If you were riding the motorbike, it would sound more natural to say "on a" or "by". Saying you arrive "with" is mostly used for things (or people) that you have brought with you. Arriving to a party with cake, or with a friend, for example. The difference is specifying the method (of transport, in this case) versus specifying possession.
If you say you arrive with a motorbike, it's not clear that you rode the bike there. So the person you are talking to could reasonably follow up with the question: "did you ride it here?"
You could also say that you arrived with eight motorbikes - they could just all be inside a larger vehicle.
Thank you for the explanation.
It's added to the list of accepted answers.
The next time you see this sentence, you can translate it using :
"on a motorbike" or "by motorbike".
If you see other awkward/incorrect English sentences, please submit a report and leave a comment in the sentence discussion.
That makes the update easier.