# Definition of "yaw" for cars?

Yaw for airplanes, I think I know what that means. When flying in crosswind, the plane doesn’t point in the same direction it is moving, yaw is the angular difference. But what does “yaw” mean as applied to automobiles?

This is from BWD…

A Yaw Rate Sensor (or rotational speed sensor) measures a vehicle’s angular velocity about its vertical axis in degrees or radians per second in order to determine the orientation of the vehicle as it hard-corners or threatens to roll-over.

By comparing the vehicle’s actual yaw rate to the target yaw rate, the on-board computer can identify to what degree the vehicle may be under- or over-steering, and what corrective action, if any, is required. Corrective action may include reducing engine power as well as applying the brake on one or more wheels to realign the vehicle.

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Same for both airplanes and cars, then. Yaw is angular rotation around the vertical axis.

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Yaw rate makes sense for automobiles as well as airplanes, just the angular rate of a turn; i.e. 4 degrees per second is stabile, 40 degrees per second, not stable. But as far as “yaw”, airplanes can be steadily pointed in a direction other than the one they are moving due to a crosswind, yet not be rotating about the vertical axis. Cars can do that too, but only if the tires are sliding.

The proper term is ‘slipping’ not sliding. And cars can drive with steady state yaw with a side wind or a crowned road. Whenever your properly aligned car drives straight while the steering wheel is slighly turned, its yaw angle is greater than zero.

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Could any of today’s automobile yaw sensors detect that? I’m thinking not. The yaw sensors used in today’s cars are only measuring the turning rates. I presume what they actually measure is the acceleration forces caused by turning. Could do the same thing presumably with a rock at the end of a string. Hangs straight down with no accel forces.

This is often called “dog legging” or “Crabbing”

There are numerous newer vehicles that can do as a feature. If the vehicle is not designed to do that, it’s bad and common causes occurs when a vehicle has been in a collision severe enough to bend the frame or a sprung frame, or a frame that’s abused with extreme driving beyond what the engineers designed for… Extreme 4-wheeling, jumping, towing way beyond allowable limits…

Back in the '60s, a friend tried to pull another friend’s car out of a snow bank, it was not working by just pulling, then the truck driver tried jerking the car out, ultimately the car cam free, but only after a lot of “running start jerking” to pop the car free.

Afterwards the truck dog legged, I do not know it the frame was bent, or the rear axle was pulled out of alignment but it sure looked stupid rolling down the road as you could see all 4 tires…

Below are a couple of examples from the internet, these trucks were not designed to do it as a feature…

All that damage and still driving on the road?

A more common failure is for the leaf spring center bolt to break allowing the axle mount perch to shift location.

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You are thinking correctly. There is no absolute yaw angle sensor.

Calculating yaw acceleration is possible with properly positioned accelerometers. Integate the accel signal to get yaw velocity. The safety systems use yaw accel and velocity. Yaw position is not as important.

Steering angle combined with yaw velocity and accel can give very useful information to the stability control and ABS systems. Electronic shock systems use it, too.

I had a 72 Chevy Impala that had a constant back and forth yaw from the rear suspension. Very annoying because the sensation is just the same as if you are driving on ice and starting to lose it. Had a similar problem with Chevy and GMC tractors. No way to adjust the location of the rear axle(s) if the hole in the frame were not drilled exactly equal from side to side, that tractor would dog track forever. Made it very difficult to back into a tight space.

Do vehicles with rear wheel steering use yaw sensors?

With active rear wheel steer, yes they do use yaw sensors. Since stability control is required on all new cars, virtually all have a yaw sensor.

The yaw sensor is usually part of a combination sensor with accelerometers and yaw sensors in the same part. Typically mounted in the console at the center of the car.

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