 2010 Jeep Wrangler Unlimited. Click image to enlarge |
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Chrysler’s 4×4 models
CFB Borden, Ontario – It sounds logical enough: you’d think four-wheel drive would simply mean four wheels driving. In reality, there are several types of 4×4 systems, and knowing which one is sending power to your wheels could be the difference between dashing through the snow (or mud), or calling your buddy with the tractor to help pull you out.
Chrysler took advantage of some wintery weather at CFB Borden near Barrie, Ontario to show off the differences. I was able to take a Jeep Wrangler Unlimited, Compass and Patriot through an off-road trail; on a snow- and ice-covered asphalt course, the company added a Dodge Journey and Ram. The various systems not only made a difference on the snowy trail, but illustrated how all-wheel systems and the vehicle’s configuration can make a difference in everyday driving in slippery conditions.
  2010 Jeep Patriot (top) and 2010 Jeep Compass. Click image to enlarge |
The company has nine systems available for Jeep vehicles (although two are almost identical), including full- and part-time systems. It’s important for drivers to understand the difference. During everyday driving on paved roads, a vehicle’s front and rear wheels don’t always turn at the same speed: bumps, potholes, or wet or slippery patches can result in one axle turning faster or slower than the other. A full-time 4WD system allows this to happen, using a centre differential that enables the front and rear shafts to turn at separate speeds. When a part-time system is set into 4WD, the front and rear axles are locked together, and every wheel turns at the same speed. This is fine on a loose or slippery surface where the tires can slide, such as gravel or heavy snow, but on dry asphalt, the system can bind, especially on turns. This has the potential for damage or excessive component wear.
Freedom-Drive I, available on the Compass and Patriot, is a full-time system that works automatically. It runs primarily in front-wheel drive, but should extra traction be required – on turns, acceleration or when the system detects slippage – it sends torque to the rear wheels. When needed, a 4WD Lock handle located on the centre console locks the system into 50/50 at low speeds. A step up is Freedom-Drive II, available on Patriot, which also distributes torque automatically, but adds a low gear selection that includes a 19:1 crawl ratio and hill descent control.
  2010 Jeep Wrangler Unlimited (top); 2010 Jeep Patriot and Compass. Click image to enlarge |
The Jeep Wrangler and Wrangler Unlimited offer two systems: the standard Command-Trac on the Sport and Sahara, and Rock-Trac on the tough guy of the bunch, the Rubicon. Both are part-time systems that aren’t meant for hard, dry surfaces. Under ordinary use, the Jeep runs in rear-wheel drive. Engaging the system in 4High locks the front and rear axles into 50/50 torque split, while dropping it into 4Low multiplies engine torque in a 2.72:1 ratio. A neutral setting also disconnects all four wheels so the vehicle can be flat-towed. The Rock-Trac system works similarly, but in 4Low, it gears down to 4:1. It also includes heavier-duty front and rear axles, and includes a front sway bar that can be electronically disconnected for additional articulation over very heavy-duty off-road courses.
There were a few systems not included in the demonstration. Command-Trac II, used on Liberty, isn’t much different from Command-Trac, other than it can be shifted into four-wheel drive at any speed, and has an electronic shift instead of the Wrangler’s mechanical lever. Liberty’s optional Selec-Trac II starts off with rear-wheel only, but when set into 4High, torque automatically switches between the front and rear axles as needed, and can be used on dry pavement. Its 4Low system locks the axles and gears down to 2.72:1.
