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embee · 24-09-08, 04:08 PM

just wrote a lengthy reply and it vapourised in the ether!!

take 2................

I've been pondering this and come to teh conclusion that there's no reason not to use 100% reciprocating balance Factor (BF), or close to it, on a 90deg V-twin.

On a single or in-line arrangement the recip BF is typically 50% since it works as a shaking force in the transverse plane (at 90deg to the bore axis) so any more and you just trade off "vertical" balance for "horizontal" shaking.

With a V-twin the recip BF from one cyl works in favour of the other cyl when the crank turns 90deg so it could be increased to 100% and not produce the shaking forces. In fact it would seem wrong not to do this.

It may not be exactly 100% since piston acceleration is different at TDC and BDC due to rod rotation and they might well split the difference, but it would be close to it.

If I were you, I'd be happy to simply calculate the drilling size for 17gms and just do it. Providing you're not affecting the overall balance strategy by serious lightening of the crank/rods and just making small changes, then you're not going to put it out by much whatever you do. Take the density of steel as 7.8, and share the mass between 2 drillings, one each web, and the mass factored by whatever the radius of the web is compared to the throw (half the stroke), for example 17gms at 31.3mm is the same balance as 10.6gms at 50mm radius. 17gms would be about 2.18cm3, so you'll be looking at 2 holes say 10mm dia and around 10mm deep once they're out at the web radius, just to check they'll be sensible sizes. Do the sums.

Remember that the reciprocating BF is arbitrary anyway, there's no "right" answer just a good compromise.

24-09-08, 04:08 PM	#6
embee Member Mega Poster Join Date: Jul 2003 Location: Warwickshire Posts: 2,802	Re: Balance crank just wrote a lengthy reply and it vapourised in the ether!! take 2................ I've been pondering this and come to teh conclusion that there's no reason not to use 100% reciprocating balance Factor (BF), or close to it, on a 90deg V-twin. On a single or in-line arrangement the recip BF is typically 50% since it works as a shaking force in the transverse plane (at 90deg to the bore axis) so any more and you just trade off "vertical" balance for "horizontal" shaking. With a V-twin the recip BF from one cyl works in favour of the other cyl when the crank turns 90deg so it could be increased to 100% and not produce the shaking forces. In fact it would seem wrong not to do this. It may not be exactly 100% since piston acceleration is different at TDC and BDC due to rod rotation and they might well split the difference, but it would be close to it. If I were you, I'd be happy to simply calculate the drilling size for 17gms and just do it. Providing you're not affecting the overall balance strategy by serious lightening of the crank/rods and just making small changes, then you're not going to put it out by much whatever you do. Take the density of steel as 7.8, and share the mass between 2 drillings, one each web, and the mass factored by whatever the radius of the web is compared to the throw (half the stroke), for example 17gms at 31.3mm is the same balance as 10.6gms at 50mm radius. 17gms would be about 2.18cm3, so you'll be looking at 2 holes say 10mm dia and around 10mm deep once they're out at the web radius, just to check they'll be sensible sizes. Do the sums. Remember that the reciprocating BF is arbitrary anyway, there's no "right" answer just a good compromise. __________________ "Artificial Intelligence is no match for natural stupidity"