Four-wheel drive (4WD) — core purpose and benefit on low-traction surfaces In light vehicles equipped with a selectable or full-time four-wheel drive (4WD) system, what is the primary purpose of 4WD in everyday engineering terms, especially when the road surface is slippery (snow, mud, gravel, wet grass)?

Introduction / Context:
A four-wheel drive (4WD) driveline distributes engine torque to both the front and rear axles. This question probes the fundamental reason manufacturers equip vehicles with 4WD: to improve traction and mobility when the available tyre–road friction is low.

Given Data / Assumptions:

• Vehicle has a functional 4WD transfer case or coupling.
• Road surface offers reduced friction (ice, snow, mud, loose gravel, wet grass).
• Tyre type and inflation are appropriate and the braking system is standard.

Concept / Approach:
Traction is limited by the normal load on a tyre multiplied by the friction coefficient. When only one axle is driven, wheel slip on that axle can limit launch or hill-climb performance. By sending torque to four contact patches instead of two, 4WD increases the probability that at least some tyres have usable grip, thereby allowing higher tractive effort before slip occurs.

Step-by-Step Solution:

Verification / Alternative check:
Comparative testing shows shorter launch times and improved gradeability for 4WD over 2WD on low-μ surfaces, all else being equal.

Why Other Options Are Wrong:

• Improved cornering on dry roads: chassis tuning and tyres dominate; 4WD alone is not for dry-road cornering gains.
• Eliminates snow tyres/chains: false; tyres remain critical for safety.
• Ensures effective braking: braking is primarily limited by tyre friction and ABS, not 4WD torque delivery.
• Guarantees better fuel economy: 4WD often increases mass and losses, slightly reducing economy.

Common Pitfalls:
Assuming 4WD defeats physics; ignoring tyre choice; confusing 4WD traction under power with braking capability.

Final Answer: