and Dynamic Balancing

by Graham Bennedick
Historic Motorcycle Club of Qld Inc.
(Toowoomba Area)

Many sources of vibration act upon an elderly motorcycle & come in the form of wheels, chains, extended arm mirrors, loose cycle parts, engine mounts & stays, movement of air over the motorcycle as it travels forward & an item reaches a resonant frequency, but the main source is engines & in particular singles & vertical twins. Vibrations come on two types, vertical & horizontal.

Think about the vertical effects on a single or 360 degree twin engine. It is obvious that the main cause is with the crankshaft & its associated parts. The crankshaft is counterweighted opposite the big end journal to counteract the forces as the crankshaft spins at rpm & can be precisely balanced to spin in its bearings without vibration. The issue becomes more complicated when the conrod & piston are introduced when significant forces come into play with each rotation of the crankshaft & rise & fall of the piston. These come in the form of vibrations which vary as to the square of the engine speed (double the speed, quadruple the resonance of the vibrations) . This can be combated to a large degree, but not eliminated completely, by adding a fraction of the piston group weight to the crankshaft. It is determined by the sum of the weight of the conrod big end & oil inside the crank being the rotating weight plus a selected percentage of the weight of the piston, rings, circlips & the little end of the conrod.being the reciprocating weight. This is the much debated "balance factor". In a single cylinder engine this is usually 50%, 500cc vertical twin (Triumph) 60% to 68%, 650cc 85%, 750cc triples 30% to 40%. These figures can be varied depending on the type of operating conditions intended for the engine. When it is to be used as a race motor at higher rpm use a higher balance balance factor percentage.

That raises the obvious question as to why not balance to 100% & all would be well. Unfortunately that's not the case as additional horizontal forces are introduced by the reciprocating & rotating masses. But, you say, the flywheel has already been balanced by the factory, you have seen the holes drilled in the centerline of the flywheel. That too is only part of the story & manufacturing methods used 50 & more years ago are different from current practice. Centreline balancing is only static balancing & does not correct for a heavy spot on say one side of the flywheel & a heavy spot on the other side. In practice this condition does exist on most older engines. Static balancing will place these heavy spots at 180 degrees opposed to each other on the flywheel rim & as these heavy spots rotate they create forces at 90 degrees to the direction of rotation of the flywheel & try to get out of the engine through the main bearings.

Balance of a rotor/flywheel comes in three forms: static, kinetic & dynamic.

A perfectly balanced rotor will come to rest in any position on its circumference after it has been spun , ie, it is perfectly balanced on its axis. An unbalanced rotor will always come to rest with its heavy spot at the bottom. This added weight at a distance from the axis causes static unbalance.

Kinetic unbalance exists when a statically unbalanced rotor is rotated. The unbalanced weight introduces a centrifugal force which tries to deflect the rotor in the direction of the force. This force increases with the square of the speed. Deflection of the rotor results in vibration of the bearings supporting the rotor.

To develop that line of thought a bit further, imagine your motorcycle flywheel as two separate plates split down the middle. Heavy spots on either plate (or flywheel rim) play havoc with the horizontal movement of the crankshaft & try to rock it in the main bearings thus creating a dynamic couple.The crank is, of course, confined by the main bearings but this movement is transmitted through the machine as vibrations, right up to that familiar hum in the handlebars.

Dynamic unbalance is caused by two equal forces acting in opposite directions in two separate planes. Rarely does solely kinetic or solely dynamic unbalance exist in a rotor. Most unbalance conditions are a combination of both. A dynamic balancing machine separates the rotor unbalance into its dynamic couple & kinetic force, enabling individual corrections to be made in any plane.

Triumph crankshaft

The dynamic couple is corrected by milling sections from the flywheel rim at different points on the rims circumference if necessary. Kinetic by correcting the centreline or the rim of the flywheel. You may well be surprised by the amount of metal that is removed from the flywheel . With one particular 500cc Triumph vertical twin it turned out to be 155 grams on one side & 98 grams on the other.

A single or 360 degree vertical twin is destined to vibrate to some extent because the balance factor is really only a compromise between vertical & horizontal forces & additionally compression/firing forces remain.

To summarise,dynamic balancing will,

- Reduce vibration significantly.
- Ensure longer engine life.
- Make the riding experience more pleasurable.

Dynamic balancing services are offered by
Charlie, DC Racing, Toowoomba. Qld
ph (07) 46341072



©2009 Graham Bennedick
Historic Motorcycle Club of Qld Inc.
Toowoomba Area
6 September 2009