FILTER TYPES
PLEATED PAPER
Standard air filters for road bikes are almost always made from either dry, or oil-impregnated pleated paper. Sometimes they incorporate a flat wire-mesh to add mechanical strength and support. When new, pleated paper filters perform their function perfectly well, but over time dust particles block the element and it becomes an ever increasing restriction to airflow. The service replacement interval is a compromise between cost/convenience and filter performance.
OILED FABRIC GAUZE
Popularised by K&N, many aftermarket performance air filter elements are made from a washable oiled fabric gauze. Although frequently pricier than the OEM item, cost savings over the long term are possible because the element can be washed out and re-oiled, returning the filter to as good as new. The thin fabric, usually cotton, gauze is held in a pleated design with wire mesh. As with paper elements, pleating increases surface area, effectively packing a large filter into a smaller space. Makers of this type of filter point out that suspending the trapped dust and dirt particles in the element’s oil, rather than within the material like a paper element, reduces pressure drop in use, allowing for superior filtration and extended service intervals. They still need cleaning though and servicing oiled fabric gauze elements takes a bit of common sense. They should not be washed in a flammable solvent, such as petrol and proper filter oil, in the correct amount must be reapplied once the element has thoroughly dried out from washing. Unless you actually want to set your bike on fire or fill your street with blue smoke.
FOAM FILTERS
Off-road machines invariably employ foamfilter elements from the factory. Mainly because these bikes are exposed to really dusty operating conditions and foamfilters have an excellent particulate retention to airflow profile – meaning they can catch a lot of dust without restricting airflow. Plus, they are robust enough to be washed out and re-used repeatedly. Foam filters are usually oiled, but some are designed to work dry. Oiling improves filtration quality at the expense of service time, not a huge problemwith off-road competition machines, but stripping your air filter out on most modern road bikes every 1,000 miles is going to get old pretty quick. Foam filters are also popular with road racing teams, a thin foam element offers adequate filtration with minimal restriction to airflow and the reduced operating time between cleaning isn’t a problem when you’ve got a team of mechanics on staff…