My son was on the US Ski jumping Team 13 years ago, blew out his ACL. They had him riding bikes as a part of his recovery and he tore out the new ACL. This caused his retirement from skiing. He has no ACL. They had him on the bike too early. He still bikes with no brace. He did a 100 mile road race last summer on a fixie, don't ask, but he says he will never do that again. He recently rode his bicycle from NYC to Vermont and back for a wedding. I blew out my knee 40 years ago and have had no ACL since then. Ny knee has no cartilage, bone on bone and the joint is made out of bone graphs. The ligaments on the inside of my knee were detached surgically and reattached on the outside to help stabilize it. I have a real big scar on the outside of my knee and lower leg. I was on crutches with no weight bearing for 6 months and then had several months of rehab, but was skiing that winter. They gave me a brace and I have had the newest versions. I never use them as they are a pain in the butt and piston down when running or XC skiing and cause chafing. I occasionally wear it skating, if I remember. For about 10 years after my knee injury I did orienteering and wore the brace for running XC through the woods (even won the National Championship in my age, which is pretty good for a one legged runner). A knee replacement is in my future, but with my type of injury they have a 20% failure rate. Anyway, with a knee replacement I couldn't abuse it like I do MB and I want to do that as long as possible. I MB bike about 30+ miles a week. Both of us have pain, but we ignore it. I am almost 69 and if you are young you should do fine, probably have some issues when you get older so enjoy your self. I ran, skied, biked but began to slow down about 20 years ago from the knee pain, but so far it hasn't stopped me entirely. Avoid clips. My sons Dr. who did his ACL repair told me that skiing is not his main source for ACL repairs anymore, it is mountain biking. He said that you come out of the clips, put your foot down toe first and the foot twists to the inside and ruptures the ACL. My son never wears knee pads when MB. I only wear them or elbow pads after a crash where I badly bruised or exploded my skin. Always wear a helmut. I split one a month ago after dropping off a the trail into a drop off and slamming face first into a rock wall. Got a concussion, and road rash and exploded fingers. The bike had a bent wheel and bars. Last Wednesday I was riding in a group and we were going over 21 miles an hour (GPS) when the person in front of me tapped the bar end on a tree and slammed back wards headfirst into a tree. The helmet was split and compressed, but no head injury, just pretty massive road rash.