Scooter Blog The Scooter Linc - Part 3 In this blog entry Linc gets technical, get ready to delve into the world of the noisy 2-Stroke...
Linc Biggins The Scooter Linc
Someone once said to me: If you go looking for noises in a 2-stroke engine all you'll ever do is work on it and never use (ride) it!
Air-cooled engines, particularly 2-strokes, are noisy. There are a lot of reasons for this but primarily an air-cooled engine uses large fins which radiate out from the cylinder (like the core of your car radiator), presenting a large surface area allowing the air passing across it to dissipate the heat from combustion via the cylinder walls into the atmosphere. These fins act like a tuning fork and produce a noise pitch which will raise and fall more or less in line with the rpm’s being produced, appropriately named ‘fin rattle’.
Combine this with rotational bearing and gear noise, throw in a bit of resonance from the exhaust pipe chamber and you get a whole cacophony of noises ‘factory standard’. When these noises get different stresses added from varying fuel quality, ambient air temperature, quality of the lubricants used, you literally end up with a symphony of changing sounds. Years later I have come to the realisation that even if you use good quality oil, fuel and keep up your servicing, then the ever changing ‘unknown’ noises from your engine can often cause you more stress then is actually warranted. I am certainly not suggesting you ignore all unusual noises, but do be aware that overall, engines are noisy.
Another noise that gets brought to my attention often following a service, comes from Constant Variable Transmissions (CVTs) which are used in all modern ‘twist and go’ automatic scooters. The CVT transmission works on a principle which for someone who has never seen one exposed is a cross between a 21 speed push bike and a car fan belt or Vbelt.
A push bike gear set works on the principle of moving the drive chain between different ratio sprockets to get the best power produced by your legs to the rear driving wheel. Anyone who has ridden a bike with multiple front and rear chain rings knows that when you get to a big hill you will not crank up it in the same gear as you will use going down the other side. The best method is of course to move the chain to a smaller front ring and/or a larger rear ring. This will allow a lot more pedal revolutions for every rear wheel revolution, therefore allowing you to climb the hill (just like selecting a lower gear in your car). Once you reach the top the best method is to go the other way, selecting the biggest chain ring on the front and the smallest on the rear and pedal like fury until the downhill finishes.
The CVT works on the same principle: the difference being that instead of using chain rings and a chain, it uses two pulleys and a Vbelt, very much like the old school fan belt on a car. Unlike the fixed pulleys on a car engine, the CVT pulleys expand and contract relative to engine power & rpms (torque is involved but we won’t go into that at present) and automatically change the diameters on the front and rear pulleys to give the best power and speed delivery that the engine can produce.
The noise I am speaking of comes about when the CVT pulleys, for one reason or another, usually normal wear and tear, go slightly out of alignment. This in turn at some point during the minimum and maximum ratios cause the drive belt to vibrate and like the ‘Fin Rattle’ tend to oscillate or thrum, particularly at higher speeds. Short of replacing the whole CVT there is not a great deal other than routine maintenance (changing your drive belt, variator rollers and guides) that can be done economically to stop (or at least minimise) it, in fact some scooters do it straight out of the crate, it is a small price to pay for such an efficient and easy to use transmission system.
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published 11/07/2011 |