Braking: One Step Up, Two Back

There is an aspect of the 5 Series that we do object to, though, and that's the braking. To be clear, the car stops well enough; it's not a safety concern. The problem is the feel of the braking, which pays the price for one of BMW's "Efficient Dynamics" provisions. Like the new 7 Series, the 5 Series employs Brake Energy Regeneration, which is a distant cousin of the regenerative braking that hybrids use to recover spent energy and use it again. In this case, there's no motor helping propel the car, but the generator (alternator) can be turned on and off with a clutch.

Driven by a belt, an alternator puts a constant load on an engine, and that hurts efficiency. This is the same reason belt-driven hydraulic steering pumps have been replaced by electric power assist in many cars, including the 5 Series. The alternator must be turned by the engine, but it doesn't have to be all the time. BMW's alternator has a clutch that releases the belt drive as often as possible, allowing the ignition and any accessories to run off the battery. The battery isn't much different from a regular 12-volt starter battery, though it's designed for deeper cycling — to discharge as much as 40 percent and then recharge. (Conventional starter batteries are kept topped off all the time.)

It's when the car is decelerating that the alternator reengages and recharges the battery. This is a clever idea that BMW claims is good for a 3 percent efficiency boost, or roughly 1 mpg in mileage terms. Unfortunately, it makes the braking feel uneven, nonlinear. As you brake, the transmission downshifts through the gears as the car decelerates, aggressively locking the torque converter whenever possible. This allows the car's inertia to turn the engine and its alternator, optimizing electrical regeneration. In a normal arrangement, an automatic transmission holds on to a high gear as you decelerate and brake, ultimately downshifting when you come to a stop. That sensation is smoother.

The problem is that the degree of engine braking varies as the 5 Series' transmission downshifts through the gears, so even though you're applying steady pressure to the brake pedal, the deceleration is uneven and you tend to come to an abrupt halt. For what it's worth, we thought this sensation was worse in the 550i Gran Turismo we tested, but it's still a problem here — and it's not what we'd expect from a company that once touted "The Ultimate Driving Machine" (recently overshadowed by the ill-advised "Joy" advertising campaign).

Given a choice, I'd want BMW to tweak the operation or eliminate the transmission's downshifting behavior during braking. I don't know how much that would hurt the mileage, but I suspect any driving enthusiast would prefer it. The driving experience is neither ultimate nor joyful. I wouldn't like it from any brand, but it's especially hard to accept from BMW.

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