Introducing new technologies or updating existing automakers face a challenge. Buyers are not particularly in a hurry to understand their device (which is understandable) and purpose. And they often demand from cars not what they are actually capable of, but what the buyer wants. Hence unsatisfied expectations, criticism, or, most dangerously, problems on the road.
It is good if the buyer understands that his new expensive car is capable of more than the driver himself. And he is ready to shell out a tidy sum for special programs where he will be taught how to properly use the arsenal of auxiliary systems. But how many such buyers? Therefore, the task of bringing knowledge to the masses falls on the shoulders of journalists. Which companies are ready to teach how to drive their cars correctly at their own expense.
It was on such a thorough acquaintance with the BMW xDrive all-wheel drive that I went to snowy Austria, where a BMW training center has been operating for many years at one of the popular ski resorts.
Immerse yourself in the BMW atmosphere
After a long and sleep-free journey from Yaroslavl to Munich (train, shahid taxi between Moscow stations, Aeroexpress and a plane to Bavaria), I did not immediately realize that a pretty blonde with a BMW logo in her hands was meeting me. And that the role of a shuttle to the meeting point with a group of journalists flying from Kyiv will be performed by a brand new "three-ruble note". And the “three-ruble note” itself is such that you will not find it in Kyiv. In a configuration close to the top, with navigation, leather interior and manual transmission. Under the hood, of course, diesel, gasoline cars are rare in Germany.
The route to Austria was laid out in advance, the points for changing drivers so that no one was left offended were marked. Overwhelmed by the desire to plop down in the 750d director's seat and sleep the whole way, I took a seat to the right of the driver and fully enjoyed the most picturesque route through the Bavarian land. Fortunately, on this day we were in no hurry and the route was laid according to the principle “beautifully”, and not “quickly”. Light sleet did not cause trouble, but on the contrary, it became a pleasant entourage of the trip.
Approximately half of the road was spent on dealing with numerous seat settings (all, of course, on electric drives). Despite my attempts to pull out the headrest with meat, he did not give in, I had, again, to look for the right button. The final victory over the seat was the found massager, which invigorated my body and spirit before I took a seat in the driver's seat.
Since childhood, I have read many times the impressions of my future colleagues, both domestic and Russian, about their tests of the BMW seventh series. And in each, the author did not fail to mention how he and his test partner argued about who would drive the car like a hired driver, and who would pretend to be a serious uncle who was being driven on important, maybe even state affairs. Sorry, but that's all, as the Americans like to say, bullshit. The BMW 7-series is more than a driver's car, which is evident from the first few miles of winding hilly roads. And the very next day we were 100% convinced of this, because we had to learn the wisdom of all-wheel drive control on the seventh and fifth BMW series. But if between the cones the dimensions and weight of the “sevens” made themselves felt, then on the road there is absolutely no feeling that you are driving a huge F-class car. It only appears when parking.
A three-liter triple-turbo diesel with a monstrous torque allows you to perform any speed-related maneuvers without hesitation. And the eight-speed automatic does everything possible to maximize the potential of the motor. And numerous electronic assistants made it possible not to get lost on the road, observe speed limits and safely get to the hotel, despite the conditional three hours of sleep on the train. Once I seemed to blink for four seconds and opened my eyes already from the vibration of the steering wheel, which warned that an uncontrolled shift beyond the marking line had begun. And the active cruise at that time monitored the distance to the front car.
Getting to know xDrive
But now, after a good rest, the next day we set off to the destination of the trip. A special training ground in the mountains, where we had to learn the principles of the xDrive all-wheel drive system. Which first appeared on the X5 crossover and gradually migrated to traditional BMW sedans and station wagons in response to customer requests. It's no joke, in Germany, every third BMW sold last year was equipped with all-wheel drive.
Thirty kilometers by car, climbing a kilometer on a snowcat that simultaneously functions as a shuttle, and finally, we are at the point of 2,684 meters, where one of the many BMW Drive Experience training centers operates on the basis of a ski resort.
Mandatory briefing on safety, proper seating and grip on the steering wheel, followed by a short theoretical part about how xDrive works.
