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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Crankshaft Balance Method
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<blockquote data-quote="greg brillus" data-source="post: 115486" data-attributes="member: 597"><p>To arrive at any actual balance factor is completely Do-able........however, "the" actual balance factor is a myth subjective to the actual engine and how it is mounted in a frame (if any) I'm pretty sure the factory spec for road twin cranks was 47% or thereabouts........perhaps as high as 50%...........How the engine is mounted is quite important to the whole idea........for example, I can guarantee anyone that if you mounted a Comet engine with the cylinder vertically, the engine would vibrate differently to normal. This is why a Vincent twin mounted in a full loop frame (Featherbed) needs a different balance factor to a stock Vincent. I had a new crank assembly for an "A" twin that was supposed to be set at 60% (it was actually 61%) I lowered it to near 55% by drilling 4 x 3/8" diameter holes opposite the big end, two of in each flywheel. These wheels were made from a much higher grade of steel than the originals which were basically "Boiler plate" and much softer. This has resulted in a much smoother engine in the lower rev range..........with that said, the same (or very similar) engine and crank combination mounted in a post war frame, that is, conventional UFM and RFM as we know, and again that engine is not to bad, this with the 60% crank balance factor .............I know you are all thinking, what the hell am i up to.........Anyways.......In the example of this discussion, the pistons are quite light verses the originals, generally around 450/460 grams....... I would start by removing some material from the top three holes as suggested by Bill, and then look at adding some tungsten or mallory metals opposite the big end. You would be surprised at how far out the balance factor can be, and the engine still be quite smooth........The flywheels on my twin racer, picture on the left, were actually out by 520 grams...........The engine was quite smooth, but it did vibrate quite badly at low engine speed. I did not set it up myself, but I did check it when the engine was rebuilt a couple of years back........I nearly fell off my chair when I found this out........The flywheels had to have four holes bored through the wheels just opposite the big end near one inch diameter each.........that is a lot of metal to remove.</p></blockquote><p></p>
[QUOTE="greg brillus, post: 115486, member: 597"] To arrive at any actual balance factor is completely Do-able........however, "the" actual balance factor is a myth subjective to the actual engine and how it is mounted in a frame (if any) I'm pretty sure the factory spec for road twin cranks was 47% or thereabouts........perhaps as high as 50%...........How the engine is mounted is quite important to the whole idea........for example, I can guarantee anyone that if you mounted a Comet engine with the cylinder vertically, the engine would vibrate differently to normal. This is why a Vincent twin mounted in a full loop frame (Featherbed) needs a different balance factor to a stock Vincent. I had a new crank assembly for an "A" twin that was supposed to be set at 60% (it was actually 61%) I lowered it to near 55% by drilling 4 x 3/8" diameter holes opposite the big end, two of in each flywheel. These wheels were made from a much higher grade of steel than the originals which were basically "Boiler plate" and much softer. This has resulted in a much smoother engine in the lower rev range..........with that said, the same (or very similar) engine and crank combination mounted in a post war frame, that is, conventional UFM and RFM as we know, and again that engine is not to bad, this with the 60% crank balance factor .............I know you are all thinking, what the hell am i up to.........Anyways.......In the example of this discussion, the pistons are quite light verses the originals, generally around 450/460 grams....... I would start by removing some material from the top three holes as suggested by Bill, and then look at adding some tungsten or mallory metals opposite the big end. You would be surprised at how far out the balance factor can be, and the engine still be quite smooth........The flywheels on my twin racer, picture on the left, were actually out by 520 grams...........The engine was quite smooth, but it did vibrate quite badly at low engine speed. I did not set it up myself, but I did check it when the engine was rebuilt a couple of years back........I nearly fell off my chair when I found this out........The flywheels had to have four holes bored through the wheels just opposite the big end near one inch diameter each.........that is a lot of metal to remove. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Crankshaft Balance Method
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