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Old 09-10-11, 04:31 PM   #1
FastJake
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Determining BB spindle length - road crank on a MTB

I tried searching but couldn't quite find what I'm looking for. I have a 1995 Gary Fisher Tassajara which I converted to single speed. The original crankset is junky and the BB is shot so I figure this is a good time to put on a set of Shimano 600 tri-color cranks I have laying around. Now, I tried test fitting them and the left arm hits the chainstay. The right arm clears, but the 40T ring hits the chainstay.

Obviously I need a longer spindle, but I'm not sure how much more I need. Sheldon's BB database says that my crankset has a chainline of 43.5mm and a length of 113-115mm, but that doesn't help me because the spindle on there now measures about 115mm. The shell width of the frame is 73mm. Any ideas on how to figure out how much more I need before I guess and order something?
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Old 09-10-11, 04:36 PM   #2
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Shimano UN54 Cartridge BB, JIS - 73x122mm maybe?? THats the longer they have I believe.

The only way to figure it out is measuring how much do you need at each side, doubt is more than 3 mm for each side, that gives you 6 mm more. round numbers 122.
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Old 09-10-11, 04:55 PM   #3
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If you know roughly how high into the crank arm (below the end of the tapers) the spindle ends (usually 2-3mm), you can place the right crank at the desired distance from the chainstay and roughly measure how short of the goal the current spindle is, and add that. It's rough and depends on your ability to eyeball what's straight.

The left is a bit easier because you can mount it and tighten is somewhat then measure the interference and how much farther out you want it.

Unfortunately, there's no theoretical conversion for road to mtb cranks because the offset built into the arms varies brand to brand and within brands over time. The trend is to much shorter spindles with more offset built into the arm, so a new mtb triple could need a shorter spindle than an older road crank.

You're left with the option of measuring as I describe, or guestimating and resorting to trial and error.
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Old 09-10-11, 05:07 PM   #4
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I would go with 122 MM ,myself that give you at least 5 MM clearance for the arm and the chainring .
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Old 09-10-11, 05:16 PM   #5
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Thanks for the help. ultraman, by eyeballing it 3mm on each side seems like enough. They almost clear, they don't need much.

Now I have another question! How do I remove the old BB? I tried unscrewing the plastic side but it crumbled almost instantly. Then I tried unscrewing the metal side but it's as if the tool isn't fitting correctly. The tool falls out of the splines anytime I try to apply pressure. I'm using a Park BB-22 tool and I'm fairly certain it's a Shimano cartridge (square taper) BB. What am I missing here?
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Old 09-11-11, 02:59 PM   #6
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You'll need to keep pressure on the tool to hold it in place while unscrewing the BB, just like you would with any other high torque fastener. However, there is a tool that will give you a hand holding the socket in place and eliminate any chance of it slipping out: http://www.universalcycles.com/shopp...8&category=215

Once you unscrew the metal side, the remains of the plastic cup should come out easily.
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Old 09-11-11, 03:06 PM   #7
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If the bb is hollow use a rear wheel skewer just as you would to remove a freewheel.
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Old 09-11-11, 04:47 PM   #8
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Originally Posted by Capecodder View Post
If the bb is hollow use a rear wheel skewer just as you would to remove a freewheel.
This is a great idea, but unfortunately it's not hollow.

I literally can't put a pound of pressure on the tool before it just drops out of the BB. The splines don't engage all the way, they only go in a little bit and then run into something. Imagine trying to remove an old 2 prong freewheel with a screwdriver

Time to give up and take it to the LBS? I really don't want to since I bought the tool and like to do this stuff myself...
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Old 09-11-11, 04:51 PM   #9
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joejack, that tool from Pedros is just what I need. Anyone know the inside threading of a BB? I think if I could just go to the hardware store and buy a bolt to thread in there that might work...
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Old 09-11-11, 05:09 PM   #10
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It's M8x1mm, not the much more common M8x1.25mm. If you can find a very well stocked hardware store (perhaps even a speciality fastener store like Fastenal) you may have a chance though. Depending on the design of the tool, you might be able to use one of your crank bolts with a large thick washer. That's what I did before I got the Pedros tool. That method only works on shorter bottom brackets though.
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Old 09-11-11, 05:23 PM   #11
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Quote:
Originally Posted by FastJake View Post
This is a great idea, but unfortunately it's not hollow.

.... The splines don't engage all the way, they only go in a little bit and then run into something.
Before you go further and possible screw yourself up, stop and try to figure out why the splines can't engage over the full depth. Something is causing the problem, possibly the ID of the tool being too small for the spindle, possibly dirt or a ding in the splines.

With full engagement you shouldn't need to stabilize the remover if you also use a long enough handle (Longer handles increase the ratio of torque to side forces). Without full spline engagement, you risk stripping out the splines even if you do stabilize the tool.
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