And here we have machines on which we have to learn practice and develop skills. Three 4WD cars (two fives and one seven) plus one rear wheel drive seven to make a difference on every exercise.
Start from a place
The emergence of xDrive in cars that do not claim to conquer off-road is BMW's answer to the desires of consumers. Indeed, with all due respect to the rear-wheel drive, which allows you to drive recklessly and cheerfully, in winter it often passes. On slippery surfaces, the safety systems do their best to keep the car stable, but when starting from a standstill in the snow, they simply choke the car, preventing it from accelerating. Otherwise, it’s impossible, it’s worth turning off DSC, as the car instantly turns sideways, even when starting from second gear. And here it is impossible not to make a lyrical digression on the topic of two systems - DSC and DTC.
DTC- Traction control system that prevents slipping and skidding. It is disengaged by a short (about a second) pressing on the system disable button and allows the driver to control the traction at will. But DSC remains on guard.
DSC- this is what is commonly called the stabilization system. That is, the whole complex responsible for ensuring that the car remains stable in any situation. It is able to help with a sharp change in the adjacent lane, keep the car on a slippery surface and prevent rhythmic skidding. Moreover, it works, if possible, proactively, analyzing information from dozens of sensors and passing it through the algorithms developed over thousands of hours of tests. Five seconds of pressing the button and the driver is left alone with the car, all electronic assistants go away. The philosophy of BMW is the driver in charge. Since he decided to turn off all systems, then they will be turned off and will not interfere with management under any circumstances.
As a first exercise, we had to practice quick starts from a standstill in four-wheel drive cars and compare with a rear-wheel drive. And by sequentially disabling security systems, watch how the behavior of the car changes. Here you can clearly see the difference between enabled and disabled DTC and DSC. With all systems turned on, the car starts straight, DTC prevents the wheels from slipping and dampens excess traction. If you turn it off, the start will be more fun, with slipping and snow flying from under all wheels. At this time, DSC and the torque distribution system between the axles will do everything possible to start the car again smoothly. And if you turn everything off, then when you press the pedal to the floor, the rear axle will still begin to skid. After all, the moment between the axles is initially distributed in a ratio of 40/60 in favor of the rear axle. But, if necessary, it can be thrown forward in a fraction of a second, so even with DSC turned off, the car quickly stabilizes with minimal steering adjustment.
What about rear wheel drive? On the snow, the mono-wheel drive BMW 740d turns from a fast-moving car into a limited-capacity car. With the security systems turned on, she hardly starts moving, not even trying to keep up with the all-wheel drive rival. If everything is turned off, then performing a U-turn from a place becomes easier than ever. A more or less quick start can only be obtained with the DTC off and from second gear. But at the same time, the driver must quickly and accurately compensate for developing skids with the steering wheel. The verdict is unequivocal, when starting on a slippery surface, xDrive steers and steers, no options.
Let's drift!
No one will deny that BMW is a driver's car. And it's just not fun to drive straight. Therefore, all-wheel drive was designed with the expectation that buyers will choose confidence and safety, but at the same time not deny themselves winter fun. Engineers have gone to great lengths to keep the character of all-wheel drive vehicles distinctly rear-wheel drive. But, if necessary, help the driver get out of a difficult situation. We had to check how they did it in slips on eights and snakes. And, again, compare impressions with a rear-wheel drive car.
In any permanent all-wheel drive, the main problem for a trained driver is the unpredictability of the car's behavior in borderline driving conditions. There is a certain and understandable behavior of a front-wheel drive car, there is, again, a certain and understandable behavior of a rear-wheel drive car. And there is a permanent four-wheel drive, capable of changing its character in a split second in accordance with how the electronic brains have decided at the moment. Which, despite the titanic work of engineers and lengthy tests, may not behave at all as the driver expects from them.
That is why we need such classes where you can understand and get used to how this or that drive works. All theoretical calculations and slides will not replace several hours on a slippery surface. This is the only way to understand and feel the car, to develop the necessary skills, so that, firstly, to avoid critical situations, and secondly, already on reflexes, work out the demolition or skidding of the car without hesitation.
