"A" twin balance factor

greg brillus

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Hi there, Is there anyone who can tell me the balance factor for a prewar twin, and if it is different to a post war twin perhaps because the earlier engines are mounted in a frame verses the later which hang from both heads. Cheers for now................Greg.
 

Magnetoman

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the balance factor for a prewar twin, and if it is different to a post war twin
Although I don't know the answer to your question, I would be surprised if the balance factor were the same. Quoting Phil Irving in 'Motorcycle Engineering' on why the balance factor is important (with examples specifically for a single), with emphasis added by me:

"Usually, a balance factor of 60-65 per cent, will be found to be correct, but it may vary widely; the factor for the MOV Velocette was as high as 85 per cent., while some engines have been down to 40 per cent... Because the eventual smoothness obtained depends so much upon the frame, a balance factor which suits one installation may not suit another... An instance of this was found in the original Vincent-H.R.D. singles. These engines ran very smoothly in the standard spring frame with a factor of 66 per cent., but the speed way versions, of which a very few were made, vibrated badly in a cobwebby dirt-track frame until the factor was reduced to 61 per cent., this not-very-large reduction making all the difference between a machine which was passably smooth and one which shook itself out of your hands."
 

roy the mechanic

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My norvin has a standard crank+ balance, bolted into the frame at both ends. The only vibration noticed was the neighbours windows. So I recon you'll most likely be ok.
 

greg brillus

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Everything is well and good when stock parts are used, this does not apply in this instance, hence the problem. I think some refinement of the balance factor as magnetoman suggests, but will it improve it or make it worse. There is an awful lot of disassembly and reassembly involved just to "Experiment around" I was of the opinion that having the engine mounted in a frame would change the vibration characteristics.
 

Magnetoman

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My norvin has a standard crank+ balance, bolted into the frame at both ends. The only vibration noticed was the neighbours windows.
You were lucky.

I've mentioned elsewhere that I'm rebuilding a 1928 Ariel to use in next year's 4000-mile Cannonball cross-U.S. ride. For that ordeal I want to balance the engine for an aftermarket piston so I need to know the original balance factor in order to do this (it has a worn non-original piston in it now). My premise is that the Ariel factory likely did more testing than I'm prepared to do in order to arrive at whatever balance factor they settled on for the frame it's in.

After a few months of searching I was lucky enough to find someone -- in Australia, by the way -- who had an original piston and rings so he was able to give me the weight. Once I have the crankshaft out I'll be able to use that information to determine the original balance factor and that will let me know by how much I have to alter the flywheels for the new piston.
 

Cyborg

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You were lucky.

I've mentioned elsewhere that I'm rebuilding a 1928 Ariel to use in next year's 4000-mile Cannonball cross-U.S. ride. For that ordeal I want to balance the engine for an aftermarket piston so I need to know the original balance factor in order to do this (it has a worn non-original piston in it now). My premise is that the Ariel factory likely did more testing than I'm prepared to do in order to arrive at whatever balance factor they settled on for the frame it's in.

After a few months of searching I was lucky enough to find someone -- in Australia, by the way -- who had an original piston and rings so he was able to give me the weight. Once I have the crankshaft out I'll be able to use that information to determine the original balance factor and that will let me know by how much I have to alter the flywheels for the new piston.

Did the piston look anything like this?
Ariel Piston.jpg
 

Magnetoman

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Did the piston look anything like this?
From written descriptions, the original piston looked exactly like that. Not to hijack this thread, but it wouldn't happen to be from a late 1920s 500cc Ariel, and you wouldn't happen to have a scale for weighing it, would you?
 
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