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Crankshaft counter weight design
Posted: 01 Dec 2024, 23:55
by Rage
I have a question that I have wondered about for a long time.
Why does not counter weight have the same design shape on every singel counter weight?
Aftermarket crankshaft have often same shape on every counter weight, but stock crankshafts does not.
Why?
See pictures
https://dmeracing.com/product/hayabusa- ... placement/
Re: Crankshaft counter weight design
Posted: 09 Jan 2025, 13:13
by Tommy Cookers
Rage wrote: ↑01 Dec 2024, 23:55
... Why does not counter weight have the same design shape on every singel counter weight?
Aftermarket crankshaft have often same shape on every counter weight, but stock crankshafts does not.
Why?
stock crankshafts will need weight checking&adjustment and have 1 (or 1 main) counterweight per cylinder
aftermarket ones are 'billet' so machined all over & have 2 weights per cylinder with little or no adjustment needed
car crankshafts have power takeoff at the end ie the Hayabusa may not be the best example
Re: Crankshaft counter weight design
Posted: 10 Jan 2025, 01:02
by Greg Locock
Are you saying aftermarket cranks aren't 2 plane balanced after machining? Wow. Anyway the one in the picture has as-cast counterweights and will certainly need balancing. I think at the top left you can see a hole in the web for balancing.
Having been part of the team that designed and put into production the best ever crankshaft for the Falcon I6 - the 12 counterweight crank - (it lasted 2 years before being cost reduced out), I'll explain a bit about designing counterweights.
First of all strictly speaking you don't need them. BUT your bearing loads would be very high and the crank would rip itself apart. So you do need them after all. In the olden days there were various manual methods for calculating them, these days we refine it by running an FEA model of the crank and block and look at the bearing loads, which partly explains why an OEM crank has different counterweight shapes than aftermarket ones.
The OEM crank will be designed for life, refinement and so on, whereas an aftermarket part might be designed to be lighter, or have less rotational inertia, and compromise a bit on life and refinement. It may well be designed for higher rpm, in which case the counterweight strategy will be different again.
Here's a thorough article on balancing
https://www.gallowayengines.com.au/arti ... -balancing