To the credit of the BMW engineers, it doesn't take long to get used to xDrive. Eight after eight, snake after snake, and an understanding begins to appear of how the car will react to the gas supply, how to work with the steering wheel and where is the line beyond which everything is already and the car with disabled security systems is making final preparations before the collision - it closes all the windows and attracts the driver seat belt. Frankly, when the car slid sideways down the slope, the adrenaline rush from the unexpectedly tightened belt was greater than from the possible touch of the snow blade.
Circle after circle, snake after snake, turn after turn, and what seemed like a huge car becomes an obedient instrument. Like an expensive violin, she reveals her soul to the driver and, like a fluff, slides like a wide fan along a snake, gently shifting from turn to turn with the desired amplitude. And now the walkie-talkie now and then begins to respond approvingly “Looks great!” after a beautifully passed snake and a spectacular semicircle with a skid to continue the exercise in the opposite direction. Here it is the drive that BMW all-wheel drive can provide.
Here are a couple of videos showing how it was, thanks to colleagues from Avtotsentr. On the first video, your obedient servant is driving the nearest "seven". On the second, it seems, on it, but I'm not sure, since we constantly changed cars, and the quality of the video does not allow us to see it accurately.
Descents and ascents - operation of additional systems
Unfortunately, all good things come to an end sooner or later. And, returning through the tunnel back to the starting point and having a rest, we went even higher, to conquer new peaks. On a steep descent, a rearrangement was prepared, on which they had to try to change the lane with braking from a speed of 50 km / h. And besides, try the descent assistance system, emergency braking on your own and with the help of an electronic parking brake, and on a reverse slope, an uphill start assistance system that keeps the car even on a steep slope.
Having grown bolder after drifting exercises on the descent and ascent, it seems that I didn’t even go into a single turn. But everything was under the supervision of DSC, there was no desire to roll three kilometers down the slope. I don’t see much point in describing individual systems, I can only say that the descent assistance system works right up to 40 kilometers per hour and allows you to intervene in the control of the car at any time by adding speed to that set by the joystick on the steering wheel, or vice versa, slowing down. The operation of the system is not interrupted.
It works interestingly on snow and ABS, blocking the wheels in the last meters of braking, “digging” them into the snow and stopping the car. Here we were once again convinced that on a slippery surface there is no need to try to be smarter than ABS. Both the schedules at the introductory briefing and our own practice showed that the most effective is braking immediately to the floor and further practicing the stop with the anti-lock braking system. Both intermittent braking and working on the verge of ABS activation give longer stopping distances.
Smart DSC also effectively helps you change lanes on snow. The main thing is not to work too actively with the steering wheel and let her understand the driver's intention. And only then the ABS will work in such a way as to accurately guide the car between the chips without the slightest wagging of the stern. If the steer is too active, then the right front (in our case, the left rearrangement) will start sliding sideways and then the car will have to be actively caught at the exit of the maneuver. Not everyone succeeded, as can be clearly seen from the bumper and hood of this fifth series. Where the adhesion of the wheels to the surface ends, no all-wheel drive can help.
The day in the mountains flew by like a moment. Having received commemorative certificates of successful completion of the course, we again plunged into the snowcat and went back to conditionally "our" cars, on which we had to make the way back to Munich.
autobahns
On the way back, our crew got a BMW 530d GT xDrive. In the back, which has not become particularly popular in Ukraine. But in vain. With forms close to sedans, the car is very practical. With a high seating position for excellent visibility, plenty of headroom and a huge trunk that is easy to access. But with us, a BMW is first and foremost a status symbol of the owner. And, therefore, necessarily a sedan or crossover. While the whole of Europe, without looking back at anyone, drives the premium station wagons of the big German trio. But this is a completely different topic.
