Slip Angle, Take 2!

[ From the captions on the photos in the first post of this series, it seems I may need to clear up a little bit of confusion regarding slip angles:  the concept applies to all four tires.  

I left the photos and captioning to my fearless editor (and great karting instructor), Davin.  His captions lead me to believe that he may have only understood part of what I was trying to convey

(Don’t worry, Davin… this isn’t an easy concept to abstract and my upcoming force diagrams will help!).  So, Take 2! ]

High Slip Angle- Take a look at the angle of the front wheels. That’s not the fastest way.

Let’s take the photo of the drift car.  The car is moving from left to right in the frame.  The car, however, is pointed about 45 degrees towards the camera.  Let’s look at the relative slip angles of the front and rear tires.

Front tires:  The front tires are pointed towards the center of the corner.  The driver is turning the wheel left, but the car is so sideways they are effectively acting as if they are turned right (if the car were pointed straight).  Eyeballing the angle, they looks to be turned inward around 15 degrees relative to the direction of travel.

This is a common slip angle, on the upper end of the range on asphalt, for a car near the limit.  This tire is working pretty efficiently, and it knows nothing about what the rear tires are feeling.

Rear tires:  The rear tires are pointed the same direction as the car… about 45 degrees from the direction of travel.  This is an extreme slip angle.  The tires are sliding as much as they are rolling and generating a huge amount of mechanical drag on the car.  The lateral grip offered by straight up static / sliding friction is minimal.

Stability is maintained by the forward (relative to the car, inward and forward relative to the curve) sliding friction generated by the spinning tires.  This action is also what is keeping the car moving forward in spite of the mechanical drag created by the high slip angle.

I’ve driven several race cars that didn’t have  enough power to keep a drift going… they would grind to a stop even under heavy throttle due to the mechanical drag.

To maintain the drift in this photo, the driver can modulate the throttle to adjust the action at the rear of the car or adjust the steering angle to keep the front end pointed the right direction.  The higher the slip angle, the bigger the challenge to smoothly maintain the arc of the track… but that is the art of drifting!

Editor’s Note: Thanks for the flattery, Andy.

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