I have 1999 Shimano XT cranks + a shimano UN51 bb . is it possible to adjust the chainline w/this setup? the crossover gear( big to big ) is way off . When I'm in the granny gear I only use the 3 largest cogs , so I'd be happy to give up the 5 smaller cogs. If it's not possible , what square taper BB's are available with the ability to adjust the chanline?

I'm not sure I understand your problem. Why are you in the big/big combination?

If the Big-Big combination is off, no problem (if everything is set up well, it likely will be off). That's why you have a middle ring and something to move the chain to it.

+1 to LowCel

Like stated above, I'm not sure what you are trying to accomplish but if you want to change your chainline, you will have to get a new BB with a different spindle width (depending on which way you want your rings to move).

Just don't x-chain.

Yeah, what everyone else said....

also, are you asking if there is an 'on the fly' adjustable BB spindle length BB? That would be interesting. Adjust the spindle out with a poploc type switch when you want to go small small, and switch it back left for the ''big big''

Or you could get a singlespeed....or just learn how to shift.

the reason I use the crossover gear is , I'm often in the Big ring and want to make use of all 8 cogs . When I am using the granny gear , I never use the 5 smallest cogs. The way things are now, I have 6 gear combinations that I don't use . The Big - Big combo is a gear that is more likely to be used than the 22x11 or 22x12 . Those gears never get used.

this is not my first rodeo ,there Bucko

Unfortunately that is just the way the bikes are designed. They aren't meant to be used with that gear combination.

Basicly with a 3x8 setup you should only use these combinations.
Big front - four smallest rear.
Middle front - six middle gears on rear (I don't follow this rule though)
Small front - four biggest gears on rear.

I know that's how they are designed, that's why I want to change it a little. I think Phil Wood BB's allow it , now if I could only find one.

Here ya go. They have them at Cambria.

It does sound like this is your first rodeo.
You should never even use the top three cogs when you are in your big ring. (Unless you have a double and a straight block)
All my Mtn bikes are set up with the chain too short to even get onto the top two cogs when I am on the big ring.
You need to use the gear combinations that give you the best chain lines and access to the best jumps from the middle ring.

Thanx lowcel , You Da Man .

that's great for you man . I use whatever gear combo that feels right at the time . Yes , I know that the crossover gear combo puts a lot of stress on components and wears out chains, chainrings, and cogs rapidly . I don't care . $hit breaks, you buy a new one. No big deal. thanx for your opinion

I see what you've got going, BoSoxYacht. Is it possible to accomplish virtually the same thing by upsizing your 2 bigger rings? I'm just wondering - I do think your solution is the better choice for many reasons, the least of which is the fact that you'll get to ditch the UN51 for a Phil Wood.

thanks gastro, the un51 has seen a lot of use and is still kicking . I've been wanting to upgrade for a while , But why , it's still smooth . It's time to get the 2 big rings replaced so I may just get new cranks + BB to match. I think I may have been a little hard on Worldwind . He prolly doesn't get much conversation at work and needs a little banter .

No problem. I still don't think it is the best idea but it doesn't really matter what I think, it's not my bike or my money. I hope the phil wood bottom bracket works for your situation.

the best thing about this attempt is, like gastro pointed out , is a new Phil Wood BB . Phil's components are simply perfect .

I haven't yet encountered a situation where I would want to be cross-chained, nor can I imagine one.

Why shift the chain up to a bigger cog in the rear when you can simply drop into the middle ring?

less chance of a mis-shift in rough terrain is why I do it . The sport is not new to me Hank . I know how "to ride" , and I know how "I ride". Sometimes these are two very seperate techniques . BTW Hank , do you read the book or see the movie? Just curious.

exactly

+1

I don't understand how being in that combo would reduce the chance for a mis-shift (or why you would be concerned about a mis-shift if your drivetrain is properly adjusted).

Being in the middle of the cassette offers more shifting flexibility, especially quicker shifts into easier gears when compared to being cross-chained. I can't think of any advantage to cross-chaining, other than having to run a longer chain which will give you more chain slap.

I've read the book a few times. I don't know of any movie adaptation.

if ur front derailleur is properly adjusted and ur teeth arent shot there is no chance of the chain popping off

very true, if ur mid cassette it is far faster/easier as well as less chance of a misshift

think about it.. if ur bigringing both.. and you need to shift rapidly.. ur screwed.. you have to send it down 3or 4 gears.. more of a chance to get chain skip..etc

i guess its personal preference.. and ur money...

as for me.. i'll stick to keeping my chain straight in the middle ring

And here I thought cross-chaining cause more mis-shifts and chain throws.

Sorry Hank , why I do this is not open for debate . I do this because it works for me. If you don't like it , then don't do it . This thread is a question about a chainline and BBs , not an inquiry about proper shifting techniques. Your opinion has been heard by all .Thank you very much