Hiya Sammy
Now for a serious answer I'm rather on the large side so have to wear something like a T shirt bra to stop em jiggling (all the pervs above will be totally confused now T shirt bra indeed lol) and if I'm on my own bike any knickers will do but they've gotta be good uns remember the age old saying off my mum what if you get run over (It happened once and was I pleased I had fancy M & S ones on
) but if i'm on the back of mr h they've gotta be big cotton ones (sorry lads) once had silky ones on and I kept sliding outta my bike trousers with the accelleration