Use the airbrakes!
When you are rounding a corner at high speed, use your airbrakes to slide more easily around the corner. To use them, hit R to engage the right airbrake, and L to engage the left airbrake, or hit both to slow down evenly. You will lose a little speed by using airbrakes, but usually you will lose more speed by slamming into the wall and taking a good chunk of damage than by using them.
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