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http://www.theacorn.com/News/2001/0405/Motoring.html
The car features BMW’s all-wheel-drive system that is used on the X5 sports activity vehicle and is now becoming available in both the sedan and wagon product range from BMW
I’ve spent a week with the 2.5 liter engine version of the 3 series sedan and it was raining, the best weather for this exhilarating sports sedan. All-wheel drive is available in three models of the 3 Series, two sedans, the 325xi and 330xi, and the 325xi sport wagon.
Using a version of the all-wheel drive system in the X5 Sports Activity Vehicle, power is split 38% front and 62% rear. This ensures that the dynamic driving character, for which the 3 Series is famous, remains undiminished. In slippery conditions, power is diverted to the wheels with greatest traction using a sophisticated version of BMW’s Dynamic Stability Control system, known as DSC-X. All three models are available with a choice of 5-speed manual or a 5-speed STEPTRONIC automatic transmission, which was in my test car.
Throughout its four generations since 1975, yet now perhaps more than ever, the BMW 3 Series is seen worldwide as an absolutely unique, highly successful vehicle concept that, by its very quality and qualities, invites imitation.
And yet, try as other marques may, the 3 Series is quite inimitable. It represents a concept that BMW originated and has nurtured for decades, one that cannot be defined by mere features or format. The 3 Series does embody certain consistent features; inline engine, rear-wheel drive and always a choice of manual or automatic transmission. And its format is just as consistent: a sporty driving machine with practical people and cargo-carrying capability. It also is, and always has been, a tightly packed bundle of muscles, built to quality standards that give it precision, reliability and durability.
At the heart of BMW’s all-wheel-drive (AWD) is a transfer case with planetary center differential. Planetary gears are so-called because they resemble a sun with planets orbiting around it (sun gear, planet gears). Almost universally used to achieve gear ratios in automatic transmissions; planetary gearsets can also be used to apportion torque to two output shafts in a predetermined way, as is done here.
This split is 38% to the front wheels, 62% to the rear wheels. Because the system includes All Season Traction – BMW’s acclaimed all-speed traction control system, here operating on all four wheels – this basic split can be modified, as required by road and weather conditions, by the All Season Traction system. Advantages of the 38/62 split are neutral handling, with mild "transition effect" when the driver lifts the accelerator while cornering. The xi models handle essentially like rear-wheel-drive BMWs under normal traction conditions.
Minimal steering effects from the front-wheel drive system (absence of the torque steer exhibited by many front-wheel-drive vehicles). Equal-length axle shafts (outboard of constant-velocity joints) also help minimize torque steer. Excellent directional stability, because more than half the driving torque goes to the rear wheels.
How it works. From the center differential, positioned just behind the transmission, torque proceeds straight back to the rear differential. Via a roller-chain drive and open driveshaft (running along the left side of the transmission and engine), torque is transmitted to the front. The front differential is at the left side of the engine pan; from there, axle shafts run – through a constant-velocity (CV) joint to the left front wheelthrough the engine oil pan (but sealed from the engine oil supply) and another constant-velocity joint to the right front wheel.
It is the equal length from these two joints out to the CV joints at the wheels themselves that constitutes the equal-length shafts that avoid torque steer.
All Season Traction functions with AWD just as with RWD, by modulating engine power and applying individual wheel brakes as necessary to control and distribute engine torque among the wheels – except here, it works on all four drive wheels instead of just the rear ones.
We don’t get the constant bad weather that other parts of the country get. It’s here that this car shines but even with our days of sunshine the feeling of safety that all-wheel-drive promotes is worth the little extra that BMW charges for the Xi models.
Take a test drive at your local dealer and pick a rainy day!
posted by 65.95.5...
, Sun, 3 Nov 2002 20:30:53
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