Why would a rider want to slide the rear tire?
We covered some of the technical info on this above but we really didn't discuss why
would we want to do it in the first place.
We have all seen the pro's do it on TV. Rear ends drifting effortlessly around nearly every corner while the rear wheel is spinning. Other than some truly impressive slow-motion videos, why is that desirable?
Well, this would be a good time to post some bike pr0n from this last seasons MotoGP races. The opening sequence you can see Marquez drifting around a corner like it was nothing. MotoGP? Indianapolis 2014 ? Best slow motion - YouTube
To begin, we need to make sure that everyone understands that when you are at the edge of your tire, your traction is at its lowest point. This means we probably cannot accelerate as hard was we would like for fear of losing the rear end. This seems like common sense but it needs to be said.
Consequently, it would make sense that in order to make full use of the horsepower of our machines, we want to get off the edge as quickly as possible and into the 'meat' of the tire so that we can give the bike full throttle.
Well, how do we control that? Isn't a corner a set radius which requires a certain lean angle to get through? Well, yes and no.
If you have managed to get through all of the crap that I have written here you probably have an understanding that racing motorcycles well has a lot to do with bending the laws of physics. This is another such case.
Behold my awesome MSPaint skills!
The yellow line is the traditional path through a turn that a motorcyclist might take. The rider tips in and his bike follows the same lean angle until he exits the turn and gets back on the gas. The line is a perfect arc and both the front and rear tires are on the yellow line throughout the entire turn.
Now, check out this next example.
Someone way smarter than me came up with the magic number of 5%. That is supposedly the optimal amount of rear wheel spin to have, meaning your rear tire is spinning 5% faster than you are traveling.
By intentionally forcing the rear into a controlled spin, it will cause the rear tire to step OFF that yellow line (red line in this example). Doing so effectively turns your bike in the direction you want to go. You are 'cutting the angle' of the turn. Once your bike is pointed in the right direction, you can stand the bike up a lot sooner than if the rear tire were still on the line. You are essentially pivoting the bike on the front tire by allowing the rear to step off the line.
Meaning that by spinning the rear a rider shortens the amount of time they will need to be on the edge of the tire, and maximizes the amount of time they will be on the meat of the tire, and thus accelerating as hard as possible.