I'm looking to replace my bottom bracket + crankset on a mid-1980's Trek 830. It has a 68mm shell with the standard 24 threads/inch, so I'm good there.

Have decided to go with a Shimano UN54 BB but am unsure about what spindle length to choose. My spindle from the original "cup and cone" BB measures ~126mm (see picture). It's a "3T" designation, from Japan, as mentioned by Sheldon Brown.

If I go with a Shimano 7/8 22/32/42 crankset, I've seen it written that a 110mm or 113mm BB spindle is required.

Which spindle length do I go with? Close as possible to the original length or the one recommended/required by the crankset?

Thanks a lot!

how much clearance did you have with the old BB?

By "clearance", do you mean the distance between the smallest chain ring and the chain stay on the drive side? If so, I'll try to measure.

a friend just got a yellow 830 for free. it has a bb cartridge- oem? how long did trek make 830s? doesn't look almost 30 years old. nice bike, though. made in usa.

Considering that the "original" BB spindle is 126mm, and the recommendation for your "replacement" is 13 to 16 millimeters shorter than that, I'd say that you should install the longer (113mm) recommendation. If there's too much clearance and the derailleur doesn't shift to the outer ring, install the shorter. If you install the shorter BB and hard parts start running into each other, you stand the chance of damaging the paint.

When combining parts of unknown specification (what's a "Shimano 7/8 crankset"?), sometimes the best answer is "Try it and find out".

The crank you use determines the spindle, but----

1st-
I'd measure the dropout spacing to see if it's the original 126MM.
(you can remove the wheel and measure inside to inside)
IF it's 126, that'll be just under 5.0"(like 1/32" under)

Modern MB cranks pretty much assume you'll be mounting it on the newer 135MM spacing and recommend the "longer" spindle length.
The shorter is recommended for 130MM spacing.
You really probably want the 108, but the 110 should work OK.

Post the model# of the crank (or a link) and maybe someone else can help confirm/deny my recommendation.

Something you can do is to back out the high limit screw of the FD and see how much "extra" travel it has. Too long of a spindle and it won't reach the big ring. That may help determine which way to go on those "extra" 2-3MM.

UN 54 is a product code, length is a separate concept, several offered.
pick up a few and test fit them .. a hands on experiment..

127.5 is the longest offered, ...

One thing to look out for on your bike; was the current spindle factory installed "backwards" (i.e. with the longer end toward the non-drive side) in the cups? I had a '93 Trek 7000 MTB that came with a Tange cup-and-cone bb and the spindle had to be installed in reverse of normal to let the nds crank arm clear the nds chainstay. When I replace the OEM bb with a UN-XX cartridge, I had to go to a 127.5 mm (the longest offered) to get both crank arms to clear.

Seems like a prime opportunity to go to a bike shop. Let them figure it out. LBS usually have a good supply of standard BB, like the UN 54 in different sizes. Buy the one BB that works.

I've seen 113 specced on mtbs with triples. If I had to venture a guess, I'd probably go with that over 110.

pete4120, As soon as I'm able I'm taking my touring bike to the LBS to fit a UN54 to a Suntour triple. Reason is because I don't want to buy two (or more) UN54s to acheive a proper chainline with a non typical installation.

Brad

Thanks for all the suggestions; look like I have some measuring to do.

Brad and Goatalope, your LBS route may be the way to go here.

Given that you're changing cranksets, the old bottom bracket isn't relevant.

I agree with this. Your life would be easier if you took the bike to your LBS to figure out. But; if you’re a hard-headed DIY’er, such as myself, you might try placing the spindle in the BB shell approximately where it lie if it were mounted; then slide the crankset you want to use onto the spindle. Then run a straight line from the center of the freewheel/cassette to the center chainring. If this line runs parallel with the bike frame, you have a decent chainline. If this line is not straight, you at least have a ballpark idea of whether the spindle should be longer or shorter.

If your upgrading your crankset to a more modern 4-bolt, they use much shorter spindles. I found this out when I built an ’03 Rockhopper frame which spec’ed with 110mm spindle. I had a UN54 113 BB that I used with an older XT 5-bolt crank and the inner ring was so tight that it rubbed against the chainstay.

The following table is from Barnetts and might be helpful.



I believe the UN54 is just a later version of the UN50/UN51.

- Mark

However, the OP is also replacing the crankset and is looking for the correct BB length with the new crank. Your chart is useful, just not relevant.

How rational and civil of you to point that out. ‘Damn you Canadians for making sense once again.’

This chainline BB length thing has always perplexed moi. I had imagined that there would be some sort of magical formula that someone would have come up with years ago. But, if there is, I haven't found it. I guess it is just one of those things that you have to play around with until you get it right. Reminds me of my wife and how we've stayed married.

OP stated that the crank he is interested in recommends a 110 or 113MM BB.
OP has a "mid 80's TREK".

Unless the frame has been re-spaced, it has a 126MM drop outs.
The 113MM is what one would recommend for a "modern" 135MM spaced frame.
The 110MM is what one would recommend for a 130MM Road Bike spacing.
ERGO, he needs the shorter BB. More likely a 108mm, but 110 should work.

I just went through the SAME thing, installing a NEW crank & BB on my 86 RockHopper With same BB length recommendations (110 & 113) for the crank.
I chose the 110MM and it's about 2MM long.

That works for me, since I tend to use the outer cogs more than normal.

I'm not Canadian. I live here: https://www.cityofvancouver.us/ , the "other" Vancouver. In fact, I grew up in California, which makes me an "invader" here.

Back in my bike shop days, customizing bottom bracket length was often discussed. The relative importance of "Q" factor, chainstay dimpling, chainline, and ankle clearance were weighed against each other. Sometimes I think we've lost some of that with the one-size-fits-all two-piece cranks.

I think I’m having a déjà vu moment from years past with you JW when I made the same mistake about your local. You have my apology for the mix-up, and some sympathy for you not living in the, what I’ve imaged as, the better Van.


That being said; I’d like to take this opportunity to ask this question about determining BB length. Leaving individualized considerations of Q factor aside, how do you bike mechanics determine the proper spindle length for a blank-slate frame to provide a straight chainline?

Tricky question, as chainlines are not always intended to be straight anymore, in the sense of the middle of the cassette lining up with the middle of the chainring set. Scope out some of the factory-correct mountain bikes we've got now... my 100% kosher Shimano setup on my commuter here lines the middle of the cassette up almost directly to the granny ring. And I'm a cross-chaining fiend, so you can imagine what my chain goes through on a 48 x 34 stoplight launch

In the days of square-taper dominance, however, I generally looked for the inner face of the granny ring to be approximately at the BB shell's face on a 68mm shell.