The rear disc brake on my son's mafia when engaged seems to pull the wheel to the left making the tyre rub on the frame. Any advice would be greatly appreciated. Cheers.
This happens because the force of the disc is stronger than the clamp between the axle and the frame.
There is a few things you could do to improve the strength of the hold.
- Rough up the drop out using corse sand paper
- Get a chain tensioner to stop the axle from sliding forward
- Get better track nuts, make sure they're compatible with the axle first.
Unfotunately, Mafia bikes make low quality builds and there is only so much you can do.
Edit: I'm assuming you have a horisontal drop out Mafia bikes jump / bmx bike.