Bicycles usually use ball bearings. Modern cartridge bearings come as a closed unit – with balls and races packed in a single unit. However, many bicycles still use older, cup and cone system, where balls can (and should) be replaced with new ones when servicing bearings. This is the case with most Shimano hubs (wheels) – they still use the cup and cone system. This post lists standard ball sizes and gives guides on how to choose a good quality ones, without the need to buy special, branded “bicycle bearing balls”.
1. Quality (manufacturing precision) grades of bearing balls
Grade defines tolerance (i.e. acceptable deviation) from a perfect sphere of a given diameter. Grades are noted in numbers, ranging from 3 to 2000. They are written in the format: GXXXX, where XXXX is a number ranging from 3 to 2000. The number represents how many 1/1000 inches is maximal deviation from nominal dimension.
The lower the number after G mark, the closer a ball is to a perfect sphere of a given dimension. Ball imperfections come in various terms:
- surface roughness
- egg shapeness
For example, a ball of a grade G1000, with a nominal diameter of 6.35 mm (1/4″), can have a diameter varying from that size for over one tenth of a milimetre. On the other hand, a G10 ball of the same nominal size will have a diameter more closely matching the nominal, with just a ±0.0013 mm maximal deviation. Similar goes for other types of deviations (surface roughness etc.). The lower the grade number, the closer a ball is to a perfect shape.
Since cups and cones in bicycle bearings have some elasticity and imperfections as well, balls of grade G25 are more than precisely enough machined (round and smooth). Going for higher grade balls (e.g. G10, G5 etc.) will not bring any performance, or durability benefits, just the balls will cost (needlessly) more.
2. Bearing ball hardness
Bearing ball hardness is usually measured and expressed with Rockwell HRC method. Values for hardened steel are usually in the range of 40 to 70. If a bearing ball is too soft, it will wear rather quickly, then start damaging the cup and cone bearing because of increased play and imperfect alignment. On the other hand, if a bearing ball is too hard, instead of it getting worn, it will start wearing (softer) cups and cones of the bearing – that are more expensive and complicated to replace. That is why it is important for the balls to be of optimal hardness. Optimal hardness for bicycle ball bearings is between 55 and 65 HRC.
3. Standard dimensions of bicycle bearing balls
Here’s a list of standard, most commonly used dimensions of bearing balls. There are other, exotic sizes, but they are extremely rare. To be on the safe side, when servicing bearings (and replacing balls then, which is always recommended), measure old balls. Best measured with a (Vernier) caliper.
- 3,969 mm (5/32″) – steerer (fork) bearings and many pedals.
- 4,762 mm (3/16″) – front wheel hubs (2 times 10 balls – i.e. 10 balls at each side most often).
- 6,35 mm (1/4″) – rear wheel hubs (usually 2 times 9 balls) and cup and cone bottom brackets (2 times 11).
- 5,556 mm (7/32″) Campagnolo Record front hubs and some exotic hubs of other manufacturers (2 times 9 balls).
It is good to replace bearing balls at each bearing service. They can be purchased rather cheaply (by 1000) from most hardware stores. Just make sure they are of proper grade (G25 or better) and hardness (HRC 55 to 65).
There is little point going over G25 grade, and absolutely no point going better than G10 (even that is an “overkill” for this application). Similarly, using balls softer than 55 HRC, or harder than 65 HRC can only decrease bearings lifespan.