1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mechanical leverage question

  1. Jan 7, 2009 #1
    Hi all, first post here. Need a bit of a hand with an issue of force. Please refer to the diagram below:


    Basically, the drawing describes the angle of two connecting rods relative to the crankshaft at 90deg. ATDC. Lines A and B describe rods of different lengths measured center-to-center, with circle of motion describing stroke.

    I'd like to ask, which rod(angle) would do a better job of pushing the crank journal down?

    Thanks in advance! :smile:
  2. jcsd
  3. Jan 7, 2009 #2


    User Avatar
    Science Advisor

    Torque on the axis depends on its perpendicular distance to the line of action of the force and the force magintude. Rod A has a greater distance and if the force is applied vertically to the top of the rods, rod A will also have a greater force acting along its axis, as it has a smaller angle to the vertical.
    Last edited: Jan 7, 2009
  4. Jan 7, 2009 #3
    If you are only concerned with the static condition, then A.T. has given you a good answer above. If, however, you are concerned with a dynamic situation, such as in an engine or compressor, the situation is not quite so black and white. The longer connecting rod will have more dynamic interaction due to its own mass and mass moment of inertia, so that there are some advantages to the shorter rod. There is also the "packaging" advantage of the shorter rod, simply that the machine does not have to be as large.
  5. Jan 8, 2009 #4
    Rod mass won't be much of an issue, as we have found a contractor who will be able to give us a much lighter set than standard. All we really need to do is check the crank case- and cradle-to rod tolerances to see how far we can go with the machine work. What we needed to know was if the longer rod had a mechanical advantage over the short; by empirical evidence we already knew that the shorter rods most manufacturers use in their stroker motors have a tendency to wear the bores, and also where in the rev range the car gets driven most of the time, which is why we decided on this approach.


    Would you be able to explain more about these "dynamic interactions" in layman's terms? I am not a physicist, I'm a tradesman and Industrial Designer LOL.

    So, as for the first question, to sum it up in layman's speak, given equal cylinder pressures for both rods, Rod (A) will exert more force on the crankshaft pin due to it being more perpendicular, if said pressure is exerted upon the top of the rod in a vertical motion(ie. down the bore) :confused:
    Last edited: Jan 8, 2009
  6. Jan 8, 2009 #5


    User Avatar
    Science Advisor

    Yes, (A) will have more force and greater lever arm respective to the axis. This results in greater torque. But for the small difference you pictured it should not be significant.
  7. Jan 8, 2009 #6
    Thanks so much for that, A.T! We're trying to eke every last iota out of this motor, so every little bit will help. We have not decided on final specifications as yet, but these are the proportions we have come up with from the initial measuring sessions; what we have now and what we envision.
  8. Jan 8, 2009 #7


    User Avatar
    Science Advisor

    I'm no expert on motors. But greater motor torque doesn't automatically mean more power. It all depends on further translations, by gears, wheel size etc.
  9. Jan 8, 2009 #8
    Non-Academic, you have asked a very hard question when you ask for a simple explanation of the dynamic interactions of the connecting rod. Let me give you a very brief description.

    The top end of the rod travels in a straight line while the lower end travels in a circle. The center of mass of the rod travels in something like an ellipse. The rod also has an oscillatory rotation. If all of this took place very slowly it would not matter, but if it takes place with any speed at all, there are significant accelerations involved (even if the crank speed is constant) and consequently there are large dynamic forces required to move the connecting rod in its prescribed motion.

    You say that you are trying to "eke every last iota out of this motor" and I can assure you that you will not do this without properly understanding the engine dynamics.

    It is interesting that you should raise this problem at this time. I am currently writing a book on the dynamics of engines and compressors, and my project for this morning is to formulate the computer code for the calculation of the loads in a V-twin crank throw. This entails analyzing all the motions of the various parts and then from that motion description determining what the rquired forces are.
  10. Jan 8, 2009 #9

    This is not a problem; we have the data from the first car that we built including gearing, tyre selection and wheel dimensions. That one ran on, per the diagram, Rod B. and revved successfully to 9,300rpm soft cut, 10,000rpm hard cut(rev limiter) over the course of three seasons.

    Would you be able to recommend some readings or references on the subject? I would also be interested in your publication once its done. What you describe are things not taught in trade school, and that are usually not considered in grassroots to mid-level motorsport(where we somehow ended up)
  11. Jan 8, 2009 #10
    I suggest that you might want to look at Engineering Dynamics by Biezno and Grammel.
  12. Jan 9, 2009 #11
    Thanks for that Dr.D, will check it out. And thanks to everyone else who replied for the help!
  13. Jan 13, 2009 #12
    Both the rods have the same lever arm (perpendicular distance between the crank center and the point of application of force by the con rod on the crank pin). And the pressure is exerted on the flat faced piston. Hence, if the surface area of the piston at the end of both the rods are same, then the force exerted would also be the same. What difference would it make actually?
  14. Jan 13, 2009 #13


    User Avatar
    Science Advisor

    The lever arm is the perpendicular distance between the axis and the line of action of the force. The lever arms are not the same.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook