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Hill start assist





11 July 2016


  • A perfect occasion to make use of hill start assist, especially if the Amarok happens to be a manual.
  • What a hill start assist system used to look like. This particular model is from an early Audi with Quattro.
  • These days hill start assist is available across various segments. From an Astra to a Chevrolet Trailblazer.
  • The 2003 Subaru Outback, which was the first modern car to reintroduce hill start assist.
  • The mechanical components of hill start assist.

Rollbacks are history

The humble hill start assist is often unfairly overlooked, but it’s an ingenious and helpful system for all road users

Reading a modern vehicle’s safety specification list can be a daunting task these days. The sheer amount of acronyms can be overwhelming. The XC90 on the cover, for example, comes with enough acronyms to fill this entire magazine.

Most of us know what the usual suspects stand for. The terms ABS, EBD, ESC and TC can be used without an explanation, but what about ACC, AFL, HUD, LDWS, PDBS, SBR, RTF and WTF?

Okay, so that last one is made up, but you get the point.

One trend we’ve noticed is the addition of acronyms starting with an ‘H’ and ending with ‘C’. There are a few other letters in between, depending on the brand of vehicle. Mostly, these stand for hill hold control, hill start assist or simply hill control.

It arrived in South Africa a few years ago, but these days it’s all the rage. You can have it in anything from a cheap and cheerful hatch to a double-cab bakkie. We’re big fans of this system. Why? Because it works. It’s one of those advancements that are worth keeping around, like Bluetooth connectivity and Emergency Brake Assist.

We’ll get to the technicalities in a bit, but the system basically prevents a car from rolling back when pulling away on an incline. It may sound unnecessary to some seasoned drivers, but it has many real-world applications.

It’s a confidence booster for younger drivers, as it gives them some additional leeway in terms of clutch control. Someone who has been driving for years would likely have mastered the process of lifting from the brake, applying the right amount of throttle and releasing the clutch to keep the car from rolling back, but a person who is just starting out may have some difficulty with this seemingly informal task.

And while many seasoned drivers might not admit to it, it’s nice knowing that there’s a system that will have your back on those odd occasions you lose concentration. It happens to the best of us.

The most important application, at least in terms of off-roading, is getting a large 4×4 to pull away smoothly on a steep incline. This is something even boffins struggle with, as we’ve seen numerous times at various 4×4 events.

This system is most useful in a manual vehicle, but it has become the norm in automatic and dual-clutch transmission vehicles too. The reason is fuel economy. Ten years ago when we didn’t care as much about emissions and such, an automatic would idle at higher revolutions, which meant it would crawl forward or maintain its position once the driver lifted his foot from the brake. With cars revving much lower in a standing position these days, a rollback in an automatic is just as much a reality as it is in a manual car. This is especially true in off-road conditions, where inclines are much steeper than those you’ll find during a daily commute.

So how does it actually work?

As is the case with so many brilliant inventions, hill hold assist is rather simple in the way it operates. Essentially, it’s nothing more than an extra few lines of coding in the electronic control unit (ECU), with no additional mechanical components necessary to make it work.

So the main components involved are the car’s ECU, a pitch sensor and braking system. The pitch sensor detects when a car comes to a halt at an angle, sends this information to the ECU, which then applies pressure to the brakes to keep the car firmly in place.

While the vehicle is stationary, the driver will likely keep his foot firmly on the brake pedal, but the important bit happens once the foot is removed from the middle pedal.
With the clutch depressed and the foot removed from the brake, a car without hill start assist would immediately roll back.

Hill start assist overcomes this by buying you a few precious seconds once you remove your foot from the brake, which gives you time to apply the right amount of throttle to get the car moving forward. It does this by still applying pressure to the brakes, even though the driver’s foot has been removed from the pedal.

It’s worth noting that this system is not foolproof, as most of these systems work on a timing basis, rather than detecting forward momentum. There are a few systems that will hold the vehicle until it moves, but the norm seems to be applied brake pressure coupled to a timer.

There is no industry standard with regards to the set time, but it ranges from three seconds in hatches, to five seconds in an off-roader like the VW Amarok.

Under normal driving conditions the allotted time is enough to allow for smooth acceleration up an incline, but it’s slightly trickier in off-road conditions. A smooth departure from a standstill on a rocky/muddy/sandy incline still requires intimate knowledge of the particular vehicle and how much rpm should be used.

Hill start assist can’t guarantee that the vehicle will move forward, but it will at least keep it from rolling back (for a limited time). If you do get it wrong, simply apply the brakes and the system will kick in again.

Off-roaders who prefer to do things manually would enjoy the benefits of this system the most, as it takes at least one task out of their hands, allowing them to concentrate more on feeding the right amount of power. Without hill start assist, it’s a balancing act between the handbrake, clutch, brake and throttle.

With the assistance in place, there’s no need to worry about the handbrake, so the driver can have both hands on the wheel and his attention entirely focused on the power delivery.

And it all started in 1937

Interestingly, hill start assist is much older than we initially thought. Our research suggests that Subaru was the first to offer it in more recent times on the 2003 Outback. It died down for a while, but surfaced again around 2010, at which point most manufacturers caught on to the usefulness of the system.

Subaru is, however, not the first. A basic mechanical version of hill start assist made its debut in 1937. It was designed by a company called Bendix Brake Company and was offered as an optional extra on the Studebaker President in 1939.

Back then there were no ECUs, but Bendix used sheer American ingenuity to overcome this problem. NoRoll, as it was known, used a ball bearing in the brake line. When the car was stationary on an incline, the ball bearing obviously rolled back and blocked the brake line. The clutch linkage dislodged the ball once the driver released the clutch pedal.

Text: Gerhard Horn