Unintentional Vibration in the Harley-Davidson Motorcycle
Before we get into the specific reasons for Harley vibration that is intentional and expected from their engine design, we need to rule out unintentional vibration. Any bike, including a Harley, is subject to harmful vibrations that suggest a problem with the bike that could become dangerous. Usually, engine vibration means something is very wrong with the crankshaft, cylinders, or transmission and could lead to complete engine failure. If it’s not from a major internal engine issue, it could be from one of these other common vibration sources.
Tires & Wheels
A continual and worsening vibration when accelerating could be a sign of problematic tires or brakes. Rule out this issue by checking for uneven tread, a bent rim or sprocket from a pothole, or a brake caliper that’s sticking. Check the rear brakes and disks for alignment and tension. Not only are tires and wheel alignment one of the most common vibration issues on a motorcycle, but they can become the most dangerous.
Dirty Engine Oil
If you haven’t changed your oil in a while, the oil properties will become thicker, dirtier, and burn hotter decreasing the efficiency of moving parts and increasing engine vibration. Changing your oil regularly is the least expensive way to prevent vibration.
Check the Chain
Your bike chain is an important but unpredictable part of your bike transferring power from your engine to the wheels and need good lubrication and tightening on a regular basis. If it’s too tight, the chain will put too much of a load on the engine causing vibration, but too loose and the chain will not align with the sprocket teeth creating engine tension and causing a vibration that you will feel from the footrest all the way to the handlebars.
Just like brakes and chains, if the valve tappets are too loose or too tight, vibration will occur. Each tappet has a specific setting that needs to be checked and adjusted on occasion.
Filters, Carburetor, and EFI
When air and oil filters get dirty and when your carburetor gets clogged, the air-to-fuel ratio can create a poor combustion process. EFI (electronic fuel injected) engines also can get dirty and clogged resulting in vibration. Cleaning the filters and carburetor thoroughly is needed to keep a smooth-running engine.
Physics of Vibration in the Harley-Davidson Motorcycle
Harley-Davidsons have always been known for their earth-shaking vibrations. In 1980, H-D finally started experimenting with rubber mounts instead of the engine directly bolted to the frame. By 1990, the Dyna line received its rubber mounts, and in 2004 the Sportster lost its rigid mounting leaving only the Softtails with an excessively vibrating ride.
But Harley’s distinctive engine shutter was inherent to its design at inception. When the engineers opted for a new and very cool 45-degree V-twin layout, the balance of that engine would be thrown off. Both pistons would have only one crankpin journal meaning the pistons would ignite at 315 degrees and 405 degrees. This is why you hear the engine sound like “Po-ta-to Po-ta-to Po-ta-to” instead of the consistent “pop pop pop pop” of other bikes at idle. The crank rotates at differing speeds because of the angle resulting in incomplete combustion on some rotations.
Many foreign bikes are designed for less vibration and more balanced moving parts using a separate crankpin journal for each cylinder. Harley uses only one crankpin that holds the two piston connecting rods. So both pistons move together as the crankpin spins around the crankshaft.
Other smoother engines place the crankpin journals at 180 degrees so that as one cylinder ignites, the other is moving in the opposite direction and fires on the next revolution. This reduces vibration as a balance occurs with pistons firing at TDC (top dead center) as the other piston reaches BDC (bottom dead center). This is impossible for a Harley engine that is at a 45-degree angle.
When one piston fires, the engine rotates only 315 degrees which is 45 degrees short of the full 360-degree rotation. The crankshaft must then rotate 405 degrees before that same cylinder can ignite again. The two rotations before firing result in uneven and unbalanced firing which can sound like a misfire. The distinctive sound and vibration are a part of the V-Twin design as pistons are firing at uneven intervals.
When Harleys get up to speed, the combustion becomes more consistent and smooths out, but at idle the crankshaft spins at differing speeds at TDC. The piston firing after 405 degrees spins slower and loses momentum than the piston which fires after a 315-degree rotation. Combustion is not always complete on every stroke, and at idle, you will often hear the ignition timing creating a bang instead of a pop.
If a Harley idles at anything lower than 900 RPMs, the inconsistent combustion is more evident with a stronger vibration resulting. The heavy flywheel and counter-weights on the crankshaft are supposed to maintain momentum, but it has less effect at lower RPMs. Thus, the staccato, uneven, signature sound and vibration will always be present at idle.
The Reason for Vibration as the Signature of the Harley-Davidson
It has been said that when creating the Milwaukee Eight engine, more care was taken into balancing the machine. After measures were taken to smooth out the vibration, riders tested out the new engine and rejected it. The engine did not have the “feel” of a traditional Harley-Davidson. And, let’s face it, the Harley name is about tradition.
Being different and standing out from the competitors meant accepting some vibration. It is said that the engineers manufactured 25 percent more vibration into the M8 engine to satisfy the die-hard faithful. People aren’t purchasing the Harley name to get that smooth, sensible riding experience meant for the boring masses. This company knows who it is and offers a bike for those that seek adventure, seek something different, and seek the feel of an old-school, but still high-tech, reliable, and fun bike. For many, fun means experiencing something no one else offers: an engine you can feel under your seat that sounds and vibrates like nothing else on the market.
About THE AUTHOR
Benjamin Rathbun has been hooked on motorcycles since 1987 when he bought his first bike, a 1973 Honda CBR450 for $300. Since then he has been through countless bikes and continued his two-wheeled hobby passing it down to his 21-year-old son who rides with him on the weekends in the Blue Ridge Mountains. Benjamin believes that nothing clears the mind faster than flying 26 inches above the asphalt on his Harley-Davison.Read More About Benjamin Rathbun