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First, I am talking about manual cars and I know this is considered impossible because of the synchronized gears, but lets suppose that you can manage to shift gears (For example you could shift into reverse when stopped and then pull the car forward with another car, or even better, roll down a hill, always with the clutch)

So, you are moving forward at a reasonable speed, for example 100 km/h, and then you release the clutch with the reverse gear engaged, what would happen then?

My theory is that the engine would be forced to rotate the other way around, wouldn't it? Then, because of that, the engine would stop, and therefore it will "generate" a lot of force to the transmission and cause the car to reduce speed (all of this, if nothing breaks, of course)

Is this assumption correct?

There's a Myth Buster's chapter that tries this but with no luck on shifting the gear. Please try to avoid answers telling that it is impossible to shift into reverse when moving forward, as I stated before, I am supposing it can be done.

  • 2
    You have found an excellent way of stripping the gear teeth off the bull gear or shaft splines ! – user16540 Apr 26 '16 at 1:33
  • A sudden skid stop before you move in reverse at low speed, at higher speeds it may lock the wheels and throw you into an uncontrolled skid, a friend of mine almost died due to this because of a faulty shifter in a t-bucket roadster, skidded off the road into a tree or two. Modern computer controlled auto transmissions will not allow you do this. – Moab Apr 26 '16 at 22:42
  • "bull gear" no such part in an automatic trans, they are called planetary gears. – Moab Apr 26 '16 at 22:46
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My theory is that the engine would be forced to rotate the other way around, wouldn't it? Then, because of that, the engine would stop, and therefore it will "generate" a lot of force to the transmission and cause the car to reduce speed (all of this, if nothing breaks, of course)

Put simply, in most cases, the motor will stall and the tires will lock. In some cases (enough speed or weight), you may force the motor to turn the opposite direction because reverse gearing is so low it would impart a high amount of torque to the input shaft, enough to rotate the motor in the opposite direction. This can happen with heavy trucks and vehicles pulling trailers.

By putting so much torque in such a small amount of time (what is called "Clutch Dumping"), provided the clutch doesn't slip, you will destroy the transmission. The reverse gear was never intended to handle that much torque. However, (not recommending this in ANY way) if you were to put the vehicle in reverse gear with enough weight pulling it forward, or if you were to slowly release the clutch on a lower compression motor (most modern motors are very difficult to rotate due to their compression ratio) you could theoretically manage to rotate the motor in reverse.

Would this cause damage? Perhaps not immediately except for the case of sudden shock to the internals. During extended operation, in engines with mechanical oil pumps, you would starve the bearings and motor of oil because the oil pump is now also rotating in reverse.

  • Engine will not stall due to the torque converter slipping as it is designed to do in reverse. – Moab Apr 26 '16 at 22:43
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    The OP is postulating a manual transmission which as you know has no torque converter. – cdunn Apr 27 '16 at 12:05
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    @Moab, please consider reading the question thoroughly before commenting. OP Indicated: "First, I am talking about manual cars". Manual vehicles do not have torque converters. – Jeep Cherokee Rescue Society Apr 28 '16 at 5:43
  • Manual or automatic...would not make a difference..the result would the same. – Old_Fossil May 4 '16 at 6:08
  • Nice answer, just one thing: A low gear multiplies motor torque and provides high torque at the wheels, but torque from the wheels is divided by the same factor and results in quite low torque at the motor. That's why you use 1st / reverse, but not 5th gear as additional parking brake. Anyhow, putting the car in reverse while driving is quite violently to the system. – sweber May 4 '16 at 7:47
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A High School auto shop teacher about 35 years as an experiment on a old shop car managed to do precisely that. Got the old car to 60+ MPH and slammed into reverse--suffice it to say it was not pretty. Idler shaft milled like a lathe, the gears....you don't wanna know...LOL

  • I wanna know. On the other hand, I expected traction to simply fail and spin the tires. – Joshua Dec 18 '18 at 18:20
  • @joshua The entire internals self destructed from the massive shear forces. Lot of parts destroyed. A lot of ground metal. – Old_Fossil Dec 20 '18 at 5:15
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This can be made to work. As a teenager I tried the technique seen from Mad Max, with an old rear wheel drive car.

While travelling forward at speed, press the clutch fully in, allow the engine revs to drop so no load on gears. Pop the brakes to briefly lock up the wheels. Then change into reverse, raise the revs to a high level (thinking nearly red-line) and dump the clutch.

