alexnharvey wrote:There's a lot of speculation that Tange Seiki made some of the square taper bottom brackets for Shimano. They certainly appear identical. The 7922 might be the Tange branded version of the highly regarded un72/73 which is no longer offered by Shimano.
The shimano UN72 bottom bracket was a purpose-made bearing unit, i.e. an axle with hardened grooves in it and a centre sleeve with precision made grooves in, plus balls between. In current Tange Seiki Bottom brackets the assembly comprises a separate axle, two (almost conventional) cartridge bearings, a spacer sleeve between the two, and cups that fit over the cartridge bearings. The assembly is thus completely different; they are certainly not the same thing.
There are many, many bottom brackets which are built the same way as the current Tange Seiki ones and they even seem all to use the same size cartridge bearings too. The Token/Tifosi one I linked to is a case in point. Some even have plastic cups. Others include ones made/branded by
- Sun Race
- RPM (FSA)
-Chin Huar
-Neco
-Genetic
etc
These generic cartridge bearing (GCB) bottom brackets are obviously not all made to the same quality and nor do they use the same quality bearings (even if they are the same size). However they all suffer from similar problems, which are not unrelated to the problems that any cartridge bearing BB assy faces. These include
a) sealing/lubrication and
b) achieving acceptable bearing preload.
Regarding b) it is a question of controlling the diametral fits whilst managing the installation loads in such a way as they don't immediately overly preload the bearings. If the BB assy doesn't have shoulders/lockrings at both ends it is pretty much certain that some of the installation loads pass through the whole assembly. Even with UN72 etc these loads are not insubstantial and they are usually enough to affect preload in the bearing. Several times I have taken a worn unit which manifested a little free play, installed it, and mysteriously the free play has gone. Other times you can feel the extra preload in the bearing once the unit is in the frame.
In units like the UN72 the (axial, compressive) installation load is resisted by a fairly thick-walled piece of hardened steel. However in GCB units there is a centre sleeve which sees the load, and this varies enormously in both stiffness and strength. In many cases it is possible to elastically deform the centre sleeve so that the bearings (which are up against shoulders on the spindle) see a disasterously large axial preload. In other cases the centre sleeve is soft enough (eg many with steel sleeves) that it can actually yield when tightening the second cup. This is both good (it can sometimes be used to 'adjust' free play out of the assy in service) but it is usually bad; to make sure the cups won't loosen in service you usually have the choice between overtightening the left cup and/or relying on threadlocking compound to hold the LH cup (and the bearings if they are not a perfect fit on the diameter) secure.
GCB units like the token/tifosi ones have a CF/aluminium centre sleeve; when I first saw this I wondered if it was mere affectation but it seems to me that this may be noticeably stiffer/stronger than most GCB centre sleeves, and that this may result in a better installation. Note that (unlike the current shimano ST BBs) the quality of a GCB installation relies 100% on the quality and alignment of the threads in the BB shell; they need to be absolutely true and the RHS of the shell needs to be faced exactly square.
Regarding a) I have seen GCB units of this type (all makes) fail after a single winter's use, because the weather has penetrated the bearings. The seals (and grease inside the bearings obviously) vary in quality but (IMHO) in all cases they are not as good as the seals on a shimano BB. In most cases you can knock the bearings out of the cups/off the spindle , remove the inner seals, and pack the bearing assy (even the whole centre sleeve) with grease. If you can get the bearings apart thusly without damaging them (pot luck if the fits are too tight or not), this (plus better seals) seems to give the thing a fighting chance of surviving the weather; I've had one such unit in service for about six years (and at least 10000 miles/lots of weather) now and others have reported similar improvements in service life.
Note that in some lengths GCB spindles are slightly asymmetric; this means that you can knock the RH cup off and reverse the spindle, thus fine-tuning the chainline. It is also possible to introduce spacers on the RHS too.
When I first saw the GCB design I wondered if it was worth renewing the bearings when they wear. I now think that whilst you might be able to do this (if you can get the right bearings) it mightn't be worth it; the units are cheap enough as they are, and unless the bearings have failed prematurely, the spindle may have had the fatigue life taken out of it anyway. FWIW some spindles seem worse than others; even though the bearings don't last that long I have seen several RPM ones break.
When my 'extra grease + better seals' experiment is over I intend to assemble a unit based on the GCB design but with the bearings themselves modified to be 'full complement' rather than use clipped balls. I may use a crushable sleeve to allow (one-way....
) bearing adjustment. The balls inside these bearings are usually (a nominal) 3/16" size so can be dealt with fairly easily.
cheers
