Ideally, mount the XGPS160 on the top of your helmet with the antenna pointing directly at the sky. This provides the highest-quality GPS data and lowers the risk of your GPS receiver getting lost or damaged.
If you can't mount it on the helmet (most often due to rule changes at your track or organization), the front fender is a good secondary location.
For full mounting instructions, including how to prep the surface before applying velcro, see the "Mounting the XGPS160" section of Getting Started With A Dual Electronics XGPS160.