I was the designated driver the other day and was driving some of my overly sensitive friend’s home in one of their cars. I pulled out of the driveway into the street and then went along on our way. But, apparently I had made a grave sin in the eyes of my non-technically inclined, non-car enthusiast friend. That sin was that I didn’t come to a complete stop before putting the transmission into drive… I haven’t heard the end of it since.
Now, it’s not like I was racing in reverse and then slammed it into drive like some sort of stunt driver. I was backing out normally, braked to less than a mile per hour, and then put it into drive. It was a very smooth transition and there were no clunks, bumps or any drama at all. If I was going any slower, I would have been stopped.
The problem is that most people don’t understand how an automatic transmission works. I believe they think that gears are actually moving into one another and that if you’re not completely stopped, you would grind the teeth off the gears. As you probably have guessed, this is not the case.
Automatic transmissions use a planetary gear set which have gears that are always meshed. Shifting gears happens by locking and unlocking the various orbits; and when not in gear, those orbits are left to freewheel. Locking these orbits is done by clutches and bands (basically brake shoes with friction material on them) which are activated by hydraulics. These clutches and bands are made to slide a little when engaging, and that’s that gives smooth shifting.
Furthermore, automatic transmissions need a method to be in gear and be able to have the car stopped without stalling the engine. This is achieved by using a torque converter which connects the engine to the transmission though a fluid coupling. The torque converter works like an egg beater mixing thick pudding in a bowl: if you beat the pudding without holding onto the bowl, the bowl will spin. But if you hold onto the bowl (like applying the brakes in your car) the egg beater will continue to turn the pudding, but the bowl (your wheels/transmission) will be stopped.
The transmission doesn’t get damaged when shifting into drive while the car is still (slowly) moving backwards because there’s no hard mechanical links between anything in the transmission. The slow engagement of the clutches and the slop of the torque converter absorb any speed differentials and allow you to do this. This ease of use of automatic transmissions is what they were designed for, and if this caused serious problems, you would hear about it all the time.