Why is it bad to use 4H or 4L on pavement?
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4 wheel drive is not the same as all-wheel drive.
4 wheel drive lacks a middle differential. This means that the average angular velocity of the front two wheels is the same as the average angular velocity of the rear two wheels.
When driving in a straight line on pavement this is not an issue. However, when turning on pavement while driving in 4 wheel mode the average of the two axles will be different due to the curvature of the turn. The wheels on each axle will have a different angular velocity when turning but this is resolved by the presence of a differential gearbox on each drive axle. In 2 wheel drive, the front axle is generally disconnected from the transmission (unpowered) which allows it to spin freely while the rear axle is connected to the transmission (powered).
All-wheel drive vehicles have a middle differential which resolves this issue, allowing all four wheels to move at different angular velocities while under power.
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