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Crankset upgrade needs shorter BB?

Old 01-31-08, 08:59 AM
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bcoppola
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Crankset upgrade needs shorter BB?

Working on my "frigid rigid" MTB (see sig) I just installed a nice lightly used Shimano LX Biopace crankset of roughly the same vintage to replace the original, worn out low end (Acera?) unit, also Biopace. Same ring sizes.

But now I can't get the front der to shift quite as smoothly. Got it working but...ehhh. Yep, played around with cage alignment, height, cable tension, etc, etc. Rear shifts great.

I really didn't measure the chainline with the old unit but a crude straightedge check seems to show the chainline is a bit to the outside. Is it likely that a simple upgrade -- same brand, same vintage -- would require a different BB spindle length?

Or maybe a gen-u-wine Hyperglide chain instead of the generic KMC "for all derailleurs" I have on now would help? Chain is new but cheap.

Derailleurs are original. Low end but in good working order w/no play.

Last edited by bcoppola; 01-31-08 at 09:04 AM.
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Old 01-31-08, 10:20 AM
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Did you get the bottom bracket and crankset torqued down well? That can make a difference in chainline.
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Old 01-31-08, 11:42 AM
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Originally Posted by Al1943 View Post
Did you get the bottom bracket and crankset torqued down well? That can make a difference in chainline.
Yes.

However, I'll put it on the trainer, pedal like heck in a big gear, and see if it loosens up & needs to be tightened more.
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Old 01-31-08, 01:06 PM
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select the middle chainring and the middle ring on the cassette, now see if the chain line is strait? you can use the rear wheel/tyre as a guide. if the chain is strait you don't need a new bottom bracket
 
Old 02-01-08, 12:33 PM
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UltraViolet's technique is the best I can think of, other than trolling the i-net for the spindle length for your "new" crankset.
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Old 02-01-08, 03:44 PM
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Thanks! I checked the chainline w/UVs method & it looks OK. Plus a little more fiddling with the FD improved it a bit. Trainer workout revealed no further problems. Triple FDs, I have heard, tend to be fussy anyway.

There's a couple of 1mm spacers I can move around on the rear hub to tweak the chainline further & see if it'll improve things a bit more or not.
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Old 02-01-08, 04:47 PM
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Originally Posted by bcoppola View Post
Working on my "frigid rigid" MTB (see sig) I just installed a nice lightly used Shimano LX Biopace crankset of roughly the same vintage to replace the original, worn out low end (Acera?) unit, also Biopace. Same ring sizes.

But now I can't get the front der to shift quite as smoothly. Got it working but...ehhh. Yep, played around with cage alignment, height, cable tension, etc, etc. Rear shifts great.

I really didn't measure the chainline with the old unit but a crude straightedge check seems to show the chainline is a bit to the outside. Is it likely that a simple upgrade -- same brand, same vintage -- would require a different BB spindle length?

Or maybe a gen-u-wine Hyperglide chain instead of the generic KMC "for all derailleurs" I have on now would help? Chain is new but cheap.
Couldn't be Acera, but if it used the same size rings, you didn't need to change cranks...coulda just changed chainrings.

You should measure the chainline. This is explained at https://sheldonbrown.com/chainline.

See also: https://sheldonbrown.com/derailer-adjustment

MTB triples on steel frames are normally 47.5 mm or so. For "road" use, a 45 mm chainline is generally the way to go.

For chains, I like the SRAM PC 870 for this sort of application.

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Old 02-01-08, 04:58 PM
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Spacers (washers) on the front chainrings can also be used to adjust the chainline.
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Old 02-01-08, 06:13 PM
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Thank you Sheldon! The old crankset (Shimano 200GS -- I was mistaken) have the rings riveted to the spider -- no easy way to remove/replace.

BTW, I was leery of Biopace and was going to replace them with a round set till I read your article. So I'm giving it a try. Hey, you were right about using the front brake only for fixed gear.

Yes, I really should measure the chainline properly before proceeding further. I plead sloth. Thanks for the SRAM recommendation.

Regrettably, I cannot make it to MA for your G&S performance. Break a leg (from a theatre major).

torque1st: As for spacers on the chainrings, only works if the rings need to be moved outwards. Used 'em on my FG conversion (from Harris of course). Mine will probably need to go in, if at all.

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Old 02-01-08, 08:07 PM
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bcoppola: Direction is only one way but it depends on your crankset setup. I worked on one the other day that would move the rings inward.
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Old 02-01-08, 09:52 PM
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Originally Posted by Torque1st View Post
bcoppola: Direction is only one way but it depends on your crankset setup. I worked on one the other day that would move the rings inward.
Right. Come to think of it, the spacers moved my FG's ring inward, not out! D'oh.

(This old 820 functions, in part, as a wrenching practice mule.)
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Old 02-01-08, 10:31 PM
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Wrenching is FUN!
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Old 02-02-08, 02:25 PM
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I just moved a GS200 crankset to my "main bike" from my "backup bike". Although it was a severe downgrade, "quality-wise", I needed the shorter 170MM cranks vs the 175MM I was using- See-
https://www.bikeforums.net/fifty-plus-50/361045-your-cranks-too-long.html

I initially used the original BB cup & spindle set up, although the races were questionable. I then changed to a new cartridge BB and noticed these differences-
On the old spindles (I had 2 different ones) when I tightened down the arms, the bolts basically tightened down immediately and I had negligible rotation as I torqued them down. On the NEW cartridge, I could feel the arms tighten down pretty much all the way. I still had that feel when I reached what I felt was more than adequate torque. I didn't have to adjust the Fder (I have friction shifters, thus more "leeway"), but I felt the more I torqued it, the more tha chain line would decrease.
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Old 02-02-08, 10:15 PM
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Sheldon, what voice do you sing? I've been in choirs since around 6 years old -- Love it!

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Old 02-03-08, 12:23 PM
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Originally Posted by Road Fan View Post
Sheldon, what voice do you sing? I've been in choirs since around 6 years old -- Love it!

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I'm a bass-baritone, been doing it since I was about 45...

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