wheelhorse

One of a team of horses which is nearest to the wheels of a carriage, as opposed to a leader or forward horse.

The off wheel-horſe a grey, named Single Peeper, ſold for 50 [guineas] […] The near wheel-horſe cheſnut, named Chance.

The driver […] neither knew any thing, said any thing, or did any thing but watch a dozen equine ears, and keep six reins taut in his hands, and coax the off wheel[-]horse with the belly of his whip-lash every two minutes, invariably accompanying the stroke with a tremendous solitary cluck.

A person who labors heavily for a particular cause, without being concerned about recognition.

[F]rom his [Benjamin Ruggles'] well-known habits of industry and constant devotion to the interests of his constituents, he was called "The Wheelhorse of the Senate."

A medical journal, it seems to the writer, should be a medium for the propagation of useful knowledge, the instruction of the profession in the best and most scientific methods of fighting disease; and who is so well qualified for the work as the old wheelhorses, the men who have grown gray in the fight?

A foot-propelled vehicle; a bicycle.

In 1830 a bold and vigorous attempt was made to utilize the wheel-horse. A French post-office official, M. Dreuze by name, brought forward an improvement on the old two-wheel velocipede, which bid fair to be successful. […] A number of the country letter-carriers were mounted on the wheel-horse, and whilst the roads continued dry and hard M. Dreuze could congratulate himself on the success of his invention; but with wet weather came bad roads, and to the wet succeeded frost and snow. A little extra labour was all that was required to overcome the extra friction of the bad roads, but the wheels refused to progress on the slippy frozen surface.

As in most other accomplishments, practice alone can make a skilful rider of velocipedes. The tyro can, however, profit by the experience of others, and I give a few rules for his guidance, as well as directions for his practice. The first point is to gain confidence in, and familiarity with, his wheel horse.