In between repairing the roof, consuming large quantities of orange tablets (Vitamin C) and thinking strange thoughts about astoundingly beautiful Thai girls, I spend the odd second or two considering the sad state of the motorcycle industry. These thoughts are usually warped by the knowledge that the first company to actually use the modern design and manufacturing techniques that are available to the actual benefit of the rider will reap rich rewards as they clean up the market. That the Japs have so far failed to make the right machines will come as no surprise to anyone who has had the misfortune to work for a large multinational company.
In my more hallucinatory moments I even labour under the grave misapprehension that I could either sell such an obvious design to the Japs or use it in a solo effort to revive the British motorcycle industry. Such thoughts are soon shattered by even the shortest exposure to the reality of UK industry, where its managers appear trained to find a 101 reasons why they shouldn't do something, rather than thinking in a vaguely positive manner.
The problem with new motorcycles is that they offer few advantages and many inferiorities to the secondhand stuff. Despite the advertising blurb, the factories really need to go back to the drawing board and redefine the middleweight motorcycle. How does 120mph, 80mpg, 350lbs and tyres, brake pads and chains that all last at least 20000 miles sound? And an engine with 5000 mile service intervals that lasts for 100000 miles. Good enough to make you think about deserting the secondhand camp and start pestering your bank manager?
To achieve such a design would be neither cheap nor easy. Certainly, I wouldn't claim to be able to produce such a machine out of my garden shed, but if I had the largest concentration of engineering talent and computer power in the world, then I'd have a jolly good try. If I were in charge of Yamaha, for instance, I'd be half way there already. Their late sixties XS650 twin could manage 110mph and 60mpg. The engine is famous for being tough and long lasting.
What it desperately needed, and never got over its twenty years of life, was a new cylinder head. Although the engine really needs a much shorter stroke to make maximum use of the cylinder head shape (the larger bore giving more room for larger valves), it'll probably be possible to get away with the stock dimensions. Valve timing would be radically altered in the search for greater efficiency. Valve clearance would be set U placing the rockers on eccentric shafts, increasing accuracy and lowering valvegear mass. Such a system would need minimal attention an would be simple to adjust. The twin carbs would be replaced by a single CV unit, making the engine smoother, more efficient and removing the tedious business of balancing carbs. The actual optimisation of all the various factors that control the efficiency of cylinder head design would soon be sorted by a pack of Jap designers using the latest in engine diagnostic equipment.
The XS engine has pistons that move in unison and produces some vibration, but the build quality of the engine means that this vibration is partially absorbed and doesn't cause bits and pieces to fall off the engine. Changing to a 180° crankshaft would decrease vibes but because of the uneven firing pulses a single carb wouldn't work properly. The rest of the engine is pretty tough, but the relative simplicity of its design would make it easy to absorb the cost of tightening up on all the machining tolerances in the engine, to increase efficiency. with its low mass and redesigned combustion chamber, it will be able to get away with using a mere four gears. The final drive chain would be replaced With a rubber belt, which, these days, have progressed to the stage where they can last for 25000 miles and need minimal attention.
These relatively minor changes to the engine and transmission will have left us with a bike capable of around 110mph and 70mpg. The rest of the performance and economy will come from careful design of the aerodynamics and drastic pruning of the mass. With a vertical twin engine it's relatively easy to design a chassis that has both a low centre of gravity and a low mass. A steel frame is preferable to alloy for a road bike as it is more resistant to the occasional off road excursion and can be bent back into shape after an accident.
Because of the low mass and low centre of gravity, frame geometry can be chosen to suit high speed riding without worrying over low speed manoeuvrability. The suspension will be conventional twin rear shocks and non-adjustable front forks. The important difference to the usual fare would be quality of construction and the high damping.
Alloy drum brakes replace discs, using the latest pad materials for improved efficiency and longer life (at least twenty grand). Alloy rims and stainless steel spokes replace cast wheels for ease of crash repair. The combination of drum and spoked wheels should weigh in rather less than cast wheels with discs saving on unsprung mass.
One of those neat alloy swinging arms with eccentric chain adjusters replaces the usual steel item, not least because an out of line wheel will quickly ruin the rubber belt final drive. To save weight and improve efficiency the exhuast will be two into one made out of stainless steel, while the silencer will be alloy.
The whole of the design can then be pulled together by an efficient half fairing moulded into the five gallon alloy petrol tank. Something on the lines of the top half of an RS BMW fairing would do just fine. The fairing will give total hand and upper body protection, while the tank will be sculptured to take the rider‘s knees out of the air stream and protect the leg in the event of the bike falling over.
The narrowness of the engine and efficiency of the shape of the fairing should allow the bike to make it all the way up to 120mph while improving the fuel efficiency to let the bike average 80mpg. Doubtless, readers would add bits and pieces to accommodate their own needs and personal perversions, but the basic machine would have so many fundamental points in its favour that it could take it all in its stride.
Even the perennial problem of tyre wear would be partially solved. The belt drive would help deliver the power smoothly, the aerodynamic fairing would reduce the actual power needed and the low mass would minimise the stresses on the tyres. The rigid frame and top quality suspension would mean that the tyre would only have to worry over adhesion and not have to be designed to stop the bike going into speed wobbles.
Electronic ignition, single carb, automatic chain tensioner and high tech valve gear would reduce regular maintenance to an oil and filter change every few thousand miles. Because it's only a twin, all the engine components can be made of higher grade alloy to increase the life of the engine from 75 to 100000 miles.
An impossible dream? I simply refuse to accept that if British Leyland can update what was basically the Morris Minor engine into the 90mph/50mpg Metro, then the might of the Jap factories can't produce a similar improvement in bike design.
Bill Fowler
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