5+ Year Contributor
- Feb 28, 2003
Engine torque has nothing to do with when the crank will reach it's natural frequency. This will be solely a function of RPM. The power applied to the crank will affect the intensity (or amplitude) of the effect. Lower output engines have less to worry about, because while still passing through RPM ranges which contribute to this, they produce less power thus keeping magnification to a minimum. Any RPM/2 point outside of these ranges will work against the cranks natural frequency.
Right, engine torque output isn't important. The torque spikes caused by each power cycle are greater than the mean torque output of the engine. They are the force the crank sees.
We don't exactly disagree, we just have two different points.
Another, better, way to say it: it cannot be made not to jumprope. There's more force being applied to the center than each end; about twice as much. Thus one reason you see why the amplitude of the 4cylinder vibration is so high. The other being the frequency of power strokes for the total length of the crank.
I'm not quite sure what you mean here. The center of the crank will have a seperate mode of vibration that will cause a higher frequency. Is that what you mean?
dsm-onster said:Also concerning what I put in bold. We're talking twisting. "twisting" is not bending of the crank arms. It is twisting the crank due to the forces exerted on the crank arms. Looking at it like this helps one appreciate how "easily" a crank twists. Yes, the leverage is greater with longer strokes. But there is still plenty of leverage there. Enough for a 4g61 to have been given a damper from the factory by the mitsu engineers.
By twisting, do you mean rotating? I'm not really sure how this ties to the sentence you put in bold for me unless you are talking about rigid body motion as mentioned in the article posted by donniekak. I'm not sure that's what you mean, though, since you talk about deflection later in your post.
All in all, we're simply arguing some of the finer points and seem to be in agreement for the most part. Use a damper/absorber/harmonic balancer. In the words of dsm-onster, "If it doesn't hurt and could very possibly help. . ."
Something else I noticed in the article above that I thought was interesting is that the author wrote that the tolerances specified for an OEM damper have a range of around 16%. That's quite a bit of leeway. No wonder the EVO guys who have stroked engines in the thread posted above see good results from even their stock damper. It appears that even the stock device has more range than I gave them credit for having. It should technically wear faster attached to an engine that creates vibrations at the limits of its range, though... not that we know what those limits are.