As much as my brain doesn't like it, the pulley speed is just the ratio of it's teeth to the chainring, multiplied by the cadence-- I'm used to having to compute gear speed for gearboxes, I guess. So with a 50T ring and a 90rpm cadence, that little guy is doing about 400rpm.
Which in the terms of a cartridge bearing, is... nothing. In the R/C world we buy bearings by the 10 or 25 pack, because it's not unusual to grenade a bearing every couple of days-- the downside to axle bearings doing 9,000rpm and motor bearings approaching 100,000rpm.
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