I came across some material on this when researching the Twichell-Twain friendship in my Twain and Male Friendship book. This is from it.....
 
Twichell was quick to take up the new activity of cycling. On 19 May 1884, he wrote in his journal:
Today I had advanced far enough in the art of the wheel to begin using the new ‘machine’ which is to be my own, having hitherto taken lessons on one furnished by the Bicycle Mfg. Co. It has been a rather rough experience till now, but I am over the worst at last and begin to see possibilities of pleasure in this modern way of locomotion.
Two weeks later, Clemens had been drawn into the new venture, but clearly with less than one hundred per cent success: ‘June 3 & 4th. On the morning of these two days M. T. and I were up at about 5 o’clock for bicycle exercise in the fresh and cool of the day…. The experiment on the whole was not satisfactory.’[i] 

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[i]. Twichell’s Journals, Beinecke.  A newspaper clipping in the Twichell Family Scrapbooks (undated but placed alongside later 1896-97 material) reads: ‘Mark Twain and Rev. Joseph H. Twichell of Hartford, who were enthusiastic bicycle riders, were pedalling along side by side one day, when the author of “Roughing It” suddenly took a “header.” Mr. Twichell slowed up and was about to dismount to aid his friend, when Mr. Clemens said: “Don’t stop, Joe; go right along. I think I’ll stop here awhile and swear”,’ Beinecke. 


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From: Mark Twain Forum [[log in to unmask]] On Behalf Of David Foster [[log in to unmask]]
Sent: 30 January 2011 01:21
To: [log in to unmask]
Subject: Sam Clemens and bicycle riding

Everyone knows that Hank Morgan is rescued by bicycle riding knights in the=
 Connecticut Yankee, but does anyone know whether Clemens himself ever lear=
ned to ride a bicycle? If so, where could I find an account of that?=20

Thanks for any leads,
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