Ok it might be wrong just did it quick,
Radius of 21'' wheel in front (21'' * 2.54 *3.14) - 1.6748m
Turns per 100km: 100000 / 16.748 = 59708.62193
Turns per second : 59708.62193 / 3600 = 16.58572831
Ups ate one 0, of course its ~ 16.6 turns per second. Which is 16.6Hz right ?