Bearings faq

Replacing cartridge bearings is not as simple as it seems.

Remove a worn out bearing
In most cases when you have to pop out a hub bearing you can only push it on its internal race. This operation will generally damage or compromise bearing races. If you are going to replace a worn out bearing this is marginal problem.
Remove a bearing that's still good
In case you have to re install that bearing later avoid to hit the internal race with the mallet, even threaded extractors or press can damage bearing balls and races. Try to warm up the hub-shell with a hairdryer, this will allow an easier pop out of bearings and softens eventual Loctite or coaxial anaerobic glue that could have been used at the factory assembling.
Insert the new bearing
The new bearing should always be inserted pressing it on the same race the hub retains it from.
Cartridge bearings are generally fitted into the hub-shell with some interference on the outer bearing race.
In this case you have to press the bearing in pushing it exclusively on its outer race. Pressure or hits on the bearing internal race will permanently damage the new bearing balls and races.
To properly press in the new bearing its necessary to use a special inserting tool (inserting tools are specifically made for each bearing size). Use only an inserting tool that is made for your exact bearing size.
Extralite offers a complete tool-set for hub maintenance, the Pro-tool kit 1 includes bearing inserting tools in every bearing sizes used on UltraHubs / HyperHubs.
Use the old bearing to protect the new one
As last alternative you could use the old bearing to carefully press in the new one as follows (use this method for emergency only)
- the 2 bearings should be perfectly aligned
- the mallet should hit only the external race of the worn bearing
- insert the bearing at small increments at 120° and keep it perfectly flat.
Remind that when you'll insert the hub axle it has to pop into the bearings with only a limited hand pressure. Forcing the axle to get it through the bearing will permanently damage the new bearing balls and races (Force amount is directly related to bearing load capacity).
Hub-shell / cartridge bearing interference
As general rule the pressing interference between hub-shell and bearing should be around 0.01mm. An intereference of 0.02mm applied on a thin bearing (ex. 6802 - 6803 etc.) can compromise the ball-races tolerance matching (on qualty bearings ball-races matching/coupling leaves less than 0.003mm of play).
If the bearing is pressed in with too high interference it can't rotate perfectly smooth and will wear out prematurely.
The opposite case of a slightly loose bearing fitting can generally be properly cured using Loxeal 83.21 / Loctite 260 or equivalent products (coaxial couplings).
Hybrid Ceramic bearings
A proper bearing and axle inserting is even more important when you are willing to mount hybrid-ceramic bearings. The ceramic balls feature a very hard surface that can dinge the bearing steel races very easily. Additionally hybrid-ceramic bearings require more precise tolerance matchings. Too much interference of bearing outer-race with its hub-shell fitting can often result into rough rolling bearing. The same can happen when axle needs too much pressure to get into the bearing inner-race.
Bearing seals and friction
Bearing seals are often the first resposable of bearing durability.
Tighter seals increase friction however do not over estimate it. The bearing friction can't be judged statically just rotating it with your fingers.
In the real use when the bearing balls rotate under load they rub and rotate between races, this makes much more friction than a well tight and protective seal that could keep your bearings spinning smoothly for long time.
Good seals are really a must on MTB use.
Bearing durabilty
Bearing durability is related to a great number of factors such as seal type and conditions, use mode and frequency, environment, washing mode and frequency, hub maintenance and lubing and more, this makes each individual case really different.
In highly corrosive environments the ceramic ball resists to corrosion and help to clean the hard steel races. In such circumstances the overall durability could be even higher than steel bearings but this is generally uncommon.

Ceramic Bearings

Bearing friction
The bearing friction should be judged under load when ball rub and rotate between races. In these circumsances the use of hybrid ceramic bearings can offer an advantage.
Today quality standards
Hi-end bearing manifacturers have now acheived much higher quality standards than only few years ago. Quality hybrid ceramic bearings are now made with incredibly hard steel races that can survive the coupling with ceramic balls for a much longer life.
Durabilty
The latest generations of hybrid-ceramic bearings as ABI Zero and HDC offer a good durabilty that is comparable to the corrispondent steel unit. Earlier bearing generations could only offer a fraction of the durability of a regular steel bearing.
In highly corrosive environments the ceramic ball resists to corrosion and help to clean the hard steel races. In such circumstances the overall durability could be even higher than steel bearings but this is generally uncommon.
Advantages / disadvantages
Do not over-estimate the advantages of hybrid ceramic bearings. Their durability is equal or lower than steel ones, price is higher, all in exchange of a slightly lower rolling resistance.
When you are choosing to use hybrid-ceramic bearings be aware that the lower rolling resistance is the sole advantage they can offer besides a very very slight weight reduction (ca 1gr. less on each hub bearing).
Nasty ginmicks
To enhance the feel of low-rolling resistance some non-serious manifacturer equip their bearing with non-touching seals and you can see them spinning for minutes, surprised.
Those bearings in reality last only few rides, then collect dirt directly into their ball races and get immeediately ready for replacement (no surprise).
Full-ceramic bearings
Among bike enthusiasts several tryed to employe full-ceramic bearings in order to save weight and further reduce the rolling resistance. Unfortunately with today technology a properly dimensioned steel bearing can not be replaced with a full-ceramic bearing of the same dimensions. Size should grow up significantly and this voids weight savings meantime requires a specifically designed hub-shell.
Additionally note that even full-ceramic bearing manifacturers are openly not recommending their products for bicycle applications due to the ceramic material fragility. Typical bicycle application involves high impact resistance at low rpm (10-100 min.) while the typical full-ceramic bearing application is at very high rpm (over 20.000 min.) and absorbs only low impacts.

Read Mountain Bike Action August 2007 article

"Ceramic Bearings 101: will they make you faster?"