Simple Wheel Slammers

I saw these a while back when they first started making them, and i had no clue what they did. But there up on albes go here to buy some. And here's a description albes wrote thats really simple and easy to understand. They also come in sizes ranging from 0.4mm to 8mm

"The idea for these has been started out a long time ago, coming from balls from looseballbearings and what not stuffed in the gap between the axle and the dropout to keep the wheel from sliding forward.

Once you have worked out how much of a gap you have between your axle and your dropout you just get that size and plug it in there."