mijome07

I'm re-building a 90s' MTB and it came with a stock 68mm BB/116mm spindle and a triple crankset. I'm going with a Shimano JIS square taper. But I only see 115 and 118mm lengths.

I'm getting either a Shimano or Sugino triple crankset and probably an 8 speed in back.

So, when in doubt, should I go up on length or the 1mm shorter spindle won't really matter much?

Thanks.

dsbrantjr

Match the BB to the crankset you choose. It may need a longer or shorter one (or the same) as you have now.

Wilfred Laurier

Cranksets generally have a recommended spindle length to give a specific chainline. That the bike came with a 116mm spindle is irrelevant if you are changing the crankset. Decide on a crank then choose the appropriate length spindle.

If you were staying with the original crank but had to replace the bottom bracket, both 118mm and 115 mm would likely work fine, assuming the 116mm spindle was the correct length to begin with.

mijome07

Silly me. I thought I had to match (or get close) to the stock length.

Thanks guys.

FBinNY 

Let your bike tell you. If you had good chainstay clearance, you have the room to go shorter, so that's a least safe. The better way would be to measure your current chainline and see which way would improve rather than worsen it. Keep in mind that precision doesn't apply because tapered cranks don't come up to the same place on all spindles, so there may still be a millimeter or two of change.

For my part, I only worry if the current chainstay clearance is very limited and I have little or no downside room.

mijome07

I just did a search and saw the cheap Shimano Atlus riveted stock crankset can go 116-121mm. They went with the 116mm on this Trek because the size of the seat tube.

Oh and I just got the frame/fork back from being powder coated. I'm replacing all the parts in this build.

Thanks again.

Bill Kapaun

Size of the seat tube doesn't matter since everything is referenced off the centerline.

FastJake

Especially with a MTB triple, I generally go with the shortest BB that will work properly without causing crank/chainring interference with the frame. This has the slight benefit of smaller q-factor but the bigger benefit of smoother running in the large/large gear combinations. I like using those but I never use any of the small/small combinations so I don't care about terrible chainline for those.

YMMV

FBinNY 

It shouldn't and didn't used to, but often does now.

Makers use the identical parts, except the band on a clamp-on font derailleur, so the position of the FD and it's range of travel changes according to the radius of the seat tube. Many modern FDs lack adequate travel to reach the outer ring on a 1-1/8" steel frame with textbook chainline, so you either have to move it in a few millimeters, or shim the FD outward.

BTW- the makers fix the problem by simply not offering 1-1/8" band FDs. Those who use braze-ons on a steel frame are left to their own devices.

Bill Kapaun

Thank you Francis.
I just work on the old stuff, so I'd assumed........

Speaking of ASSume----
As an old boss said to me-
When you ASSUME, you make........
I replied, speak for yourself sir....... but that's a different lifetime ago.

I appreciate your insight.

mijome07

Just received a Sugino triple 42/32/20T crank set. Sheldon said to use between 110-113mm spindle (square taper). Since this set has a 20T, should I choose the 113mm BB so there is plenty of clearance for the chain stay?

That 20T sure is small. Or maybe I'll be fine with the 110mm. I'm going with a 11-32/34T cassette. I've seen another BF member using a Dura Ace FD with this crank set and there was enough reach for the outer ring.

I don't know what spindle he was using. Yes, I messaged him and never got a reply.

FBinNY 

You're not visualizing this well.

The chainstays run at an angle from within the width of the BB shell, and spreading as they go back to the rear axle. So the smaller the granny, the closer it can be to the frame.

In any case, it's rarely the grannies that don't clear, it's the outer, or middle depending on their relative size. (see below)

mijome07

Okay. Just found this photo with the same crank set on the 20T. He's using a 113mm spindle. That chain sure looks close to the bottom of the FD.

LesterOfPuppets

Looks like the chain is also headed towards a pretty small rear cog. It's a tradeoff between wanting a low gear and wanting to use most of the cassette.

LesterOfPuppets

With an 11-30 cassette, the mid ring range overlaps the little ring range back to the 20x18, so no biggie.

In other words, 32x28 is almost equal to 20x18. I left off the 32x30 in case that gear is noisy.

This is with 9 in back, but 8 would be similar, just a little more spacey somewhere.

https://www.gear-calculator.com/?GR=D...&SL=2.6&UN=KMH

mijome07

Thanks. Gonna have to think about which cassette to get.