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Torque Reaction Force of Transmission and Drivetrain

  1. Apr 5, 2015 #1

    As I understand it there is an equal and opposite reaction for every mechanical action. In the case of a vehicle drivetrain the engine generates a torque at the flywheel or flex plate which becomes an input to the transmission and ultimately the wheels.

    The rollong resistance of the wheels determines how strong the reaction is. If the vehicle has four flat tires it will be a lot. If the vehicle is coasting at 100 mph it may be very little.

    If all brakes are applied the full torque of the engine will be reacted back and should be like a force that provokes the engine to spin in the opposite direction.

    Normally because of gear reduction and low vehicle load and engine weight the engine would not spin about the drivetrain.

    Unlike a drill, if the drill bit seizes in the work piece it can twist the whole drill around taking the operator with it.

    Impact wrenches decouple the torque reaction by design or the force would probably injure people.

    I believe the engine mounts act as springs that absorb and resist some of the torque reaction under load.

    I am concerned with this idea however. While the engine is rotating in place and deflecting its mounts the drivetrain does not turn or turns more slowly.

    Once the mounts hit their mechanical limits the engine has more resistance to torque and stops rotating, power builds and the drivetrain gets going and develops inertia. The mounts become unloaded again. Now as the load builds on the engine it begins to turn and compress the mounting system.

    In this way if the mounts are not sufficiently rigid a kind of stick slip or oscillation of engine output power can occur which will be bad for friction clutches.

    If a hooke's joint is used the deflection of the mounts may change the front driveshaft working angle and cause a binding or oscillating torque perhaps.

    How about it? What is the role of engine mounting in the task of powertrain clutch engagement?


  2. jcsd
  3. Apr 5, 2015 #2


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    This is true.

    Insofar as the reaction of a vehicle to the torque of the engine is concerned, rolling resistance has nothing to do with how the car structure responds. There will be a reaction generated even if the car is out of gear and standing still.

    Not sure what you're getting at here. It's rare that the brakes are applied and the engine is left in gear while it runs flat out. This seems like a good recipe for smoking tires and leaving a lot of rubber on the road.

    As long as the engine is running, a torque and corresponding reaction will be generated. Whether or not it's noticeable is a different thing.

    In cars with rear drive and live rear axles, there will be a difference in the force applied to the tires on each side of the car due to the engine wanting to spin the axle around the drive shaft. Cars with independent suspensions do not suffer from this, as the torque of the engine is absorbed by twisting the structure of the car.

    Yes, you're right. A car is not like a drill.

    In addition to keeping the engine from dragging on the ground, the mounts serve to transmit the torque of the running engine to the car frame/structure. The static loads from supporting the engine will be altered so that a torque reaction is generated to oppose the torque from the engine.

    Don't be so concerned. This is not true. The crankshaft is still free to turn regardless of how the engine mounts are loaded.

    Not sure what you are trying to say here.

    Not sure what you are trying to say here.

    Universal joints are installed into the drivetrain to allow for relative movement between the transmission and the differential in vehicles equipped with live axles and rear drive.

    This also occurs in vehicles with frame mounted differentials, to allow for ease in removing the driveshaft, I suspect.

    The mounts keep the engine from dragging on the ground and turning into an anchor.
  4. Apr 5, 2015 #3
    Putting a car in gear and holding the brakes and full throttle is a common way of checking the mounts, drive or reverse. Excessive movement indicates a weak mount that needs replacement.

    Likewise with automatics the test has been used to check maximum engine power output and compare to see if the clutches in the trans can hold and withstand it. All the slip should be in the torque converter.

    My question is related to the fact that the engine rotates under a braked torqued load.

    It goes back to the concept of equal and opposite reaction. When a cylinder fires with the piston a little after top dead center the resulting gas pressure pushes in all directions.

    Doesnt that mean the force pushing the piston down is equal to the force pushing the cylinder head up?

    The head is part of the engine and transmission structure so the force applies to them all.

    If the forces are in equal and opposite directions, and engine generating a torque will receive a torque in the opposite direction of rotation.

    So if the crankshaft is seized hard enough by lets say the brakes, the engine may actually bounce backward from it.

    Supppsing it is not an automatic but a manual. As the clutch engages the torque begins to transfer to the drivetrain. As the drivetrain loads the engine it should create a corresponding reaction torque that tends to rotate the engine a bit.

    Whenever the engine rotates around the driveshaft it may be like the driveshaft losing speed?

    Engine mounting systems have mechanical limits so they cant spin very far. But I have heard of oscillation back and forth on clutch engagement with weak mounts for this reason.

    My terms and concepts will be mixed up because I have no education in physics.

