Crank balance

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Onder

Onder

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Guidance here please: suppose your crank is balanced to a certain % but you change out the pistons to ones that are heavier or lighter where does
this leave you in regard to moving the sweet spot in the rev range?
 
Solidly mounted or in isolastic mounts, as that will make a difference too.
 
If your combined piston mass is heavier it will lower the "factor" that the crank is balanced to - and vice-versa of course!
The quantum depends on the difference in mass.
Not necessarily a bad thing - depends on what your balance is now.
Cheers
 
I have never seen a table of balance factors versus least vibration at X revs. Only individual experiences.
However, with Isolastics, it is less critical, but still improved with getting the balance factor to original spec ( check dry or wet !) and dynamic balancing also slight improvement for Commandos, and then getting proper spec Iso rubbers etc. ( this is mainly from reading many, many posts over th e last 5 years!
 
We are dealing with solid mounted engine. New rods are heavier than the old ones and I want to know where I will
be if I don't rebalance . How much does it matter which end of the rod is heavier?
 
Onder said:
We are dealing with solid mounted engine. New rods are heavier than the old ones and I want to know where I will
be if I don't rebalance . How much does it matter which end of the rod is heavier?
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I believe a change in mass at the small end will have a greater effect than vice versa as the small end is reciprocating while the big end is rotating . Why not static balance it to original spec ( or at least check it staticky ?
 
Why not indeed...no lathe...:-(
But I agree about where the weight is.
 
Onder said:
Why not indeed...no lathe...:-(
But I agree about where the weight is.
Click to expand...
You don’t need a lathe to static balance - just a simple pair of parallel and dead level straight edges - the sharper the better to minimize friction.
 
Yes they are in some old manuals but then I'm without lots of stuff since I moved...no surface plate, no pillar drill, no lathe, not even a welder anymore. Price of upping-stakes I suppose.
 
The higher your your small end weight is the more stress you will put on the engine due to reciprocating mass, and vibration may increase. see JS motorsport website for more detail on balance factors ETC. Or maybe Jim will see this and reply. My 750 commando has a balance factor of 64% using Maney crank, and Jim,s rods & pistons.
 
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