When you lose your sense of balance and can't keep the bike going in an approximately straight line anymore. That seems to happen to many folks in their mid-90s. I'm 74 and still riding as hard as I can, which is almost as hard as I used to ride, just not as fast. 41 miles on Sunday, 51 minutes total in zone 4. 50° and raining for the last 20 miles. Only 1 PR. Fun ride anyway.
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Results matter