Engine Rotation Direction
12-09-2005, 03:59 PM
#21
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Originally Posted by Adnectere
I need to say a big thank you to everyone for replying so well and quickly. You guys have cleared some up, but I still have one question: If the crank & driveshaft both move ccw when viewed rear->front, why do their torque reactions go the opposite way. Up front, the chassis responds to the force put into it by lifting the left and planting the right, but in the back the axle responds to the force put into it by lifting the right and planting the left. The forces are rotating in the same directions and planting opposite sides!
Try this (thought) experiment. Start a (CW or RH) 1/2 inch drill bit into the center of a 12 in. long piece of 2 x 4. About 1/2 way thru you hit a knot (or nail) which stops the drill bit from spinning. Which way will the 2 x 4 try to go? Which way will the hand drill try to rip off your arm if the 2 x 4 is anchored or restrained? The 2 x 4 represents the axle, and your hand holding the drill is the chassis. You may need to try this to grasp the concept if the thought experiment isn't intuitive.
Actually the engine is attached to the chassis, and the rear axle isn't except thru the springs and anti-roll bars when considering rotation. No magic here. Reaction is equal and opposite to action.
Last edited by Old SStroker; 12-09-2005 at 07:40 PM.
12-09-2005, 04:25 PM
#22
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Originally Posted by Adnectere
Good replies again. OldSStroker, that's very insightful about the hand-cranking. I like that explanation, it makes good sense.
Maybe you're (Treyz28) right about the sum of the forces. I was thinking of two individual forces:#1 the torque on the engine mounts and the chassis lifting up front and then #2 the driveshaft's torque on the axle in the rear. Maybe they're both just part of the same sum. That makes pretty good sense when I think about it holistically.
Something in my brain isn't clicking though. -Lashes the gerbils to run faster- If the chassis up front responding is because of the engine mounts being torqued, then that opposite/equal/reaction should finish there. There isn't torque through the engine mounts being applied to the axle.
The transmission mounts have no torque because of the driveshaft? It doesn't move? Or is it just stuck via the flywheel to the motor so tight that they move together and the force goes through the engine mounts (but not the tranny mounts?)?
Maybe you're (Treyz28) right about the sum of the forces. I was thinking of two individual forces:#1 the torque on the engine mounts and the chassis lifting up front and then #2 the driveshaft's torque on the axle in the rear. Maybe they're both just part of the same sum. That makes pretty good sense when I think about it holistically.
Something in my brain isn't clicking though. -Lashes the gerbils to run faster- If the chassis up front responding is because of the engine mounts being torqued, then that opposite/equal/reaction should finish there. There isn't torque through the engine mounts being applied to the axle.
The transmission mounts have no torque because of the driveshaft? It doesn't move? Or is it just stuck via the flywheel to the motor so tight that they move together and the force goes through the engine mounts (but not the tranny mounts?)?
you are
12-09-2005, 06:32 PM
#23
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Ok, that drill example made great sense. I'm good with how the chassis responds up front with the lifting and also how the axle does and why. Is this the same force that acts on each? The motor puts force into the chassis through the motor mounts and twists the chassis, is this the same force that affects the axle? Or are they two separate forces? I'm inclined to think they're separate. The weight and force of the motor spinning will be put out as a force to flywheel and mounts; the force of the spinning driveshaft will be put out on the axle. So are these two separate forces? Is this why if the tranny is in neutral and you rev it, only the front is affected and not the rear? (Obviously the two are separated and driveshaft is not spinning, but if the same force that torques the motor mounts moves the axle, then it would do it if the tranny is neutral - but is the neutral example proof that the two are separate?) Is the tranny affected through its mounts? If the engine/driveshaft put out two separate forces then the transmission must be affected, at least by being attached on one end to the driveshaft.
12-09-2005, 07:48 PM
#24
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Originally Posted by Adnectere
Ok, that drill example made great sense. I'm good with how the chassis responds up front with the lifting and also how the axle does and why. Is this the same force that acts on each? The motor puts force into the chassis through the motor mounts and twists the chassis, is this the same force that affects the axle? Or are they two separate forces? I'm inclined to think they're separate. The weight and force of the motor spinning will be put out as a force to flywheel and mounts; the force of the spinning driveshaft will be put out on the axle. So are these two separate forces? Is this why if the tranny is in neutral and you rev it, only the front is affected and not the rear? (Obviously the two are separated and driveshaft is not spinning, but if the same force that torques the motor mounts moves the axle, then it would do it if the tranny is neutral - but is the neutral example proof that the two are separate?) Is the tranny affected through its mounts? If the engine/driveshaft put out two separate forces then the transmission must be affected, at least by being attached on one end to the driveshaft.
