if you use a hammer. Parts are often soft aluminum and can be easily damaged using these highly advanced hammer techniques. When attaching the crank, some people recommend greasing the bottom bracket spindle, and do not. I fall in the latter category. If you have the low profile setup, and a regular square bracket spindle, that crankset you linked to should work fine. If you got shimano stuff, stick to shimano. Good luck.

  3. #3
    Senior Member
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    Thanks a bunch Skimr2 - I took a look at my bike and it looks like I have a taper bottom bracket. If I look at the crank arms, I can see that they run parallel to the bike from the pedals towards the center for about 3/8ths of the way, then there is a shallow taper/run where the crank arms run at an angle into the plane of the bike. It's a shallow angle, but it definitely looks tapered to me.

    I think I'm going to go with the Nashbar crankset. Thanks again for your help.

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