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  1. #1
    Thinks it's still 1991. 1987cp's Avatar
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    More cartridge BB ponderings (in two parts)

    I'm getting ready to do some cartridge conversions on my older road-ish bikes, and already have two older Shimano cranks (an Exage 300 in my possession and a Deore MT60 on the way), and Sheldonbrown.com's BB chart lists both models and recommends a specific BB length for each. And doesn't note an asymmetrical requirement. And I know that the BB requirements are really set by the crank and not by the frame. So far so good.



    Confuzzlement, Part 1:

    On the Rivendell website, they sell the Sugino XD2. Per Sheldon's chart, that crank uses a 113-118mm BB. No explanation for why the range on that when no range is shown for my Shimanos. But Rivbikes.com notes on their BB page:

    Sizing: Non-Rivendell Road bikes with three chainrings and no bowed-out chainstays: 110mm. Any bike with bowed-out, but not ultra bowed-out chainstays: 113mm or 115mm. All Rivendell models take a 107mm with the XD2 crank.
    So if I were getting an XD2 (which I'm not) for my old road-ish frames with straight chainstays, and were to follow their advice, I'd get a BB which is 3mm shorter than the minimum width Sheldon's chart shows, and evidently according to them I should be happy. If I were to try to transfer that advice to my Shimanos (which I assume both came originally on MTBs), that would mean I should again get BBs slightly shorter than Sheldon's chart shows.

    BUT, and this may be the key question, is Riv's advice likely to be based on having extra clearance with straight stays, having a more-inboard chainline preference on a roadish frame, just getting the narrowest "Q factor" practicable for a given combo, or what? Or should I quit overthinking it and just get what Sheldon's chart recommends and call it good?




    Confuzzlement, Part 2

    So the UN55 is obviously better than the UN26 because it comes with a metal left-side cup and is apparently a bit smoother, and some claim it'll last longer. And really, $25 for a BB isn't all that expensive. But especially since I'm looking at getting two at the same time to use on really cheap bikes, the UN26 is really tempting since I'm really cheap and it's cheaper still. However, I've noticed some sellers listing a "UN26E" and a "UN26K" (and maybe one more, in addition of course to the plain "UN26" with no letter after it), with no explanation of the differences. I've seen a picture of one that seems to show a spacer included, but pics aren't always reliable. Any experienced input to explain or guess what the letters mean?

  2. #2
    coprolite fietsbob's Avatar
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    Range is often due to seat tube diameters,, oversize-big tubes move the FD outward,
    so to get stroke right the crankarm-chainline moves outward too..

  3. #3
    Andrew R Stewart Andrew R Stewart's Avatar
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    "E" BBs are for the ft der mounting plate (not clamped on ST). Usually found on mountain bikes, often with rear suspension.
    Where the rings sit in relation to the chain stays is my first BB axle length concern. This can be near impossible to figure out as it's dependent on many factors. How is the crank arm designed (rings to axle hole), axle hole design, the actual manufacturing tollorances,Inner ring size and chainring bolts protrusion, chain stay dimensions (and any dimpling or not), BB shell's placement of the stays WRT the shell's edge.

    Many times I have calculated out the replacement size only to have real life fitting need one size up or down for best results. I wouldn't assume that the two cranks would use the same BB axle lengeths, untill after confirming. This is why shops service departments exist. they will have a range of BB sizes in stock, be willing to test fit till the best is known and only charge you for the one used. Then you can test fit the second crank on the "right for the first crank" Bb and see how that works. Andy.

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