I usually carry a small pistol with me. It’s good to get there attention, but if they think you’re bluffing and have to use it…. well. Next time you’re riding carry some pepper spray. If you get confronted again use it on them. Don’t wait, just blast. Hesitation will only get you hurt. Now, If you cannot carry spray, Attack the knees, hips, basically any low joint. They’ll go down. If you have a good grasp of the skeleton, in the front of the pelvis there is a split held together by cartilage, Striking that hard will break the cartilage and they will not get back up for quite a while. Again, DO NOT HESITATE. Remember he has a motor you don’t he can drive himself to the hospital, kinda hard to ride with a broken bone.
Are these useful tips in a book, or was it in some issue of Mountain Bike Action magazine that I missed 
😎