What do the different sizes mean. Is all the additional width on the chainring size? I was thinking 113, might be OK for a touring bike with a road tripple. On the other hand I want up to 42 teeth on the middle ring for halfstep gearing. Should I go to a 115?
Q factor does not interest me, I'm bigger than the average cyclist, so standard qs probably aren't a fit anyway.
It's an off-brand BB -- a recognized brand but I can't remember which, and I'm not at home with my parts bin.... It's probably close in quality to a Shimano UN-52, but definitely not above that. If I remember, it's about as heavy, too; maybe an ounce heavier.
I got a couple as backups. Even used one on my wife's bike for 2800 miles last summer in some pretty nasty conditions. It held up well. Since it has sealed cartridge bearings, I don't think you have to worry about bearing quality/adjustment, or the seals -- they may not be as bullet-proof as a UN-73 but I doubt they'd fail in normal use. Nashbar had them on sale for $9.99 when I got them, I figured you can't go wrong at that price: 3 for the price of a UN-73.
The correct BB length is determined by the frame maker. The maker will generally tell you a couple sizes, depending on what crank you get (double or triple). But if you get within a couple mm of ideal, it'll work fine. The difference in length between the sizes isn't necessarily all on the right (chainring) side; this is because "fat tube" bikes need longer BBs, and of course the cranks have to be positioned outward on both sides.
If you don't have access to the frame maker's info, measure what you've got, or someone else's identical frame. Or go to Sheldon's site for help.
i have not tried that bb. last time i needed some bbs, shimano un73s cost me $18 and nashbar were $16. its probably a TH industries aka FSA bb, they have a few sold by nashbar labelled "power pro" or something like that. i bought a nashbar isis bb for ~15 bucks, got ~7k on it, no problems. so great deal.
you should be interested in q factor.
cheap BBs allow you opportunity to experiment with q factor. for me (and many racers, which i'm definitely not) a narrower q factor crank pedals better. i cant tell you why this happens, but i did have the distinct impression it pedalled better. i set a monthly mileage record right after i changed the bb, so this MUST be true. a shorter spindle will deflect less so more efficient energy transfer. i suggest you try a narrow bb.
component makers tend to go with wider q factor because it makes their job easier, NOT because its technically the best choice. this philosphy in fact is what drives most all of the so called technical improvements on bikes (with easier meaning lower cost / higher profit). later the writers come up with a sexy spin.
threadless headsets are good example of this. makers used to have to stock 4-5 (steerer) fork sizes to accomodate range of frame sizes, now only one. writer spin: saves weight. stiffer. more aerodynamic.
the width of the bb spindle is supposed to be centered even across width of bb shell. unless you intentionally shift it with a spacer, which probably is not ever going to be advantageous.
i'm running a 108mm isis bb on a mtn triple, works fine. i think it calls for 113. so chainline off 2.5 mm. which means works slightly better (less chain bind/friction) in the hill gears and worse in the downhill/tailwind gears. can't really tell a difference since i average 12 mph.
i also have a 110mm sq. taper setup on another bike. works fine except neoprene booties occasionally rub chainstay at heel. so be aware more potential for crankarm / chainring / shoe interference with frame on narrower q factor (another reason makers go wide).
bear in mind that the bb, cranks and pedals together determine the q factor, not just the bb spindle. shimano made some hollow deore cranks a couple years ago, i bought some from nashbar for ~$32, figured worth it for the chainrings alone (64/104/4 arm). turns out those deores had a huge q factor, about 20mm wider than my xt cranks on same bb. also, theres a difference in the required bb spindle width depending on crank interface type, ie sq taper, isis, shimano splined. just to further muddy the waters and increase the probability that you'll surrender and visit your LBS. well if that dont stop you the search for the right bb tool and crank arm puller will. this is actually a good reason to patronize the LBS. tools cost more than a single (or two) visits.
sorry for the rambling, i had a near death experience riding tonight, wound up.
333, Every day is a near death experience, I hope you feel reasured soon!
Thanks for all the info, guys. I'm the frame builder, and you would think I would know about this stuff, but making a frame is certainly one of those things that exposes all your little information divots. I have a badly busted up ankle from a plane crash I was in, and my right foot is out of position a little. So the heel would tend to wack the seatstay compared to a normal person. That's why I don't care about Q, it may be importan but I have other priorities. It's a good point that these uints are so cheap I can afford to be wrong. I actually want to get a Phils, just for kicks, but I'l try one of these first, and see how it turns out. I'm using a square bottom bracket,