I changed my driving partner for the test a few kilometers before leaving for the autobahn. On the windshield, where the navigation system data and automatically readable speed and overtaking limits are projected, the long-awaited sign “the end of all restrictions” lit up and the car, joyfully roaring with a six-cylinder diesel engine, rushed to the mark of 210 km / h, which we were not recommended to exceed by an eloquent sticker with a speed index tires. A little less than 30 minutes and behind you were 100 kilometers to the BMW press park. At the same time, I did not experience any particular stress for such a long ride at a crazy speed for Ukraine. Gentle turns, a minimum of junctions of other roads, a clear understanding that there will be no unregulated pedestrian crossing ahead, and the road on both sides is clad in bumpers, guaranteeing that neither a person nor an animal will accidentally wander here. And the drivers giving way to the left lane as soon as you appear in their mirror. About the quality of the coating, I think, it is not worth mentioning.
At the same time, the car did not require any extra effort to maintain a speed of 200-210 km / h. The engine rpm was kept around 3,000 and the average fuel consumption was 13 l/100 km. The usual speed of 130 km / h, the 530d GT is able to maintain at just 1,500 rpm and consume 7 l / 100 km at the same time. And the power and torque of a three-liter diesel engine (245 hp, 540 Nm) is more than enough for all cases in which fast acceleration is necessary. It is difficult to understand for what purpose it is worth choosing a more powerful engine for this car or even for the seventh series.
Eventually
Three days with xDrive on asphalt and snow gave an unequivocal answer to the question of why it is worth buying an all-wheel drive BMW. It provides more than enough "fun" for those who like to test their not-quite-civilian riding skills from time to time. The car remains rear-wheel drive in nature, but at the same time has all the advantages of all-wheel drive. Giving confidence in the winter and significantly pushing the line beyond which counter-emergency driving skills may be needed.
All photos from the BMW xDrive test
We express our gratitude to AVT "Bavaria" for such a detailed acquaintance with the all-wheel drive BMW xDrive
This all-wheel drive system was developed by the BMW concern and can be attributed to permanent all-wheel drive systems. Depending on the driving conditions, the system can provide stepless, variable and continuous torque transmission. This system is installed on sport utility vehicles and passenger cars.
There are four generations of the xDrive car system:
1. First generation - installed since 1985, the ratio of the transmitted torque is 37:63, there was a blocking of the center differential and rear wheel differential with a viscous coupling.
2. Second generation - installed since 1991, transmitted torque in the ratio of 36:64. Blocking of interaxal and rear interwheel differentials with a multi-plate clutch. It is possible to redistribute the torque between the axles from 0 to 100%.
3. Third generation - since 1999, torque distribution in a ratio of 38:62. Interaxal and interwheel differentials of free types were used, the interaction of the system with the system of exchange rate stability is possible.
4. Fourth generation - since 2003, the torque is distributed in a ratio of 40:60. It is possible to redistribute torque between the axles from 0 to 100%, electronic differential lock, interacts with the exchange rate stability system.
Unlike the system, the classic rear-wheel drive transmission served as the basis for the xDrive all-wheel drive system of cars. The distribution of torque is carried out by "razdatkoy". It consists of a gear train that is controlled by a friction clutch. In the transmissions of sport utility vehicles, a chain gear is installed instead of a gear.
Transfer box diagram
xDrive interacts with the DSC stability control system. The system also includes electronic differential locks, DTC traction control and HDC descent control.
The interaction of xDrive and DSC is provided by the ICM integral control system, and it also provides communication with the AFS active steering system.
How BMW xDrive works
The operation of the xDrive system is determined by the friction clutch operation algorithm. The system has the following modes:
1. Start from a place
2. Driving with understeer and oversteer
3. Driving on slippery surfaces
4. Parking
Start BMW from a place - if the conditions are normal, then the friction clutch is closed, the distribution of torque in the ratio of 40:60, this allows you to develop maximum traction during acceleration. When reaching 20 km / h, the torque begins to be distributed depending on the driving conditions.
Driving with oversteer (rear axle skidding) - the clutch is closed with more force, more torque is transmitted to the front axle, the BMW begins to behave like a front-wheel drive car
Almost all automakers have all-wheel drive versions in their model lines. For the most part, only crossovers and SUVs have all-wheel drive. But there are also manufacturers who offer an all-wheel drive system on ordinary passenger cars - sedans, station wagons. It is noteworthy that only branded companies, including BMW, are engaged in the release of such models.
