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Re: Why BMW 325iX and 525iX sucks?
Egons Bush <email@example.com> wrote
> I've heard a lot from Audi Quattro fans that BMW 325iX and 525iX AWD
> sucks. The only argument they use is that Audi knows how to make AWD
> cars and BMW does not. That's great, but that would be very
> interesting to hear some more technical information.
I am not familiar with the design of the 525iX, which isn't available in the
USA, but I can give a quick description of the 325iX that was available here
from ~1987 to ~1991.
The car is an E30 BMW just like a standard 325i of the time, but the front
suspension was modified to accept the front drive. It shared the same 165hp
(could be easily chipped to 180hp) 2.5l straight six of the 325i with a
different oil pan. Basically, power comes out of the transmission into a
transfer case where torque is split 35% front, 65% rear by means of a planetary
gear set (viscous locking). An ordinary driveshaft sends power to the rear
differential (viscous locking also) and a "metal chain-belt" links the front
driveshaft to the center differential.
This front driveshaft runs to the front of the car alongside the transmission
into the front open differential. This front differential is housed in the
alloy oil pan and one driveshaft goes directly to the left-front wheel and the
other goes thru the oil pan (under the crank) to the right front wheel. These
front driveshafts are equal length to avoid torque steer. The front subframe,
the front a-arms, and front suspension geometry were all redesigned to accept
the AWD hardware.
With rear torque bias, it tends to behave like its RWD brothers, something that
was very important to BMW. Torque split also closely matches weight
distribution when accelerating from a stand still. Since the center and rear
differentials are of the viscous locking type, there are no diff locks on this
car - they "lock by themselves" when wheelspin occurs, and then unlock after
traction is regained. Also, since there are no mechanical diff locks, the ABS
works all the time.
I theory, it was a great system. Works automatically, doesn't deactivate the
ABS, and has close to the same driving feel of the other BMW's. In practice
however, it was more complex than quattro and not as bulletproof. Having so
many parts specific to that model alone, repairs can get expensive - especially
the transfer case that is lubed by only a quart of ATF. At least the locking
differentials are maintenance free. It also had a higher center of gravity,
ride height, and curb weight. In stock form, it could give a quattro a run for
its money, and probably beat it (200q20V, S4, RS2, etc. excluded), but power
gains are limited by the ability of the drivetrain to handle it and repairs and
maintenance can get pretty expensive.