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What is the effect on conrod forces?

  1. May 1, 2015 #1
    Quite a number of years ago I was speaking to a friend (Greg) who did engine machining work for some semi professional race teams and individuals. I was questioning him on how best to balance conrods. ( From what I had gathered off the internet from a multiple of different sites, and after learning how the different harmonics influence 'engine balance' I learned to balance the rods end for end. After multiple different trial and error attempts at balancing ends of the rods I had, I was struck by something which has stuck with me till today. Its the effect of weight located passed the the centre point of the eye or big end of either end of the rod in question.
    If I had a rod on the scales measuring the weight of the little end being supported at the 'eyes' centre, if I added exactly 1 gram to the very farthest point on the little end, the effect of that 1 gram 'should' reduce the big end weight by a very small amount, and vice versa.)

    So the question I posed to Greg was do you balance the big end caps separately first, then do the rest of the rods? He said "No, it makes no difference."

    I can't help but think that it must make a difference, even if it is minute. Mind you, he was balancing his 'engines' to within 1 gram end to end, and the setup I had was repeatable to less than a 1/2 of that. So possibly the results were not showing up in his measurements.
    So, my question is, even if a rod is balanced end for end exactly, does it make a difference to anything dynamically (other than stress inside the rod) that the mass is not evenly distributed? If the end weights are the same, the centre of gravity would be in the same spot, so in theory the crank wouldn't notice any difference, or am I missing something?

    Thanks guys.

  2. jcsd
  3. May 1, 2015 #2
    BTW, for a little reference, I had been using a set of paint mixing 'teetering scales' which made the process akward but doable. I have just purchased a set of 2kg with scale down to 0.01g which I would like to get use out of, and I hope to be able to do better than just roughly balance them.

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