The Benefits of a V Motorcycle Engine


Greater performance has been the relentless quest for manufacturers and riders since the start of the wonderful era of motorcycling. The first motorcycle had just one cylinder but the V-twin followed hard on its heels. The obvious reason is that, with the axis on the crankshaft across the frame, you can have twice the engine cylinder capacity and performance for very little increase in width. A narrow engine is very important on a motorcycle because you have to bank over to go round bends without being tipped off by touching the engine on the road. A narrow motorcycle is also desirable to keep the frontal area small for lower wind resistance.

To me a wide motorcycle is an anachronism and although you could cite the Honda 250 straight-six as an extremely successful machine, it was only 250cc and therefore had only little cylinders to stack across the frame. The most elegant engine ever made, at least in my opinion, was the Moto Guzzi V8, for its width was only 30mm wider than its world championship winning Bialbero 350cc single. Of course, Moto Guzzi also came up with its trademark arrangement with the cylinders protruding from each side of the motorcycle; this must be one of the longest running series engines, beginning its life in the 1930s as an agricultural.

The extra width of a V-twin engine of twice the capacity of a single cylinder is just the width of one of its con rods. Similarly, a V4 750cc is only wider than an equivalent 375cc parallel twin by the width of one of its con rods.
There are downsides to the V-configuration, of course, because the extra cylinder, or bank of cylinders, has to be stowed along the length of the motorcycle but this is an easier problem to solve on a motorcycle than extra width. Air-cooling the rear cylinder was a problem but not today with liquid-cooling. A distinct advantage with the 90 degree V4 configuration is that out of balance forces are better than for a straight four. The primary forces (those with a frequency the same as the engine speed) cancel out completely on both types of engine. The secondary forces (those at twice the frequency of engine speed) for the V4 are 70% of those of the straight four and in a direction that bisects the vee. However, there are unpleasant rocking couples at both engine speed and twice engine speed. This is due to the one crankpin up, one crankpin down shape of the crankshaft. The good news is that balance shafts can virtually eradicate both these disadvantages, albeit at the cost of stresses inside the crankcase increasing as the square of the engine speed. However, it is this feature, and the added weight implications that make the V4 engine a relatively rare bird in the engine world.

The Honda race engines are the best V4s and their success was on the back of the basic motorcycle need to be compact and narrow.

In recent decades the cost, weight and general complication of the rotating balancer shafts rotating at twice crankshaft speed have become popular to cancel out those pesky, damaging second order out of balance forces in many engines. They have become acceptable on singles, twins and fours in order to compete with six- and eight-cylinder engines in terms of absence of vibration and addition of refinement – but they serve no function but that. So, for me, the most elegant and most cleverly conceived engine is the Honda RC211V- five-cylinder 990cc MotoGP engine. It did not need balance shafts to remove vibration. The elegant perfection of this engine was that this was a 75.5 degree V4 with an extra – fifth -crank, con rod, piston and cylinder inserted between the front pair of cylinders, which performed the duties of the balancer to the V4. I think of this in the same way as the reciprocating balancer in the BMW F800 or the Ducati Mono but in this case it is actually doing work. This is again so well specified to the purpose for which is was meant, being not only compact but also taking notice of the ergonomics of the motorcycle, i.e. someone has to ride it. The rear bank of two cylinders allowed the frame to be narrow and the rider's legs to be close to the motorcycle.
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