Filters are the most basic of motorcycle consumables – but without air and oil filters, your motorcycle's engine wouldn't last more than a couple of hundred kilometers. So what are those filter on our motorcycles?
Cleaning your motorcycle is important, but there are two areas where cleanliness is absolutely vital – the oil in your engine and the air in your airbox. If either becomes contaminated with foreign objects, it can spell the end for your motorcycle engine in seconds. For this reason, booth are filtered, as you'll know if you've ever had your motorcycle serviced.
Oil filters first. There are two types of oil filter in general use: 'spin-on' and 'cartridge' types. As the name suggests, spin-on filters are the ones that poke out the front of the engine and refuse to unscrew-ever. Cartridge filters are simply made from the filter material and fit into a chamber in the engine.
Although the packaging is different, they work in the same way, passing the oil through a cellulose paper filter element. This paper is designed to trap particles above a certain size – given in microns (thousandths of a millimeter). To give you an idea of size, the thickness of an average human hair is usual around 100 microns. Depending who you listen to, filters will typically trap particles from 100 down to 5 microns. As well as retaining particles, it's important that a filter doesn't restrict flow too much. This is where cheaper filters can often fall down, and expensive ones excel. To safeguard engines, all filters systems have a bypass that allows oil to flow past the filter if there's too much resistance, so your engine remains lubricated even if the filter is blocked.
Resistance to flow, or lack of, is an important part of the filter's design. It's one reason why you should be careful when replacing your filter with anything other than the approved type, and why it should be replaced at service intervals.
Air filters work in much the same way. They stop particles such as grid, sand and stones entering the combustion chamber, where even small particles can damage the metal because of the velocities involved. Like oil filters, air filters generally use a cellulose 'paper' as standard, although oil-impregnated foam is a common aftermarket replacement.
When it comes to air filters, restriction to flow is seemingly more important, as it directly affects how much power the engine is a capable of producing. To increase the flow-rate, both oil and air filters are normally made concertina-shaped to increase the surface area, as well as using perforated metal to catch large objects and support the paper element.
While oil filters are disposable items, air filters can be cleaned using compressed air, blown in the opposite direction to the filtered air. However, they will eventually become irretrievably clogged. Foam air filters can work more effectively than paper filters, but require regular maintenance to stay in top condition.