‘Steering feel’ has been discussed in automotive industry circles for decades and much has been written about it by road testers. In this interview, Matthew Beecham talked with Prodrive’s chief dynamics engineer, Matt Taylor about what steering feel means and how electrically powered steering systems are opening opportunities for manufacturers to satisfy the individual requirements of different customers, or brands that share a common platform, with nothing more than software or calibration changes.
From your perspective, what does steering feel mean?
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‘Steering feel’ is the communication of tyre forces and vehicle dynamic states to the driver, via the steering wheel. It is quite distinct from the vehicle’s response to steering inputs but many people confuse the two. For example, the perceived accuracy of a vehicle’s steering – the tendency of the vehicle to follow the driver’s chosen path – is influenced more by the vehicle’s response, determined by many factors, than by the precision of the steering. From the vehicle manufacturer’s marketing perspective however, steering feel can be something completely different as it is often one of the main factors used by vehicle buyers to categorise the vehicle.
Dynamics specialists must put to one side their own view of what constitutes technically excellent steering and deliver a feel that the end user believes is excellent for the category in which the vehicle competes. But we cannot compromise safety. Good steering feel matters because it can make the difference between noticing that a road surface is slippery or not, even to a driver who has no idea that the information is coming through the steering wheel.
To what extent do definitions of ‘good steering’ vary from one region to the next or from one customer segment to the next?
‘Good steering’ requires a combination of two things. The first is good feel, which is a subjective characteristic. Familiarity plays a big part – most drivers want their new car to respond in a similar way to their previous ones, which can be a lesson to the developers of new systems. Many customers believe that they just want low steering effort. Most will ask for more feel up to the point where the forces at the steering wheel become unacceptable and this certainly varies from region to region, as it is influenced by the road quality and patterns of use as well as by personal taste. A good example is the Middle East where cars parked in extreme heat develop subtle flat spots on the tyres and, once started, are immediately driven at high speed on good road surfaces. The users complain of unacceptable shake through the steering, yet the systems are perfectly satisfactory in other markets.
The main regional difference is steering effort. In North America and Asia, for example, low effort is considered a sign of luxury, where as in Germany it is seen by many drivers as the signature of an over assisted, poorly engineered system. In most European countries, high steering effort is considered ‘sporting’ although it has no relationship with feel or steering quality.
The other contributor to ‘good steering’ is the linearity of response, which is not subjective but clearly measurable. If you ask a chassis engineer, you will be told that good linearity is important, by which I mean that each additional degree through which the steering wheel is turned will produce an equal, incremental response from the vehicle. This makes the vehicle predictable and gives the driver confidence. A linear response also reduces driver workload and fatigue on long journeys, even though he or she may not realise it.
If you ask a marketing man, you may discover that from a commercial perspective, good steering on a particular vehicle requires a non-linear response so that the initial input produces a higher response rate, to make the car feel nimble and therefore more appealing to the target customers. Several cars that are well regarded for steering response use this approach and it is the engineer’s job to tailor the system to suit the tastes of the market.
Do you see certain OEMs in certain countries upgrading / modifying, say, sports model steering feel in order to make them ‘fit’ better in that market and to make them more premium?
This is a very common requirement but changing the specification of an expensive component like the steering rack conflicts with pressures to maximise commonality and manufacturing volumes. A more common, and sensible, approach is to change the rate of some of the suspension bushes, or even just the brand or specification of the tyres. This affects the vehicle’s response to the steering, as we discussed earlier, and can significantly change the driver’s perception of how the vehicle feels and behaves.
Modifying the systems after Type Approval must present an interesting set of challenges? How do you set about achieving that?
Our work tends to be carried out during a vehicle’s development phase, prior to Type Approval, so this isn’t really a concern for us. All the components that affect steering feel and response, such as springs, dampers and roll bars, would be subject to re-approval in most markets, if changed from the original specification.
Is it easier or more difficult to deliver the required feel with the increasing uses of EPS? Are there new opportunities that EPS presents?
EPS opens up huge opportunities for manufacturers to satisfy the individual requirements of different customers, or brands that share a common platform, with nothing more than software or calibration changes. Features such as light steering for parking, with a heavier feel at speed, or integrating steering feel with suspension or traction control through a “sport” button, become much more affordable, as do a number of haptic safety functions, such as nudging the steering wheel to warn of lane departure, or to wake up a drowsy driver. These are additional to the primary benefits of improved fuel economy, lighter weight and lower cost, compared to hydraulic systems, and the ability to work in an HEV without a mechanically-driven hydraulic pump.
The remaining challenge for EPS is to deliver the fidelity of steering feedback to the driver that conventional hydraulic systems can. The debate about HPAS vs EPS has a parallel in the arguments many years ago concerning assisted vs unassisted steering. Nowadays, few people accept reduced steering feel as a consequence of power assistance and I’m sure EPS will get there.
What is Prodrive’s role? What can you do to add value for your customers?
The remainder of this interview is available on just-auto’s QUBE research service
