Discussion in '2-Stroke Engines' started by Spitfyre, Jun 29, 2009.

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3. ### loquinActive Member

The major source of vibration in any MB is the engine, and vibration originating there can be greatly reduced by milling off small, carefully selected amounts of metal from the crankshaft. Ref this site for details.

However, there are other sources of vibration. Chain on sprocket. Tire/wheel imbalance. Tread pattern (Mountain bike tires with knobby tread can vibrate badly, for instance.) What can happen in certain cases, is that the motor vibration can be in sync with tire vibration at particular speeds. When that happens, the vibrations can reinforce each other and result in a very 'strong' vibration.

Maybe your sprocket has gotten a bit worn, and it causes the chain to vibrate like a guitar string as the chain rollers slip into position. The length of chain, and the tension it's under, will make the frequency of the vibration different. This is called the resonance frequency. If the number of teeth and sprocket rotation speed cause a vibration that happens to be at the same frequency as the engine or tire resonance frequency, you could get a heavy vibration at certain engine RPMs as you accelerate, but only at certain speeds. And, if this happens to match the engine resonate frequency (or be an even multiple of it) you could get even worse vibration. Maybe adjusting the chain tension will help. In bad cases, you might need to change the rear sprocket to one with a different number of teeth.

You could even get the 'perfect storm,' where engine, tire, and chain resonance frequency all reinforce under certain extreme conditions. It's possible that you could get enough vibration to throw a chain, crack a frame weld, or even lead to early engine damage! Try to figure out where the vibrations are coming from, and change the conditions that cause the frequencies to reinforce each other.

As an example, suppose you had a knobby tire with knobs approximately every half inch. You count the knobs around the tire, and there are 160 of them around the circumference. This means that at 300 RPM at the rear tire (apx 23 MPH) the knobs are coming into contact with the road 48000 times a minute, or, 800 times per second. If your engine RPM happened to be running at 4800 RPM, at this speed, the vibrations would tend to reinforce each other, as 48000 is exactly ten times greater than 4800... So, you would want to reduce tire vibration (change tires to get a lower vibration, or preferable, none at all,) add rubber motor mounts to reduce the engine vibration that is 'coupled' to the frame, or, pay someone to mill the crank to balance the engine. Unlike auto tire balancing, where you can add weights to the wheel to get better balance, with engines, you need to shave weight off the crank.

Last edited: Jul 1, 2009