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July 28, 2014

How Hill-Hold or Hill-Start Assist Technology Makes Steep-Road Starts Safer for Drivers

Hill_hold-iStock_24852251_SMALLIt's a scary moment for any driver when you move your foot from the brake to the accelerator on an incline and the car begins to roll backwards: Nobody likes the anxiety of that panicked glance in the rearview mirror to see "just how close is that car behind me?" And wondering "How fast can I pull away on this steep hill?" isn't much fun either.

While sturdy torque converters used to keep automatic-transmission-equipped cars in place on an incline, today's newer automatic transmissions are more fuel-efficient and tend to rev lower, which means they can slip more. And so these days, we're often experiencing that anxious rollback moment, even in automatic-equipped cars.

While a coordinated clutch/brake-to-gas/handbrake roll-off is a familiar maneuver for those who prefer to drive stick, not wanting to face hilly or inclined starts can be one of the primary reasons many people prefer an automatic gearbox -- and might explain why only about 5 percent of the cars sold each year are equipped with manual transmissions.

Enter convenient auto technology that is becoming more common: hill-hold, or hill-start assist technology, which makes navigating hilly terrain effortless, allowing drivers to get started without rolling backward or forward.

Hill-hold technology essentially offers an “extra foot” to manual transmission drivers to help novice and veteran clutch-shifters better manage hills, but we're also seeing it more frequently on automatic-transmission cars as well. “As fuel-efficient CVT transmissions with low-creep torque and low internal friction become more common, so does rollback on inclines,” explains to Nate Berg, Manager of Vehicle Testing for Mitsubishi Motors North America. “Drivers of manual-transmission-equipped cars are better prepared for such rollback, but drivers of automatic-equipped cars can find it quite disconcerting,” Berg says.


How Hill-Hold or Hill-Start Assistance Works

In a vehicle equipped with hill-hold, a pitch sensor detects the tilt of the body when the car is stopped on a slope and sends a signal to the vehicle's brake hardware to keep the wheels clamped for a few seconds after the driver releases the brake pedal. This can be helpful whether your car is pointed uphill or down.

It’s especially helpful on sloped roads on a rainy day, in slow traffic on long inclines, or when the person behind you inches too close in traffic. Hill-hold control just makes moving away after being stopped on a hill a total no-brainer by retaining braking force on the wheels for a couple seconds. That’s usually long enough for the driver to safely apply the accelerator and let out the clutch for a smooth start.

Hill_hold-iStock_86772367_SMALL“With hill hold assist on a CVT or automatic transmission, the brakes hold for approximately two seconds and then seamlessly blend their release with the pull of the engine,” explains Berg. “This gives the driver sufficient time to confidently accelerate away from the incline, regardless of whether the vehicle might want to roll forwards or backwards.”

We're finding this convenient technology across a variety of vehicle types, including the inexpensive and super fuel-efficient $14,800 Mitsubishi Mirage that we recently piloted around the hills of San Francisco; it’s even available in such luxury cars as the sporty $76,000 Porsche Panamera sedan.

The hill-hold function became standard for Porsche when single- and dual-clutch transmissions like the Porsche Doppelkupplung (PDK) found their way out of their racing vehicles and into street cars. Although shifts feel automatic, inside the PDK is a full-blooded manual transmission with two computer-operated clutches, unlike a normal automatic transmission with a torque converter that connects the engine to the transmission and slips when at idle speed (and keeps the car from rolling back on moderate inclines).

Heading-up-a-hillSo, when the PDK-equipped car comes to a stop, the computer disengages the clutch, effectively putting the car into neutral -- and if the car is on a hill (because it is technically a manual transmission) it will roll (either forwards or backwards). With the hill-hold function, the car recognizes if the car is going to roll and applies the brakes when the footbrake is released.

Once the technology was developed for the PDK, it was natural for the company to include it on Porsche's manual-transmission cars. The Cayenne was the first, and the Cayman and Carrera sports cars followed soon after.

If the driveway to your home or parking garage is sloped, or if you live or drive around an especially hilly area, you may want to inquire about this technology the next time you’re shopping for a new car.

Mitsubishi is quite proud to boast that it offers such a high-tech feature in its CVT-equipped Mirage, and we expect high-tech from a luxury performance brand like Porsche, but you might be pleasantly surprised to find convenient hill-start assistance available on such models as the Audi A3; Chevy Sonic and Equinox; Dodge Durango; Fiat 500; Ford Explorer; Kia Soul and Sorento; Honda Pilot; Infiniti QX; Jeep Grand Cherokee; 2012 Lexus RX, GX, and LX; Mercedes-Benz C-Class and M-Class; Nissan Cube, GT-R, Xterra, and Frontier; Subaru Forester and Impreza; and Volkswagen CC, Eos, Golf, Passat, and Tiguan; and many others.



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