On the sensors there is a U shaped channel on either side of the sensor. The o-ring sits into this u-shaped channel, wraps around the fork or chainstay and then hooks into the u-shaped channel on the other side. The problem is that the o-rings get old and start to crack and then they will break and there goes your sensor. The cadence sensor allows you to use zip ties as an alternative but the speed sensor does not. You can buy the o-rings at any local hardware store, so as long as you catch them before they break it is not an issue. It was rumored at one time that Sigma was going to redesign the speed sensor to use zip ties but to my knowledge they have not done so as of yet.