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Balancing Act

The Beach Boys and old motorcycles have one thing in common... Good Vibrations, or depending upon your tastes in music, Bad Vibrations.

Engine vibrations cause stress and fatigue, not just with the rider, but perhaps more impor­tantly within the components of both the engine, frame and other cycle parts. This leads to pre­mature wear and failure in a myriad of different parts. It can be the main culprit responsible for broken crankshafts, disintegrating pistons, split fuel tanks, fractured oil pipes, number plates falling off and many other related occurrences.

Cost and technology limited the degree of balancing which could be achieved before a bike left the factory. Static balancing of the crank between centres was most often employed. Whilst conrods and pistons were constructed to relatively high engineering standards for quality marques, they were not individually weighed or adjusted for balance.

Dynamic balancing for everyday use was as yet still to come, as were some of the aero technology engineering practices which ensured almost 100% consistency in manufactured compo­nents, at an affordable price.

Dynamic vs Static

Basic static balancing can be a bit hit or miss. It often relies on just spinning the bare crankshaft between centres and noting where it comes to rest. If it rests in the same position every time the chances are that it’s got a heavy spot.

If you remove metal from this point, you should start to achieve balance and if you continue to apply this practice over several spins you should eventually achieve a better balanced crank­shaft.

However, there is still some friction from the centres which will always affect the final balance. There are also bound to be other variables as this is a manual process and relies heavily upon the skill and experience of the operator. If the conrods, pistons, gudgeon pins, circlips, shells, etc., fitted to the newly balanced crankshaft are uneven in weight or balance, then you can quickly negate much of the previous effort.

Dynamic balancing is an entirely different kettle of fish and there are three fundamental differences between this and the static method.

Firstly, it no longer relies upon gravity to determine the heavy spots, relying instead on sophisticated electro-mechanical devices which very accurately measure the balance variations as the crankshaft is spun under power.

Secondly, these devices measure the imbalance at varying points along the shaft in order to ensure that it can be accurately balanced along its entire length and not just at one point as an average.

Thirdly, each component of the shaft (e.g. pistons, rings, circlips, rods, shells, trunions, nuts, etc.) are weighed and individually balanced where possible, or taken account of in the overall balancing formula.

At the end of the day, balance is achieved in both static and dynamic methods by the removal of metal, but in the dynamic mode the final balance accuracy is far in excess of anything that could ever be achieved in a static mode.

The Calculation

Each particular engine (or family of engines) has a crank balance factor. The factor for AJS & Matchless 650 twins for instance is 65%. Without explaining this figure in detail it should suffice to say that the factor relates to the degree of balance required which will minimise disruptive vibrations through the rev range up to the projected road cruising speed for that model.

It is almost impossible to avoid all vibration without using fairly sophisticated counter balance shaft mechanisms and some is therefore likely to be present in varying degrees either before or after the projected cruising speed.

If you vary the factor, you simply move the vibration period and intensity lower or higher within the rev range. If you cannot remove very disruptive vibration at a particular rev range by balancing, you might consider re-gearing either the primary or secondary drive in order to move those vibrations beyond the normal cruising speed (but beware of making the machine untractable in certain road conditions).

The Results

Most cranks come back from the dynamic balancer with a significant number of deeper or extra holes in the flywheels (over and above those that the factory drilled). The pistons and conrods will have also been balanced and matched for weight on twins to allow fitment to either crankpin. Additional crank fitments like alternator rotors will also have been taken into consideration.

A note of the balance factor achieved and the individual weights of pistons and rods etc. should accompany the newly balanced crank to allow new pistons etc. to be brought to the correct weight in the event of a future rebore being required, thus retaining the original crank balance factor (see photo).

Overall, when the crank is fitted it would be almost impossible not to appreciate the work done. For a cost in the range of £50 plus VAT and with all other factors such as carburation and ignition in perfect order, you might even be able to balance a thrupenny piece on the tank with the engine at tickover (for readers with short memories that’s a pre-decimal hexagon sided coin of the British realm).

Whilst it’s always a balancing act to decide where to spend the money on a rebuild or a restoration, 50 quid spent here will pay dividends during many years of comfortable and reli­able riding to come.

And finally...

Eddie Cox, probably one of the best and most experienced balancers in the country lives at Bassett Down Balancing near Swindon in Wiltshire and can be contacted on 01793 812331.

I say in the country, but I suppose I should indicate further afield given the number of international bike and car racing teams that use his services worldwide. His customer list reads like a who’s who in international motor sport, not to mention a bevy of well known profession­al bike restorers in the UK.

Eddie can turn around a twin crank in a couple of days or, by arrangement, can do it while you wait.

From Jampot February 1997 original submitted by Chris Read.