June 14th, 2012 by
In this day of four-stroke domination, many riders will never have had the experience of wringing out a high-revving two-stroke quad. This is a shame because two-strokes created a type of power delivery that even the peppiest four has difficulty reproducing. Best of all it managed to do it by weighing less, containing far fewer moving parts and at a fraction of the cost to manufacture/ maintain.
Even though brand-spanking-new 2-stroke ATVs exist only in the hallowed halls of rider memory, the technology that makes them tick still deserves closer examination. One of the bits unique to the “two-smoke” is the reed valve (or as its often called, “reeds”).
A reed valve is actually quite simple by design, consisting of flexible petals mated to a block connecting the carb to the top-end of the engine.
In its simplest form, the reed acts as a check valve; keeping fuel & air from being pushed into the intake tract. In other words the reed valve opens to allow fuel to enter the top end from the carburetor then closes up shop so that the fuel & air mix stays put to be combusted (rather than get spit back when the piston comes sailing up).
While this sounds like a fairly demanding job, the actual process occurs quite naturally. Rather than rely upon mechanical means of operation, the reed is all about using something that surrounds us at all times: atmospheric pressure.
When the piston plummets down its bore, a low-pressure zone on the engine side is created, causing the reed petals to flex open. Fuel and air then pass through the reed block. Once the piston returns upward, that lower pressure finds itself suddenly to the intake tract side and as such the petals are again forced to close tight.
Early reed petals were constructed of metal but like most components, have evolved to incorporate advances in technology, which have made them both lighter and stronger over time: From stainless steel to fiberglass to most recently, carbon fiber.
Modern systems like Moto Tassinari’s VForce3 reed valve system have gone to great lengths toward becoming a true ‘bolt-on’ modification (opposed to an engine tear-down affair like in years past). With technological advances like snap together construction, reed petal replacement incredibly quick and easy. This is certainly a good thing considering your reeds open and close 133 times per second at 8000 rpm!
April 23rd, 2012 by
My quad’s rear shock feels really springy- it still seems to be working but the rear of the machine bounces more than I remember over rough terrain. My friend says it means the shock is blown and needs to be rebuilt. I thought a blown shock wouldn’t work at all. Please help.
Sadly we’re inclined to agree with your friend on this one. To understand why a blown shock turns even softer and bouncier than a properly functioning one, let’s take a moment to examine what’s going on inside your ATV’s shocks.
Shocks absorb impact by using the flow of fluid (oil) through stacks of washers that act as valves. When a shock is new (or freshly rebuilt), the system works smoothly. However, like all oils, shock fluid begins breaking down over time. As shock fluid degenerates, it becomes thinner and hence passes through the valving with less resistance than it should.
This is critical because the damping circuits (valves) inside your shock are what provide the resistance not only to impacts causing the shock to move in the first place but also to keep the shock’s spring from compressing and expanding wildly. So why have a spring at all if it’s oil doing the absorbing? The answer is rebound; or the force that allows the shock to extend after it has been compressed. Without a spring, your shock would only work the first time it was compressed.
In addition to the shock’s fluid breaking down inside, the outside of your ATV’s suspension is dealing with all sorts of nasty conditions each and every time you ride. Dust, grime small rocks and sand gather up on the shock bodies then get pushed into your shock’s rubber seals. Acting as gritty sandpaper, these unwanted materials breakdown seals and wipers over time as well, often resulting in the shock’s internal fluid leaking out.
The reason a blown shock feels suddenly softer is that the fluid-driven damping circuit is being bypassed entirely due to one (or both) of the situations described above. In other words, your machine is in fact riding on the spring and the spring alone. No oil damping absorbing the terrain (compression) and likewise no oil damping to slow down the spring’s return expansion (rebound).
Higher end shocks can be rebuilt (oil change, fresh seals etc.) but many budget-friendly ATVs (including minis) offer non-rebuildable units as a cost shavings measure. If that’s the case with your particular quad, replacement is the only option.
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