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 a V-twin engine with a 90 angle between cylinders. These crankshafts are balanced in a different way that can only be done with computerized equipment. Offset crankshafts cannot be balanced on knife edges using various weights, as they must be balanced rotationally and axially (from side-to-side).

The only truism is that each journal, and therefore each cylinder in an offset crankshaft 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. All modern Triumph twin engines are offset crankshaft using what they call a 270 degree crankshaft. Cross-plane, offset or staggered journal crankshaft technology is not new. It was first used for a 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, right, 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 for 50% of the balance for one crank journal while on the other side a counterweight is welded to the flywheel. Note that no holes are ever drilled in an ED G Crank for balancing!

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 always take over; the balancers add or subtract weight as required to get the optimum counterweight distribution possible in relation to each crank journal.

“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. All of this information applies to stock 360 degree crankshafts. 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 bad vibration come from?

With a typical 90 degree offset crankshaft engine, with the 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 to 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. We have almost 20 years of experience with offset crankshafts in 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.

“What else can be balanced?”

If reducing vibration is critical for either long-distance riding, or racing, then a crank can be balanced with all drive and timing gears, and 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 of these components. 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, to satellites, water and air turbines and large mining industry ball mills that are 30 feet long and 14 feet in diameter.

More detailed balancing information can be supplied by ED G Cranks; send us a message and we’ll send it along.