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Good steering is what makes a car a precision machine and effortlessly driveable. It has a significant impact on sheer driving pleasure.
BMW Department Head Steering System Development
There are numerous factors that must come together to make the driving experience a sheer pleasure – the design, the engine, the chassis and the steering. Wait, the steering? Yes indeed: if a car’s steering doesn’t perform well, it’s no fun to drive – but what exactly does precisely tuned steering involve? And what makes a BMW drive like only a BMW can? Our two experts give you an insight into the tuning process: Steffen Koch, BMW Group Head Steering System Development, and Christian Heiss, BMW Driving Dynamic Development
The aim of every steering engineer: the total package, comprising the car steering and all other chassis components, should provide the person behind the wheel with everything they need to be a better driver.
BMW Unit Head Steering System Development
A
Absorption:
Steering absorption ensures that the steering wheel never turns too quickly.
Assistance:
Is delivered by the power steering. Mechanical steering is when only the force exerted by the driver on the steering wheel turns the car.
D
Directional stability:
The ability of the vehicle to continue on a straight path without moving the steering wheel.
R
Returnability:
Returnability ensures that the steering wheel always returns to center, guaranteeing that the vehicle continues on a straight path (➜ directional stability).
S
Stability limit:
The narrow range in which a car drives through a curve, for example, without swerving or becoming uncontrollable.
S
Steering power:
The effort that the driver must exert in order to move the steering (➜ assistance).
S
Steering response:
The ability of a vehicle to cleanly and directly perform steering commands while driving on a straight path.
How car steering is developed
The most basic parts of a car’s steering are the following: the tie rods, the steering gear, steering wheel and the steering column. There is also an optional feature called rear-wheel steering, which is available in certain models. In order to achieve the steering feel typical of a BMW, it is important to work closely together with all the suppliers. They send the steering system components to BMW in modular form. BMW then does the fine tuning. The engineers and test drivers adapt the steering system parts to the chassis, absorption, springs and brakes.
Assistance, absorption and returnability are the three pillars that define the steering experience.
BMW Driving Dynamic Development
Assistance, absorption and returnability are the three pillars that define the steering experience (see also “A little steering primer”). These factors all depend on the vehicle’s speed, steering speed and the forces that are exerted on the rack in the steering gear. All these parameters are adjusted for individual preferences in each variation of the BMW model program. This means that even within a model family, the engine and body variation differ with regard to their steering tuning – which involves relevant expenses.
A car should be intuitive to drive.
BMW Unit Head Steering System Development
What makes steering perfect
Well-tuned steering makes a correction for the unevenness of the road and mistakes made by the driver without the driver even noticing. It simplifies car driving, makes it more relaxing and therefore safer. This is according to expert Koch, who goes on to offer an example: “If you’re driving your car on a road that narrows down a great deal due to construction, you have to correct your direction often by turning the steering wheel. A car’s steering should be tuned in such a way that this correction is not necessary.”
At the same time the steering serves to convey the condition of the road to drivers, so that hitting the curves is just as enjoyable as “going to the supermarket,” states Heiss. Good steering must also cover the entire spectrum of driving situations. How a BMW handles must be predictable for every type of driver – regardless of whether a professional or a beginner is sitting behind the wheel. Or, as Koch puts it, “It must be predictable and linear. And the vehicle must do what the driver expects of it no matter the speed.”
Of course the steering power must be adjusted primarily for the type of vehicle and its weight with regard to direct deflection (the car reacts directly to steering commands, if it didn’t, it would be sluggish). If the steering is too loose, the driver overcompensates, i.e. the driver unintentionally uses too much force. In this case, the adjustment involves finely tuned components such as the steering gear, etc. – the better developed these are, the more options there are for fine tuning.
The trick is to adapt the wide range of tuning options available to the model in question. With a BMW Z4, for example, the focus is on providing both direct contact with the road and driving pleasure, while for a BMW 7 Series, comfort is more important. In this respect, each model has its own individual “feel”, explains Heiss, which determines the priorities when tuning the steering.
- Power steering: This assisted steering is one of the greatest evolutionary steps in steering technology. It boosts the driver’s steering power hydraulically. Before power steering, parking a car was an involuntary workout. Only with the advent of the power steering system could larger and heavier vehicles be offered to a greater pool of customers, notes Koch.
- Servotronic: The next development in hydraulic power steering was Servotronic, which makes it possible to adjust the amount of steering assistance to suit the speed of the vehicle, explains Koch. Since Servotronic assists with maneuvers, little effort is required from the driver. And it provides stability at higher speeds. In contrast to plain power steering, both are now possible. Koch adds, “So not only is it easy to park, it also delivers sharp handling when driving fast, with the focus on driving pleasure.”
