Tuning Benefits of Bis Crank

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Tuning Benefits of Bis Crank

I'm still trying to learn more about balancing before I make any more decisions about it, but finding information specific to a twin cylinder is proving tricky!

Unfortunately this is one aspect of tuning that I am intrigued about and your postings have drawn me into wasting many hours scouring the internet in the same "vein ". :D

I found an extended German discussion about the BIS crank and the relative balance weights and ratios for vertical and horizontal engines. It does seem that the heavier crank in combination with significant lightening holes in the flywheel is necessary in the BIS, mainly in response to the lack of gravity assistance on the downward stroke of the piston.

If I read it correctly they were saying that the vertical engine responds better to having a proportionally lower percentage of the piston and conrod mass built into the crankshaft counterweight. If the BIS crankshaft counterweight is heavier than the standard version, this implies that it would need significant machining before being safe to use in a standard-spec engine or in one with lightweight internal components.

It is only fair to say, (as did one of the German commentators), that my thoughts are based only on cramming myself with online information gleaned over the past few days. :)
 
It does seem that the heavier crank in combination with significant lightening holes in the flywheel is necessary in the BIS, mainly in response to the lack of gravity assistance on the downward stroke of the piston.

If I read it correctly they were saying that the vertical engine responds better to having a proportionally lower percentage of the piston and conrod mass built into the crankshaft counterweight. If the BIS crankshaft counterweight is heavier than the standard version, this implies that it would need significant machining before being safe to use in a standard-spec engine or in one with lightweight internal components.
. :)


I didn't realise the Bis Flywheel had more weight removal that an Aircooled, although I've never seen one so I had nothing to base it on! That's an interesting point about the bis crank needing weight removal to match the aircooled engine. Toshi mention in an earlier post that there is 400g difference. I assumed that the extra mass of the larger pistons was the main reason for this and that if you were to fit oversized pistons that were heavier, then the bis crank would be a better place to start because it is much cheaper to remove crank weight during balancing. To add weight they use a tungsten alloy which costs around £100 per slug.


Now that I have a few engines in pieces I will weigh a 500 and 126 crank. The 126 pistons are bigger, but there is much more material on the 500 pistons. My theory is that the 500 & 126 pistons weigh the same and as a result the 500 & 126 cranks also weigh the same.
 
I didn't realise the Bis Flywheel had more weight removal that an Aircooled, although I've never seen one so I had nothing to base it on! That's an interesting point about the bis crank needing weight removal to match the aircooled engine. Toshi mention in an earlier post that there is 400g difference. I assumed that the extra mass of the larger pistons was the main reason for this and that if you were to fit oversized pistons that were heavier, then the bis crank would be a better place to start because it is much cheaper to remove crank weight during balancing. To add weight they use a tungsten alloy which costs around £100 per slug.


Now that I have a few engines in pieces I will weigh a 500 and 126 crank. The 126 pistons are bigger, but there is much more material on the 500 pistons. My theory is that the 500 & 126 pistons weigh the same and as a result the 500 & 126 cranks also weigh the same.
You are probably right about the piston weights being the same. But even if they're not, the science of this seems to involve a lot of guesswork and approximation.
But I think the BIS flywheel is heavier than the air-cooled but appears to have some asymmetrical lightening on the periphery. If so I suspect some very complex interaction with the horizontal thrusts, which we don't need to worry about
 
You are probably right about the piston weights being the same. But even if they're not, the science of this seems to involve a lot of guesswork and approximation.
But I think the BIS flywheel is heavier than the air-cooled but appears to have some asymmetrical lightening on the periphery. If so I suspect some very complex interaction with the horizontal thrusts, which we don't need to worry about

Hi Peter, I note your comment "seems to involve a lot of guesswork and approximation" and that is precisely why I have entrusted my mish-mash of parts to the professionals, even though I am using standard BIS pistons the rods are much lighter, hence the BIS crank had some extensive work (lightening) done to it to get everything running smoothly. I agree that it is all too easy to assemble an engine without getting it right and after a visit to my engineer chap I have found that even the standard crankshaft end float is regarded as too tight for a modified engine and will need to be adjusted and that's from an engineer who is in his 70's and has a lifetime of experience ?????
Ian.
 
Hi Peter, I note your comment "seems to involve a lot of guesswork and approximation" and that is precisely why I have entrusted my mish-mash of parts to the professionals, even though I am using standard BIS pistons the rods are much lighter, hence the BIS crank had some extensive work (lightening) done to it to get everything running smoothly. I agree that it is all too easy to assemble an engine without getting it right and after a visit to my engineer chap I have found that even the standard crankshaft end float is regarded as too tight for a modified engine and will need to be adjusted and that's from an engineer who is in his 70's and has a lifetime of experience ?????
Ian.
That's the sort of guy I would trust as I would also trust the reasoning behind such specifications. I didn't mean to suggest that anyone in particular is guessing about balancing, but a certain amount of the "science" itself does rely on approximately formulae.[emoji3]
 
I accept that I will probably never get to understand to the science re balencing of a 500/126 engine and especially why a Giardiniera / Bis engine requires the quite large Bob weights on the flywheel and pulley. Complete "brexitology" to me and I am sure it can't be because of gravity as has been suggested just because the cylinders are horizontal rather than vertical. Might be relevant with a beam engine but not one that will run at 6,000rpm.
So every time this emerges it nudges a brain cell. I can understand the idea of balencing the crankshaft flywheel etc but where do the piston and con rod come in as they do not rotate. Trying to understand this I thought of a cannon firing. Assuming the charge in the cannon was the same each firing then the ball would carry the same force. If the cannon ball varied in weight then the force would vary. If the piston & rod were not the same weight with equal forces from combustion the force at the crankshaft would be different so would that mean some sort of unbalance ?
 
Balancing is done without taking into account the force created during combustion. Imagine the engine was rotated with a large motor attached to the flywheel, plugs removed. As the pistons reach TDC their own mass is trying to pull them out of the top of the engine. The crank is exerting a force in the opposite direction to try to pull the crank out of the bottom of the engine. The combination of upwards and downwards force should balance out in an ideal world. If the mass of the pistons or con-rods increases, so does the upward force. The crank needs to match this additional force to remain balanced.
In reality the issue with our engines is when the crank is at 90 and 270 degrees. The crank is trying to throw the mass to the left or right when there is no piston movement to balance this.


This is worth a read as it's one of the few pieces I have read that tries to explain why the mass of the con-rods should be treated as a reciprocating mass rather than an end to end mass. It's heavy going though!


http://victorylibrary.com/mopar/crank-bal-c.htm


I have yet to find a descent publication that covers balancing of a two cylinder engine. A friend said I was trying to fond a general book about a specific subject! I think he has a point there!
 
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