In firm hands all round
BMW's active steering proves that something is still possible with steering systems.
A Swiss company believes it has made the mechanical quantum leap.
The functional principle of PSS steering, made easy to understand / Automobil Revue
The search for a steering system that is as direct as possible, yet stable and smooth-running, has caused many an inventor to ponder - not least in Switzerland (Wandfluh steering system, cf. AR 10/1999 and 27/2001).
Werner Bless and Erwin Rott in Rüti, Zurich, have applied for a patent PCT/EP 2006 /000559 for an invention that has met with great interest from Ford, Mercedes, Opel and Porsche as well as the steering specialist ZF in Friedrichshafen. But they have not yet made the breakthrough, despite low manufacturing costs.
Test drives and parking manoeuvres have shown that PSS steering (Progressive Safety Steering) has future potential. Around 30 instructors from the Touring Club Switzerland (TCS) and the Road Safety Centre Veltheim (VSZV) had the opportunity to put their experiences with PSS steering on record. The specialists were extremely positive about the new technology, which they "experienced" with a Porsche Boxster. They described the increase in comfort when parking as outstanding, driving in the city as noticeably more pleasant, the overall increase in safety as noticeable to good and the familiarisation process as problem-free.
After the first test drives with the Boxster, which has meanwhile been aged by around 40,000 km, AR can agree with these assessments.
Parking with the PSS steering in particular is an "aha" experience, evasive manoeuvres on motorways were problem-free despite the very spontaneous reaction, and getting used to it was hardly worth mentioning. From stop to stop, the PSS steering requires only 1.5 turns, so overgripping is never necessary. Because no major effort is required, the invention comes very close to the ideal steering.
The reason why none of the above-mentioned manufacturers achieved a breakthrough could also be due to the fact that the invention is a purely mechanical one, while many car manufacturers are currently focused on electronic solutions. For Bless, there is also a generation problem: people often work intensively on electronic assistance functions (such as automatic parking assistance), and there is a belief that they can achieve something similar on this track as with the new mechanical design from Rüti. Bless assures that his design is compatible with ESP and other assistance functions and works with both hydraulic and electrically operated rack-and-pinion power steering systems. The space required for the PSS steering gear is no larger than for a conventional gearbox with rack and pinion.
Simple operating principle
The functional principle looks simple - not to say simplistic - but someone had to come up with it first. The system with racks rising from two sides and halved, straight-toothed pinions mathematically enables the ideal form of progression of a steering system with the highest degree of efficiency. However, it was not the first version that Bless and Rott devised.
The AR was already in Rüti above Rapperswil on Lake Zurich in 2004, driving an Opel Astra with a new, purely mechanical steering gear. Even that one worked perfectly, but with only a single turn of the steering wheel from stop to stop. Parking manoeuvres took a lot of getting used to and, according to Bless, the system was not "suitable for racing"; moreover, it had an undesirable moment of inertia from the centre position. The secret of the first PSS steering was a pinion with an additional eccentric cam.
After a lot of hours and francs had been wasted and principles had been thrown overboard, the developers spun in a new drawing board. Thanks to practically identical construction effort compared to conventional rack-and-pinion steering systems, the solution now available seems more promising than BMW's considerably more elaborate system, for which customers have to fork out an extra 1810 francs (in the 5 Series). Further information: www.a-win.ch
You can find the article in issue 48/2006 of "Automobil Revue", which you can of course also subscribe to online.
AR 48 of 29.11.2006