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It is my understanding that ball bearings for hubs have different sizes and grades.I've looked at the tech docs for Shimano XT hubs and Ultegra hubs to see what size bearings they take. I've looked online and seen that there are different grades of bearings.

According to sheldon brown, Grade 25 bearings are the best bearings to be used w/ bicycle applications. What is the benefit of using a higher grade ball bearing over a lower grade ball bearing? Also, how often should ball bearings be replaced?
 

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Bearing systems

mjlarocco said:
It is my understanding that ball bearings for hubs have different sizes and grades.I've looked at the tech docs for Shimano XT hubs and Ultegra hubs to see what size bearings they take. I've looked online and seen that there are different grades of bearings.

According to sheldon brown, Grade 25 bearings are the best bearings to be used w/ bicycle applications. What is the benefit of using a higher grade ball bearing over a lower grade ball bearing? Also, how often should ball bearings be replaced?
You typically will not find specifications supplied for loose ball bearings in bicycles. Manufacturers are constantly changing the size of the bearings they use in hubs, pedals, bottom brackets, etc. so there is no standard size. You will even find rear hubs that use one size bearing on one side and a different size on the other side. It's brand/model/year specific.

Sheldon Brown is (was) very knowledgeable about bikes, but not 100% correct. Grade 10 bearings are readily available from bearing supply houses, and Grade 5 bearings are commercially available, though you have to buy a LOT of bearings. At one time, Campy was actually hand selecting lots of bearings such that their effective quality was Grade 1. I don't know if they still do this. Plus, you can't look just at the bearings, but at the entire system. A set of races (or cups and cones) that are uneven or of poor quality, mounted on an axle that is flexible, will eliminate any advantage potentially gained by using higher quality bearings. High quality bearings with high quality cups and cones on a stiff axle result in a smoother rolling unit and MUCH longer life if properly maintained.

My mental image is of the loaded bearing being forced through a "tube" that is the cup and cone; if the bearing is less round or the cup and cone uneven, then when things don't line up right, friction and damage are soon to follow.

IME, you replace bearings when they no longer shine like they did when new. Others will tell you to replace bearings every time you overhaul the unit, but I have put 90,000 miles on a single set of hub, pedal, and BB bearings with no visible loss of that shiny appearance, and therefore no reason to replace them.
 

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mjlarocco said:
It is my understanding that ball bearings for hubs have different sizes and grades.I've looked at the tech docs for Shimano XT hubs and Ultegra hubs to see what size bearings they take. I've looked online and seen that there are different grades of bearings.

According to sheldon brown, Grade 25 bearings are the best bearings to be used w/ bicycle applications. What is the benefit of using a higher grade ball bearing over a lower grade ball bearing? Also, how often should ball bearings be replaced?
For cycling purposes there are 3 or 4 grades of bearings. Grade 25 is the highest grade, 300 the lowest, with maybe grade 100 and 200 in between. Generally speaking, the lower the number, the harder the bearings. Considering even grade 25 Chromium bearings cost only pennies each there is no reason to use anything else. Common bearing sizes are 1/4" for bottom brackets and rear hubs, 3/16" for front hubs and some headsets and 5/32" for other headsets. Always measure your old ones before ordering.

As far as how often to replace? I'd change that to after how many miles, because wear matters much more than time, unless you've stored your bike in a damp place for 5 years. If you're going through the trouble of cleaning and repacking the assemblies, and considering a bag of 100 grade 25 bearings run about $4.00, I'd replace them at that time. How often is that? For me it's around 8,000 miles.

Where to buy? Here's a good source:
http://www.loosescrews.com/index.cgi?c=Bearings&id=736863222396
 
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