Switching from Reverse to Drive - always stop completely?

Question

I've been told by a couple people that it's really bad for a car to be shifted from Reverse to Drive while the car is still moving backwards. Is this true?

In my experience:

On automatic cars, this could be true - most cars I've tried this on (not often, mind you, since I don't want to destroy cars that aren't mine) jerk strongly when switched.

On manual cars, however, I've noticed the opposite; rather than a strong jerk, there feels like what I'd call a 'winding'-like sensation - the car slows down and then moves forward smoothly.

So, again, is this a bad thing to do? On all cars or just some? If just some, is it a make-and-model kind of thing, or an automatic/manual thing?

Answer 1

A lot of newer cars are smart about shifting (they have electronic solenoids to control the hydraulics). I can put my 2001 Nissan Pathfinder in reverse at 50 MPH, and it's smart enough to not engage, it goes into neutral. However, at speeds below its cutoff point (I've done it at about 15 MPH and regretted it), you can put a lot of stress on the drivetrain if you shift into drive from reverse (or vice-versa). Even more so if you shift and apply the accelerator.

But when you are moving slowly, like 5 MPH or less? No, no harm--at least in the automatics I have experience with. The transmission will shift through neutral first, relieving any preload. Then when it shifts to drive or reverse, the torque converter will take the mismatched speeds (RPM), that's what it's designed to do. The problems arise when the torque converter engages strongly due to a high RPM difference, and this can shock the drivetrain and break stuff.

In manuals it all depends on how long you slip the clutch. I could make a change from reverse to 1st at 20 MPH be smooth. At least, as long as the clutch and synchronizers last.

Answer 2

In the automatics I've been in where people do that, it's a rather dramatic bang/clunk if shifted from Reverse to Drive while still rolling backwards. I hate riding with people that do that, makes me cringe everytime. I hear newer cars are smarter about it, but it still sounds like a bad idea. I hate to risk my transmission on a sensor that might fail when it's so easy to just not do that...

On (typical) manuals, it doesn't cause any large loads, but you have to slip the clutch more to get moving again (as well as the synchros having to do a bit more work). So, it does cause slightly more wear. Beware if you have a race transmission with straight cut gears and no synchros though, that would be bad news going reverse to drive while rolling backwards. Most manuals do not have synchros on reverse, so rolling forward and shifting into reverse is a no-no on they typical manual.

Answer 3

I have always thought changes of direction while moving was a bad idea. It doesn't matter if it is an automatic or standard shift, small car or large truck. Doing this puts tremendous stresses on the drive train. All mechanical parts are built with clearances between moving parts. When you ask them to change direction while moving the parts act as hammers as they are accelerating prior to contact. If you pushed a stationary drinking glass with a hammer it would not break. If the glass was rolling and you wanted to change its direction with a hammer I think the out come would be obvious. It may seem like a less abrupt change with a manual shift because the clutch is slipping and absorbing the energy. So instead of shocking the U joints you are wearing out the clutch.

Answer 4

Since electric cars will eventually displace all forms of internal combustion engines, the answer to this question will change over time.

I have developed a shifting technique on my 2016 Toyota RAV4 hybrid, where when reversing from the parking location I switch to "D" when slowly moving rearwards. The shift is extremely smooth and the result is that the backwards momentum continues to decrease and eventually turn to forwards momentum. The reason this works is that Toyota's hybrids do not have a transmission. They have a power split device that is used to electrically adjust the relationship between engine RPM and wheel speed.

On all purely electric cars, you probably can do the same. There is no reason an electric car would not do a smooth shift between "D" and "R" at speed. Of course, the electronics may disallow the shift if the speed of the car is over a certain threshold.

However, what about non-Toyota hybrids? It's anybody's guess, basically. Some hybrids do actually have a conventional transmission, and therefore, may not allow shifting between "D" and "R" at speed. I am aware of at least some non-Toyota hybrids that use a construction similar to Toyota, namely a power split device where one shaft contains the internal combustion engine (ICE), another shaft contains the motor generator 1 (MG1) and the third shaft contains the motor generator 2 (MG2) and is connected to the wheels. This construction is sometimes called an electric CVT (eCVT). On all hybrids of this type, it is possible to shift between "D" and "R" at speed.

Answer 5

Shifting from reverse to drive, or vice-versa, while the car is still moving can cause increased stresses to the driveshaft u-joints/constant velocity joints which can lead to breakage.