At the same time, each of these manufacturers has its own patented all-wheel drive technology. For Bavarians, this is the xDrive system. It is worth noting here that this is not something special and unparalleled. The general concept of all-wheel drive is identical for all cars, and the patenting of certain systems only secures the right for some specific design solutions.
General concept
The first BMW models equipped with all-wheel drive appeared in 1985. At that time, such a class as a “crossover” did not yet exist, and this manufacturer was not engaged in SUVs. But having appreciated the success of all-wheel drive versions of Audi, the Bavarians decided to install all-wheel drive on cars of their two series - 3 and 5. Such a system was optional. That is, of the entire rather extensive line, only some versions were equipped with all-wheel drive, and even then - for a surcharge. In order to somehow designate cars with such systems, an “X” index was added to their name. Subsequently, this index grew into xDrive.
It is noteworthy that the xDrive all-wheel drive is not aimed at increasing the cross-country ability of the car, because an SUV will still not work out of a station wagon and a sedan. Its main task is to provide better handling and stability of the car.
xDrive all-wheel drive device
The overall concept of all-wheel drive in BMW is classic, that is, it consists of:
- transfer box;
- drive shafts;
- Main gears of two bridges.
The list did not include differentials, because they are not so simple. BMW designers have constantly improved this type of drive, refining it and abandoning some design solutions in favor of others.
Drive designation
In general, with the advent of all-wheel drive versions and to the present, 4 generations of systems can already be counted. But the official name xdrive" she received only in 2003, with the release of the 4th generation, and before that, all all-wheel drive models were designated with the “X” index. In 2006, the xDrive system became the main one, all others were abandoned. But the “xDrive” designation has completely taken root, which is why many motorists even call earlier generations xDrive all-wheel drive.
It is noteworthy that with the release of each subsequent generation, not only the design changed, but the type of all-wheel drive itself gradually changed.
The xDrive system is positioned by the automaker as a permanent all-wheel drive (“Full Time”), but it is not, it is just a marketing ploy. It already belongs to the “On Demand” type, that is, with the automatic connection of the second axis if necessary. But all previous versions were "Full Time", but they were used on a limited number of models, while xDrive is available for almost the entire line of models, from sedans to full-size crossovers.
1st generation
As noted, the first all-wheel drive BMW appeared in 1985. The 4WD used then provided a constant supply of torque to the wheels of two axles, while the system was asymmetric, the distribution along the axes was 37/63.
Separation along the axes was carried out by a planetary differential, to block which a viscous coupling was used. This design made it possible, if necessary, to apply up to 90% of the traction force to any of the bridges.
The rear axle differential was also equipped with a blocking viscous coupling. But ahead, no locking mechanisms were used, the differential was free.
1985 4WD iX325 Model
Despite the supply of thrust to both axles, models with such a drive system were considered rear-wheel drive by default, since the torque was supplied directly to the rear axle. The rotation of the front axle was carried out due to the power take-off by a chain-type transfer case.
The “weak point” in the first all-wheel drive system used by BMW was the viscous couplings, which were far inferior in reliability to the Torsen locks used by Audi.
The first generation systems were installed on the 3 Series E30 325iX sedan, station wagon and coupe. Their production continued until 1991.
2nd generation
In 1991, the 2nd generation of the drive appeared - asymmetrical, with a 36/64 distribution. The Bavarians began to install it on sedans and station wagons of the 5th series (E34 525iX). At the same time, in 1993, the system was modernized.
Model E34 525iX
Prior to the system upgrade, an electromagnetic clutch controlled by the ESD system unit was used to lock the differential installed between the axles. The front end was also not equipped with any kind of blocking mechanism. The differential of the rear axle was blocked by an electro-hydraulic clutch. Due to the use of two clutches, it was possible to almost instantly distribute thrust between the axles with a ratio of up to 0/100.
After the upgrade, the design of the system has changed. The electromagnetic multi-plate clutch, which was controlled by the ABS unit, continued to be used as a central differential lock.
