BMW xDrive all-wheel drive system explained
BMW’s xDrive all-wheel drive system is one of the most sophisticated and effective on the market. Here’s how it works.
BMW’S ALL-WHEEL DRIVE system is called xDrive and is fitted to a range of BMW’s soft-roaders which at present are, in order of increasing size, the X1, X3, X4, X5, and X6. The odd-numbered cars are soft-roaders, with lifted suspension and larger tyres than the equivalent-sized road car. The even-numbered cars are sporting versions of the lower odd model, so X3 – X4, X5 – X6, and these have a coupe-style body with handling and power tuned for an even more sporty performance than the already sprightly base vehicle.
The SUV variants always do well for manufacturers and the the X series is no exception, which is why BMW is always adding new variants with a seven-seat X7 in the works. There are also a few M variants, which are cars tuned by BMW’s in-house performance arm, such as the X5M and the X5 M50D – yes indeed, BMW has created a 4×4 diesel performance car. Drive it and you’ll see why it deserves an M-badge.
The xDrive system is also available on selected roadcars such as the 3 Series wagon (but not here in Australia) and now the BMW M5 which you can read more about HERE.
What’s the point of xDrive?
The aim of xDrive is not to produce a serious off-road vehicle – you just need to look at the clearances and angles on any of the BMW xDrive cars to see that, let alone more subtle designs such as air intakes and underbody protection – so xDrive is more about improved traction on slippery surfaces and improving handling.
The xDrive system starts with traction control, familiar to 4×4 drivers as the system that brakes a spinning wheel to send traction to its partner. There’s also stability control, BMW’s nomenclature Dynamic Stability Control (DSC) which detects the car running wider (understeer) or the back end overtaking the front (oversteer) and applies the brakes individually to correct the course. There are even more sub-programmes, all designed to maximise traction such as the ABS enhancements of EBD and EBA, but let’s focus on what makes BMW and xDrive different.
Read more: traction control explained.
How does xDrive work?
This starts with a 40:60 split of the torque from front to rear. This is instantly good news, as most all-wheel drive systems merely power the front wheels and let the rears trail, driving them only when the computers detect a loss of traction (with the exception of Subaru). Those systems are never, ever as effective as the likes of xDrive because if all wheels are driving there’s less chance of slip in the first place, and for enthusiast drivers, there’s the sharper handling of a rear-drive biased car because the front wheels have more traction available for steering.
Interestingly, this 40:60 split also means the centre clutch that distributes torque front:rear is always working. Other systems which only occasionally drive the rear wheels have centre clutches designed to run for short periods only, which means they overheat after a bit of use, leaving you in 2WD and most probably, stuck. This irritating design flaw shouldn’t be a problem for any xDrive vehicle.
But the 40:60 ratio is not fixed, and the computers can send 100% of the torque to either the front or the rear axles via an electrically operated centre clutch. Imagine a hillstart with both rear wheels on ice and fronts on tarmac and you’ll see why that’s important.
But xDrive is clever enough to distribute torque even without wheelspin, for example during cornering. Let’s say the car is starting to understeer, which means running wide, caused by more grip at the rear than the front. Simple way to fix that is to ask the front tyres to do less driving, leaving more grip for turning, so that’s what xDrive does, simply biases torque to the rear. Same deal for oversteer where the front has more grip than the rear, it biases torque to the front, leaving the rear tyres more grip for turning.
How does it know to do this? Like any modern car there are sensors everywhere. Wheel speed, steering wheel angle, throttle and brake position, yaw and now even pitch and roll, all of these together form a torrent of data the computers use to determine what the car is doing, what it should be doing and in a new development, what it is going to do.
For example, you’re hammering into a corner and hit the brakes. The car can work out what sort of traction the surface affords from the rate at which the car slows down relative to the amount of braking force against wheelslip and other factors. Then you decide to turn into the corner. The car now knows how tight you wish to turn, the speed you want to turn at, and the traction of the surface. It can decide how easily you can make the corner, presumably rating it on a scale of “no worries” to “panic stations” (not BMW terms, mine). If the answer is up at the panic end of the scale then the computers can start to bring in various programmes such as CBC (corner brake control) which brakes individual wheels to help the car turn and much more, even before the car has begun to slip and slide. We have now entered the realm of proactive electronic driving systems, as distinct from waiting until there is an actual problem and then putting out the fire.
Another example of proactive tech is the suspension. BMW’s xDrive can instantly change the stiffness of the dampers, for example under hard braking it’ll stiffen the front, when cornering left it’ll stiffen the right and loosen the left.
There’s also Torque Vectoring, which is something that BMW have fitted to every X6 and are now slowly rolling out throughout the X-car range. A normal car has a differential which distributes torque 50:50 left and right, and when the inside wheel slows down, the outside wheel speeds up by the right amount to keep pace. Torque vectoring is a kind of slew-steer – when cornering a wheel is deliberately driven slightly faster than it would otherwise be in order to aid the turn.
For example, when cornering the outside wheel would be ‘over-driven’ to help the turn, but where the car is oversteering then the inside wheel would be driven more quickly. This is all done through a clutch pack that operates planetary gearsets, with the computers figuring out from the sensors what needs to happen when.
So that’s all the theory. In practice, it means the vehicle is always making the very best use of the available traction, and going where you point it. Everything works automagically, there’s no skills to learn, just point and drive.
Does xDrive actually work?
Yes, without doubt it’s better than a non-electronic-assist car, and even better than a more basic traction control equipped vehicle. Of course, BMW, like all manufacturers, pretends its technology is better than anyone else’s. In many ways that’s not true because it’s all the same basic gear underneath, and there’s nothing in xDrive you can’t find elsewhere – but only on BMW’s premium rivals such as Mercedes, Audi, Land Rover and the like, you won’t find this gear (yet) on many Korean or even Japanese brands.
Where I will give BMW points is in the overall calibration of the system which they do very well as their “driving machine” ethos is important; avoiding a front-biased drivetrain. Another advantage it’s keen to talk up is a weight distribution of close to 50:50 front-rear. Now, I know this ratio is much revered, and I disagree that it should be so but that’s another article.
Anyway, BMW is proud of the fact it doesn’t need a transaxle in the front because of their rear wheel drive architecture. Let me translate that. A transaxle is a “transmission axle” integrated combination often with the differential, and it’s what you find in front-wheel-drive cars. Like, ahem, Audi who BMW are so keen to contrast with. The transaxle is right at the front of the car, is great for space-saving and weight saving, but it does put a lot of weight over the front axle, especially as the engine has to go in front of the transaxle.
BMWs on the other hand are natural rear-wheel drives (for the moment, anyway) so they put the transmission behind the engine, so the engine can move further back so you get better weight distribution, and the weight distributed more centrally. This is all worthy engineering, and it’s design like this that starts to separate the good from the great.
The video below shows a bit of drifting and a split-mu traction demonstration with a BMX X3 and a Toyota Hilux.