I think you should just post them into c/urbanism, there isn't enough activity for our to matter very much, in my opinion. There no rule that a comm can only have posts that at directly related to the name of the comm (we aren't reddit)
Cluttering up c/urbanism is my concern. A post like "What car should I buy?" or "Why is my engine making this noise?" doesn't have an urbanist angle to it and doesn't fit the theme of a sub-forum that's about removing car infrastructure. A similarly technical post about bike maintenance or a new model of cargo bike doesn't belong there any more than it would c/technology. Having those non-urbanist bike posts in one place just makes for more tidy browsing.
Maybe turn Urbanism into Urbanism & Transportation if there's really a need for labelling like this.
Agreed, we should make the coms more general instead of adding more. We're spread too thin as it is.
I have nothing to add but that I agree with this take.
doesn't fit the theme of a sub-forum that's about removing car infrastructure
It seems to me that you've defined the theme of the comm more narrowly than you need to. I think its totally worth a meta post where all of us who browse c/urbanism can discuss and decide on policy related to e-bikes together. This decision making process, no matter the outcome, builds a stronger community.
c/cars is for:
low occupancy transit
So wouldn’t bikes fit in there? 
Because under communism all bikes would look like this:
Does that look low-occupancy to you?
What?!? I hate communism now. 
You WILL pull the train. You WILL pedal the community bike.
Community and moderator requests and nominations for unmodded communities.
No bulk requests. That means one comm request per post. This is so we can sort out their interest individually.
Community request posts with enough upbears and a volunteer for moderation willing to submit an application will be created during mod drives.