The use of locks on the main gears was completely abandoned, the differentials both front and rear were made free. But there was an imitation of a rear axle lock, the role of which was performed by the ABD (Automatic Differential Brake) system. The essence of its operation is very simple - by means of wheel speed sensors, the system detected slippage and activated the brake mechanism to slow down the slipping wheel, thereby transferring the moment to the other wheel.
3rd generation
In 1998, the 2nd generation was replaced by the 3rd. This type of all-wheel drive was also asymmetric, distributing power in a ratio of 38/62. They were equipped with models of the 3rd series (E46) in sedan and station wagon bodies.
This generation of all-wheel drive was different in that all differentials (center, wheel) were free. At the same time, there was an imitation of blocking the main gears by the system.
In 1999, the first crossover X5 appeared in the BMW model line. Its design also used the 3rd generation system. At the crossover, all differentials were free, but the cross-axle ones were blocked by the ADB-X system, in addition, the downhill motion control system, HDC, was also activated.
The 3rd generation of all-wheel drive on the 3rd series models was used until 2006, but it was replaced on the crossover in 2004. With this, the era of differential 4WD "Full Time" for BMW ended, and they were replaced by xDrive.
4th generation
The main feature of this type of drive is that the use of a center differential was completely abandoned. Instead, a multi-plate friction clutch was installed, controlled by a servo.
The xDrive transfer case with drive gears is used on passenger cars
In normal driving mode, the distribution of traction is carried out in a ratio of 40/60. But in a fraction of a second, it can change up to 0/100. The system operates in fully automatic mode, and there is no option to turn it off.
How xDrive works
Rotation is constantly fed to the rear axle, that is, a car with such a drive is actually rear-wheel drive. At the same time, the servo drive, due to the system of levers, presses the friction discs of the interaxle clutch, which makes it possible to take power and supply it to the front axle drive shaft.
If necessary, the servo drive changes the degree of pressure of the discs, changing the torque split. It either fully compresses them, providing a 50/50 transmission, or releases them, interrupting the supply of torque to the front.
xDrive chain drive transfer case for crossovers
The operation of the servo drive is controlled by a whole complex of systems, which ensures the redistribution of thrust between the axles in very short periods of time - 0.01 seconds.
For its work, xDrive uses the following systems:
- Chassis control ICM. Its task is precisely to synchronize the drive with other systems;
- Dynamic stabilization DSC (exchange stability). It not only manages the division of traction between axles. The system also "manages" and imitation of the differential locks installed on the main gears, slowing down the slipping wheels.
- Steering AFS. It provides stabilization of the car during braking, in which the wheels move on surfaces with different coefficients of friction.
- Traction control DTC;
- Help when driving downhill HDC;
- Redistribution of traction between the wheels of the rear axle DPC. It carries out "steering" when driving corners.
The main advantage of xDrive is its comparative design simplicity. The absence of mechanical differential locks greatly simplifies the drive unit and makes it very reliable.
Also, to change the functioning parameters, it is not necessary to alter something in the design, it is enough to make changes to the software of the systems that control the drive.
The main advantages of the xDrive system in operational terms are:
- Variable stepless torque sharing between the axles;
- Constant control over the behavior of the car and instant reaction to a change in the situation;
- Ensuring high levels of car controllability;
- High accuracy of functioning of the brake system;
- Vehicle stability under different driving conditions.
Thanks to the used friction clutch with an electronic control system, the xDrive system has a number of operating modes that adjust the drive to the driving conditions:
- Smooth start of movement;
- Entering corners with oversteer;
- Driving in corners with understeer;
- Moving on a slippery road;
- Parking in confined spaces.
Each mode has its own specific features. So, at the start, the friction clutch provides a redistribution of moments between the axles in a 50/50 ratio. This provides a dynamic set of speeds. But after reaching 20 km / h, the system begins to change the ratio depending on road conditions. The average ratio is 40/60, but this can quickly change if the electronics detect a change in conditions.
When the rear of the car begins to skid (oversteer) when entering a turn, the servo instantly compresses the clutch discs, providing 50% and more thrust to the front, due to which it begins to “pull” the rear axle of the car out of the skid. If these measures are not enough xDrive starts to use other systems to stabilize the car.
