Riding over a log is a basic mountain bike skill, which unfortunately few riders seem to have. Notice I am not saying “log jumping,” you don’t really jump it as much as ride over it. When you ride and see logs across the trail you probably notice chain ring marks on the log; that is a sure sign of poor technique. Often there are cheater stones or small logs in front and back of the log you are trying to go over. Those just make it too easy and keep riders from learning the proper skill.
When approaching a log you want to be in a standing position on the bike with the pedals level. You don’t want to be going too slow, ideal speed is maybe 5 to 10 mi/hr although you can be going faster. This is all timing and technique. The first step is to push down on the handlebars to compress the shock and then during the rebound lift the handlebars to bring the front wheel up off the ground. This first step will occur just before you get to the log; the distance from the log when you start the lifting process will of course vary according to your speed. You can practice this first step on level ground. Particular front shocks compress and rebound at different rates, so it is important to practice with your own bike to perfect the timing.
Now the very important second step. You DO NOT want to throw your front wheel over the log; if you do the chain ring will simply slam in to it. What you want to do is set the front wheel on TOP of the log!! Of course since you are moving forward the wheel will not stay on top of the log very long, but we will deal with that in a moment. You do not want to do the front wheel raising process to soon or the front wheel will come down before you get to the log and you will simply crash into it, with bad results. When you are first learning the technique you will be scared of crashing into the log and invariably throw the front wheel to far forward and over the log. Like I said, it is all technique and timing and requires practice. The pictures below show a rider just as he is setting the front wheel on the log. Lets call the rider Sam (since that is his name).
In the picture on the left Sam is just reaching the log and on the right the front wheel is sitting on top of it. Don’t ask how many times I made poor Sam ride over the log before I got a picture with the wheel sitting on top. Notice Sam’s feet are approximately level. In the picture on the right he is getting ready for the final move.
The third step is the most important and takes the most practice to learn. The front wheel is on top the log and you are going to use it as a lever. Jump up slightly and use your feet to left the rear wheel. In the next picture below Sam has just completed this all important step. Notice how far above the log the chain ring is, it will clear the log by several inches! In fact the rear wheel will also clear the log without touching it. Notice the log has not moved, in fact it has not even wobbled. There is no forward force on it.
So to review and add a bit of discussion. There are three steps, the first is compress the fork and lift the front wheel, the second is to set the front wheel on top the log (rock, hiker, whatever) and the third is to lever the rear wheel into the air. As we mentioned above the wheel is not on top the log very long, especially if you are traveling at any speed. You will find that the levering action to lift the rear wheel actually begins an instant before the front wheel touches down on the log; this causes the front wheel to just tap the log as you ride over it.
So all it takes is practice, practice and practice! In the pictures Sam is riding over one of the hollow artificial logs we used for mountain bike clinics. Since the logs weighed very little it was pretty safe if you lifted the front wheel too early and crashed into it; the log just skittered out of the way. You might wish to make such a practice log, just use some plywood. We had two of them shown in the photo below. I wish I still had them. We had mtn bike clinics once a month in the summer. The final picture shows some of the stuff set up on the gravel parking lot during one of the clinics. As you can see we started them young.