God is in the details, as Mies Van der Rohe said. Subaru’s recent WRX STi has attracted my attention with an engineering choice that deserves respect.
I have two reasons for this article. One is the subject itself, Subaru’s devotion to steering quality, and two is to make up for our neglect of the brand. During our recent foray into engines, DTW failed rather spectacularly to mention Subaru who have championed boxer engines on the grounds that these make for a car with a lower centre of gravity, to the benefit of handling among other boons.
As LJK Setright noted, lowering the centre of gravity is a more effective means to ensure stability than widening the track of a vehicle. Furthermore, Subaru have developed a boxer diesel engine in order to reap the economy advantages of diesel, while still retaining their signature engine layout. This is worth noting in these days when Fiat throw their engines into Alfa Romeos and PSA have no opinions on engines at all.
I will have to return to that boxer diesel at another time because, in this post, I’d like to consider Subaru´s decision to engineer a hydraulic rack for the Subaru WRX STi despite it being a very specialised model. The numbers involved are in their thousands, a fraction of the overall sales of the WRX generally. They could so easily not have bothered and still have had a good car. But they did bother and I think that’s worth a closer look.
Why did they do this? While the standard WRX’s electric rack is already on the good side, Subaru felt that a hydraulic rack would help further improve the steering feel and precision. For a car as focused as the WRX STi, to have not have made this decision would have been inconsistent, given the long list of other modifications that set the STi apart.
What is steering feel? Philip Harnett’s PhD thesis (2002), ‘Objective Methods for the Assessment of Passenger Car Steering Quality’, has a section which is a good introduction to the matter of steering quality or “feel”:
“Reynolds  describes steering feel as one of the automobile’s most elusive and abstract properties. He emphasises its importance when saying: ‘Of all the things I want to know about cars, understanding steering feel is very near the top of the list’. This comment is representative of many individuals, as steering feel is a term used copiously in connection with the evaluation of passenger cars by the industry, press and the public. Almost everyone involved with or enthused by the automobile will have an opinion on what steering feel constitutes and what represents ‘good’ steering feel.”
Harnett goes on to say: ”Zaremba, Liubakka & Stunz  …write that steering feel is effectively defined by the steering wheel torque the driver senses during steering manoeuvres and by the vehicle response to steering inputs.”
In the light of the importance of steering quality, Subaru made a set of revisions to the STi version of the WRX to maximise this aspect of the driving experience. As something of a steering fetishist, this struck a chord since, regardless of your speed, you always have to steer. Why not make the only constant point of contact (other than the seat) as good as possible? If there is an easy (and strangely overlooked) way to make a car pleasurable to drive, it’s through tuning the steering system.
I asked Subaru’s UK press office for a little information on this so as to see what Subaru did to offer a better set-up for the 2015 STi’s hydraulic rack. The changes included a quicker gear ratio than on the standard car, stiffer hydraulic power steering torsion bars, optimized power steering assist characteristics (presumably the rate and amount of assistance) and an increased spring constant for steering gearbox mount bushings (up by 400%).
A quick look at Hooke’s Law will remind you that the spring constant is a value for each spring determining the amount of force needed to stretch or compress it a given amount. In layman’s terms, it means stiffer bushings, and that reduces play in the rack when high forces are applied.
According to Subaru “a quicker steering gear ratio and more rigid system have reduced response lag from lateral G-forces when steering for steering precision and made for a more rigid feel.” At high speeds and under tricky conditions the response lag becomes all the more important. The idea is that the driver inputs should be matched by proportional and direct outputs, and presumably that signals sent back from the wheels should be accurately communicated to the driver too.
Response lag muddies those signals. Ideally the phase lag should be in the range 0.0 Hz to 0.4 Hz . Above that, things get confusing. The steering gear ratio was reduced, the more so on the RHD models where it is now at 12.7, which requires less input for the desired steering output. As you can see, this all builds up step-by-step to create clearer and more linear controls.
Road & Track´s views of the changes to the steering are as follows: “So precise is the steering that you can select an inch-square hunk of apex pavement and hit it lap after lap. Steering this quick should be darty, nervous, prone to kickback. That it evinces none of those traits is evidence of rigorous development.”
I’ve focused on the steering rack and its effect on steering feel, but for a more general review of the car, you can read Road & Track’s views by clicking here. I note that Road & Track reports that the a-pillars have been made narrower – is this a first in recent years? The other thing I like to do when steering a car is to see what’s in front of me. Thick pillars don’t aid that goal.
Given that the main focus on automotive design is in the gadgets category these days, it’s heartening to see Subaru carrying on with putting engineering first. The next step for steering technology is to try to put back what electric steering has lost. I can see the economy advantage of electric steering for many cars but, at the same time, it reduces involvement and, in a way, detaches the driver from the driving.
The cars I have liked best are the ones whose steering I can remember and I think that’s not a coincidence. You can see why Subaru has such a loyal following.
A full set of details of the car can be found here.