In the event of a drift of the front during a turn (lack of understeer), the drive, on the contrary, reduces the moment on the front axle up to its complete shutdown and, if necessary, still activates the stabilization systems.
When driving on slippery surfaces, xDrive makes the car all-wheel drive, providing up to 50% of traction to the front and including auxiliary systems.
In parking mode, as well as when driving at very high speeds (over 180 km / h), the servo cuts off the rotation to the front, making the car completely rear-wheel drive. This has its drawbacks, especially during parking. Due to the disengagement of the front, the car cannot always overcome even small obstacles (curbs) if the coating is slippery and the rear slips.
The disadvantage of xDrive is that it takes time to connect the axis, albeit a little. That is, the system turns on the front axle only after the skid has already begun. This can be somewhat disorienting for the driver, and he will take the wrong action.
The "weak" point in the very design of the xDrive all-wheel drive is considered to be a servo. But the designers took care of this by placing this unit on the outside of the transfer case, which allows you to quickly replace or repair.
Finally
The xDrive system has proven itself so well that it is offered for the entire model range - versions from 1st to 7th series, a number of cars equipped with 8-cylinder power plants (550i, 750i), and is also installed on all crossovers of the X series. ".
Note that for sedans, station wagons and coupes, the system is structurally different from the crossover drive. The difference between them lies in the transfer case. In passenger cars, it is of a gear type, and in crossovers, it is of a chain type.
So far, the Bavarians are in no hurry to change the xDrive drive, because it is really good and works great. Therefore, all the developments regarding the drive are just performance improvements, the design is not affected, because why remake something that functions perfectly.
AutoleekModern BMWs got all-wheel drive back in 1985. This was long before the advent of crossovers, so the Bavarians optionally equipped with such a transmission only the 3rd and 5th series, which received an additional letter x in the index. A razdatka with a center differential was attached to the gearbox, from which the drives went to the front and rear axles. In the systems of the first two generations (1985 and 1991), couplings of different design blocked the center and rear axle differentials.
In 1999, the BMW X5 crossover entered the market, equipped with a third-generation all-wheel drive transmission. Its fundamental differences are: all clutches are abolished, blocking of interwheel differentials imitate brake mechanisms under electronic control, the interaxle differential is completely free.
And in 2003, xDrive appeared on the X3 compact crossover, which subsequently registered on all all-wheel drive BMWs. The system has already undergone several upgrades, but its basis and principle of operation have remained the same.
BASIS OF BASICS
With all the innovations, the current xDrive has retained the fundamental architecture of its predecessors. An electronically controlled friction clutch helps to distribute the moment more efficiently between the axles, which, in fact, replaced the center differential and its lock. In addition, the X-Drive arsenal has an electronic system inherited from the first X5 that imitates the blocking of cross-axle differentials (ADB-X): it grabs a slipping wheel with brakes, allowing you to realize more torque on the other.
The redistribution of torque between the axles depends on the compression force of the clutch clutches: at the command of the electronics, they compress or diverge - depending on the situation. The clutch compression is controlled by a servomotor. The cunning lever (shown in the diagram below, position 2) converts the rotational movement of the electric motor shaft into its axial movement, which compresses or unclenches the clutches.
When the clutch is locked, part of the torque is removed from the rear axle and transferred to the front - through a transfer case with a chain or gear drive. Differences in design are due to the layout of the central tunnel. There is more space in crossovers, so they use a unit with a chain, and in cars - a more compact version with gears.
BMW is being disingenuous when it calls the xDrive transmission permanent all-wheel drive. In normal modes, the moment is distributed in a ratio of 40:60 in favor of the rear axle. In this case, the clutch is almost completely clamped (with full blocking, a rigid connection between the axles is provided, the moment is divided equally). If the clutch is disengaged, then all the moment goes to the rear axle. That is, in fact, we have a permanent rear-wheel drive with an automatically connected front axle.
