Hi Geo'!
You asked...
First question does anyone know what size BB an RST takes?
Not me, not definitively. I'm going to defer to Dave Whittle on this one, since he works with them daily and can tell you for sure.
Second there seems to be a big (5x) difference in price between Shimano UNxx and those by Royce, Wood etc -are the higher prices justified in performance?
There is a big price difference, but I have also found a performance difference wrt longevity under severe conditions (blowing talc-fine desert sand, general use in rain, lots of uphill work, high use, etc). Whether that cost-benefit ratio pencils out for you depends on a number of factors, including where, how, and how much you ride and whether or not you do your own work (labor costs). In helping friends with their bikes, I've found the recent bottom-line Shimano sealed BB cartridges seem to have taken a dip in quality and don't last as long. Previously, the broad range of Shimano units were indeed a good value that lasted long enough to more than justify their *low* cost (and the proportionally greater labor cost needed to replace them).
In my own experience and that related across a number of user groups and Fora, it seems a square-taper (internal) BB is generally the best value-for dollar in terms of long lifespan and reliability. I've had Tange units (subcontractor on some higher-end Shimano units) last in excess of 25,000mi. My Phil Wood units have exceeded 30,000mi and are still spinning away happily. They cost a lot back when I bought them, but far less than current models due to inflation over the last 25 years or so they've been installed. On balance, I have found the premium units justified their higher cost and then some, becoming bargains over time. Though I don't have deep experience with them, SKF's internal square-taper units are also well-regarded by many.
I'd say if you ride a lot and have a history of trashing BBs, then a premium square-taper unit would be a good choice that could be expected to last "longer" than a cheap version of the same.
Thirdly, I've only heard bad things about outboard BBs -anyone like them?
Again, taken as a survey across user groups and Fora as well as my own experience, external BBs don't seem to last as long as internal versions, all things being equal. Unfortunately, things aren't equal, and setup during installation seems to play a huge role in longevity, as does a correctly faced BB shell.
Unlike internal units, the bearings inside the cups of external ones tend to be smaller and spaced further apart. My autopsies show some were not adequately lubricated from new (they come sealed, so lubrication can't be readily added). Some are not well sealed against water and dirt intrusion and lubricant loss. All these things do indeed shorten bearing life. It is more critical for EBBs to have a squarely faced BB shell to thread into. It is *really* critical they not be excessively preloaded during installation. Much like a threadless headset, it is really, really easy to overload the bearings with the preload ring, and I think this affects EBB bearing life more than anything else. That preload ring is really intended to remove extra play between the spindle and the bearings, nothing more. Unfortunately, just the other day, I saw someone crank down on the thing like there was no tomorrow. A spin of the cranks showed they still turned with remarkable smoothness, but likely not for long; the design is not very tolerant of high preloads.
Within the EBB world, there are also two broad grades: Factory standard in a variety of ranges, and aftermarket premium models.
In my own use and with careful installation and attention to preloads, I found Shimano Deore units were not as long-lived as my experience with square-taper internal (sealed) models. I have since switched to a Phil Wood EBB unit on the Nomad and everything is working well at use far exceeding what I got from the Deore EBB units. You can see photos of my install here:
http://www.thorncycles.co.uk/forums/index.php?topic=4523.msg43281#msg43281The Phil unit is considerably heavier than the Deore, but the heft is put to good use, as the stainless billet cups offer better support than the aluminum used in the Deore units I had. External dust shield clearance is tighter, and there has not been the lube loss I experienced with the Deore. So far, so good, but I have not yet accumulated enough use on the Phil unit to say it is better, worse, or as good as their internal units.
My fast-light pedaling style may be a factor in lifespan, but I have not yet determined how. As a spinner, I am putting in lots more turns, but the cyclic loading is less and more even than if I were a masher.
Though obvious, it is worth keeping in mind: Cranks intended for internal bearing BBs aren't compatible with external bearings, so a change in BB would also require a fresh set of cranks, adding to the cost of conversion.
I have detected a few rumblings from down below which could be a few things including the BB.
You're right; the rumblings could be due to many things. Here's a few leading candidates worth checking before splashing out for a new BB unit of whatever kind:
• Chain wear (stretch)
• Chain tension, lubrication, and cleanliness as well as a stiff link or two
• The state of the chainrings (wear)
• Pedal bearing wear
• Tightness of the BB retaining cups
Hope this helps.
All the best,
Dan. (...who thinks brackets may be at the bottom of it all)
