The figure below shows how balance weights are placed for a typical single-cylinder engine. With an offset crank, counterweights are placed in a similar manner, with 50 percent of the weight on either side of each journal in a manner that is completely different from the other side. One side will be the crank cheek while the other side will be a counterweight welded to the flywheel as close as possible to the journal.

In relation to an ED G Crank, the balancers do their best to ensure that 50 percent of the weight to counterbalance each cylinder is on either side of the journal. In some cases this means that weight is removed or added to either side of one journal. Philosophy creeps in when someone says "add more weight to the crank cheeks." Science takes over as the balancers only add or subtract weight as required to get the optimum counterweight distrubtion possible in relation to each crank journal.

"I've heard that I should use balance factor XX" The problem with balance factors is that the relate to a combination of the engine and the motorcycle frame. Triumph spent many years changing balance factors to suit the 'new' unit construction frame in the mid sixties, partly to fix vibration problems, but also to improve acceleration. Alot of data exists on the web for balance factors to use with particular frames, engine displacements, compression ratios and engine mounting methods. None of this information applies to offset crank engines. ED G Cranks has tested various balance factors, in different frames, for various engines, from 45 to 68 percent.

With a 90 degree offset, 50 percent seems to work best for all of them (BSA, Norton or Triumph) no matter what frame is being used. 40 to 45 percent works well with Norton 80 degree offset cranks. Isolastic-mounted engines are far more tolerant of balance factor variations that are a few percent off while solid-mounted engines are not. Exceptions only occur for pure racing, Bonneville Salt flat engines where a narrow RPM range is required and with some pre-unit BSA and Triumphs in some years. The 1961-1962 double-downtube Triumphs may require a different balance factor but we have no experience with them or extreme high RPM-only engines. We can report experience soild and isolastic mounted Norton engines, with pre-unit BSA engines in swingarm frames and pre-unit Triumphs in both swingarm and rigid frames and 50% works with all of them.

"Can you build a crank that I can tune?" Yes, it is possible to make a crank that can be tuned, but only for testing purposes. A 'tunable' crank implies there is some way to bolt weights to either the crank cheeks or the flywheel. There are two problems with this; the bolts must be strong enough to hold counterweights in place (most are not) and you can't tell what effect changing weight in one place will have on the entire crankshaft. This process may work on a stock crank because all journals are on the same plane but it won't work on an offset crank - remember the 50-50 rule? Manufacturing tolerances of orginal components, and in the ED G Cranks process, mean that computerized equipment is required to determine accurate balance.

"What about adding slugs into the wrist pins?" If you were spending the time and money to build an all-out racer or a salt-flats record breaker it is possible to use slugs in the wrist pins of each piston to tune the crank for the best balance factor. The advantage of using this process is that there is equal weight distribution because weight is being centred on the journal. The disadvantage is the amount of weight that can be added or subtracted. Even with copper, aluminum or tungsten slugs the balance factor can only be changed by a small amount. Once the optimum balance factor is found the engine should be stripped and the weight of best slug should be used by the balancer to calculate and adjust counterweights to replicate the experimental balance factor that was found.

The best balancing is done with computer-controlled equipment, with the entire crank being taken into account this makes a tunable twin-cylinder crank more of a myth then a practical reality. My balancers also report that the balance factor range that can be adjusted on the flywheel or the wrist pin will be less then 3 percent either way. We have had to re-balance cranks for some customers because people changed pistons or because the balance factor they requested was too high. Even when building a special-purpose racing engine its best to start at 50 percent, then strip the engine and re-balance at a lower factor (such as 45 percent) with the crank mounted in a computerized balancer. If you are wrong, its easy to weld material on the crank to add weight to return it back to the way it was before, then go lower if required.

"What else can be balanced?" If reducing vibration is critical either for long-distance riding or racing then a crank should be balanced with all drive and timing gears, the alternator rotor and nuts and clips installed. If a customer desires this service it does not add to the cost of your crank but you must ship all components to Ed G Cranks for this to be done including pistons and connecting rods. At an additional change we can balance your clutch, drive sprocket and rear brake drum and sprockets (yes, separate from the wheel) to reduce any imbalance inherent in the manufacturing process. It makes a noticable difference!