Bicycle Mechanics - matching bottom bracket to crank to frame
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08-08-06, 10:27 AM
I"m replacing the bottom bracket and crankset on my Trek 4300. The bracket is 73x116. For the life of me I cannot find a decent replacement. The only bracket i've found is the Shimano XTR M952 which apparently only works with XTR cranksets that cost more than the original value of the whole bike! Any suggestions?
08-08-06, 10:40 AM
I'm assuming you have a square taper crank here, which I think is a safe bet for a Trek entry-level mountain bike.
You *can* get 73 mm square taper bottom brackets if you look hard... Nashbar has a few UN-53 models: http://www.nashbar.com/profile.cfm?category=73&subcategory=1096&brand=&sku=8104&storetype=&estoreid=&pagename=
Unfortunately, it looks like they're out of all but 73x110 right now. You might call and see when they'll get other sizes in. Performance Bike carries the higher-quality UN-73 model: http://www.performancebike.com/shop/profile.cfm?SKU=158&subcategory_ID=5140, again you might call to see when they'll get the right size in stock.
Harris Cyclery carries a variety of them: http://sheldonbrown.com/harris/bottombrackets.html#shimano ... scroll down to "73 mm width". If you can afford it, I would go for the UN72/73 over the UN52/53, as they're lighter and more durable.
Any competent Local Bike Shop should be able to get you a 73x115 bottom bracket, either they'll pull it out of the parts bin or special order it. By the way, a 73 mm bottom bracket is identical to a 68 mm bottom bracket in terms of threading. The only difference is the width. There are various shenanigans that you can do to get a 68 mm bottom bracket to fit in a 73 mm shell, you may want to search the forum for these if you're feeling adventurous :)
PS- In case I wasn't clear, it's okay to replace a 73x116 BB with a slightly longer or shorter spindle, e.g. 115 or 118 mm. Those sizes are more common. You'll have to make a few adjustments to the front derailer after changing the spindle length, since the chainrings will be slightly closer or further from the frame. But that's pretty easy to do!
08-08-06, 10:56 AM
I don't need to get another square tapered bracket, any will do. I'm suprised at the different types; square, Ocatalink, ISIS, shimano splined.... add the unusual 116mm spindle length and I can't find a set. I've heard different opinions on getting a length that is different than the orginal. So my first choice would be to get a 116 but if that's not possible...
08-08-06, 11:10 AM
Well, you *do* need to get the same type of bottom bracket spindle if you plan to keep your current crank.
There is absolutely nothing wrong with square taper BBs/cranks, they are widespread and reliable. It's just that the newer splined designs are slightly stiffer and lighter, and thus becoming more popular. There are essentially 3 such splined designs: ISIS (standardized, many manufacturers make ISIS cranks + BBs), Shimano Octalink version 1, and Shimano Octalink version 2.
I don't think 116 mm is a common spindle length unfortunately. I have replaced many bottom brackets with spindles that differed slightly in length, and I can assure that it's not a problem to tweak the chainline by 1 or 2 mm. There are basically only two major problems that you could run into: if the spindle is way too short, the inner chainring could whack into the chainstay, and if the spindle is way too long, the front derailer might not be able to reach all the way out to the outermost chainring. A 1 or 2 mm change in chainline is pretty darn unlikely to cause either problem.
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