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Four Wheelers explained:

Four Wheeler: Also called an ATV or all terrain vehicle is a small, open motor vehicle having one seat and three or more wheels fitted with large tires. It is designed chiefly for recreational use over roadless, rugged terrain. The term "all-terrain vehicle" is used to describe a number of different, small open motorised buggies and tricycles designed for off-road use. The 4-wheeled versions are also called quad bikes or more often quads. Models with 3 wheels are called all-terrain (motor)cycles or ATCs. 6 and 8 wheel models exist for specialized applications. The rider sits on these models just like on a motorcycle, but the extra wheels make them more stable at slow speeds.

Sport models are often built with a mind towards motocross racing, either in the design of the machine or at least in the marketing of it. To be successful at motocross racing, a quad must have light weight, high power, good suspension and a low center of gravity. There are many other types of riding and racing such as GNCC (woods racing), Baja (desert racing), hill climbing, and trail riding.6 wheel models have a small dump bed, with an extra set of wheels at the back to increase the payload capacity. They can be either 4 wheel drive (back wheels driving only), or 6 wheel drive.

Some of the most popular manufacturers of four wheelers/ATVs are:
Honda
Kawasaki
Bombardier Recreational Products
KPX
Yamaha
Suzuki
Polaris Industries
Cannondale
ATK

Important four wheeler Safety Tips:

Follow these safety tips to make riding both fun and safe!

Before You Ride Your four wheeler:
-Familiarize yourself with your four wheeler before you ride. Read and understand the instruction manual and observe all safety and warning labels on your moped. Check the controls.
-Wear the proper clothing/pads including helmet, elbow pads, knee pads, wrist guards, gloves and shoes.
-Check the equipment including the nuts and bolts, steering, brakes and controls, frame (for stress and fractures) and tires (for wear). Inspect your four wheeler thoroughly before each use for loose or missing parts.

When Riding You should:
1.Always wear a helmet and other protective gear.
2.Never ride on public roads - another vehicle could hit you.
3.Never ride under the influence of alcohol or other drugs.
4.Never carry a passenger on a single-rider vehicle.
5.Ride an ATV that's right for your age. The guidelines are:
a. Age 6 and older - Under 70cc
b. Age 12 and older - 70cc to 90cc
c. Age 16 and older - Over 90cc
6.Supervise riders younger than 16; ATVs are not toys.
7.Ride only on designated trails and at a safe speed.
8.Take an ATV RiderCourse; Call Toll-Free at 800.887.2887

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Four Wheeler Q&A's, Tips, Troubleshooting Section:

You’re on a steep hill and you rapidly lose momentum. You’re already down in second gear and there’s still a whole lot of incline ahead. Time to drop her down into first?

Don’t! You’ll flip your machine back on top of you. Better to stop and reconsider. Use the front brake—don’t touch the back. If possible, ease off the brake to let the quad roll slowly backwards, turning the handlebars so that it ends up at right angles to the hill. As you do this, move your downhill leg over to the uphill side of the quad, kneeling on the seat. This places more weight on the uphill side of the quad and allows you to jump clear if it starts to roll. Once the quad is in position, you can turn it around and head back downhill for another attempt at the incline, or find another route around it.