Never takes someone's grinding challenge when they have clearly left themselves room to 180
Ride the outside of the action, grinding most of the way, after the start up. When it gets crowded you will often box in people who were looking for quick kills.
Set people up for 180s, most players will let their ego play too big of a role in their playing
For closer grinds, just 180 everything, they don't have to be too tight, but they usually help. Of course that's a high ping player talking.
After some dies, check to see if their wall will lead to an easy box in when it disappears on an unsuspecting opponent.
Use your brakes, they're good for you
Set up a local game with the settings of your favorite server and add about 10 comps in a small arena with infinite walls to work on your skills in confined areas.
Watch for which players are outgrinding everyone, don't push your luck.
If someone is boxing you in, but they haven't sealed off anything yet, change directions and go the opposite way to avoid it.
Realize that the starting positions for the bikes are not completely random, you cannot turn immediately and be able to cut off someone next to you.
Finally, I'm going to have to agree with Digital Logic and say, use your glance keys all the time. You have to know when someone is coming up on you.
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ProfessorPap 
JohnnyEnzyme 
QuirkyCrow