  5. Apr 5, 2015 #4

    Randy Beikmann

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    I've got a few comments:
    - Remember that Newton's third law only holds for one pair of action/reaction forces at a time, which act in equal and opposite directions. Examples are a tire pushing down on the road, while the road pushes up on the tire; gravity pulling down on the car and upward on the Earth.
    - It's instructive to look at the bearing forces in a transmission if you want to understand the "reaction torque" from it (unfortunately this usage of "reaction" is not a statement of Newton's 3rd law and I think causes much confusion). If a transmission has a gear ratio of 3:1 and 150 N-m is applied at the input, 450 N-m is produced at the output, and 300 N-m acts on the case, through the bearings, to tend to make it rotate. If the gear ratio is 1:1, there is no torque acting on the case.
    - If you combine the engine and transmission as one structure, and recognize that 150 N-m of mean torque on the crankshaft from combustion is accompanied by 150 N-m on the engine block in the opposite direction (this is per Newton's third law), all torques on the engine/transmission unit would balance out except for the 450 N-m reverse torque from the driveshaft on the transmission (in the 3:1 case). This is where engine mounts are needed to counteract the torque and keep the engine/transmission in equilibrium.
    - If you suddenly engage the clutch, Newton's 3rd law applies everywhere, but that doesn't mean torques balance and everything is in equilibrium. The sudden application of torque will rotate the engine structure and deflect the engine mounts, which will "spring back." In fact, the engine will oscillate, or vibrate, in rotational motion.
  6. Apr 5, 2015 #5
    Awesome reply Mr. Beikmann.

    How will the oscillation or vibration affect drivetrain clutch engagement?

    Won't it lead to a rough stick-slip condition until the load is fully coupled?

    Clutches dont have their full clamping force from the start, the engagement would be very harsh if they did. Unpleasantly so.

    A good system then would be one that smoothly engages over time, and for this I believe a good structural support and springing system for the powertrain is needed. The mounts also dampen fine vibration from the powertrain to the body.

    I mentioned driveshafts because hookes joints need nearly identical angles or the driveshaft will whip in a twisted motion. Besides that Hooke's joints have an oscillating velocity that is proportional to the working angle.

    If the velocity changes as the joint speeds up and down the load torque should oscillate as well. A driveshaft at a bad angle would seem to cause a fluctuating load. This is bad for smooth clutch engagement perhaps.

    When considering a hookes joint the torque transmitted through it and the changing speed result in fluctuating power in the physics sense of it. Is that right, wrong?

    So called constant velocity joints of the tripod or rzeppa type probably have better angularity tolerances. Tripod type are not true constant velocity I think.

    One thing that is another load varying issue is differing traction at the drive wheels depending on the differential type. When a wheel breaks loose from the ground it provides almost no reaction torque (?), with an open differential this may cause it to spin wildly fast until it hits the ground again, causing shockloads and power oscillation.

    And actually intelligent systems for launching race cars or for traction control do balance the wheel torque and available traction.

    In severe cases with a RWD particularly live axle powerful vehicle the LR wheel may lift if not restrained well enough by spring stiffness and shock absorber....

    What do you think?

    Many thanks to everyone for indulging my curiosity

  7. Apr 6, 2015 #6

    Randy Beikmann

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    Andrew, you bring up many important issues for noise and vibration in powertrains, any of which can't be done full justice in the forum. But curiosity is good - it what drives you to learn. I've worked for years on exactly the areas you're discussing, and I still have to consult other experts when I work on them. Always.
    One major thing I have learned is to learn to walk before you run. I still look in detail at a simpler version of a complicated problem I'm working on, and gather some intuition about it. Then I add in more elements of it (more degrees of freedom, non-linearities, etc.) to see if/how they interact. It might take me weeks! But when it's done, I not only have an answer, but I know why, and how to do it faster next time.
  8. Apr 6, 2015 #7

    jack action

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    This is most likely the problem caused by engine mounts that are too soft.

    All Hooke's joint angles are «bad» angles, in the sense that input/output speed start varying as long as it is not equal to 0. This is not a problem when you put 2 Hooke's joint together (in the correct phasing) and run them at the same angle: The variations cancel each other out and you get a constant-velocity assembly. But if your engine mounts (as well as your suspension mounts) are so soft that you cannot keep both Hooke's joint at the same angle, then you will get torque variation that will translate into wheel hop of difficult clutch engagement.


  9. Apr 6, 2015 #8
    Thanks Jack,

    The suspension design of some rear wheel drive vehicles I have looked at with trailing arms on both sides has an interesting effect.

    It causes the rear axle to rotate as the suspension travels so that the nose angle at the pinion shaft changes symmetrically with the joint at the driven end.

    If there is too much play in the mounting its conceivable to me that the front joint would go out of matching angle.

    Or as youve said, if the rear suspension doesnt hold the axle horizontally- the trailing arms handle the sort of vertical or axial alignment.

