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Applying mechanics to a mechanic

  1. May 20, 2008 #1
    Dear smart people,
    I am not so smart but consider myself above the average mechanic. However i have come across information from highly respeced racing mechanics that i can't make sense of. I have researched this subject for a while and it seems everyone gets it but me
    It involves the rotating assembly(fly wheel, pulley, crankshaft,connecting rods, and pistons) on any piston driven internal compustion engine. They say when you replace the pistons of an engine with a different weight piston you must rebalance the crankshaft by machining it on the counter wieght ends. An example is switching from the low compression pistons in my v-8 to an aftermarket lighter alloy high compression piston designed for high compression. They say the counter weight balance balances the weight of the connecting rod and half of the weight of the piston. They also say the resulting imbalance of not rebalancing the crank would create vibrations that would eat the bearings.
    This is where I have the problem, to me it seems the crank shaft counter weight should only balance half the connecting rod weight and none of the piston weight as none of the piston weight is spinning. Further more it seems to me the force of the weight on the crank from the piston travel would pale incomparison to the forces of compresing the air and fuel, the exploding air fuel ratio, the almost free travle of the piston pushing the spent gasses out, and the suction of sucking new air and fuel in.
    Now like I said. I'm probably not as smart as most of the forum users as i have no education beyond highschool. So if I realy am thinking backwards, take this oprotunity to flame me I will take it for wasting you time on such a large post. I may also cry a little and devolpe an anti-technology personality and insist on making my wife use the out house with me. BUT if i have stumbled on a mass missunderstanding of basic phsyics, the next time you see a lowly grease monkey in conversation with another motorhead. Feel free to but into the conversation and try to expand their conversation beyond the black magic they know as hot rodding into true mechanical enginering aplication.
    Love SSVPV8 :smile:
  2. jcsd
  3. May 21, 2008 #2
    My guess, and it's only a guess.
    But, yes the forces are incredible with pistons, but the mass is relatively small. When you make that mass (the piston mass that is) smaller or larger, the counter weight will not be optimally positioned to get the most "bounce back" force out of the spin it has.

    It also might have something to do with momentum, but I will not try any other shots in the dark.

    I am interested to hear the answer to this question though.
  4. May 21, 2008 #3


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    Well, you have a head-start on me by having finished high-school. I'm a bit of a gearhead myself, though.
    While I've never actually looked into it (and wouldn't understand the math anyhow), my 'gut feeling' about it is that the counterweight is simply to balance the forces acting upon the crank journals. Those forces should vary with different reciprocating mass, but also with the compression ratio and even valve changes. There are a few guys here who know a lot more about it than me, though. Hang tight and they'll show up before long.
  5. May 21, 2008 #4


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    The pistons are definitely part of the rotating assembly, even though they are reciprocating. Their connection to the crank means that both their weight and the forces they create due to their motion all get transmitted to the crankshaft.

    What might be causing a bit of an issue is you may be thinking of things in terms of what is known as a static balance. That is where you would put something on a flat plate or on v-blocks and roll it a bit to see if it wobbles and the heavy side goes to the bottom. That is OK, but the other thing to consider is dynamic balancing. That means you now have, in the case of the crank shaft, very high forces due to large accelerations due to the rotation of the engine. The higher the rpm, the higher the dynamic forces. The other thing to remember is that the imbalance force is not going to be in the same direction as the forces due to combustion. The combustion forces, by design, create torque on the crank. Imbalance forces create a bending or pushing force perpendicular to the shaft axis. That is why it is so detrimental to bearings. The shaft may bow and all of that force has to be taken up by the bearings eventually.
  6. May 21, 2008 #5


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    Perfect post from Fred.

    Ssvpv8, forget about combustion forces for a second, and think about the engine spinning (on its own) at several thousand rpm. Try to imagine the forces (due to the piston's inertia) acting on the gudgeon pin, trying just to hold on to the piston at TDC. You'll realise that these forces are directly transmitted to the crankshaft. Hopefully it becomes clear that changing the mass of the pistons requires due consideration for rotating imbalance.
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