Quick and Dirty
There is only a very small opportunity that a bearing will be overloaded on a bicycle. For this discussion, ‘overloaded’ means actual plastic deformation (dimple within the bearing). To frame the discussion, let’s say we have a 250 lbs mountain biker going down a mountain. A normal mountain bike is going to have a minimum of 2 bearings per hub, this means the weight of the rider (and fractional weight of the bike) is going to be assumed to be spread over 4 bearings. This leads us to simple math of 62.5 lbs per bearing. A 608-2RS PEEK P63 has a radial load capacity of ~273 lbs static load capacity.
The 608 bearing size was chosen because it is both incredibly common – rollerblades and skateboards – but also that this bearing should clearly be too small for a mountain bike hub – feel free to contact us to discuss the multitude of reasons why.
Even if a 608 bearing was used for all 4 bearings for both front and rear hubs, this 250 lbs. biker could fall at over 4 G’s and not dimple the races. As a point of reference, this is over double that of the full acceleration of a Bugatti Veyron and the Space Shuttle usually does not clear 3G’s.
A 2200 lb white rhino riding a bike with four 6903 radial ball bearings will not dimple the races due to the combined static load rating of 2292 lbs, but of course the bike frame, wheels and tires would fail long before the bearings.
Engineering a Bearing Load
First and foremost, there are engineering assumptions above and below to make the discussion easy. As in other discussions on this website, tire cushion, tire pressure, bike flex, etc. all are potential energy absorption and discharge (spring/capacitor) aspects of a bicycle that can reduce the impact force of the bicycle. A low-pressure tire will absorb some of the landing force and then re-elevate the biker similar to a shock absorber on the mountain bike, both of these will decrease the maximum impact force that the bearings see. Other aspects that we are letting go because of large effort vs. actual return are; crank arm deflection, bottom bracket absorption, etc. If one has the time to do this level of analysis, would love to see it! However, the results of that will be of minimal effect per the large amount of safety factor that is built into most bicycles (if not all) as far as load is concerned.
Bearing Load Capacity
An interesting point on bearing load is that the load capacity has a lot to do with the ball diameter. This can greatly affect the contact ellipse of the ball to the races which will in turn greatly affect the load capacity. As an engineer knows, contact area is a great determinate of load capacity. This is the reason why some 200 series bearings can have a larger load carrying capacity than even 6000 series bearings with a larger ID. This can be seen with a 6201 bearing vs. a 6002 bearing. Even though the 6002 has a larger ID and actually more balls for more points of contact, the larger ball of the 6201 outweighs both of those aspects and has a larger radial load capacity.
Also, quite interesting is the actual interaction of the radial load on the bearing. Most, excluding higher precision, radial ball bearings will not have a press on both the ID and the OD, and will not have a pre-load. A pre-load is an axial force against a bearing that insures the balls are in contact with the races at all times. A pre-load is absolutely required for angular contact bearings because of the inherent design philosophy. But this is not required for radial ball bearings within the bicycle industry. With no press fits and no pre-load, the inner race and outer race of the bearing are allowed to move independently within the internal clearance and clearances to the shaft and housing in which they are installed. These three dimensions of space can add up so that the radial force, usually vertically down on the frame, pushes the force to the top of the bearing as shown in the picture.
As one can see, not all of the balls will see the force at one time. The load sharing will change while riding the bike with potential side loads, getting air, etc. This non-equal load sharing of the balls is taken into account for general purpose load ratings, but interesting nonetheless.
Bracket Bearing Sizes
Back to more real numbers. For most bikes, hub and bottom bracket bearing sizes are 6902, 6904, 6000, 6001, etc. sizes that have sufficient load capacity for just about anything a person can throw at a bike. For this reason, we would not worry about load capacities unless some bearing part numbers are seen that include 605, 606, 607, etc. that are very small bearing ID’s (less than 8mm).
For most bicycle applications within general design parameters, load should not be of a concern. Only with heavier ‘payloads’ and dropping off of cliffs at 4G’s or the opposite of having too small of bearings, will load ever be an issue.