Balance shaft removal

These engines have a 270 crank. The 270 crank configuration is supposed to be the optimal set up for a twin in terms of limiting the vibration.

So... why do they need a balance shaft...?

If the cranks were properly dynamically balanced, could the balanced shaft be removed?

I have no specific plans to do so, I’m simply hypothesising.

Is 270 optimal? What about 180, the pistons are moving in opposite directions. What about 72 (or is it 74), one piston is changing direction the other is at maximum acceleration. I'm sure there are other configurations to consider.

Ok, it’s close to optimal. The idea being that one piston is at maximum velocity (mid stroke) whilst the other is stopped (at either TDC or BDC) thus cancelling each other out.

180 is definitely not optimal, not in theory or practice. I tried it in practice and there wasn’t much between it and a 360 when both had been dynamically balanced.

Anyway, please let’s open a new thread if anyone wants to debate this ad infinitum. I’d like to keep this one on the balance shaft topic please.

Sorry, wasn't trying to hijack the thread. As for the balance shaft, it adds weight so therefor I am sure Norton feels there is a benefit having it. Is dynamically balancing a crank more expensive than adding balance shaft and all the parts that entails? I have READ of situations where the weight of a balance shaft was more than compensated for reducing weight in the frame because it did not have to endure as much vibration. I'm not an engineer, just throwing out ideas.

Fast Eddie said:

These engines have a 270 crank. The 270 crank configuration is supposed to be the optimal set up for a twin in terms of limiting the vibration.

So... why do they need a balance shaft...?

If the cranks were properly dynamically balanced, could the balanced shaft be removed?

I have no specific plans to do so, I’m simply hypothesising.

Click to expand...

Nigel,

The conservation of crankshaft momentum on the 270 crank design does help reduce vibration, but there are 2 other problems specially for a vertical twin.

First, the distance between bore centers on a vertical twin will produce a rocking couple on a 270 degree engine, or any crankshaft layout other than 360 degrees.
On other 270 degree designs, like Ducati 90 degree V twins, the distance between bore centers is minimal with the rods right next to one another on a since crank pin.
This reduces the rock couple significantly virtually eliminating the effect at normal engine speeds.

The other issue for primary balance is the effect of unbalanced crank flywheel mass.
We all know that the crank flywheel mass balances the reciprocating mass of the piston/wrist pins/rings/rod as they move up and down in the bore.
However, the flywheel mass is not balanced by anything when the flywheel moves forward or backwards as the crank rotates.
This acceleration of the flywheel mass produces a harmonic that pulses forward and backwards in a vertical plane.
Left unchecked the harmonic produces serious vibration. The balancer shaft is designed with lobes that counteract the unbalanced mass of each flywheel.
The balancer shaft rotates in the opposite direction from the crank, and the lobes are timed to exactly (or closely) counteract the mass of the opposing flywheel mass.

Fair points guys.

As you say oldbeezer, the rest of the bike, inc frame and bracketry etc, will likely have been designed around expected levels of vibration WITH a balance shaft.

And as you point out BritTwit, whilst some types of vibration can be dynamically balanced out, some cannot.

I guess my hypothesis is/ was that with the modern trend towards ANY mechanical feel whatsoever needing to be engineered out, perhaps the need for a BS could be avoided in a 961 if other measures were applied.

Probably not something that fits into the ‘just give it a go’ category is though eh?!

An alternative to balancer shafts would be the reciprocating mass balancer like the type designed by BMW (or Rotax depending on who you believe) on the F800 motor.
Very nice design requires a special crankshaft design with a third connecting rod that operates a synchronized reciprocating mass which performs the balancing duties.
This requires a bit more sophisticated engineering.

BritTwit said:

An alternative to balancer shafts would be the reciprocating mass balancer like the type designed by BMW (or Rotax depending on who you believe) on the F800 motor.
Very nice design requires a special crankshaft design with a third connecting rod that operates a synchronized reciprocating mass which performs the balancing duties.
This requires a bit more sophisticated engineering.

Click to expand...

Designed by BMW or Rotax ??

There was a Commando fitted with one of these in the ‘70s...!

Let's not forget the Ducati Supermono of the mid '90s

Fast Eddie said:

Designed by BMW or Rotax ??

There was a Commando fitted with one of these in the ‘70s...!

Click to expand...

If I recall it was based on a 750 Commando.
I used to have a pdf file of the magazine article with photos of the bike.
The crankcases were 'modified' to allow for a small cylinder to protrude out from the motor under the cam area between the cylinders.
There was a hollow piston that moved fore and aft in that bore, driven by a rod connected to the center of a special one piece crank.
The piston's movement canceled the mass of flywheels and thus provided a very smooth 360 degree twin.

I'll see if I can find the file with the article.

BritTwit said:

If I recall it was based on a 750 Commando.
I used to have a pdf file of the magazine article with photos of the bike.
The crankcases were 'modified' to allow for a small cylinder to protrude out from the motor under the cam area between the cylinders.
There was a hollow piston that moved fore and aft in that bore, driven by a rod connected to the center of a special one piece crank.
The piston's movement canceled the mass of flywheels and thus provided a very smooth 360 degree twin.

I'll see if I can find the file with the article.

Click to expand...

Like so many issues that I can’t quite resolve to my satisfaction, the story of the modified 750 commando with a third “balancer” piston kept bouncing about inside my skull and would not go away.
Today doing another search I came upon the Andover Facebook page and - Viola there he was, Jake the 3 Cylinder Twin with the protruding chin. Originally developed by Doug Hele, Classic Cycle did a road test article on it in 2014.
It was supposed to be the smoothest Commando ever.
Well at least to Classic Cycle anyway.

Very cool concept, with unique aesthetics.

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If my bike was that smooth, I probably wouldn't want it. That twin rumble is a major part of the attraction.