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Rack-and-pinion on the horizon

Posted: August 1, 2014 by Rolf Lockwood

ZF Lenksysteme’s Servoline rack-and-pinion steering system (sitting behind the beam axle).

None of today’s trucks are exactly primitive, but there’s still rich ground for improvements in some of the fundamental technologies they depend on. Like the chassis, where some very useful suspension work has been done over the years but not too much more. We’re about to see that change, including further suspension offerings.

Rack-and-pinion steering is definitely on the horizon, for example, though it’s already available on several heavy-duty Freightliner models. That’s an industry first, a mechanism developed in-house and tested on Mike Ryan’s Pikes Peak race truck among others. Germany’s ZF Lenksysteme, a 50/50 joint venture of Robert Bosch and ZF Friedrichshafen, is a steering specialist and is hot on the same trail. It’s developed a system for the North American Volvo VNL that I tested recently.

ZF has also been working on an independent front suspension for heavy trucks (they already produce one for buses), and I got to play with that as well at a test facility developed within a huge, mothballed German army training camp near Stuttgart. It includes a two-lane road around the outside of its many acres, 36 km long with twists and turns and hills and dales like any rural road. But no traffic, so it offered a great chance to test the various steering and suspension technologies that ZF was showing off to the world’s press.

In fact, the engineers at Dana have also developed a prototype independent front suspension fitted to a Peterbilt 387 in the Roadranger test-truck fleet. It’s been around for a year or so, but I first came across it at the Eaton proving grounds in Michigan back in July. It wasn’t part of the program, but I took the truck for a low-speed tour of the track’s ultra-rough-pavement section. A brief test but enough for me to marvel at the smooth way it soaked up punishment. Coming to market? Maybe. But let’s go back to steering.

The Rack Advantage
Pretty much since time began, trucks and other heavy vehicles have used a recirculating-ball steering design. A variation on what’s called a ‘worm and sector’ system, it means that when you turn the steering wheel, you also turn a large screw or worm gear that meshes with another gear–or gear sector–that is then forced to rotate on its axis. The Pitman arm, attached to the gear sector, is itself forced to move, and since it’s also connected to the steering linkage, the wheels turn. Ball bearings between the teeth of the worm and the screw inside the steering box reduce friction.

It offers mechanical advantage and robustness, making it right for big vehicles, but it falls down somewhat in terms of speed and precision. Ideally, a given movement of the steering wheel translates exactly into a movement of the wheels on the pavement. But in a recirculating-ball system, there’s often a dead spot on center, sometimes leading to a certain vagueness and in the worst cases to the need for continuous adjustment at the steering wheel. It can be very tiring to drive such a truck. In practice some systems are better than others, and the same basic hardware may perform better in one truck model than another because factors like suspensions and tires and the general geometry of the truck’s front end come into play, too.

The system offers relatively little feedback, which can be both good and bad. The bad side is that vagueness, but the good one is that the steering wheel is less likely to jump at you when the truck hits a nasty pothole or a serious bump of some sort.

That slight glitch aside, just about every car and light truck on the road now sports rack-and-pinion steering. It has the advantages of simplicity–fewer parts and pivot points–and light weight as well as superior control and thus reduced driver fatigue. Freightliner’s rack-and-pinion system consists of two main components: the horizontally mounted rack and a vertically mounted pinion. The assembly is attached to the front axle by two brackets and connected to the steering column by an intermediate steering shaft. When the steering wheel turns, the pinion spins, moving the rack to the left or right and thus turning the wheels.

The real advantage to the driver is enhanced road feel and something that’s hard to put in words… let’s call it a closer connection to the driving process and to the vehicle itself.

Freightliner claims other advantages:
* Reduced maintenance–the system remains cooler, improving the durability of seals and hoses.
* Increased payload–it’s 45 lb lighter than a conventional system.
* Improved engine access–by moving the steering gear and universal shaft from the left frame front, engine access is greatly improved.

ZF Developments
Germany’s ZF Friedrichshafen AG is not exactly a household name over here but it’s in fact a major automotive component supplier. You’ll likely know ZF best as the manufacturer of the automated FreedomLine transmission sold here by ArvinMeritor, but the company is much more than that. With 122 plants in 26 countries, including a steering-gear plant in western Ontario, its focus is driveline and chassis technology.
Amongst the 26 vehicles arranged for the press to play with in Germany recently, there was that made-in-America Volvo VNL tractor. It was sporting ZF’s Servoline rack-and-pinion steering with the rack sitting neatly behind the conventional beam axle. The truck will find its way back here this fall as ZF tries to sell Volvo and other domestic truck makers on the rack-and-pinion idea.

The former army camp offered that long and narrow country road plus artificial prairie crosswinds (really), and I had the chance to spend some time in the Volvo at speed on good surfaces and bad. It was terrific, and you’d be impressed by the steering precision, the strong centering, the effortless tracking, and no ‘bump steering’ effect. Still a prototype, ZF says we might see it here by 2009.

ZF refers to Servoline as “linear steering” and says it’s superior to conventional rack-and-pinion systems. It consists of a mechanical rack and pinion with amplification by way of a hydraulic cylinder in parallel with the rack. It’s said to be light, small, and can be installed with either rigid axles or independent suspensions. Its light weight and short length means less self-steering (or bump-steering), and packaging issues are eased by its compact size, leaving space for the larger cooling packages demanded by EPA-compliant engines. The modular design allows for hydraulic steering cylinders of different sizes, according to truck size and anticipated application.

After the Volvo, it was on to a tall MAN cabover, which also had the rack-and-pinion system. It went still further, having been treated to the ZF independent front suspension as well. The ride? Superb. The handling? Very, very good.

And then there were the artificial crosswinds–by way of several huge fans–to help us test ZF Lenksysteme’s Servoactive steering gear that combines hydraulic and mechanical actuation with electronic control and an electric motor that superimposes a second gear and thus changes the steering ratio as need arises. The result is automatic compensation for crosswinds and grooved pavement, among other benefits. I tried that in another MAN cabover, and sure enough, those monster fans tried to push me off the road but the system reacted in an instant and kept me straight–with no movement of the steering wheel itself. I tried it with the system turned off, too, so I knew the power of that fake crosswind.

It forms the basis to do much more without driver intervention, like automatic tracking, obstacle avoidance, automatic reversing, even what ZF calls “autonomous driving.” More conventionally, and perhaps more likely to find favor with users, it also offers speed- and load-dependent steering assist and a speed-sensitive steering ratio. It’s already being successfully used in BMW 5-Series cars.

ZF is conceiving all this and more as a package, calling it the “truck chassis of the future,” with many of the components already proven in buses and other non-highway commercial vehicles. As well as the technology above, ZF already offers something called airspring-damper modules equipped with a continuously variable electronic damping (shock absorbing) system named Continuous Damping Control or another called Pneumatic Damping Control. Used now in many commercial vehicles, including trucks, it ensures that the right shock-absorbing force is set for the corresponding load.

The advantages of all this, ZF says, are simplified installation for the vehicle manufacturer as well as ease of handling, increased agility, and ride comfort for the driver. There should be safety benefits too, not to mention preservation of road surfaces. And let’s not forget payload gains.

Bottom line here: the truck chassis is about to gain serious sophistication with potential benefits for just about everyone concerned. Sooner or later some pretty interesting steering and suspension technologies will be routine stuff on just about every medium-duty truck and on-highway tractor. And on heavier equipment? Well, you might have to wait a little longer.

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