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Gas-Powered String Trimmers (page 2 of 4)

by Ken Morrison

Trimmer Basics

At the business end of the trimmer are the head and the monofilament line that spins from it. A shaft delivers the engine's power to the trimming line, and of course there is the engine, often with a clutch.

Line and line heads. To cut, all string trimmers use rapidly spinning monofilament line, and its flexibility offers a degree of safety unmatched by bladed trimmers. When the string hits anything hard, it doesn't break or chip; it just wears away slightly and more is fed out. How fast the line wears depends upon its thickness and the material it's hitting.

This monofilament resembles a heavy fishing line, and its diameter is one way trimmers vary. Thicker lines don't necessarily cut heavier growth, but they cut faster and are more durable. As line weight increases, engine power must go up, too--it takes more power to spin a heavier line.

Most heads spin a single line, but some have two lines, one from each side. More engine power is also required for two-line heads, which cut faster (but usually not wider) and vibrate less than single-line ones.

Line-advance systems come in three types: manual, tap-to-advance, and automatic. Manual heads, the most basic, require that you turn the engine off, then advance the line by hand. This type of trimmer is all but extinct. Tap lines are better; bumping a spring-loaded knob on the ground advances the line. Automatic is most convenient and costly. It works by sensing the increase in rpm as the line wears and gets shorter, then releasing line until the rpm return to normal.

However it's advanced, the line eventually runs out and needs replacing. Some heads employ easy-to-use pop-in replacement spools, while others require some disassembly and hand-winding.

Power shafts. Shafts come in three basic styles: straight, curved, and split.

Straight-shaft trimmers offer the most power, comfort, and flexibility. Their shafts are longer, which gives them more reach for easier cutting under shrubs and fences. Reduction gears lower the spinning speed and thereby increase the power to the line.

Most straight-shaft models have two-line monofilament heads that take wear-resistant 0.095-inch-diameter line.

Many straight-shaft trimmers have solid steel driveshafts supported by vibration-reducing bushings. Others rely on flexible drive cables in plastic liners. Both are reliable, but solid driveshafts are smoother and more durable. If you plan on cutting heavy brush with a blade, favor units with solid driveshafts.

Curved-shaft trimmers are the lightest and most economical. They deliver power to the cutting head by a flexible-cable driveshaft. They are perfect for trimming around flower beds, fences, and paths found on a typical city lot.

The more powerful units employ dual-line heads. Gear reduction is not possible on a curved-shaft trimmer so the head always spins at engine speed.

Curved-shaft models are shorter than straight-shaft models, making them easier to store and transport. But, if you have to bend your back for the trimmer to reach the ground, it may be too short for you.

Split-shaft trimmers take straight-shaft utility one step further. The two-piece shaft accepts various attachments such as a blower, blower vacuum, cultivator, edger, hedge trimmer, pruner, and even a snow thrower. Changing attachments is as easy as pushing a button or twisting a knob.

Engines. Nearly all gas-powered trimmers have electronic ignition systems and primer-type carburetors to make starting easier. Most gas-powered trimmers rely on two-stroke engines, but a few four-stroke overhead-valve models are available.

Two-stroke engines are simpler and lighter, produce more power for their size, and can operate in any position, even upside down. Their disadvantages are relatively high fuel consumption, greater exhaust emissions, and the necessity of mixing oil with the gasoline.

Four-stroke engines are rare on string trimmers but promise to become increasingly significant. They are somewhat quieter and cleaner, and they use their unmixed fuel more slowly. They also have more power at low speed. But compared to two-strokes, they suffer from slower throttle response, greater mechanical complexity, more weight, and frequent oil-check maintenance.

For gardeners who like the four-stroke's environmental advantage but prefer the two-stroke's utility, new, low-emission two-strokes are now entering the market. Two manufacturers, Tanaka and RedMax, have unveiled designs that will meet the most stringent small-engine emissions requirements.

Trimmer engines fall into two performance categories: those with single-counterweight crankshafts, and those with dual-counterweight crankshafts. As a rule, single-counterweights are less expensive.

If you prefer premium tools, look for dual-counterweights. They run smoother, are more powerful, last longer, and are more expensive, but their higher quality makes repairs easier and more cost effective.

Engine size in cubic centimeter cylinder displacement (cc) is another way to compare engines. Though greater displacement generally translates into more power, in the range of engine sizes used for trimmers that's not an absolute. In practice, a 23-cc engine might outperform a lower-quality 30-cc engine. Horsepower would be a better way to compare engines, but manufacturers seldom publish those ratings.

A clutch is another indicator of trimmer quality. A clutch disengages the cutting head when the engine idles. On clutchless trimmers, the cutting head never stops spinning. To check whether a trimmer has a clutch, pull slowly on the starter rope. If the head spins, there is no clutch.

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