not true. i came from stock to an arrow shorty. they hear me riding behind them, and when riding next to them on trafic jams they hear me comming closer so they make room to let me pass. when riding stock under 5k they didn't hear me comming. unless u can ride faster then aprox 330m/s they will hear u
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How do you know the cars that move over for you heard you and didn't simply see your headlights coming? If traffic is very slow or stopped, and the drivers don't have their radios on or some other form of distraction, then yes there is a chance they'll hear you. But driving at speed, especially on the freeway, they won't hear you until you are passing them.
Haven't you ever been driving then are surprised at the loud ass bike that is suddenly next to/passing you? Happens all the time. I found the following online which explains it pretty well:
It's rooted in physics. Unlike an earthquake, which emits waves in a sphere away from its epicenter, sound waves can be directed. (A good example of this a megaphone: if sound waves were not directionally sensitive, megaphone users would all be deaf.) The pressure wave emitted from an exhaust pipe is pointed away from and behind the motorcycle. To hear it from anywhere other than directly behind the bike, you rely on the resonance of the pipes and reflection of the sound waves off of other objects, like cars, signs, buildings, etc.
In other words, the only time the loud pipes draw attention to a motorcycle in traffic is when the bike is facing away from the motorist. In most cases, facing away from the motorist means riding away from the motorist: increasing the separation of bike and potential hazard. The only possible situation in which a car can pose an external threat to a motorcyclist when the bike is facing away from the car is if the motorist is backing up or (following) behind the rider. The threat exists, but only marginally: the Hurt Study found that a whopping three percent of accident hazards come directly from the rear (or the "six o'clock" direction).
Conversely, Hurt found that 77% of all accident hazards approach the motorcycle from in front of the rider (specifically, the eleven to one o'clock range). To have any chance of alerting those motorists to the presence of a motorcycle with exhaust noise would require that the tailpipe be pointed forward. The chances of rearward-facing straight pipes making a bike more obvious are very slim. The reality of the loud pipe is that all you'll succeed in doing is irritating all the people behind and beside you who don't pose much of a threat anyway.
I know this is a highly debated topic, and for the record I'm definitely not against aftermarket pipes. I just don't believe they significantly improve drivers ability to hear you coming. To each his own......we can agree to disagree
Rudehugh> yep, definitely below 8k for cruising.