Bottom Bracket Symmetry

[ravanzino]

I’d like to replace the bottom bracket on my 30-year-old, 10-speed, Centurion Le Mans road bike with a cartridge-type assembly such as the Shimano BB-UN26. The bottom bracket shell has standard English threading of 1.37 in. X 24 TPI with a 68-mm width, and the spindle is stamped “3S2.” The spindle takes nuts on the ends to attach the cranks, but the cartridge bottom brackets all seem to take bolts. I measured the distance from the ends of the square taper to get a spindle length of 124 mm. The existing spindle is non-symmetrical with the longer side on the right (chain side). When mounted on the bike, the “protrusion distance” from the edge of the shell to the end of the square taper is 29 mm on the right and 27 mm on the left.

I need to select a spindle length for the new bottom bracket so that the protrusion distance is approximately the same on the right and the same or longer on the left. But, I don’t know whether the Shimano spindles are symmetrically mounted. If they are symmetrically mounted, I can calculate the protrusion distance by subtracting 68 mm from the spindle length and dividing by 2. A 127.5 mm spindle would have a protrusion distance of 29.75 mm on both the right and left sides. This is fairly close to what I need. However, if the Shimano spindles were not symmetrically mounted, I’d need to know the exact protrusion distances for the right and left sides.

Does anyone know whether the spindles are symmetrically mounted in the Shimano BB-UN26? If they are not symmetrically mounted, what are the protrusion distances for 122.5 mm and 127.5 mm spindles?

[RBurrelli]

Can't answer your question, but check out this link. The 3S on the chart may correspond to what you have mentioned. There is a lot of info.

Hope it helps.

http://www.sheldonbrown.com/bbsize.html

[ravanzino]

(10-07-2010, 04:08 PM)RBurrelli Wrote:  Can't answer your question, but check out this link. The 3S on the chart may correspond to what you have mentioned. There is a lot of info.

Hope it helps.

http://www.sheldonbrown.com/bbsize.html

I've seen that chart. That chart is for "Traditional Cup and Cone" bottom brackets. That's what I have now. I know that my spindle, like the 3S one on the chart, is asymmetric. What I want to know is whether a Shimano BB-UN26 cartridge bottom bracket is symmetric or asymmetric.

[DaveM]

Your right, it's hard to find any confirmation of this online. I believe most square taper cartridge BB's will have a few mm of offset (longer on the drive side) since this was pretty standard on square taper cranks. Since the difference in your measurements are all very small, I'd guess you'd be fine with either 122.5 or 127.5 unless your cranks were very close to hitting the chain stays with the old BB. Narrower is generally preferred, but even 127.5 is only 3.5 mm longer than what you had. Doubt you'll notice the difference.

You can also just call whatever shop you plan on buying from. They can probably tell you for sure one way or the other.

[Bill]

Nice information. I never seen that chart before, but is pretty useful! Dave I am wandering did you use to work in a shop at one time?

[DaveM]

(10-08-2010, 09:04 AM)Bill Wrote:  Dave I am wandering did you use to work in a shop at one time?

Yes, for a good chunk of the 90's. Unfortunately, my knowledge base drops off very suddenly for tech that came out after then. Don't know much about...disk brakes. These days I volunteer in a bike coop that helps people fix their own bikes. So we get a lot of old bikes being brought back to life and people who just want something functional. Very few carbonophiles.

[Bill]

Ok I'll private message ya I don't wanna high jack this thread, sorry should've done that in first place.

[trevgbb]

These are the Techdocs for UN26, but they don't give the dimension you wan't!!
http://techdocs.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/BB/EV-BB-UN26-2438A_v1_m56577569830694234.pdf
http://techdocs.shimano.com/media/techdocs/content/cycle/SI/SI_0078A/SI-0078A-001-ENG_v1_m56577569830677388.pdf
the LH is not as critical as the right, as long as it is not shorter as this could cause the LH crank arm to foul the frame.
May have a un26 in the workshop, if so will check it out.