Post by GrumpyUnk on Mar 28, 2020 9:34:57 GMT -5
To add just a smidge to what Thai has stated, all engines have 'harmonics', not harmonicas, but multiple rpm values where vibration of specific pieces occurs and is MUCH more measurable/able to be felt than at other rpms.
The vibra-tach likely shows those rpm ranges where that occurs.
Just as a 2T exhaust is tuned for a specific rpm range, the balance of a reciprocating engine(piston) will be tuned for vibrations in multiple directions, with compromise being always there to hold your hand.
The pistons go up and down, the crankshaft spins, and the counter weights that try to balance against the piston up and down have a side-to-side motion all their own. You get at least two sets of vibrations going on at one time. The 'felt' vibration (bag tingle) will come and go based on which set of vibration is the most prominent at that rpm.
The angle that the piston moves on, horizontal, vertical, or some where in between makes a difference in how things are felt. Fore and aft- horizontal- will minimize piston up and down as the piston is not moving thataway. You will feel the crankshaft counterweight vibration in the up and down axis as that is 'unbalanced' weight in that range of motion. The crank weights counter piston motion that is not present, so you get/feel their contribution to vibration more. Change the angle of the piston to vertical, and the counter weight vibration goes fore and aft... you won't feel it except likely through the handlebars(maybe?).
It is all a compromise. Some engines are naturally balanced, I think, or at least a lot more than others. A flat four, opposed, such as a VW, balances out piston motion by having another piston going the opposite direction at the same time. It will have crankshaft vibration, and others, which I don't know how to explain.
A straight six, such as the Chevy, Ford or ChryCo slant six, are naturally balanced beyond what I can explain,l as is a 90 degree V8. Read up if interested. You may be able to find the SAE proceedings online somewhere. As a kid, I read some from the 1950's that were bound that overwhelmed me with tech terms, but I still found them interesting. Back then, making hydraulic lifters work on 'modern V8 engines' was a difficult task as the lubes were in shear, and their molecules got mangled by the cam:lifter interaction. Packard and Cadillac, Oldsmobile (and Studebaker) all had interesting papers.
tom
The vibra-tach likely shows those rpm ranges where that occurs.
Just as a 2T exhaust is tuned for a specific rpm range, the balance of a reciprocating engine(piston) will be tuned for vibrations in multiple directions, with compromise being always there to hold your hand.
The pistons go up and down, the crankshaft spins, and the counter weights that try to balance against the piston up and down have a side-to-side motion all their own. You get at least two sets of vibrations going on at one time. The 'felt' vibration (bag tingle) will come and go based on which set of vibration is the most prominent at that rpm.
The angle that the piston moves on, horizontal, vertical, or some where in between makes a difference in how things are felt. Fore and aft- horizontal- will minimize piston up and down as the piston is not moving thataway. You will feel the crankshaft counterweight vibration in the up and down axis as that is 'unbalanced' weight in that range of motion. The crank weights counter piston motion that is not present, so you get/feel their contribution to vibration more. Change the angle of the piston to vertical, and the counter weight vibration goes fore and aft... you won't feel it except likely through the handlebars(maybe?).
It is all a compromise. Some engines are naturally balanced, I think, or at least a lot more than others. A flat four, opposed, such as a VW, balances out piston motion by having another piston going the opposite direction at the same time. It will have crankshaft vibration, and others, which I don't know how to explain.
A straight six, such as the Chevy, Ford or ChryCo slant six, are naturally balanced beyond what I can explain,l as is a 90 degree V8. Read up if interested. You may be able to find the SAE proceedings online somewhere. As a kid, I read some from the 1950's that were bound that overwhelmed me with tech terms, but I still found them interesting. Back then, making hydraulic lifters work on 'modern V8 engines' was a difficult task as the lubes were in shear, and their molecules got mangled by the cam:lifter interaction. Packard and Cadillac, Oldsmobile (and Studebaker) all had interesting papers.
tom