Mountain Biking (MTB) is an increasingly popular outdoors activity which offers a unqiue connection to nature along with the health benefits of cardiovascular exercise. Yet, complex MTB technique is an entry barrier that often prevent novices from enjoying the sport. Developing interactive systems, which can support developing MTB proficiency can augment the outdoor experience and make the sport available to a larger group of users. To that end, we designed, implemented and evaluate MTBalance - a system which provides body posture feedback for beginner mountain bikers. Based on inertial tracking, MTBalance informs the user about how to correct their posture to improve MTB performance. We conducted a study in which we compared different feedback modalities for MTBalance. We observed that the system increased perceived balance awareness. Our work provides insights for designing body awareness systems for outdoor sports.