Thanks for the video. Fast track. A lot of attrition.
Just a note, I break in the Diff on the bench by running for 30 seconds holding one wheel. It is ready to go and does not change once its on the track. No polishing or sanding needed. Carbide balls are very nice.
Boomer I am not sure what groove you are talking about, but the annular bearing channel is shaped to take axial loads. No new groove should be cut by the balls. The contact area is not decreased only the angle of contact is changed. I found the math on this but it was not very clear. Grease is nice to prevent ball to ring contact. A seal is very nice. The $1 dollar bearings with shields instead of seals don't last long at all.
The diff rings do get a groove because they are not a bearing in the typical sense where metal to metal contact is to be avoided by using the proper lube. The diff rings rely on metal to metal contact which makes the groove for the friction required to drive the axle forward. The balls drive the diff rings.
To each his own method, but on all the other RC cars that I have owned the 30 second breakin recommended in a manufacturers manual works fine as it does in the 1/10 pan car. They pretty much all include the carbide balls now. There are different types of thust bearings present, but all seem to fail at about the same rate. I agree that failure of this bearing is a common source of gritty diff action.
Here is a link to the
Slapmaster Bearing. Losi used a similar design for a while, but it was well shielded from dirt entry. Failed about as fast as the Associated flat washer thrust bearing. What seals this puppy.
John