V-Twins & Parallels | Motorcycle Engines
A motorcyclist from California asks: Why does a V-twin feel different from a parallel twin?
Great question! Almost every motorcycle manufacturer has some version of the two-cylinder engine. In the case of the V-twin, the term covers a multitude of engine cylinder configurations from 42 to 90 degrees, the most famous being the Harley-Davidson Motor Company’s 45-degree engine that I am intimately familiar with.
Ducati’s twin is sometimes called an L-twin because its 90-degree arrangement looks like an L from the side. A quick check of the roadracing record books shows the success this Italian manufacturer has enjoyed with the design. There are numerous other manufacturers of V-twin engines, but for the sake of space, we’ll stop here.
Critical to understanding the difference between engines is the basic knowledge of how an engine works. What follows is a simplification of the way most motorcycle engines work.
All four-stroke engines require two full revolutions (720 degrees) of the crankshaft to complete one cycle of operation. The first 180-degree segment-the intake stroke-begins with the piston at the top of the cylinder (top dead center) and the intake valve open. As the crankshaft rotates, the piston moves downward, creating a vacuum in the cylinder and sucking the unburned air and fuel mixture in.
At the bottom of the stroke (bottom dead center), the intake valve closes and the piston begins travelling upward on its compression stroke. All of the air-fuel mixture in the cylinder is compressed into a tiny area by the time the piston reaches the top of this stroke. At that point, the spark plug fires and the now-volatile mixture is ignited.
The resulting ignition creates an explosion within the cylinder, and the expanding gas rapidly forces the piston back down in the cylinder on its power stroke. At this point, the exhaust valve opens and the piston rises through its exhaust stroke, pushing spent gases out of the cylinder into the exhaust pipe.
The exhaust valve now closes, the intake valve opens and the piston begins its downward travel back on the intake stroke again. Consider that each cylinder in an engine must experience all four strokes in their entirety in order to operate, the only difference being the synchronization between when each cylinder is going through its cycles.
In-line parallel twin motorcycle engines are two-cylinder designs running beside each other in separate bores either in 180-degree (one piston up, one piston down), or 360-degree (both up or both down, but firing the opposite cylinder each time the engine reaches top dead center) configurations.
Triumph is also currently using a 270-degree arrangement on the Thunderbird, which essentially mimics the feel of a 90-degree V-twin even though its cylinders are side-by-side. Remember, with a cylinder offset of 90 degrees, the next firing pulse will come 270 degrees after the first pulse, if using a 360-degree crankshaft.
There are advantages to each type of engine. The compact front-to-rear size of a parallel twin makes life easier for engineers when designing a chassis because the engine can be moved forward or rearward, as needed. This allows quite a bit of leeway in establishing the center of gravity.
Because the bores and cooling fins can be staggered, V-twin engines are characteristically thinner and longer in profile. This gives designers less leeway because the larger footprint, but allows for a narrower motorcycle between the rider’s knees.
There are also some misconceptions about power, torque and vibration when inline parallel and V-twins are compared to each other. Engine configuration alone does not affect power and torque output; tuning and effective engine management are far more responsible for the result that actually reaches the rear wheel.
And now to get to the question on vibration-most modern day engine manufacturers are very concerned with the comfort of the rider and vibration is a big issue with all twins. In a recent issue of Ultimate Motorcycling we discussed how Harley-Davidson uses rubber mounting systems or counterbalancing to reduce unwanted vibration from their motorcycles. Most modern parallel twins are counterbalanced, making vibration much more manageable than in previous versions.
Compare a vintage Triumph Bonneville with its modern sibling; although similar 360-degree parallel twins, the modern 865cc Bonneville T100 shakes only a fraction of what its lower output 40-year old ancestor does, largely due to the twin counterbalancers.
The Ducati, however, has perfect primary balance because the 90-degree stagger of the cylinders on the block creates a situation where the movement of the opposite piston is always counteracting the vibration that comes from the firing of either cylinder.
Your best bet? Test ride a cross section of motorcycles you are interested in and compare the feel of the engine, the width of the powertrain, and the sound of the exhaust. There is no right solution for everyone; it is simply down to rider preference.Google+