DOS,

Good questions.

First, there's an intrinsic reason why AC bearings have higher ratings for radial loading service. seb1466 described it. If you think how deep groove radial bearings have to be made its this: Inner and outer races are precision ground with grooves for the balls. You can't just make the race concentric and shove in a retainer full of ball bearings, for the same reason that the balls don't fall out in normal use. To install the ball bearings you must make the inner and outer race non-concentric (you move one race so that it touches the other race). This leaves a crescent-moon shaped gap that the bearings will fit in. Note that you can't fill the races totally with the ball bearings touching each other in a DGR. At some point the balls force the races to become concentric and no more can be installed. Once you get the maximum number of balls that fit the inner and outer races are close to concentric. You complete the proces by adding a spacer that keeps the balls apart and equally spaced. The point is, with DGR bearings you MUST have spaces between the balls. The only way to fit more balls into a DGR is to make the "deep groove" not so deep. This makes the next problem discussed much worse.

If you've ever repacked an older hub with cups and cones (that is, AC bearings) you can see that with such bearing you can fill the race with more balls. More balls for the same size bearing mean more area to support load means higher ratings. You could still put a spacer in, but the spacing can be tighter for AC than for DGR bearings.

The increased number of ball bearings explains higher load ratings in radial service.

Now, look at how the different designs look:

https://3.bp.blogspot.com/-6SWSy16Ot...e-bearings.jpg


Note that the line that intersects the bearing contact points for AC bearings is angled. This means that a side thrust on the angular contact bearing can be resisted by design. The DGR bearing contact line is perpendicular to axial (side) loads. This means that theoretically the DGR can't resist side loads at all! Practicality intervenes. There is some built-in clearance in the DGR, so (for example, in the pic above) the outer cone shifts less a little bit and the line of contact angles a little bit. For geometric reasons, though, the force on the bearings created by side loads are much higher than in the AC bearings. This can distort the races a bit. Both the clearance and the distortion act to make the DGR bearing a bit more "angular contact" so that it can resist side loads. But again, a side force in a DGR creates way more force on the bearing races than one sees in an AC bearing. And if the groove in the DGR is less deep, the problem is way worse. The upshot is that if you over-preload a DGR bearing its gonna fail faster than its AC counterpart. Also, if you encounter a lot of side forces on the DGR it will fail faster.

Sometimes industrial applications use DGRs when there will be no side loading. Electric motors often have DGRs. But for apps with radial AND side loads you'd always use an AC bearing. For really heavy duty stuff (car, truck, and locomotive axles) you use tapered roller bearings. The loads and speeds on bikes don't require that. For your amusement, a train axle bearing using opposed tapered roller bearings:


PS In rail service, some bearings have spring-loaded devices that give a contact side load on these bearings, to improve service life and peformance.