Crankshaft balance

From Paul Dunstall's tuning notes: Atlas 750cc 84%.

Hope that helps, Gordon.

John Hudson Notes - Crankshaft Balance Factor

The balance factor to which all twin cylinder cranks were balanced, atBirmingham, was 70% of the reciprocating weight, but at Woolwich at an Atlasengine number, which I do not know, they increased this percentage to 85.

I certainly have no proof that this figure was or is an improvement and in aconversation with Doug Hele quite recently I was discussing balance factorsfor the twins because another friend had obtained a Nourish crank for a 500cctwin for racing. I therefore asked Doug If we should raise the 70% factor to80 which is the figure to which flywheel assemblies on Manx, G50 and 7Rengines are balanced. (Incidentally the Norton International engines had theircrankshafts balanced to 70% and not 80.) However Doug would not recommend a figure higher than 70% because of the greater distance the main bearings areapart compared with the much shorter distance which separates these bearingsin a single cylinder engine, and a higher factor on a wide crank assembly fora twin would be likely to contribute to a flexing of the assembly at high rpm.

As I understand it there should not be a mechanical reason why the 750cc Atlasengine should vibrate more than its 650cc counterpart. I would doubt if thehollow crown 7.3:1 Atlas pistons weigh appreciably more than the almost 9:1pistons in the smaller engine.

It is interesting to note that the Triumph Bonneville balance factor of 74% issimilar to the 70% which we used on all the pre-Commando twins, both featherbedsand non-featherbed frames, except that at Woolwich they tried a factor of 85%on the 750 Atlas models but I myself am convinced that this was not right.

The original Commando balance factor of 52% of the reciprocating weight waschosen to give relatively smooth running at low rpm because the isolastic

The original Commando balance factor of 52% of the reciprocating weight waschosen to give relatively smooth running at low rpm because the isolasticmountings are best suited to give smooth running at higher rpm.

Don't forget that, to achieve effective balancing, you will have to take crank, pistons, and con rods. You need to decide which crank to use initially, then get the one for the spare engine balanced, separately, with whatever rods, and pistons you are going to use.