The diameter of the ball head is 16mm, which is a standard size. It can be perfectly matched if you buy a panical cup holder and a water cup holder. You only need to tighten the nut and it will not slip. The reason for the slippage is that your water cup holder does not match my ball head, and the data size of your water cup holder is not very accurate, because they are all matched with the ball head sold by the cup holder merchant you bought, so there is a difference Please preview this text before purchasing, and read this message by default after purchasing If you encounter your own water cup holder matching my ball head slipping, we refuse to return and refund If you have any questions, you can send us a message