ball-bearings.
Gotta cycle story fer ya -
An old friend, who cycled everywhere in Berkeley, incl. up/down all the hills -- was in for a health check. They put him on a test cycle, at one stage.
He kept pedalling; they kept cranking down on the calipers ... he kept pedalling.
There was smoke, IIRC...
Finally the guy asks, ~ 'Umm, this is most unusual.. how's come?
[I mean.. couldn't he *see* that the guy has calf muscles like an ostrich !?]
David played him along for a while, then confessed.
Keep on 'peddling it' all over town ;^>