|Balancing: Fact and Fiction|
All the balancing stories, balance factors and experiments you may have heard in relation to British twins do not apply to offset crankshaft engines. Offset crankshaft engines are similar to V-twin engines with a 90 angle between cylinders.
The only truism is that each journal, and therefore each cylinder in a vertical twin, must be considered as if you are balancing a single cylinder engine. With the advent of computerized balancing equipment, science now allows us to go farther then that; a crankshaft can now be balanced for rotation and from side-to-side to reduce the effects of any rocking couple. Yamaha now calls their offset crankshafts cross-plane technology, as used on their latest 4 cylinder sports bikes but also used on some vertical twins. Triumph also sells one version of their vertical twin (The Bonneville America) with what they call a 270 degree crankshaft. Cross-plane, offset or staggered journal crankshaft technology is not new; it was first used in Cadillac straight-eight engine in 1908! Whatever the crankshaft form is called balancing science must take over from this point, not philosophy and not opinion based on here-say rather then fact..
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 approximately 50 percent of the weight on either side of each journal in a manner that is completely different from the other side. One side the pork-chop shaped crank cheek is used to balance the crank journal while on the other side a counterweight is welded to the flywheel as close as possible to the journal.
In relation to an ED G Crank, the balancers ensure that 50 percent of the weight to counterbalance each cylinder is on either side of the journal. This means that weight can be added or removed on either side of each journal. Metal may be welded to the pork chop to increase weight if required or a Mallory metal slug may be added. Philosophy creeps in when someone says "add more weight to the crank cheeks" - they don't know what they are talking about! Science must take over; the balancers add or subtract weight as required to get the optimum counterweight distribution possible in relation to each crank journal. Weight is added, or removed, near the crank journal, from the flywheel counterweight, or one crank cheek according to information displayed by the balancing system's computer. It is not possible to balance an offset crank on V-blocks of using any manual method.
"I've heard that I should use 77% balance factor." The problem with balance factors is that they also relate to a combination of what's best for 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. A lot of data exists on the web for balance factors to use with particular frames, engine displacements, compression ratios and engine mounting methods. Some of this information applies to racing engines where they only care about smoothness for a small, high RPM range. 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 66 percent. We know what works, what doesn't, and what happens if you decide to change pistons from ones that were originally 400 grams to new forged pistons that are only 275 grams. Where did that vibration come from?
With a typical 90 degree offset crankshaft engine, with engine solidly mounted to the frame, 50 percent seems to work best for all of them (BSA, Norton or Triumph). 40 to 45% may be used for high-rpm racing engines to reduce vibration at high RPM at the expense of low RPM smoothness. 60% is wonderful balance factor for low rpm smoothness (below 4500 rpm) but vibration may be worse then a stock 360 degree engine above 6000 rpm. Isolastic-mounted engines are far more tolerant of balance factor variations that are a few percent off while solid-mounted engines are not.
Exceptions may occur for pure racing or Bonneville Salt flat engines where a narrow RPM range is required. The 1961-1962 double-down tube Triumphs may require a different balance factor but we have no experience with them or with extreme high RPM engines in those frames. We can report more then 15 years of experience with solid and Isolastic mounted Norton engines, with pre-unit and unit BSA engines in swingarm and rigid frames and unit and pre-unit Triumphs in both swingarm and rigid frames. 50% seems to work well with all of them using camshaft profiles designed for the street and compression ratios below 9.5:1.
"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 'tune-able' crank implies there is some way to bolt weights to either the crank cheeks or the flywheel. There are many problems with a tune-able cranks; 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 at all on an offset crank - remember the 50-50 rule? Manufacturing tolerances of original components, and in the ED G Cranks process, mean that computerized equipment is still best to determine accurate balance.
"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. For an additional change we can balance your clutch, drive sprocket and rear brake drum with sprocket installed (yes, separate from the wheel) to reduce any imbalance inherent in the manufacturing process. It makes a noticeable difference!
"Who does your balancing?" All balancing done by D. Garland & Son of Scarborough, Ontario, Canada. This company can balance anything that rotates from the crank in a tiny model aircraft engine, satellites, water and air turbines and large mining industry ball mills that are 30 feet long and 14 feet in diameter. Contact Geoff Collins, this site's owner, for more information about industrial and automotive balancing.