well in my opinion, by bikes emphasizes the mode of transportation which you've used in getting to the place of you own specification. Ex. I came to china by plane. "On" emphasizes on where "it" is right now. Ex. Where;s the ball ,its on the table

in sentences like they went to the store on bikes they went to the store by bike it does but it sounds better to use the first one... I hope i helped, first timer on this site : ) sorry if i didn't

"By bike" describes HOW you travelled. "On bikes" describes WHERE you were while you made the journey. Arguably the result is the same, but I recommend using "by bike" as it describes the action and is a little clearer. To use Blakbatt's example, "They went to the store on bikes" can mean two things: 1) they used bikes for their journey, or 2) there exists a store that sits on bicycles. ie. who or what was on the bikes?? Common sense tells us to assume the first meaning is correct, but really the whole point of grammar is to save us from guessing in the first place... don't you agree?

Exactly the same meaning! Both are good choices.