Between riding in the woods, and supermoto, I’ve been spending a lot of time with my KTM lately. So much so that I feel like my other bikes may be getting jealous. This has left me with a lot to think about in terms of how to turn a bike in different situations.
I started riding on the farm in Saskatchewan with no instruction, at a time when the internet didn’t exist, and I don’t believe there were any motorcycle riding training facilities in the province. The farm may have had a total of ten feet worth of elevation change from one extreme to the other, and the roads around Saskatchewan don’t have a lot of curves.
As a result of this start, I was completely unprepared for riding in British Columbia. I rode for years without dealing with hills, or having even the most basic understanding of how to turn a motorcycle properly. It was pure luck that I managed to fumble through until I started to learn the physics of riding. However, I was only riding on the street at the time, so I only focused on learning about riding on pavement.
While I understand the right way to turn a dirt bike in the woods, I am left confused about the actual physics about it. I recently had the opportunity to talk to Chris Birch about the physics of riding in the dirt, and even he didn’t really know why what works does, just that it does.
This bothers me. Chris is an incredible rider who really knows his stuff and has a tendency to over analyze things at least as much as I do. The fact that he doesn’t actually know the physics of cornering in the dirt left me feeling fairly defeated……..
At the most basic level, cornering requires a shift of the center of gravity to the inside of the balancing point, and a balancing of the forces of gravity and centrifugal force, all while not over reaching the limits of traction. On pavement the traction is fairly predictable, and the faster you go around a turn, or the tighter the turn you want to make, the more the centrifugal force wants to push you and your bike wide.
To travel even faster around a given radius turn, the center of gravity needs to move farther to the inside of the curve. This can be done by leaning the bike over more, moving your own weight to the inside of the turn, or both. Gravity and centrifugal force work together to push your tires down into the road and provide you with plenty of traction.
Now all this goes out the window when it comes time to ride in the dirt. The bike does still have to lean to make a turn, but we have to keep our weight above the wheels to keep from losing traction and having the centrifugal force push us to the outside and causing the bike to slide out from under us.
This is similar to low speed riding on pavement, except in that situation we need to keep the weight over the wheels to keep gravity from pulling the bike down to the road.
This is the source of my confusion. Why is it that we ride in the same fashion in two different scenarios, but failing to do so causes opposite results, and what does this mean for riding supermoto?
Is the best way to ride a dirt bike with street tires to ride it like a dirt bike, or like a street bike, or a combination of the two, and how much does the geometry of the bike play into all of this?
I plan to tackle these questions and elaborate further on the physics in the near future.