Motor Scooters: US Government Technical Specifications

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Quality Mobile
Vespa Service In
Southern California

The D.O.T. (US Department Of Transportation) requires certain critical parts on all vehicles to be tested, very similar to UL testing on household appliances. Normal testing is the part is taken to catastrophic destruction, real or estimated, then 1/3 that speed, RPM, load, etc. is established as the safe operating range. On a motorcycle the critical tested components are:

Front forks failing positive dynamic stability. You can figure out what that is.

Tires and rims. Acceptable rotation speed and weight.

Brake ability. With the disc brake now common this is very rarely a factor.

Final drive connection/linkage. When the chain, or drive belt in your case, breaks or tries to crawl off due to centrifugal force.

Should any of the parts on your bike fail to meet the testing standard for the bike at the maximum speed the engine can theoretically push it, a speed limiter will be required, installed by the manufacturer, before DOT certification.

To get down to the reality. There is a chance your tires fail to meet DOT. While tires aren't expected to pass times 3 testing and have other safeguards built in, they are usually what limits how fast the DOT will accept the top speed of the vehicle. As a perfect example of this, the Concord jet that crashed had tires safety certified at 190 MPH. The tire speed exceeded 200 MPH during take off. The rest is history.

Put the bike up on a stand so it's rear wheel is off the ground, put it in high gear and rev the engine. If there is a limiter you will hear the engine resist acceleration past a certain point.

What you need to watch for is a loss of positive dynamic stability with front forks, compounded by your tires. Above a certain speed the tires change shape. As you know, a tire tries to climb up the side wall, a smaller distance around. With loss of dynamic stability the tire will start to give a little tug to one side rapidly followed by a tug in the opposite direction. The geometry of the front forks automatically tends to dampen this effect. If the combination of tires and speed exceed a certain degree, the forks will no longer be able to dampen the motion, the front forks 'fishtail' and the bike goes out of control.