While cleaning and lubing my 15 year old bike's front hub I discovered that one of the ball bearings had failed (see photo).

The rusty, pitted bearings had scored the races but these revived the nearly 30 year old hubs.

I used these to replace the old bearings in a set of vintage bicycle hubs, packed with new grease, and now the hubs roll like a dream

If it helps anyone else, I have Bontrager Duster 29" wheels with a Shimano Free-Hub.

Repacking your hubs and BB (and cleaning/lubing your chain) is the best way to make ANY bike more efficient