Crank balance

Onder · Jul 27, 2021

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?

cjandme · Jul 27, 2021

Solidly mounted or in isolastic mounts, as that will make a difference too.

robs ss · Jul 27, 2021

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

jan nelder · Jul 28, 2021

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!

Onder · Jul 28, 2021

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?

Richard Tool · Jul 28, 2021

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?

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 ?

Onder · Jul 28, 2021

Why not indeed...no lathe...:-(
But I agree about where the weight is.

Richard Tool · Jul 28, 2021

Onder said:
Why not indeed...no lathe...:-(
But I agree about where the weight is.

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.

Onder · Jul 28, 2021

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.

outawack · Jul 28, 2021

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.

Onder · Jul 29, 2021

This is a good basic approach to get started:

Crank balance

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