Motorcycle exhuaust baffles

[Forrest Addy]

Hm. I thought mine were pretty clear. Not necessarily germane - a segue in fact - but crystal clear.

 

[speedsport]

it's getting harder and harder to keep my mouth shut. "Quiet pipes almost cost me my right eye".

 

[Motomoron]

I have to confess that the Suzuki DRZ400 super motard I ride to work through about 5 miles of Washington DC gridlock is deathly,
ear-shatteringly, horribly loud. It's got a gorgeous titanium Akropovic (ah-krop-eh-vich) exhaust system which complements the 39mm Keihin flatslide carb nicely. Funny thing is, on singles, there are no performance exhausts which aren't obnoxious. I'll say this though, the pedestrians glued to their cell phones are WAY more likely to glare at me than to walk aimlessly off the curb directly in front of me.

My 50cc 2 stroke scooter's pretty snarly sounding too. I do have an old BMW R100 CS thats whisper quiet. I always said that in my crowd, having a quiet bike is the most rebellious thing you can do.

 

[jayhawkman]

Talked to a guy I know today that has built exhaust before. Its not as complex as I thought. I will let you know how it comes out. Thanks

 

[Racer Al]

Jayhawk --

Sorry about the signal-noise ratio on this one. I came in pretty late.

Both Michael Moore and Motomoron are friends, good folks both, and competent fabricators.

I have built several custom systems for both bikes and cars, and actually prefer working in stainless.

With regard to your question, the above-mentioned Smith/Morrison book goes into great detail about the design of mufflers.

In a nutshell there are two types: absorbtion (packed, straight-through baffles) and what I describe as dissipation or serpentine path mufflers. Smith/Morrison refer to this as a back pressure muffler, but there is more to it than that. The serpentine path mufflers do not rely soley on packing to reduce the sound, but they may rely on it to alter the tone. Most OEM exhausts are this type of combination, while most aftermarket exhausts are absorbtion using a straight-through silencer.

There are several variables in the absorbtion silencer design: clearly, the diameter of the muffler core, and the length of the core are the two main ones. Diameter has the greatest effect on both back pressure and silencing.

Length of the whole system is how you "tune" a pipe to enhance certain RPM ranges. Long pipes tune to lower RPM, short pipes to higher RPM. Not that this applies to your question, but I don't understand how these under-seat sport bike exhausts work because they're so long. Definitely a trick in there that I'm not aware of.

The other features that many people ignore are the perforation diameter and spacing (I roll all my own cores from flat perf sheet) and muffler core taper. There are certain MFGs that use a very small perf on a very wide space -- these sound harsh and raspy, which seems to be stylish on sport bikes. My basic rule of thumb is use a large, tight perf, and wrap it in copper window screen wire to keep the packing in. This yields a nicely high-freqency-attenuated "thump-y" tone.

Tapering the core may or may not aid performance, i.e. gas flow, but it's a cute idea. It seems to also capture a wider range of higher freq tones, leading also to a "thump-ier" tone. Speaking of gas flow -- don't confuse gas flow with sound energy.

 

[jayhawkman]

Thanks great imput