[naylon]

How does 550i xdrive transfer power to the road?

On a poor surface, if there is wheel spin, does traction control come ON before power is transferred to the front wheels?

Or traction control is not triggered most of the time and power is transferred to the front wheels if the ECU detects wheel spin?

[SC300T]

When launching there is always power applied to both axles. The transfer case just decides the torque split percentage based on algorithms stored. It isn't a cheapo Honda "slip then grip" setup that is generally 2wd and has to detect slippage before torque is applied to the other axle. X-Drive is a very good setup for rain and putting the power down. If the front starts slipping first, it will move more torque to the back and vice versa. When cruising down the road both axles are driven. I forget if the default was 40/60 or 50/50, but it is somewhere in there. It does use individual brakes to transfer torque from left to right on slippage, and I think it also brakes an inside wheel when cornering to help rotate the chassis.