hey guys,
I currently have Lindskog Dynamic Balancing doing a crankshaft for me (cb750), and I was going to have him lighten it 10%. They asked me about the rod and piston weight compared to stock, and how lightening the crank would affect vibration at higher rpm. Before this phone call, i was always under the understanding that on an inline 4 cylinder (2 up, 2 down) crankshaft, bob weight didn't matter so much as long as they balanced each other out. This is true for the primary balance, which can be balanced. Then i started looking into what secondary balance is, and how the counterweights have an effect on it. Also noted is that inline 4's have more secondary imbalance than an engine where the pistons are out of phase with each other.
i would assume different engines are effected differently with this issue, and wondered for anyone that has lightened crankshafts, if they have noticed vibrations at higher rpm's, and if this is something that can be hazardous to the crankshaft. I seem to recall that a lot of the performance cb750's typically used factory crankshaft weight. My primary basis for lightening the crank was to have a motor that rev's a little more freely.
Lindskog recommended I weight the original rod/piston setup and compare it to what I am going to use. I will figure this out over the weekend, and am thinking I will do whatever Linkskog recommends, as they have a lot more experience than what I have.
I always appreciate this forums input and experiences.
Thanks,
Daniel