Rolling resistance is the result of the compression of the wheel, suspension system if you have it, and or the ground. Rolling resistance on a bicycle is determined by how much energy is required to move over the trail. A bike with no suspension is ‘unsprung’ and must be lifted over any imperfections found on the trail in order to move forward. These imperfections vary from rocks, tree roots, divots in the trail, and many more obstructions that are found on our path. On average, a bike and its rider weigh around 175 pounds. So, all of this ‘unsprung’ weight must be lifted over these imperfections. With suspension however, the majority of the weight is ‘sprung’ and imperfections are absorbed by the suspension. So in this case with suspension, only the unsuspended portion of the bike (wheel and lower frame) and a small portion the riders weight needs to be lifted, which amounts to a mere 35 pounds. Common sense tells you that it would take a lot less energy to lift 35 pounds versus trying to lift 175 pounds. This helps to explain why most all high-end mountain bikes feature full suspension and why full suspension bikes are easier to ride. (And all this time you thought full suspension was just for comfort!)
Air resistance is a big factor when bike riding, but not so much for mountain biking as it for road biking. Therefore, I will spend little time describing what it is, but I still feel it is important enough to include on this page. Air resistance is self explanatory in the fact that it can be defined for our purposes as the bikers' resistance to motion caused by air. To give an example of air resistance, think about driving in your car and placing your hand out the window (this is not advised, but I’m sure we’ve all done it at one time). If you turn the palm of your hand towards the direction your driving, you will feel a much greater force than if you turn your hand palm down towards the ground. What you feel is the affect of air resistance.
Wind Resistance can add to air resistance if traveling against the wind, but it can also aid a biker in fighting air resistance, if we happen to be traveling in the direction of the wind. (Which seems to never happen!)