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Iron Butt Association
May 17, 2017
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Exhaust Noise Standard

In the interest in maintaining good relations with law enforcement agencies and the general public, the Iron Butt Association prohibits the use of obnoxiously loud exhaust systems on any motorcycles participating in the Iron Butt Rally. As described below, the exhaust noise standard is 105 dBA based on the SAE J1287 test procedure. Except for riders with motorcycles granted a written exemption, motorcycles that exceed this level will not be allowed to be used in the Iron Butt Rally.

Motorcycles with original equipment exhaust systems in good repair will be exempt from testing unless, in the opinion of a designated Iron Butt Association official, they appear to be excessively loud. Motorcycles with non-original equipment exhaust systems, or without a written exemption from the rally chairman, will be required to pass the noise test described below.

Detailed Discussion of the Test Procedure and Standard:

Noise standards that apply to OEM motorcycles are based on a test procedure developed by the Society of Automotive Engineers (SAE) that measures vehicle noise levels during acceleration at a distance of 50 feet. Although this is the most representative test procedure available, detailed requirements for the test course make this test impractical without a test track. In recognition of the practical problems of running the drive-by test, SAE has developed a much simpler procedure suitable for use at the roadside or in a parking lot. This test, SAE J1287, involves measuring the exhaust volume of a stationary motorcycle running at 50% of its maximum allowable engine speed while in neutral. To limit the influence of other noise sources in the vicinity, the procedure specifies measuring the noise level at a distance of only 20 inches to the rear of the exhaust outlet. Because of the short distance between the exhaust outlet and the sound level meter, the absolute noise levels measured with the J1287 procedure are higher than on the drive-by test; however, the ranking of vehicles is similar.

The standard that will apply to entrants in the Iron Butt Rally is 105 dBA (decibels, A-scale). This standard is based on the highest noise level recorded using the J1287 procedure from a broad range of motorcycles with original equipment exhaust systems. Most motorcycles equipped with original equipment exhaust systems meet this standard by a wide margin. For example, late-model Honda Gold Wings have been measured at only 85 dBA on this test. Most late-model BMWs score between 90-94 dBA. In contrast, some Harley-Davidson models with OEM exhausts are as high as 105 dBA, which is the basis for the standard.

Because the decibel scale is logarithmic, a level of 105 dB sounds twice as loud as 95 dB and four times as loud as 85 dB. As a result, there is likely to be a four-to-one range in noise levels for motorcycles participating in the Iron Butt Rally. However, most people will not consider motorcycles at the loud end of this spectrum objectionable.

Some aftermarket exhaust systems will meet the 105 dBA standard. The "touring" versions of the Staintune and BMP systems for BMWs have been measured below 105 dBA. For comparison purposes, tests of motorcycles with straight pipes (no muffler) have produced noise levels of approximately 125 dBA on the J1287 test. This is four times louder than the standard and painfully loud to individuals in close proximity. Certain aftermarket exhaust systems using mufflers with limited baffling will also fail the test. For example a BMW equipped with a Staintune with a sport baffle and several popular aftermarket systems for Harley-Davidsons have been measured at about 115 dBA.

Noise testing will be performed at the beginning of the Iron Butt Rally along with other required inspections. To minimize the risk of last minute disqualification, entrants with non-OEM systems should obtain a noise test as early as possible. The noise meters used for the official testing will be certified to meet certain industry standards. Do not rely on test results performed using just any meter (for example, recent comparisons with a popular Radio Shack model showed error rates approaching two dBA). Official noise testing will usually be available at the annual IBA dinner during Bike Week in Florida (typically the first week of March).

If you have questions about this policy, or need additional information, please contact IBA Tech Advisor Tom Austin at austin@ironbutt.com, or just use this form.

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