- Electric power steering: Koch calls electric steering the “Big Bang” in steering history. It offers the advantages of Servotronic, while saving on fuel because the EPS (Electric Power Steering) motor only uses electricity when it is actually used to steer. As cars spend most of the time driving straight ahead, this plays an active part in reducing consumption, which translates into fuel savings of a good three percent. It doesn’t sound like much, but the impact is enormous. EPS makes all steering functions possible, from parking assistance through to autonomous driving.
- Rear-wheel steering: This further assists the agile driving performance of a vehicle. It enhances the vehicle’s lane stability at higher speeds (the rear wheels turn in tandem with the front wheels) and improves the car’s maneuverability at lower speeds (the rear wheels turn in the opposite direction to the front wheels). This significantly reduces the turning radius. Rear-wheel steering “drops” the car handling feel by one car class. BMW offers rear-wheel steering for the new BMW 5 Series and other models.
The engineer’s favorite tool: the “Popometer”
“Popometer” is a fun word that Germans use to describe gauging the driving dynamics of a car by feeling rather than by calculating. In German, “Popo” is what little children call their buttocks. So you could call it a “buttometer.” All the computers in the world cannot replace the reactions and experience of a test driver. Or, as BMW expert Heiss puts it, “For us, the Popometer is the most important tuning instrument bar none.” At least in the final step of tuning the steering. Once the vehicle arrives at the applications department following steering development, it’s all about the feel of the car.
So, of course, measuring technology is used here. The first 30 percent of this process of developing components is done in advance with the help of computers, adds Koch. In the mid-term, up to 80 percent of development work is expected to be done on a computer. But the last 20 percent, the fine tuning, or as Koch calls it, “the ideal way to define the typical steering feel,” can only be achieved by humans. Heiss says that computers are used to examine the results and identify any unresolved issues. But then it goes back to the experience of the development engineers in charge. For the time being, Koch does not see any changes forthcoming because computers are not yet able to take over these tasks from humans.
For us, the Popometer is the most important tuning instrument bar none.
BMW Driving Dynamics Development
The best possible comfort and safety: pull-drift compensation
BMW believes that a finely tuned steering system is also the safest. Compromises are never made in terms of safety, emphasize both Koch and Heiss. But ways to improve comfort are constantly being researched. This is how another of BMW’s steering features, pull-drift compensation, came to be.
So what is pull-drift compensation? Koch uses this example to explain: lanes are generally sloped to one side (to enable rainwater to drain off, etc.). When driving straight ahead, drivers have to constantly compensate for this slope so they don’t go off to the side. This creates long periods where it is necessary to maintain this force, which is tiring. But the system recognizes that the driver intends to keep driving straight ahead, so the steering assist provides greater force. Drivers still have to compensate, but not nearly as much, which reduces the force on their hands.
There is no compromising when it comes to safety.
BMW Unit Head Steering System Development
What the future will bring us: the impact of steering on autonomous driving
According to Koch, the features of a highly developed electric steering system are the key to all future developments – and especially autonomous driving. Electric steering will make it possible for engineers to allow computer systems to autonomously drive the car (➜ Read more: The Path to Autonomous Driving). The system is already used today in parking assistance (➜ Read more: Overview of the Main Driver Assistance Systems).
Drive-by-wire belongs to this same area. It’s an overarching term for systems in which there are no longer any mechanical or hydraulic connections between the devices the driver operates (steering wheel, brakes and gas pedal) and the systems that control the vehicle. For the steering system, steer-by-wire means that the connection between the steering wheel and the front tires is purely electronic. The electric steering gear no longer takes over just the power assistance, but also maneuvers the axles independently based on signals from the steering wheel.
Tuning the steering of an autonomous vehicle is the pinnacle of vehicle applications.
BMW Driving Dynamics Development
The big advantage of steer-by-wire is that the wheels and steering wheel can turn independently of one another. This is also important for autonomous driving: the steering wheel no longer needs to rotate (as with parking assistance). Safety systems present a particular challenge because the driver must be able to control the vehicle if an error occurs even if there is no mechanical connection.
Letting a car drive straight on shouldn’t be a big problem. But in reality it is because there are a number of processes involved in this simple act. “And this is very challenging for the computer,” notes Heiss. “A human will react intuitively by compensating in order to stay in the lane, such as if the lane slopes slightly to one side.” But computers and intuition do not go together quite as well. In short, technicians have a lot of work ahead of them trying to train computers to achieve the amount of human intuition involved in driving. Or in other words, the Popometer has set the bar very high.
Author: Nils Arnold; Illustrations: Señor Salme
