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Crankshaft balance factor for Dommie 99

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Hi,

I want to get the crankshaft of my Dommie 99 project dynamically balanced as I’ll be running Thunder Engineering conrods. I can’t find the correct balance factor for a 99 crank anywhere. I even asked Thunder Engineering but they don’t know. Any advice and guidance would be much appreciated. I’m going to use this bike on the road at legal speed limits so don’t want a ‘racing’ tuned crank.

Thanks

Regards

Tony

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According to P.L. Garratt in Norton Motorcycles (5th Edition) on page 100 the Balance factor for all twins 650cc and below is 70%. If your engine is an alternator model be sure to fit the rotor before you send it to be balanced. On wide (twin) cranks the side to side factor is as important as the percentage I am told.

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I suspect you will have to fit the engine sprocket as well . I have heard that an extra weight to compensate for the oil in the sludge trap  can be added too. Would be worthwhile to balance the pistons against each other . Our Atlas (Commando) ones were well out to each other and vastly heavier than the orriginals. I spent ages  knife edging the bottom of the skirts and removing bits of casting flash ,but the extra thick heavy pins defeated me a bit.

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Hi. I’ve been having a bit of a think about this .... I would have thought that, as the rotor and engine sprocket are basically circular and are fitted axially along the line of rotation, they would have no effect to the crank’s balance as their centrifugal mass is the same at any point around the crankshaft. But that’s just a guess. I’d love to hear if it’s wrong as the guy who does my crank balancing has never asked for the rotor or sprocket. I can see that an extra weight to replicate the oil in the sludge trap would make sense.

Regards

Tony

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No way of knowing if a rotor is balanced already,and I think you might need the sprocket to have something to tighten the rotor up to?.

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Hi Tony,

   With any parallel twin, single or V-twin, the design makes it almost impossible to achieve a vibration free engine through-out the rev range. Dynamic balancing allows you to pin-point the rev range where you expect to use the bike most. You will probably only have around a 1,000 rpm window that the engine is at it's best balance so it depends on what type of roads you use. If you use motorways and dual carriageways you will probably want that window between 3,500 and 4,500 rpm. If you ride mostly on A and B roads and urban areas you will be better to aim for 2,500 to 3,500 rpm. When you take it to be balanced, tell the guys what you are looking for in the way of cruising speed and how the vibration will be either side of this. I think it will be best to balance the rotor individually so it is equally balanced and does not affect the crank. I don't think the sprocket will have enough mass to cause a problem.

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Hi Tony

no need to include the rotor or sprocket as you rightly say, complete piston and conrod assemblies plus crankshaft only when I had mine done. The balancer can also do a wet crank calculation

My 850 Commando was balanced at 63.5% dry to match the combination of heavier Carrillo steel conrods and lighter forged flatop pistons (which were both identical in weight complete with gudgeons, circlips and rings) This is close to the standard Mk3 850 specs of 63% dry, 52% wet and works fine. I can,t say if this relates to your bike but Bill Molines post of 70% sounds reasonable for your solid mount motor

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Thanks Neill,

I’ll probably go with the 70% balance factor as it sounds like that’s what was ‘normal’ for a stock Dommie straight from the factory. The only thing I haven’t done yet is compare the weight of the new Thunder rods with the original Norton ones. If the Thunder rods are heavier, I might ask the engineering company to lower the balance factor to somewhere between 65 and 70. It all seems a bit of a black art to me so I’ll just see what I end up with.

Regards

Tony

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You probably don,t even need to do that, go with the standard value. In 2007 I just took all the bits requested by the Bassett Down guy and he balanced it to 63.5%. Apparently ideal for all round use and so it has proven on my bike. I had no idea what the correct factor was. Chris Applebee in Essex is another who may offer advice, he balances Nortons and will be rebuilding and balancing my Z1000 crankshaft

 The RGM Iso rubbers fitted in 2016 definitely introduced a slight buzz under power around 4000rpm but as with the low rpm rumbles, too busy enjoying the ride these days to worry about it. Sounds like an interesting project you have there

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Generally most people who got to know him accepted that John Hudson knew what he was talking about. The attachment is from his Notes and covers Norton Engine crankshaft balance.

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It's a bit of a black art to me, but I thought the point of any balancing was to balance what you have, regardless of the weight/mass of the reciprocating and rotating components.  I thought that was why an engine needed to be re-balanced with whatever pistons, bearings, flywheel etc. you used but I did assume the factor would remain the same to suit the desired normal cruising speed. 

 

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