    Thats how it seemed anyway. The driveshaft angles have to match even when the vehicle is heavily loaded with a trailer for example.

  10. Apr 7, 2015 #9
    Hey Mr. Beikmann,

    I think our methods differ because we have different goals. As an engineer working in powertrains I think the scope of work is more narrow and the quality and specificity is much greater.

    But I am a technician so the details are less important than the broad concepts. Because I dont have to design systems I do not have to know each piece in great depth, but I benefit from a complete working understanding of all components and aspects.

    So by analogy I have a jigsaw puzzle on the kitchen table and every day year after year I collect the pieces and slowly find where they go. As they accumulate into small masses it becomes clearer what they are or where they go. What I need are not pieces in super fine high resolution print, but to connect the pieces so as to see what can be seen when they are placed correctly

    Like a large kitchen table I have a long attention span that collects these things bit by bit and it is very curious about powertrain mounts.

    They serve a few purposes that I know of. They support the weight of the powertrain. By design they contribute to the safety performance of the vehicle during impact or collision. Some are mechanically redundant, some are soft core, some have a cable strap for redundancy. The mounts dampen vibrations, apparently the body of a vehicle resonates at around 20hz which is close to the firing frequency of an idling engine.

    Ive found there are both elastomeric mounts and hydraulic mounts. And the one hydraulic mount I opened had an inertia track, and maybe a decoupler.

    Besides doing all of the above the mounts also have some springing ability and often have integrated bump stops. They may have V shaped structures that have non linear stress/strain but thats above my head. Beyond that they align the powertrain correctly.

    So nothing is ever as simple as it seems. Diagnosing a vibration at idle and finding the source can be tricky. The mounts can "ground out" when they collapse or sag.

    If the transaxle and engine assembly is too far to one side or the other it can cause torque steer related to the differing axle lengths.

    Clutch chatter with a manual is often blamed on the clutch but may be caused by bad mounts, not the clutch assembly.

    I seek a complete, rather than a thorough understanding. I am called "overanalytical" and "passionate", but hey, I enjoy this stuff.
  11. Apr 7, 2015 #10

    Randy Beikmann

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    It sounds to me like you have a good overall knowledge of powertrain mounts, and their general tasks. And you're right, my emphasis is (now) on designing the system before any hardware exists, so I have to dig into the theory deep enough, so that the system is is good enough, so that it only needs to be "tweaked" once the car exists.
    It sounds like you're more concerned with trouble-shooting/diagnosing, and in that case, you are taking more care than most. The advantage you have in trying to understand the "root cause" of an issue is that you develop instinct, so when the standard "fixes" don't work, you may be able to imagine what's going on in the parts, and improvise.
    That part is not that different from what I do, and in fact it's how I learned to design the systems. And in fact, I try to have a "not too deep" knowledge in the overall systems (which is what I think you're looking for), and then deeper in whatever specifically needs to be done at the time.
    If you are troubleshooting problems with this much diligence, I think that's awesome. These systems are engrossing. And it doesn't hurt to give yourself a leg up!
  12. Apr 7, 2015 #11
    Yes you have it exactly. What I am after is the "core logic" that the systems as a whole were built to address. I want to understand components through their role in a general framework so that I can apply reasoning and critical thinking skills to unfamiliar problems within the domain.

    With a good understanding of the general background I can focus my attention and efforts on the specific thing that is aberrant.

    I receive criticism at times for over-studying or learning things without direct dividends but I shrug it off. For everything that I begin learning about so many doors and insights open. The relationship of firing orders and engine balance and crankshaft design sparked by an interest in the source of vibration was a real eye opener and illuminated the use of balance shafts.

    I had to let that topic rest for awhile it is fantastically complicated.

    After studying engine mounts I came to appreciate the wide variety of bushings and isolators between the Driver and the road and powertrain. In the steering system alone there is a flex disc in the steering shaft, rubber bushings for the rack mounts, inner and outer tie rod polymer (?) bushings inside.

    A good look at fasteners give you an idea of where the high vibration locations are. Shock absorbers have nylon locknuts or deformed threads typically. Flex plate bolts are more prone to loosening. Many drive shafts have flex disc couplings to help isolate road vibrations. Others are two section designs - length is a big factor in vibration I think.

    Even shock absorbers as semi active dampers and vacuum controlled engine mounts. Newer vehicles have active shock absorbers and even sway bars.

    In places there are tuned masses for damping (?) like large counterweights on shift linkages and bushings there.

    Every part has a purpose. Understanding a component failure is often important.

    Once the background is adequately established it is easier to rapidly understand the situation at hand and address it. And this is what I love.

    Right now I have met my match with a vehicle that has a used engine, used transmission, remanufactured PCM and possible wiring faults. It shifts erratically and I have to determine Why. A few other people tried and failed already.

    Regards Mr. Beikmann. I am happy as a tech :-)
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