SHIMANO BOTTOM BRACKET

[MIKETHOMAS]

I've just worn out my second set of bearings (mountain type hollow units on a conventional touring bike). These have lasted about 1600 miles.. The frame was prepared for these bearings i.e. BB housing faced square. When these came onto the market I thought they were an engineering dream with the bearings well spaced. In reality they are rubbish. What's the answer, go back to taper & cartridge? Or is there an alternative?

[Brucey]

quite a few people have trouble with these bottom bracket units and complain bitterly about them. I don't think the seals are that good, they don't have enough grease in from the factory, and the gap through which crud can get in is unfavourably oriented. If you look inside the bearings and they are full of brown water you know what has happened.

Note that;

1) campag use similar bearings with two sets of seals, not one.
2) SRAM GXP BB units use similar bearings (with a different seals, retention and clamping system) and recommend that you overhaul the bearings (by popping the seals and regreasing) after every 'very wet' ride.

Shimano don't propose any maintenance at all, or make provision for it. I would certainly encourage a squirt of GT85 to displace water after every wet ride, and I would also heartily recommend regreasing the bearings (which can be done with a bit of fiddling about) every month or so if you ride in the winter weather.

I am sure that some such bearings fail because even though the frame has been faced, the BB threads were so skewed that the cups don't bear against the faced BB shell anyway. I'd suggest checking the width across the installed cups with a vernier caliper. A good tolerance would be +/- 0.1 mm. Much more than that and the bearings will likely be shot in no time.

I am equally sure that many of these BB units fail exactly the same way most A-head headsets do. You won't necessarily want to hear this but if as many external BBs are adjusted as badly as headsets are, it would account for all the failures and then some. The bearing preload with shimano H-II BBs has to be spot on, which is a tiny tiny torque on the end cap. Plenty of cranksets are fitted with way too much preload (just like headsets often are) and then the bearings fail 'because they are no good'.

I have a notion that if the preload is too high, it may also have the potential to distort the seals, so the water gets in more easily; double whammy....

I'd be interested to hear how your bearings failed. I guess the saving grace is that they are not too expensive to replace.

cheers

[Erudin]

I managed to replace the bearings in my bb (Link), definitely worth packing them with grease as mentioned above. I check the bearings are not side-loaded by spinning the cranks with the chain off, the cranks should spin freely for quite a while.

Good article about "sealed" bearings: http://sheldonbrown.com/brandt/sealed-bearings.html

[PT1029]

I don't use them (my old stuff still runs). The word I get is to replace the external bearings/cups with Hope ones (colour choice too!).