A string trimmer should be comfortable to hold and operate. It should be sturdy, but not so heavy that you have to strain to lift and hold it as you work. It should feel balanced as you hold it, which makes it easier to control during use.
Trimmers have three kinds of shafts – curved, straight or split shafts.
Curved-shaft trimmers are usually lighter, economical, and better balanced. The curved design requires a flexible cable drive shaft, which limits most such machines to using nylon trimmer lines. They are good for trimming around flower beds, fences, and paths found on a typical home landsdape. Curved-shaft models are shorter than straight-shaft models, making them easier to store and transport; more attractive to shorter yardeners. The test is if you have to bend your back for the trimmer to reach the ground, it may be too short for you.
Straight-shaft trimmers offer the most power and flexibility. Their shafts are longer, which gives them more reach for easier cutting under shrubs and fences; making them attractive to taller yardeners. Reduction gears lower the spinning speed and thereby increase the power to the line. Some yardeners think straight-shafted trimmers are harder to maneuver and control, but they are more durable.
Split-shaft trimmers are sometimes called combination-trimmers - 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.
Trimmer Engines - The motor should not be too noisy. Gasoline powered trimmers emit noise levels of 95 to 110 decibels or more; electric models are generally less noisy than gas powered ones.
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 especially the necessity of mixing oil with the gasoline.
Four-stroke engines 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.
Each manufacturer offers a variety of additional features which can contribute to greater customer satisfaction with the product. Such features include:
See-through gas tank so you can see how much fuel you have.
Anti-vibration padding in the handle, with comfortable foam grips, to avoid nerve and circulation damage to the fingers during prolonged use.
Muffler shield to protect against accidental burns.