"Front" and "Back"
There's too much regional variation to describing on road traffic direction when riding in groups.
No one has ever misunderstood either of the above. #heresy
'Front' and 'Back' are the British Cycling recommended terms, I think.
'Up' and 'Down' are easily misconstrued. In Olde Essex cyclist's slang
up is the direction you're travelling, others may mean up (at the front - ie against you). 'Oil' (= motor), 'Big Oil' (= lorry/bus etc), from the early days of motoring when steam vehicles were fairly common and presented different hazards. Still use those terms habitually ...
I like 'Nose' and 'Tail' - from my days riding with Ayrshire CTC.