Custom Crankshafts ?

[Mc Juicy Hairy Balls]

had my v12 crank balanced and apparently no need for bob weights malarky as crank is already naturally balanced??
i was in no position to argue...

[Alan M]

Question for the gurus here - what advantage does using the "bobs weights" give you ?

I'm aware of the balance factor for stuff that isn't "naturally balanced" (my uncle taught me all about that when we were rebuilding a single cylinder vintage bike engine and the new piston was considerably heavier than the old one), but for, say, a 4 cylinder crank what do the bobs do ? Assuming you get the weights for the rods / pistons matched, would you not be able to just balance the crank as-is without needing bobweights ? Or are the balancing machines good enough to balance at different places on the crank (ie ensure that although it is balanced, it isn't balanced "unevenly" (if that makes sense) - an example being pushbike pedals, they could be balanced as a whole but obviously each "side" is unbalanced.

Inline even cyl number (4,6,8) cranks don't use bob weights to set them up to balance, but bob weights must be used on V type engines, with a balancing quotient. A single, or parallel twin (Triumph Bonneville where both pistons go up at the same time, one exhausting one on compression) though would be set up with a particular balancing quotient depending mostly on the rpm to be used. An old motorcycle single might be around 58% (I think), where a parallel twin will be more like 85% although Triumph couldn't make up their mind on this either.

On inline even cyl number cranks, the rods and pistons are balanced separately, but on a V8 if a set of aluminium rods were replacing a steel set, then the crank would have to be rebalanced.

[morerevsm3]

Benincas Alfa sports sedan (with Nissan X trail motor destroked to 2 litre) has a custom crankshaft the machined themselves, seems to live happily at 10,000rpm...

[gxxr]

Nice one. What lathe do you have at home?

Not moi. I'd end up with $1000 worth of scrap steel after a short while even if I knew what to do on counterweighting! I seem to remember working on the mill while that thing was generating ker-thunk, ker-thunk and half expecting it to jump out, roll across the floor and attack my ankles. It's just a manual lathe with dro, 400x1000, there's several other cranks that are lying around in various states of finish too.