The attraction of better traction

BMW’s Dynamic Performance Control raises the bar even higher for slippery driving.

ARJEPLOG, Sweden–For much of modern motoring history, four-wheel drive in passenger cars has been a brand identifier primarily for Subaru and Audi.

More recently, everybody’s trying to get into the act – or get out of the snowbank. I was personally surprised to learn that 25 per cent of BMWs sold last year were four-wheel drive.

(Point of clarification: like a few other recalcitrants, I continue to use “four-wheel drive” to indicate a car whose all four wheels have power sent to them. Hence, there’s no difference between four-wheel drive and all-wheel drive; the former term is just more accurate.

I mean, if you have a three-wheeler and they’re all driven, that’s all-wheel drive too, right? The marketing dudes use all-wheel drive to indicate systems that divide power front-to-rear automatically, without driver intervention.

But if that’s what they mean, why don’t they just say so? Oh yeah, I forgot – they’re marketing dudes.)

It was primarily Audi that showed four-wheel drive wasn’t just for getting out of snowbanks, when a couple of quattro sedans blew Trans Am sedan racing apart in 1988. (Needless to say, they were banned by the rule-makers.)

Dividing the available traction between four tire contact patches instead of just two allows the car to put more of the power to the ground, more of the time.

The challenge for the engineers is to make sure just the right amount of power gets to each contact patch. The devil is in the details.

Pardon the up-coming drivetrain primer, but this may be necessary to understand what’s going on here. For me, I mean.

In a corner, the front wheels of a car must go farther than the rears, and the outside wheels must go farther than the inside. Thus, each wheel must turn at a different speed.

If the wheels on a given axle are not driven, like the fronts on a rear-drive car or the rears on a front-driver, no worries: they just turn as fast as they are pushed or pulled.

But if you try to put torque through them, then you have a problem. A differential, something you may have only ever seen as that big ugly lump in the middle of the rear axle of a pick-up truck, attempts to solve said problem with a combination of spinning gears that is truly bewildering. Heck, I’m an engineer and I can barely figure out how a diff works.

But if it didn’t work, the axle would bind up and the car would skitter all over the place.

This introduces another problem: that is, the mechanism that allows the outside wheel to spin faster also means that if you try to put too much power to that axle, the wheel with the least amount of grip can spin freely, thereby defeating the purpose. There’s that snowbank again.

In cases where traction is of the utmost importance, such as sports cars, racing cars, and any car on a wet or snowy road, various forms of slip-limiting differentials have been devised.

These diffs are good to a point. But all are in effect negative devices, because they reduce the amount of power available for the task.

What if you could figure a way to force the outside wheel to spin faster, thereby helping pull the car through the corner, rather than simply slowing the other side down?

You wouldn’t necessarily use all this power to go faster; you could at the same time increase the car’s steering sensitivity, making the car handle better and more safely.

Improve dynamics, driving enjoyment and safety?

Win win win.

The theory is old; J. Walter Christie was working on it in the early 1900s. But the combination of age-old mechanical systems and fast-acting and reliable electronics have finally made this goal a reality.

Honda introduced its Super Handling front diff on the Prelude in 1997, but not much came of that.

The same company’s Super Handling AWD system is currently offered on the Acura RL, and it works a treat, making that car surprisingly dynamic and quick, yet safe as church.

And now BMW has shown its latest development, something called Dynamic Performance Control, or DPC. (Sometimes I think BMW spends as much time developing the three-letter acronyms as it does the technology.)

Adding an additional planetary gear set and a pair of electronically controlled multi-plate clutches to the differential in effect can direct more power to the outside wheel of the axle.

In action, it is the perfect illustration of the corollary of Murphy’s Law, which states that any sufficiently advanced form of technology is indistinguishable from magic.

What makes DPC unique is that it functions whether there is power supplied to the wheels or not, while Honda’s system only works under load.

Theoretically, DPC could be applied to either the front or rear axles of a car, or both; and to front-, rear-, or four-wheel drive vehicles.

At the moment, BMW sees it as a further development of the xDrive four-wheel drive system, which in itself automatically divides torque between front and rear axles as required by driving conditions.

DPC in the rear axle then takes whatever amount of torque is available to the rear wheels and divides it left-to-right as appropriate.

Arjeplog, just 56 km south of the Arctic Circle and home to BMW’s new cold weather test facility, offers a wide range of driving conditions to test something like DPC: Snow-covered hills; plowed frozen lakes; closed-to-the-public, snow-laden winding forest trails; special low-friction surfaces so slippery you can barely walk on them, let alone drive.

Pairs of otherwise similar BMW models, one equipped with conventional xDrive, the other with xDrive plus DPC, were pitted against each other under a variety of conditions, and it really was no contest.

The DPC was not all that much faster through a slalom course; the biggest difference was that far less steering input was needed to negotiate the serpentine. The real-world advantage here is that a less-than-brilliantly-skilled driver would be more likely to get through an emergency back-and-forth steering manoeuvre with the DPC car.

One of the downsides of most four-wheel drive cars is a tendency to understeer, that is, plow straight ahead in a bend.

By its very nature, DPC reduces this tendency markedly, meaning the car is much more entertaining to drive, a major issue for a company whose motto is The Ultimate Driving Machine.

The most definitive test was an impromptu ice drag race on a “split-mu” surface – one side of the car on regular ice, the other on the polished low-friction surface.

Just like in the 1950s teen movies, the technician / starter held out both arms, then dropped his hands (Natalie Wood dropped her hanky, if memory serves …). The DPC car pulled away with ridiculous ease.

Likewise on the hills, skid pads, winding roads. It simply works, invisibly and effectively.

Now in one sense, any test set out for a four-wheel drive vehicle is doomed to failure. If the vehicle passes the test, the test wasn’t tough enough.

But if the vehicle fails the test, the vehicle isn’t good enough; understand that there are situations too difficult – hills too steep or too icy; mud too deep – that no car could possibly surmount.

The trick is to set the tests tough enough that your vehicle passes them, while the competitors can’t.

In this case BMW was competing with itself, and the new technology clearly wins.

The question now is, does it matter? Do you really need more traction than you can already get with today’s excellent chassis control electronics and modern winter tires?

The pick-up drivers often say that all four-wheel drive does for you is get you stuck deeper, farther from home – usually out of cellphone range.

The passenger car equivalent is that four-wheel drive can just let you have your crash at a higher speed. Even with DPC, four-wheel drive can increase acceleration and cornering ability, but doesn’t do a whole lot for braking.

Also, four-wheel drive tends to overstate the amount of grip the driver perceives, unlike rear-wheel drive – the most extreme example – which instantly tells you by virtue of the rear end fishtailing all over the place that you’re out of grip and should back off.

Still, “backing off” could mean backing off into the ditch, something devoutly to be avoided.

The above caveat could be applied to any advanced technology, as it has to ABS brakes, for example.

But engineers, by their nature, believe that a sharper knife is better than a sharp one. Their job is to develop sharper knives; our obligation as consumers is to learn how to use them properly and safely.

There is no doubt that BMW has developed a sharper knife with DPC.

At this stage the company is not saying when DPC will be made available to the motoring public.

Judging from the thinly disguised upcoming new-generation 7 Series luxury sedans that were scattered about the Arjeplog test facility; the fact that new technology is usually introduced in the top-end vehicles before trickling down to the lower classes; and the fact that unlike the 7’s uber-competitors, the Mercedes-Benz S-Class and Audi A8, the biggest Bimmer has never had four-wheel drive, that’s where I’m placing my bets.

See you in Frankfurt this September?

Jim Kenzie

Toronto Star May 05, 2007