This was one of the first designs that I made for my QAV250. They fit great and the design keeps them locked in place so that they are always pointed forward. The hole for the bolt lines up perfectly with the hole for the landing gear in the frame. Use a 10mm M3 bolt with a nylon locking nut and they're very secure. However, they didn't turn out quite as good as I expected. The main issue with them is that they are a bit "bouncy". They do seem to absorb some impact from light crashes and I've yet to break one made from ABS. (I've broke several made of PLA.) The second to last picture shows one of the ones that survived a hard impact. And as you can see from the very last picture, they are still useable even after that impact. I can still compress it fully and it springs back. The other issue was that printing the "ball" on the bottom wasn't easy. The lower edges tend to curl up a bit when printed on it's side. It's a part that is designed to be skidded across concrete and asphalt though, so that's not a huge deal, IMO. For now, file this one under "work-in-progress". Maybe someone can give me an idea of how to improve these? I'm including a version that I tried with print support (to help with the edges of the "ball" from curling up) AND a version for the new style frames with the missing cutout in the arms.