Clear as mud?
12-09-2005, 11:01 PM
#25
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Ok got it. The drill just torques the other way on its own. And the motor is doing this torquing on its mounts. The axle is a result of the driveshaft rotation; so the two are separate forces. Sounds good, thanks a bunch Old SStroker!
12-10-2005, 11:21 AM
#26
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Now that all is clear, let me muddy it up again! So far, we've been talking about a live rear axle. Let's now consider an independent one, like a Corvette. The center section is mounted to the frame, so any reaction torque applied to the frame through the motor mounts is counteracted by an equal torque through the center section mounts.
So what happens when we load the engine up against the torque converter? Answer: Nothing. The car just sits there. (maybe twists it's frame a little bit)
How about the fact that there's a gear reduction between the crank and the drive shaft? (The axle ratio doesn't matter at this point, because the torque applied to the axle is resisted by the brakes and is not part of the picture.) The transmission gear reduction times the output torque from the converter is the torque acting on the frame both in a CCW direction (seen from the front) through the motor mounts and in a CW direction through the axle center section mounts.
PS: This also tells us that it is in fact the same torque acting on the motor mounts and the live or otherwise rear axle. When you wing it in neutral however, engine torque alone is reacting momentarily against the inertia of the crank/flywheel/converter, so the reaction transmitted through the motor mounts is unopposed, causing the left side of the car to rise.
So what happens when we load the engine up against the torque converter? Answer: Nothing. The car just sits there. (maybe twists it's frame a little bit)
How about the fact that there's a gear reduction between the crank and the drive shaft? (The axle ratio doesn't matter at this point, because the torque applied to the axle is resisted by the brakes and is not part of the picture.) The transmission gear reduction times the output torque from the converter is the torque acting on the frame both in a CCW direction (seen from the front) through the motor mounts and in a CW direction through the axle center section mounts.
PS: This also tells us that it is in fact the same torque acting on the motor mounts and the live or otherwise rear axle. When you wing it in neutral however, engine torque alone is reacting momentarily against the inertia of the crank/flywheel/converter, so the reaction transmitted through the motor mounts is unopposed, causing the left side of the car to rise.
12-10-2005, 02:43 PM
#27
In the drill analogy, it is the magnetic force that transmits the equal and opposite torque between the rotating and stationary parts of the electric motor. Is it safe to say that electromagnetic force in the drill analogy is like the force between the pistons and the cylinder walls that is equal and opposite to the side component of the force on the angled rod on the power stroke?
12-10-2005, 02:55 PM
#28
Originally Posted by MadBill
How about the fact that there's a gear reduction between the crank and the drive shaft? (The axle ratio doesn't matter at this point, because the torque applied to the axle is resisted by the brakes and is not part of the picture.) The transmission gear reduction times the output torque from the converter is the torque acting on the frame both in a CCW direction (seen from the front) through the motor mounts and in a CW direction through the axle center section mounts.
12-11-2005, 05:16 PM
#30
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It's a tough experiment to control, and the container you are draining needs to be very large and very accurately made
I wont spoil it for those who haven't; you can almost get away with a similar
experiment in the kitchen sink.
Fill the sink fairly high, and then let the water settle for a good 30-45 minutes.
Pull the plug and notice the drain of the water (it would be better to pull
the plug from beneath the sink (IE: just below the drain hole) to avoid any
disruption of flow.
Sorry, back on topic now (maybe we can take this to PM?)...
12-12-2005, 05:51 AM
#31
Most outboard boat motors(not all) and diesels being 2 stroke can run in either direction as they are piston ported (no camshaft). The piston makes power everytime it comes to TDC which is why 2 strokes rev so much quicker than 4 strokes. If you want a 4 stroke to run in the opposite direction your going to have to design a cam with the lobes flip-flopped with each other and install a starter motor that spins the wrong way. I used to have to 2 stroke street bike that I installed a high performance ignition on. Sometimes when the idle speed would drop way down at a red lite I would notice that the tach RPM value would drop to ZERO, but the engine would still be running. When this happens you don't what to let the clutch out or you'll find yourself sitting on the hood of the car behind you. The spark was so strong that when it would arrive before the piston got to TDC it could actually stop the crank dead in its' track and force it the opposite way. The 2 stroke engine doesn't care and it would run just fine spinning backwards. I had to raise the idle speed up high enough that the spark could no longer stop the piston and shove it back down the cylinder bore before it got to TDC.
