Ive never fragged one. I have had them creak in a carbon fiber frame however. I fixed this with teflon tape around the ends and put it back in. You must be a bike mechanic or have a rainmanish memory to be able to know that much about bottom brackets!
current weight: 224.2
Fitness Minutes: (439) Posts: 3,641 3/4/11 4:56 P
Seeing that my last post about grease brought out some serious technical discussion I decided to post something about my thoughts on bicycle bottom brackets. Sorry it turned out so long but I'm still interested on what other people think about them.
Bottom brackets have been problematic for a long time. To give you some idea of the disarray the world of bottom brackets are in I'd like to bring up the traditional Italian threaded bottom bracket. Italian bottom brackets have right hand threads on both sides. Because of procession the cups of a bottom bracket actually tend to turn the oposite direction that we pedal so a right hand threaded fixed (right hand) cups tends to unscrew with use. French threaded bottom brackets had the same liability but French threaded bottom brackets are now obsolete. The solution to this situation as dealt with by the Campagnolo tool kit is a tool that can tighten the fixed cup so tightly that not only won't it unscrew but you might not be able to remove it with any other tool (Putting the right cup in a vise and using the entire frame as a lever often works though). In a bizare combination of inch and metric measurements Italian cups are 36mm in diameter but have 24 threads per inch. To make maters worse Italian bottom brackets use long obsolete Whitworth threads that as far as I know are no longer used anywhere else. One advantage Italian bottom brackets had is a slightly larger diameter than English bottom brackets. When an English threaded bottom bracket got stripped or cross threaded it could be reamed then threaded in Italian threads; this trick saved many English frames. One problem is that while Italian bottom brackets are 70mm wide English are only 68mm wide. The resulting bike required Italian bottom bracket cups but still used an English bottom bracket spindle. Now that bottom brackets are cartridge I don't know how practical this trick is but I'll bet Phil Wood could still provide a bottom bracket for this need. Speaking of widths the 68mm and 70mm widths of bottom brackets were decided on back when bikes only had a single cog in back. This is really too narrow for modern bikes. Longer spindles can produce correct chain lines but the longer the unsupported spindle is the more likely it will break and stiffness suffers. With triples especially on mountain bikes the situation only gets worse. There is a new wide standard for English bottom brackets of 74mm but this hasn't caught on and only addresses one of the problems with English bottom brackets. Bottom brackets used to be durable back when we used cup and cone bearings in them. High quality bottom brackets with 9 to 11 1/4" balls almost never failed unless you neglected them too long. When bottom brackets went to cartridge bearings there really wasn't enough room for large enough a bearing to be durable. Back in the day bottom brackets were expected to last indefinitely today they are a consumable. When your bottom bracket wears out you simply replace it with a new unit. The situation got worse when bike part makers started to use larger hollow spindles to both increase stifness and reduce weight. With the larger spindles there is even less room for bearings. What we are left with is a totally illogical standard that only exists because of tradition. There is an old joke, "How did God create the universe in only 7 days? Easy, he didn't have to worry about an installed base." Some component manufacturers have come up with an ingenious way around the current standard, external bearing bottom brackets. These have cups that screw into the frame with large bearings on the outside of the bottom bracket shell. I have to admit that this is a reasonably elegant solution; one I might even be willing to support if we could do away with Italian threads. American bikes long used 1 piece "S" shaped steel or iron cranks; to snake the crank through their bottom bracket shell needed to be rather large with a 2" inside diameter. These bottom brackets are unthreaded with press in cups. These cranks and bottom brackets also have a lovely name "Ashtaula" (osh-tah-BUE-la) that rolls off the tongue. It is also 68mm wide like English threaded bottom brackets so if it was widened it might not be such a bad standard to adopt. It would have plenty of room for hollow spindles and robust bearings. Cannondale produced a standard based on this called BB30. It uses a 30mm spindle (standard was 24mm) and widened the shell to 73mm. I don't mind that this shell is threadless but it also uses press in bearings which I'm not crazy about. If the bottom bracket shell is damaged through poor assembly or maintenance and the shell wallowed out or distorted it may be impossible to repair the frame. BB30 is also an open standard meaning while Canondale holds the patent for it everyone is free to use it without paying royalties. Sadly nobody refers to these cranks and bottom brackets as Astabula. Of course Shimano isn't going to stand still; they have introduced their own standard. Their bottom bracket is officially called "Shmano press fit" but it is widely called BB86, BB90, or BB92 depending on the shell width. This shell has a inside diameter of 41mm giving it room to hold larger cartridge bearings than the old standard but the bearings are hosued in a plastic or aluminum carrier rather than pressed directly into the frame as in BB30. As much as I've disparaged Shimano over the years (Positron 2 and Front Freewheel anyone?) I have to admit I have a slight preference for the Shimano approach over BB30. I don't know if even mightly Shimano has the muscle to force a new standard onto the cycling world though. Trek uses a standard of their own also called BB90 on their Madone frames but I'm still not clear if this is an adaptation of BB30 or Shimano press fit. A few years ago headsets were the Acheles' heel of bicycles but now that 1" threaded headsets have largely been replaced by 1-1/8" threadless headsets the Bottom Bracket is the next area for improvement on bicycles. Personally I'd like it if some real engineers threw out all the existing standards and produced a clean sheet design that addressed all the issues giving cyclists a durable, stiff, light, easily maintained and reasonably priced standard.
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