Tips on reducing noise and making it quieter!


Local time
4:28 AM
Jan 22, 2008
Essex, England, UK
After a while the attention begins to get tiresome! By riding "under the radar" you can just get on with riding, and the best way to do it is by stealth, making very little noise. Theres loads of info all over the place on reducing noise so I've tried to put it all in one place for everybody.

Any body else have any good suggestions?

Reducing engine block noise
When the fuel combusts in the cylinder it releases a lot of energy, much of this is kinetic energy, the movement shakes the engine block. These shakes happen very fast and happen up to 6,000 times per minute. This high frequency acts in a vibrating motion; vibrations cause sound. So a large amount of the noise from the engine is directly transferred through the engine block causing the fins to "flap" or resonate up and down. To reduce the noise, the key is to reduce the vibrations of the engine block. The engine is mounted onto a metal frame, which conducts these vibrations causing the whole bike to shake; this isn't just noisy but can be very uncomfortable. So to dampen these vibrations firtt insert rubber strips (bits of old tyre) in between the mount and the frame. This will absorb some of the vibrations making the bike quieter and more comfortable, also protecting your frame from fatigue and scratching it up. The other main source of vibrations is through the cylinder head. I have experimented with using rubber strips in between the fins to absorb some more of the vibrations and stop the fins from shaking as much (I used cut up bits from an old fanbelt). There are two problems with this however; the fins are used to spread out the surface area to help the engine cool better. So you will have to be careful to not use too much as this could cause it to overheat. This leads to the next problem, the engine gets extremely hot so you must use a material that is capable of withstanding very high temperatures (the rubber compounds from an old fan belt are ideal).

You want to wedge the bits of rubber in, but remember that these engine blocks are cast aluminium so are very brittle. Be careful not to wedge them in too hard as the fins can easily break.

Heres a link to a thread where this has gone into much more detail:
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Other methods
One of the biggest noises is the drivetrain. I have found that the cluch is loud when not engaged. I will put a small length of heat shrink tubing over one of the three pins that hold the clutch plate. The tubing just fills the small gap and eliminates the clattering noise. Its a huge improvement. Try it! Gear whine is also loud on the bike. So I will use a lot more grease (specifically bearing grease).
Reducing Noise through Exhaust pipe
The noise you associate with exhaust is due to the rapid expansion of air during combustion and to the turbulence of its high speed exit from the engine. Smooth out these pulsations, reduce the turbulence of the exiting gases, and this will silence the exhaust noises. The usual way of reducing the noise of engine exhaust is through the installation a muffler or silencer.

A muffler works by providing a large chamber in which the sound of combustion and turbulence are reduced. The quality of the silencng, therefore, depends on the size of this chamber and on the downstream resistance the gasses hit.

You could modify the silencer by putting fibreglass packing inside the muffler, this would need replacing every once in a while though as the oil will clog the exhaust up and slow the engine down.
Noise through the intake
Apparently this is the most noisy part of the engine (as the ports are opened the noise can still come out through the very simplistic intake).

There are a few ways to silence this.

Make the airbox as large as practical. Maybe aluminum and pop rivet construction. Make the airbox as rigid as possible. Point the intake down and as close to the bottom of the bike as you can.

The idea being to dampen (dull) the intake sound.

How about adapting an existing automotive ot motorcycle airbox?
Changing the chain
The existing chain on chain drive happy time bikes can be replaced for a BMX chain, one guy said it was faster, doesnt stretch and is 70% more quieter. They are also more lightweight.

This is a popular replacement: KMC 415 bike chain
I have also noticed that when my bike is running, if I put my hand on the clutch cover, it quietented down considerably.

I'm considering getting some cork gasket, making a cork clutch cover gasket, and lining the inside of the cover with cork as well
I have also noticed that when my bike is running, if I put my hand on the clutch cover, it quietented down considerably.

I'm considering getting some cork gasket, making a cork clutch cover gasket, and lining the inside of the cover with cork as well

Theres some advice on this here:

thick rubber glued on to clutch cover with a cardboard pad on the the inside. From a distance the mechanical whine is the loudest thing on these engines.