Many Motorcyclists Try To Cut Down Noise

Ask most outdoor recreationists to recall an off-road encounter with a motorized vehicle, and they’ll probably mention the noise.

Off-highway motorcycle noise, in particular, seems to raise the hackles of even the shared-use crowd. But understanding how the noise is produced, measured and abated can go a long way toward easing outdoor tensions.

Most off-highway motorcycles are powered by two-cycle engines. Power is generated in a two-cycle each time the engine turns a revolution. Such a setup produces more power per cylinder than a four-cycle motor, and the engines can be made smaller and lighter. The trade-off is that a two-cycle engine is less efficient and produces more pollutants.

The reason a two-cycle is dirtier is that the spent fuel and oil gases are exhausted at the end of the power stroke. The end of the power stroke also happens to be the beginning of the intake and compression stroke. Thus, there’s always a little blow-by. And any time an engine exhausts unburned fuel, efficiency is reduced.

Motorcycle noise can come from two sources: the engine or its exhaust. A put-putting engine itself is not too noisy, but at high rpms, a constant chain of cylinder combustions adds up. Recalling how two-cycle engines operate, remember that the exhaust valve is actually open a bit before all the power has been transferred in the power stroke. That’s what gives the machines their distinctive popping-type sound.

Engine noise can be vented another way, without going through an exhaust system. The device is called a compression brake. Think of it as a vent that opens directly from the cylinder to the air. A motorcycle rider can open the vent to bleed off compression and power to gain more control of the bike. Unfortunately, this also vents a tremendous amount of noise.

But let’s just say everything is working fine and engine exhaust is sent through the factory exhaust system. Many people still mistakenly believe that power can be increased by altering the exhaust system. The shade-tree theory says that by removing the muffler and decreasing back pressure through the exhaust system, additional fuel and air will be pulled into the intake system.

According to the Motorcycle Industry Council, that’s not necessarily true. In fact, they say in a number of their publications that noise doesn’t equal horsepower. Inadequate back pressure can actually mean less power that results in engine damage. The council strongly recommends that off-highway riders maintain their motorcycle’s exhaust systems as they came from the manufacturer.

Spark arresters are an integral part of today’s motorcycle exhaust systems, too. Spark arresters prevent heated particles from leaving the engine and starting wild fires. They do not affect power and should always be left intact.

Motorcycle noise is measured with a decibel meter held at 20 inches from the exhaust.

A decibel, or dB, is a unit of measure of sound intensity. Zero dB is considered the lower limit of what people can hear. The loudest sounds humans can tolerate are about 120 dB.

More than two years ago, Charlie Morey, editor of Dirt Rider magazine, began Project Stealth. Project Stealth was a massive public information campaign directed at changing off-highway riders’ attitudes toward machine noise. Morey’s goal was to ensure that off-highway riders understood noise production and the negative effects it has on the public.

Riders have responded well, according to Nora Hamilton, dirt bike ranger on the Mendocino National Forest in California. Hamilton oversees 32,000 acres that is host to more than 25,000 riders each season. The forest has set a 101 dB limit and Hamilton says the riders have responded with enthusiasm. She says cyclists have even altered mufflers to make exhaust more quiet. Now it is not uncommon to see 250 to 500cc bikes registering less than 85 dB.

Most of the riders making modifications have found that a length of radiator hose fitted tightly between the muffler and spark arrester works wonders. If the hose has a right angle bend in it directing the exhaust flow down into the spark arrester, even more noise was eliminated.

Like any modification, however, riders should consult their dealer or a local off-highway motorcycle club before attempting such changes.