Joined: Aug 2017 Posts: 2,748 Threads: 31
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Location: Auckland, NZ
Hi Alan, (but worded for others)
I guess it is questionable whether crank wobbling in flywheel or vice versa is the factor!
Unless especially requested and some precision mandrel provided, the crank and flywheel usually balanced together, in which case any dynamic balance within flywheel not likely to be corrected at source.
A colleague recently broke irreplaceable 2 bearing crank in a 4 cyl jowett 10, not long after supposed crack test! With hindsight, excess flywheel runout was suspected...
Joined: Aug 2017 Posts: 1,341 Threads: 34
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Location: Cheshire
Car type: Race Ulster, 1926 Special, 1927 Chummy, 1930 Box
Yes that’s how mine are done. But a lot of people assume the crank is in balance as it comes - they rarely are. Then they static balance the flywheel assembly. That isn’t enough. You have to eliminate the couple imbalance because I think it’s that that causes cranks to break.
Alan Fairless
Joined: Aug 2017 Posts: 3,417 Threads: 107
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Location: Darkest Bedfordshire
Normal practice these days to balance crankshaft and flywheel individually so any flywheel can go on any crank. The precision mandrel is a problem which might be overcome by first balancing the crank then once again with flywheel fitted (but strict instructions not to touch the crank).
Joined: Mar 2015 Posts: 5,466 Threads: 231
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Location: Scotchland
Have any of you dynamically balanced a crank and flywheel separately and then put them together? The results may surprise you, as they did me.
Joined: Aug 2017 Posts: 988 Threads: 6
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Location: Scottish Borders
How the crank taper fits the flywheel will have a big effect. Might not be perfectly square of perfectly central.
Jim
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Location: Scotchland
Precisely my experience Nick - I suspect for the very reasons AustinWood suggests.
Joined: Aug 2017 Posts: 3,417 Threads: 107
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Location: Darkest Bedfordshire
That's interesting. I'm not even going to try & guess why, dynamic balance is a complex thing.
The 'modern' way is doubtless based on the relatively fine tolerances / consistency of modern (and new) components, and the imperative to minimise cost of production and maintenance; might not work so well when applying to 1930's parts or copies thereof.
