That's the how. The "why" is that every crank has a torsional 1st mode (resonant frequency) somewhere in the operating rpm of the engine and the tuned damper minimizes the angular deflection that happens. The smaller engines I dealt with deflect around 3-6 degrees. Some nitro hemi folks say that the crank deflects around 20 degrees during running which then the cam guy has to account for in the grind.Warp Speed wrote: ↑Mon Nov 04, 2019 6:10 pmTypically with encoders on the crank measuring twist.
By varying o ring design, number and durometer, along with diameter for a given application and rpm.
Cranks are made of stiff pieces (crankpins) and not as stiff pieces, namely the rod pins and pork chops and when put together they will have a spot between the snout and rear flange that is susceptible to twisting. Sometimes the twisting happens near the same time a bending frequency is excited but that's not very often. (A bending mode that lives for any amount of time in a crank is a really bad thing. If you find the thread where the sbf crank broke you can see the beach marks are straight across the fracture which means nearly 100% bending failed it.)
In simple terms, the heavier the balancer and clutch/drivetrain (think inertia), the lower the torsional resonant frequency and vice versa. The "balance" is inserting the resonant frequency where it's only a "pass through" and spends little no time there.