Figuring out BB length

[d_douglas]

Hi folks,

I am in process of retrofying my Vicious 29er and have hit a snag. The frame is an older one of unknown vintage (maybe 2008?) so doesn't have massive tire clearance, FYI.

So, I am trying to fit a Ritchey Logic double crankset with 46x36t rings and a 107mm XT cartridge BB. I assumed that the Logic cranks are pretty narrow, so I used this BB (because it was sitting around) and now see that there is no way it'll work. It's not even close. Even the NDS arm solidly hits the chainstay. so it needs to be at least +5mm both sides as a first guess.

I need a new BB and wonder if anyone knows how to determine length without buying endlessly. I know I can take it into my LBS and ask them to find a UN54 (or so) and test fit until it works, but is there a way to determine length beforehand, so I can simply go to a little used shop and grab a $10 BB and do it myself?

Sheldon Brown's site does not have such info (as far as I can see), but I have looked up Ritchey BBs on das web and it seems that there is a wide range of sizes from 107mm-122.5mm.

Also, if I could determine the correct length, I might try to hit people up on the Forum for a Phil Wood BB :)!

Any thoughts?

[wombatspeed]

Since you have already installed the 107mm BB just measure how much more clearance you would like on DS and NDS (and how much more you need the chainrings to move outwards for a proper chainline).

Then simply add that difference to the 107mm --> and you will have your ideal BB length.

That's the way I have been doing it...

Good luck

[d_douglas]

Yes, i thought of that, but the rings impact the stays, so I cannot tighten them! I could do it for the NDS, though.

Are BBs asymetric normally? I think so. What I mean is: if I can see that I need 7mm extra on the NDS, do I just assume that I can use a cheapish Shimano BB that is 14mm longer than I currently have?

[gdw]

The Ritchey bottom brackets for that crankset were 120mm and that's what you'll need to try but it still might not work.
http://www.bikepro.com/products/bottom_brackets/ritcheyp.shtml

Your frame was probably designed for cranksets with 94 or 104mm chainrings. The bend in the chainstays might be too abrupt and they might flair out too quickly to run a 36t inner chainring and maintain a decent chainline even with the correct bottom bracket.

[Mark McM]

Yes, i thought of that, but the rings impact the stays, so I cannot tighten them! I could do it for the NDS, though.

Are BBs asymetric normally? I think so. What I mean is: if I can see that I need 7mm extra on the NDS, do I just assume that I can use a cheapish Shimano BB that is 14mm longer than I currently have?

Asymmetric BBs were common a few decades ago, but today the norm is for symmetric BBs. So unless a BB is specifically spec.'ed as asymmetric, you should be able to assume it is symmetric.

Here's a suggestion to figure out the how much extra BB length you need:

Measure the distance from the inner surface of the outer ring to the inner surface of the inner ring. Next, remove both chainrings, and install the driveside crank onto your current BB. Now, install the smaller chainring onto the outer chainring position, and measure the clearance between the chainring and the chainstay. Subtract this distance from the original measurement you took from the distance between inner surfaces between the chainrings, and this should give you the absolute minimum extra length of BB you need on the drive side. (If there isn't clearance for the small chainring in the outer position, estimate the amount of chainring/chainstay overlap, and add this to the inter-chainring measurement). Then, add a few millimeters to this value for a little extra clearance. Finally, multiply this number by two to account for the extra BB length on the other side of the symmetric BB.

Example: The distance between inner surfaces of the two chainrings measures 6mm. After installing the driveside crank onto the BB and installing the small chainring in the outer chainring position, there is only 1 mm clearance between the chainring and the chainstay. Subtracting 1 mm from the 6mm inter-chainring measurement gives a value for the minimum extra driveside BB length of 5mm. Adding 2 mm for some extra clearance gives the extra BB length on the driveside of 7mm. Multiply this by 2 to find a total BB length increase (both left and right) of 14 mm. So, if the original BB was 107mm, then in this example you need to get a new BB of at least 121 mm.

[Peter P.]

I have another homegrown solution.

Install the 107mm BB but don't tighten down the fixed cup. Leave the fixed cup tool hanging on the cup or axle.

