骑(qí) - to ride (astride) in Mandarin

骑 (qí) is the verb for sitting astride something and travelling on it: a horse, a bicycle, a motorbike. The body position is what the word encodes, not the vehicle.

That is exactly where English speakers slip. We say "ride" for a bus and a taxi too, but Chinese does not: you 坐 (zuò, sit) in anything you sit inside, and you only 骑 what you straddle. 坐车 is taking the bus, 骑车 is cycling, and swapping them sounds odd rather than merely wrong.

骑车 on its own is understood as cycling in almost every context, so you rarely need to say 骑自行车 unless you are contrasting it with a motorbike.