Where can I ask questions about these two topics:
- Motorbikes [should I ask it on Bicycles SE?]
You seem to mean "motorcycles". In that case there was a proposal which has been closed. It never seems to have come out of the commitment phase. However, it's been reopened as Motorcycling to give that community a second chance.
However, as suggested by @random, the Mechanics SE seems to house some questions tagged "motorcycle". So perhaps have a look there.
The bicycles SE is not on-topic for questions about motorcycles it seems.
A proposal and beta for an Astronomy SE didn't make it. It was closed in public beta. It seems the number of visits and questions per day was not sufficient for it to survive. As it says on that page, you can download the dump here And it has been merged into the Physics SE, so perhaps you can try your luck there. I don't participate though, so check the faq first.
For bicycle questions that is Bicycles, which is described as the site for, "people who build and repair bicycles, people who train cycling, or commute on bicycles." If your question fall under one of those categories, it should be welcome, if it is not too broad, or constructive.
For astronomy questions, the proposal didn't graduate, and the site has been merged with Physics.
For motorcycles questions, there is Motor Vehicle Maintenance and Repair, which is described as, "for mechanics and DIY-enthusiast owners of cars, trucks, and motorcycles."