BMW's xDrive all-wheel-drive system is permanently engaged and has a built-in rear-wheel bias. On smooth, dry roads, 60% of the power goes to the rear wheels and 40% to the front.
When xDrive detects wheel slip, it automatically sends more torque to wheels that have greater traction. In some situations, all power can temporarily go to either the front or rear. When parking, 100% of the power goes to the rear wheels.
- 2013 BMW 328 Pricing and Research (Cars.com)