As long as you keep the revs high, what happens is the tyres immediately spin so you have very low torque. This is twitchy, but manageable. You must not let the revs drop until the car is stationary, as you don't want the engine to be driven backwards.

Disclaimer - the second time I tried this I killed the engine/transmission...

  • Hahahaaaaaaaa of only we could have watched!! – Bevan May 4 '16 at 10:32
  • Local pro drag racer some time ago mistakenly punched reverse rather than neutral on his pushbutton shifter at the end of a run. He died. – TomO Sep 5 '17 at 20:09
  • "Then change into reverse..." This won't work. You can't shift into reverse at all at high output shaft (wheel) RPM. It isn't a matter of rev matching to the engine, the countershaft and output shaft RPMs are just too far apart to mesh. Not to mention they are turning in different directions. I'd like more information how you did this and how fast you were actually moving. – geoO Jul 31 '18 at 16:10
  • Got a nice bit of lockup, banged it into reverse. Maybe going 30mph. Like I say, it worked once... – Rory Alsop Jul 31 '18 at 16:40
  • Edited the post to include that. – Rory Alsop Jul 31 '18 at 16:41
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Yes and no. Yes the engine would in theory be forced to rotate in reverse. No in practice because the extreme torque shock would simply strip the clutch shaft splines or even completely screw the shaft in two. Please don't try this at home...

1

First of all, even in a synchromesh transmission reverse isn't typically a synchronized gear. Another assumption you make is that you even need to engage the clutch at all. If we are pretending why not assume a clutchless shift?

But let's play along: a shift involving use of the clutch and let's also assume the trans simply does not pop out of reverse back into neutral. Let's also assume we've disabled any lockout feature of the car intended to prevent this very situation! But if you did manage to engage reverse at some forward speed I can think of four outcomes based on the possible points of failure in this system of opposing torques. It's quite a neat thought experiment!

  1. Massive clutch slip, at least initially. The clutch may fail to grab because the flywheel is driven by the engine one way and the wheels drive the clutch the other and you get a sliding and glazing, a freewheel phenomena as the drive wheels simply aren't driven by the engine any more. Maybe as you slow and the clutch finally grabs you'll end up with one of the following problems.

  2. If the clutch does hold and nothing breaks then what you have are drive wheels spinning backwards at some speed and a vehicle moving forward at some speed. Ugly. If traction with the road is low (say the drive wheels are on ice) you'll slide for a long time as the tires can't find traction.

  3. If you do have clutch friction and sufficient tire traction something else has to give. You may shear off transmission gear teeth, maybe the idler or reverse gears. If this happens you will freewheel again since the engine is no longer driving the wheels. The broken teeth might float around in the transmission case but really what more damage could you do?

  4. If the clutch holds, tires have traction, AND the transmission lives, the crankshaft would have no choice but to be driven backwards within the crankcase! A broken crank seems the most likely scenario here.

What a show!

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What Will Happen If You Switch to Reverse Gear While Moving Forward?

Let’s say, you accidentally or intentionally switch to reverse gear when cruising through the highway at the fifth gear. It will make the lever engaged to the fifth gear to disengage and move toward the reverse gearwheel to engage with it.

It will be quite impossible since there will be no synchronization between the main shaft and the gears at such a high speed. The reverse action will set in motion when your car finally achieves the synchronization speed. To simply put, the reverse gearwheel will make Gear 1 to rotate anticlockwise (when it was rotating clockwise) and Gear 2 to clockwise (when it was spinning anticlockwise). This forced action will result in a number of catastrophic consequences that may even lead to the total damage of the gearbox. The issues you can face are:

  • The gears end up in a deadlocking position.

  • Breaking of the teeth of gears that have to engage

  • Stalling of the engine

  • Serious damage to several engine parts including crankshaft, connecting rod, and other components.

What Will Actually Happen?

Well, none of the dangerous things mentioned above will happen because the transmissions in modern cars are designed to avoid this situation. The automatic cars have a function called Reverse Inhibit that nulls your request of putting the car into a reverse direction. It will only activate the command when the vehicle gets down to the proper speed. Even if you try to do it at a slow speed, the gearbox will make some bizarre sounds, which seem like a warning that you have done something wrong.

If you put the reverse gear command in an old stick shift, the gearbox will make an abhorrent buzzing sound. The gears will try to engage in a reverse motion but the high speed won’t let that happen. So, it will be quite anticlimactic. The car won’t stop on a dime and change the direction into the opposite or the transmission won’t explode, like what you see in a movie!

  • Have you even read the first paragraph of my question? – Pablo Matias Gomez Feb 1 '18 at 13:08

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