Here is what I say to my daughter when she starts worrying what her mates are thinking of her:

Most of the time people are so worried about what everyone else is thinking that they really aren't spending a whole heck of a lot of time thinking about you.

Or, to put it bluntly: honey, the girls aren't looking at your butt, they're too worried about what everyone is thinking about their own!

It sounds like you get some ribbing.
Take it for what it is - ribbing.

I think, knowing guys, they probably think it's pretty cool that you tore your bike up yourself and put it back together, and that you're not buying in / selling out, and that you're trying to do something good for your body. That's admirable!

And also, knowing guys, they generally only rib the girls they feel the most comfortable with. It's a form of flirting. Take it as a compliment.