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Old 01-17-06, 11:59 AM   #1
BubbaDog
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Shimano UN73 Bottom Bracket - What Cranks Fit Spindle

I am going to be building up a new road bike on the cheap in the not too distant future and can get a great deal on a Shimano UN73 sealed bottom bracket with a standard square taper. Two questions:

- Is this a decent bottom bracket? I know it's not the lightest at 289 grams, but the price is right for $13 brand new. I'm just looking for a good quality middle range bracket, and can upgrade to lighter/better later when finances permit. I don't care about bling, anyway you can pretty much tell any lie you want about a bottom bracket once it's in the frame .

- The two spindle lengths that are available are 115 and 122.5, I don't have a clue as far as what cranksets these lengths are proper for, any insights?

TIA for the help, opinions, harassment, etc....

B'Dog
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Old 01-17-06, 12:03 PM   #2
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115 is probably a little long unless you're looking to run a triple maybe. I just got a 107 for my old triple mountain cranks, though. What drivetrain setup would you like to run?
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Old 01-17-06, 12:07 PM   #3
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Quote:
Originally Posted by ImprezaDrvr
115 is probably a little long unless you're looking to run a triple maybe. I just got a 107 for my old triple mountain cranks, though. What drivetrain setup would you like to run?
I'm still waffling between a double and a triple. A double 'cause there's not too many hills in Houston, maybe a triple 'cause I can steal the BioPace off my touring frame until I get additional finances. I was kinda thinking that the spindle lengths were the reason they were being cleared out so cheap....

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Old 01-17-06, 01:11 PM   #4
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UN53/UN73 work with current Tiagra and older non Octalink 105 and higher. Check eBay for older (8speed) cranks which will work with 9speed drivetrains. Lots of 105s out there like this:

http://cgi.ebay.com/Shimano-105-cran...QQcmdZViewItem
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Old 01-17-06, 02:04 PM   #5
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Even a Sugino XD triple (square-taper) crank ideally goes onto a 107mm bottom bracket spindle. I've got my XTR triple (running on a road bike) mounted on a Dura-Ace 102mm square-taper bottom bracket. 115 is pretty wide for anything on a road bike, unless you've got an older crank that used further-apart bottom bracket spindles.
However, for building up a new bike, or upgrading your current one, I'd recommend that you pick the crank first, and then the bottom bracket. Just because you can get a $30 bottom bracket for $13 isn't a reason to limit your choices of cranks.
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Old 01-17-06, 03:24 PM   #6
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I'll concur with the notion that you may end up not saving much in the long run. I ran a 130 or so BB spindle on my frankenbike before getting the 107 on last week and it's much more comfortable. With the longer spindle I was riding with my toes pointed out from the bike. Felt very strange indeed. And you're also right that there's a reason they're trying to get rid of these two spindle lengths, and it's not a reason that should motivate you to buy one. You can get BBs cheap to begin with, just wait and get the right size for the crank you end up running.
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Old 01-17-06, 05:27 PM   #7
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a 115 will work with many older higher-end shimano cranks, pre-94 campy cranks. 122 is pretty common for older MTB cranks and some current low-end MTB cranks. but like others have said, it's best to have the cranks first and go from there.
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Old 01-17-06, 08:48 PM   #8
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The UN73 is a very durable bb. The 115 mm was originally speced for 8-speed Ultegra double cranks but works fine on my 105 8-speed triple cranks too.
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Old 01-17-06, 09:48 PM   #9
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Thanks to everyone for the inputs. I'll have to measure the spindle on the Phil Wood that's on my current bike to see what it measures out at. It currently has my BioPace triple and is comfortable for pedaling so I guess I can use it as a general guideline for moving forward on the new ride. Since I am doing this on the cheap to start with, getting the bottom bracket first vs. getting the crankset first may not be that big a deal. Ultimately, I'll probably go with a used crankset (or steal my BioPace) and upgrade later so can replace the BB then too if need be for the best fit.

On another note, being an old school 5 speed rear cluster kinda guy can someone explain why cranksets are now classified by the number of speeds they support in the rear?

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Old 01-17-06, 10:04 PM   #10
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Quote:
Originally Posted by BubbaDog
On another note, being an old school 5 speed rear cluster kinda guy can someone explain why cranksets are now classified by the number of speeds they support in the rear?
The crankset "speed" designation is mostly a marketing thing, but with some substantive reason thrown in. Basically, as you get more cogs on your rear wheel, the chain needs to be narrower to work with the narrower cogs. So, cranks designed for 5-speed might have chainring teeth too wide for newer (8/9/10-speed compatible) chains. 10 speed chains are narrower than 9-speed chains are narrower than 8-speed chains. But to my knowledge, the internal width (the part that fits around the chainring tooth) is the same for 8/9/10 speed chains. It's only the outside diameter of the chain that varies.

Anyway, cranks designed for 9 or 10 speed have the chainrings spaced a bit closer together than cranks designed for 8-speed, etc. You can use a crank designed for 8-speed chains with a 9-speed drivetrain just fine. Slight, slight danger the chain gets caught between the chainrings, but in practice it's virtually impossible that this will happen. I use a crank designed for 8-speed on my 9-speed drivetrain.
Mainly, the deal is a marketing thing, so that people think they need to upgrade their crank when they get more cogs on their rear derailler. Usually, the only thing that would need changing is
* narrower chain
* new cassette cogs
* new shifters, if you're using indexed shifting
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