The invention relates to a bicycle adjusting device, in particular an adjusting device for adjusting the height of the front wheel suspension, the rear wheel suspension, the saddle or of the steering mechanism of a bicycle, e.g. of a mountain bike, a bicycle with at least one bicycle adjustment device, as well as a bicycle-steering head bearing system, whereby the adjustment has at least one double-acting piston-cylinder assembly with a first cylinder space (14a, 34a) and a second cylinder space (15a,35a) in each case, and whereby the two cylinder spaces of the two adjustment devices are interconnectable by means of a fluid duct fitted for that purpose.