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.
During normal driving, all the wheels of a car rotate at slightly different speeds when you make turns or drive over uneven pavement.
4H and 4L locks the front and rear axles together so the front and rear wheels HAVE to turn the same speed.
Off-road it’s ok because the wheels can slip a bit to compensate for any differences in wheel speed, but on pavement the tires grip the road too well so there’s a lot more strain on the drivetrain, which can damage things.
This video does a great job of explaining why 2wd often means 1st and 4wd often means 2wd. It’s all about the wheels needing to turn at different speeds. [differential video](https://youtu.be/yYAw79386WI)
When you make a right turn, the right wheel is closer to the corner than the left wheel, so the left wheel has to spin faster than the right wheel does to make the turn smoothly ..
4h and 4l usually lock the system so they have to spin at the same speed so the left wheel hops and drags which wears out the tires..
4h and 4l are good when ur trying to get unstuck or pull stuff or climb stuff..
When you turn, the wheels on the outside of the turn have to travel a little further than the ones on the inside of the turn, because a large circle has a bigger circumference. This requires the outside wheels to turn faster than the inside wheels.
A car’s wheels are able to rotate at different speeds thanks to a special device called a differential (get it, “different”). When the wheels rotate at different speeds it’s referred to as “slip” and depending on the car you might have a limited slip differential, open slip, or locked (locker) differential. Its kind of hard to explain, it’s a lot easier to see demonstrated in the following old video.
[https://www.youtube.com/watch?v=yYAw79386WI](https://www.youtube.com/watch?v=yYAw79386WI)
When you use a locked differential, or 4H/4L, then the wheels can’t rotate independently anymore. 4H/4L lock the front and back axles together, while a locked differential locks the wheels on the same axle together. So now when you turn or go around a curve on dry pavement, the wheels want to rotate at different speeds but the can’t, and you end up with one of the wheels getting dragged along and also putting a lot of strain on the differential as it tries to stop it from spinning fast enough.
On slick pavement, sand, mud, snow, etc it doesn’t really matter because the tires can slip against the loose surface. But obviously getting power to all of the wheels is sometimes very helpful offroad especially when one wheel is stuck in the mud or something and the open or limited slip differential would not transfer power to where it’s needed.
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