A Buyer’s Guide: Choosing the Right Driveline — Part 2 — Four-Wheel Drive

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Four-Wheel Drive “Rock Crawler”

Control-Trac, Auto-Trac, X-Drive, and 4Matic, it seems that each new car manufacturer has their own terminology for multiple driveline systems. To the normal consumer, it can get really confusing, so, how do you determine which driveline is right for you? Front-wheel drive, rear-wheel drive, on demand four-wheel drive, or all-wheel drive, but which one best meets your requirements? Weather conditions, geographic locations, and driving habits must also be considered prior to making your purchase. If you make the correct choice, you may enjoy years of dependable service; make the wrong choice and get stuck with a vehicle that will not consistently get you from point A to point B. This guide is intended for potential new and used car buyers with an emphasis on helping you to choose the right driveline.

Part two of this guide, on drivelines, will cover on-demand four-wheel drive applications. Check out Part 1, which deals with two-wheel drive (both front and rear) vehicles and look for Part-3 on all-wheel drive systems, as well.

Four-Wheel Drive

Four-wheel drive vehicles can be classified into two categories; on-demand four-wheel drive and all-time four-wheel drive (also called all-wheel drive). The principal variation between the four-wheel drive system and the all-wheel drive system is the capacity of the former to convert from a two-wheel drive to a four-wheel drive system and back.

On Demand Four-Wheel Drive

how 4WD works
On-Demand 4WD System

The on demand four-wheel drive vehicle typically spends ninety-nine-percent of its life in two-wheel drive mode, but when required it can be converted into a four-wheel drive vehicle. Typically, a four-wheel drive vehicle is pushed by the rear wheels during normal operation. When four-wheel traction is required, the front wheels are “locked-in.” Early models used manually locking hubs in the front. Manually locking hubs require that someone exit the vehicle and physically turn an actuator on the front hub, locking the front axle into the hub and allowing the front wheels to pull in addition to the rear propulsion. Modern vehicles use electronic actuators to lock the hubs; many will even lock without bringing the vehicle to a stop.

On-demand four-wheel drive vehicles use a transfer case, which is driven by the transmission, to transfer power to the front and rear wheels. Two-wheel drive and four-wheel drive modes, as well as high and low gear modes are controlled via the transfer case. Some vehicles utilize a manual shifter for mode selection of the transfer case, while many late model vehicles use an electronic mode select system. Two drive-shafts are used, one for the rear differential and another for the front differential. The front driveshaft remains in neutral until the transfer case is placed into four-wheel drive mode at which time it also uses power transferred from the engine, through the transmission and transfer case, to drive the front wheels.

This type of four-wheel drive design is normally used in trucks and SUVs. Most sports cars and mini-vans, which use a form of four-wheel driveline, are equipped with an all-wheel drive system that varies from the on-demand system by remaining constantly under power from all-four wheels.

The chief benefit to an on-demand four-wheel drive system is the ability to engage the front wheels only when needed, preventing unnecessary wear and tear on front end and front driveline components.

S.M. Darby

S.M. Darby

I am a freelance author with over 25 years of experience as a professional, ASE certified automotive technician and shop owner, muscle car enthusiast, avid street racer, and classic car restoration specialist.

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