Here's another publicity stunt. The manufacturer claims that the clutch can throw forward up to 100% of thrust. This will happen if, with the clutch fully locked (both axles are rigidly connected), the rear wheels hang in the air or end up on absolutely slippery ice, and there is dry asphalt under the front wheels. Then 100% of the torque can really be realized on the front axle, because the rear wheels do not have traction, that is, the torque on them is zero. But there is no magic in this - the laws of physics rule the ball, and not the unique design of the clutch. This task is up to any hard-locking differential. In addition, the situation described under normal conditions is unrealistic: even if the rear wheels are on mirror ice, the grip of the coated tires, albeit very slight, will still be, and with it an insignificant fraction of the transmitted torque will appear. Therefore, xDrive cannot transfer 100% to the front axle.
And yet xDrive is really effective and at the same time structurally simple. It is perfectly complemented by the electronic stability control DSC, which allows you to realize all the advantages of all-wheel drive: it improves dynamics and handling, while taking care of safety and does not infringe on the ambitions of the driver.
PLANNED MODERNIZATION
With the advent of the second generation of the X5 crossover in 2006, xDrive was also slightly updated. We limited ourselves to finalizing the control electronics, giving the system of exchange rate stability even more rights.
Before constructive changes, it came two years later. On the X6 model, an active rear differential with electronic control DPC (Dynamic Performance Control) was introduced into the X-Drive scheme. It is able to redistribute the moment between the rear wheels - this saves the car from understeer and allows it to take turns at a higher speed, remaining on the trajectory set by the driver.
The DPC has stepless blocking, up to 100%. Structurally, this is implemented by adding two planetary gears and a pair of multi-plate friction clutches controlled by electric drives. For the first time, such a scheme was demonstrated by Mitsubishi Lancer Evolution VII. At BMW, it is only available on the X5 and X6 crossovers. For younger models, its simplified electronic counterpart, Performance Control, was added as an option. This function is built into the stability control system: in a turn, it brakes the inside rear wheel to add momentum to the outside.
The absence of other changes to the design of the xDrive transmission speaks to the reliability of the system. Representatives of BMW claim that for the entire time of its existence, it has not caused serious problems. According to statistics, apart from oil seals and anthers of drives, the clutch control servomotor most often fails. But this happens closer to 300,000 km, and only every third, or even fourth owner rolls so much. In addition, the location of the assembly outside the transfer case simplifies the replacement procedure, and the price of the motor is low.
MOUNTAIN ANNIVERSARY
BMW decided to celebrate the 15th anniversary of its crossover line with high mileage on the winter roads of Montenegro. The route did not provide for off-road, but abounded in mountain serpentines. Actually, in such conditions, the capabilities of the xDrive system should be revealed in all their glory.
Before me is the entire line of crossovers, except for the younger X1. Cars are shod in winter non-studded tires. The temperature difference between the flat and mountainous parts of the route is from a slight minus to +15 ºС.
Limiters in the speed of driving on serpentines were only common sense and the instinct of self-preservation. Far from everywhere the width of the road allows you to freely pass with oncoming cars, and most of the turns are blind.
To be honest, driving for a long time at the limit of tire grip was scary and physically difficult. But under these conditions, xDrive never made me nervous, and sometimes pleasantly surprised. The older brothers X5 and X6 with an active rear differential fervently screwed into the studs. In sports mode, the stabilization system made it possible to misbehave a little and, under the addition of gas, exit the studs sideways. And in rare running and open turns, the older Xs, with increasing speed, more confidently relied on the outer wheels, as if the turn turned into a profiled one.
More restrained X3 and X4 provoked less active driving. But X3 still managed to please in one potentially dangerous situation.
Before the long-awaited open turn, the asphalt in the braking zone was covered with frost. The brake pedal vibrated frantically, and the speed dropped frighteningly slowly. But there was no need to take emergency measures: the X3 fit into the turn with a margin without losing stability. Well thank you xDrive!
The price of freedom
Free (open) symmetrical differential has a serious drawback. It always shares the torque equally. When one wheel loses traction, the other stops. For example, if we hang just one wheel on a four-wheel drive car with three free differentials in the transmission, it will spin helplessly, and the car will not budge. And in order for the car to drive, various differential locks are used to transfer part of the moment to the wheel (or wheels) with better grip: these are self-locking differentials, various clutches or their electronic imitators, which work under the control of the stability control system.
