Overdue update: I swapped out the old Shimano UN40 BB (68x113) for a new UN54 68x107 today. I gained a steel leftside cup, and a much better chainline on the Quickbeam. Back in early November, when I found the old cracked cup, I just put in a new plastic cup. It promptly cracked upon install, but at least stayed tight. I was preprared to live with it, until I got to checking my chainline and realized the 113 bottom bracket was unnecessarily wide. While looking for the Tange unit Garth mentioned, I found a Shimano at the LBS, along with some thin chainring spacers to help me nudge the chain over.
I brought the chainline down from 50+ mm in the front to just 45 mm, pretty close to the 42 mm at the rear. I could get it closer with thicker chainring spacers, but dem photos in RR41 scared me (page 16), so I'll ride with a 3mm offset between front and back for now. Maybe I can find a quality front ring with a lateral offset at the teeth? Funny thing, RBW's website says all Rivs can use a 107 BB, and no question about it, the Sugino XD cranks fit very nice and close with the 107 unit. Hey, I just gained some pedal strike clearance! But it came assembled from Riv with the UN40 113 unit. - Andrew, Berkeley On Mon, Oct 31, 2011 at 5:44 AM, Garth <[email protected]> wrote: > Yes, you likely have a UN54 BB in there ... and it has a plastic cup. The > reason they usually break is people overtighten them. The need not be > torqued as the misguided instructions say! Just snug is fine. But hey, > plastic is just a bad idea period, for anything in the BB.. and Shimano's > use of it is inexcusable. > > You can get a new plastic cup, but you may be wondering if it will crack > too... not really fun. Another option is a new aluminum cup by Problem > solvers called an Up-Cup #54. But, for the cost of the Up-Cup($15-20), > you'd be better of just getting a new BB. Shimano now has UN-55's which use > a steel cup, but I'd get a Tange LN-3922 from Tange. It's the same > internals as the one Riv sells but is all steel. The Tange's do not have > that silly lip on the cup that the UN55/54's do, therefore allowing more > chainline adjustments than with the Shimano. A second choice would be the > IRD steel cup BB, easier to find. I've gotten the Tange's from bike shops > via ebay and online. > > > -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