Install the right crankarm.

Using the captive fixed cup tool, unscrew the fixed cup 1 turn at a time until you get the clearance you seek.

Measure the length in millimeters you unscrewed the fixed cup. Double it to accommodate the non-drive side crank arm.

That should be the axle length you need.

Any holes in my idea?

[gdw]

Trivia
Check out the 1992 Bridgestone catalogue on Sheldon's website. The MB-1 was equipped with your crankset and was supplied with a 120mm bottom bracket.

[Mark McM]

I have another homegrown solution.

Install the 107mm BB but don't tighten down the fixed cup. Leave the fixed cup tool hanging on the cup or axle.

Install the right crankarm.

Using the captive fixed cup tool, unscrew the fixed cup 1 turn at a time until you get the clearance you seek.

Measure the length in millimeters you unscrewed the fixed cup. Double it to accommodate the non-drive side crank arm.

That should be the axle length you need.

Any holes in my idea?

Yes, that could work too, but with this caveat: Depending on the BB tools required, you might not be able to leave the fixed cup tool on. For example, many splined BB tools cover the end of the spindle. Even if the end of the spindle is exposed, the tool may protrude so far that the crank can't be fully seated on the tapers. On the other hand, if the fixed cup is only partially installed, you may still be able to turn it by hand. Also, this method only works if the drive side spindle & bearing is captured by the fixed cup - this would be more difficult to do with cup and cone bearings.

But if the BB is a cartridge bearing, with the cartridge captured by the fixed cup, this might be the easiest method. In which case, I'd suggest that the order be that the fixed cup be installed only a few turns initially, the crank installed onto the spindle, and the cup screwed in (by hand) until the desired clearance is achieved. Before inserting the fixed cup, put a rubber band (or a piece of an old inner tube) around the lip of cup so it can be gripped more easily (slide the rubber band off before measuring the gap).

[Peter P.]

One other thing to consider: The Vicious 29er chainstay design may be configured for recent vintage triple drivetrains and certain width tires.

That means a 36T inner ring may not clear the chainstay but a 32T will, for example. If you adjust the chainline with the correct length axle for your 36x46 chainset, you may run into chainline and shifting/rubbing issues.

For example, I remember reading of someone who bought a Richard Sachs cyclocross frame second hand and tried fitting a 39x53 chainset on it. The 39T ring was hitting the chainstay or darn close to it. Richard explained the bike was designed for the smaller chainring combos usually found on 'cross bikes.

This is more common than you think.

[d_douglas]

Update:

I realized that my wife's bike had a long axle BB, so I pulled it and threw it on my Vicious. Bad news is that at 118mm, it was STILL too narrow. The 36t ring still bottomed out before ever being tightened. I am guessing that I need at least 5mm additional to clear the stays (crankarm and chainring)

Why is this? I used fr same rings on an XTR M900 crankset (107mm bb I think) on this frame last year, so why am I needing a 125mm BB? Granted, I am going from a bash/36x24/ to a 46x36, but are ritchey doubke cranks that much 'wider' (narrower?) than XTR?

I don't think there'll be chainline issues, but it just seems weird.
What companies out there make a 125-128mm cartridge system that would ideally be asymmetric?

[PacNW2Ford]

deleted, rethinking things

[oldpotatoe]

Update:

I realized that my wife's bike had a long axle BB, so I pulled it and threw it on my Vicious. Bad news is that at 118mm, it was STILL too narrow. The 36t ring still bottomed out before ever being tightened. I am guessing that I need at least 5mm additional to clear the stays (crankarm and chainring)

Why is this? I used fr same rings on an XTR M900 crankset (107mm bb I think) on this frame last year, so why am I needing a 125mm BB? Granted, I am going from a bash/36x24/ to a 46x36, but are ritchey doubke cranks that much 'wider' (narrower?) than XTR?

I don't think there'll be chainline issues, but it just seems weird.
What companies out there make a 125-128mm cartridge system that would ideally be asymmetric?

shimano UN series..127mm

[d_douglas]

Yeah, I guess it's that simple. Thanks OP