by Charles Owens on 05/01/2013
The Gatekeepers of Power
An Offroad motorcycle’s overall engine performance (it’s ability to achieve consistent acceleration response and peak horsepower), is directly linked to the performance of it’s reed petals. Two-stroke MX/Offroad engines receive air and fuel mixtures through intake tracts, or reed cages. Typically, reed cages are located between the carburetor or throttle body and the engine, and are designed to work in conjunction with a system of delicate, pre-mounted reed petals. Together, the reed cage and the reed petals create the heart of a 2-stroke bike’s intake assembly. The small, flexible reed petals are primarily responsible for controlling the inlet of air and fuel by the simple process of opening and closing against the surface of the reed cage. Because reed petals are responsible for the ongoing regulation of air and fuel into the engine, they are the single-most crucial component within the intake system’s delivery process. They are the flexible component of the reed cage that “reacts” to the engine’s cycling piston – making them the “Gatekeepers of Power”.
Quantifying the lifecycle of an Offroad Reed Petal
To regulate the incoming air and fuel in proper ratios reed petals are designed to “reactively pulse” as the engine’s pistons cycle through their intake and exhaust strokes. This reed petal pulsation, or “FLEX CYCLE” occurs at a one-to-one ratio in unison with the upwards and downwards movement of the engine’s pistons.
To better illustrate the relationship between piston movements, engine RPMs and a reed petal’s “flex cycle”, let’s first establish a quantitative baseline…when an engine turns 8000 rpm, it’s reed petals will reactively open 8000 times per minute. At 6000 RPMs a bike’s reed petals will endure a staggering 360,000 flex-cycles per hour. After 50 hours the amount of reed petal flex-cycles will total approximately 18,000,000!
Now that the concept of reed petal flex-cycling has been illustrated and given some real-world usage numbers, we can turn our focus to the specifics of how all reed petals naturally degrade over time. Very rarely will any reed petal last forever. Over the lifespan of every 2-stroke engine it is very likely that it’s reed petals will begin to deform, chip, or experience edge micro-fracturing. Throughout its life span, a reed petal flexes millions of times, and its individual composite fibers start to lose their memory, becoming slow to react in unison with the engine. The petal’s modulus of elasticity decreases so that in equal engine conditions the petal will deform and open farther than specified. This condition is termed excessive reed “hang-open” and has been proven to contribute to a rich condition in the engine, poor horsepower output, poor fuel efficiency, and inconsistent power delivery performance.
Furthermore, within each flex-cycle, the reed petals are designed to flex open in unison with the piston to allow air and fuel into the engine. The petals then briefly close, precisely “kissing” against the surface of the reed cage to stop the internalized air and fuel from backwashing out of the engine. This process is commonly referred to as “Reed Petal Trapping” and is one of the performance tuning areas that can either positively or negatively affect a reed cage’s ability to supply the engine with what it needs to maximize power and acceleration response. In a static state (when the engine is NOT running), the reed petals typically maintain a slight gap between the surface of the reed cage. This gap is normal and does not translate into a problem once the engine is running because the piston-induced back-pressure forces the reed petals to make a positive seal against the surface of the reed cage. Typically, this piston-induced back-pressure; amounting to approximately 5-10 PSI (back pressure varies with each model bike). Although this natural back-pressure allows the reed petals to achieve proper seal against the reed cage surface, overtime, it may cause soft, over-flexed petals to deform, or experience edge chipping. In fact, it has been seen that many plastic stock reed petals tend to get soft after a month or so of riding and begin to further degenerate quickly thereafter as a result of the engine’s back pressure through the system.
When reed petal degeneration occurs, most, if not all reed petal designs, regardless of materials used, are unable to properly trap the air and fuel charges in unison with the engine’s pistons. In this regard, failing to eliminate air and fuel “reversion” is a known cause of decreased engine performance and can easily be remedied by changing to a new set of reed petals.