Piston Slap: Rackin’ Up The Benefits Of Steering Racks

Jay writes:

Sajeev,

What are the downsides to recirculating ball, old-school steering (with a steering box)? How is rack and pinion better? Seems more complex.

Sajeev answers:

While it is more complex, rack and pinion has serious advantages in a world of ever-improving powertrains (i.e., we have more power) and tire compounds (i.e., more grip) as time progresses. You simply want more precise steering as you continually push the performance envelope. And there’s a story to be told here, as steering innovations in the last 40+ years deserve the oxygen of publicity.

Perhaps the best means of shedding light on these two methods of steering comes from picking on a rack and pinion “laggard” like the BMW 7-series. While most platforms from Lexus, Infiniti, Cadillac, and Lincoln introduced in the last 40 years had rack and pinion steering, BMW (and Mercedes-Benz, to be fair) stuck with steering boxes for their flagship sedans for a bit too long.

I spent many an hour in a low-mile, well maintained 1996 BMW 750iL (E38) and loved every minute of it. BMW clearly worked with Satan himself to make antiquated steering boxes perform shockingly well, but that long-wheelbase example still didn’t turn in like other rear-wheel drive luxury cars of the era. (Looking at you, Infiniti Q45 and the Lexus LS 400).

That sin was addressed with the 2001 BMW 7-series (E65), as BMW made a new platform that accepted rack and pinion steering like the 3 and 5 series before it. BMW’s press release, highlighting the benefits of this model’s new rack and pinion steering:

• Precision of road feel
• Lack of “free play” at the steering’s center position
• Lower weight, because there is no center arm.

They are right, and I’d include the potential for a lower center of gravity. Many steering racks are slender enough to be mounted lower in the chassis than a steering box. That may not make a notable impact on its own, but concurrent innovations with other subsystems have an additive effect (so to speak) on a vehicle’s dynamics. Consider the weight saving benefits of plastic intake manifolds, aluminum suspension components, etc. as innovations that can really highlight the lower center of gravity with rack and pinion steering.

Back on topic, that same press release attempts to explain why BMW took so long to add the benefits of rack and pinion steering to their flagship luxury sedan. And we should let them do just that:

“BMW’s 3 Series has long used rack-and-pinion steering; the Z4 Series and Z8 have it too, as do 5 Series 6-cylinder models and all X5s. Now, careful development has produced a rack-and-pinion system that brings the above advantages to the 7, while meeting the highest standards in isolating road shock from the steering wheel.”

Forget about the aforementioned Lexus, even the front-wheel drive, rack-and-pinion-equipped 1985 Cadillac DeVilles did a great job “isolating road shock” from the steering wheel. And the steering feel wasn’t half bad considering the target demographic, too. So let’s find out how much “careful development” was really needed on BMW’s part to make this design worthy of a 7-series.

“To enhance the system’s fundamental weight advantage, the rack-and-pinion housing is made of aluminum.”

That’s on par with the aluminum rack and pinion housing in the flagship Lincoln Continental for 1982. Which was basically the same unit from the Ford Fairmont of 1978. Come on BMW, you can do better. And they did:

“Power assist is also variable, via the Servotronic vehicle-speed-sensitive system. This reduces steering effort in parking and at very low road speeds more than the engine-speed-sensitive system of most other BMW models, as is appropriate for BMW’s largest, most luxurious cars.”

Now we are talking! Time has sped forward from 1978, to what may have been the first speed-sensitive steering first, seen in the computer controls of the 1988 “Essex” Lincoln Continental. Now I own one of these Lincolns, and it’s painfully clear Ford Motor Company was on the bleeding edge of this technology. Of course, ’88 Conti’s speed-sensitive steering isn’t as linear as a modern system. Perhaps the computers were too rudimentary back then, or maybe the valving was aggressive enough to forgo linearity for outright shock value.

BMW clearly benefitted from waiting for other automakers to make their mistakes on the technology available at the time. So let’s get into the best reason for rack and pinion steering, as offered by the press release.

“It also includes a feature of the 6-cylinder 5 Series models and Z8: a variable steering ratio. The rack-and-pinion gear teeth are profiled to make the steering ratio 10.6% ‘quicker’ as the wheel is turned away from the center position. This fine-tunes steering response according to the situation one is in, be it on an Interstate at today’s high cruising speeds or maneuvering into a parking space.

Variable steering ratio and variable power assist are not the same thing; they are two separate features, here optimally complementing each other.”

I am certain most American and Japanese automakers did variable steering ratios in the same manner ever since the early 1980s. (My 1982 Ford Shop manual says the aforementioned Fox-chassis Lincoln had a variable steering ratio inside its aluminum case.)

But the difference is in the details, as BMW assessed everyone else’s efforts to make a rack and pinion steering system with modern technology. They took the lessons learned from the 1980s and 1990s and honed it into something with just a little more engineering finesse for a flagship BMW.

Combined with a more revolutionary suspension innovation (hydraulic anti roll bar), the rack and pinion’d E65 7-Series performed admirably in the corners. Granted the E65 was larger, taller and generally reviled by BMW enthusiasts, but the steering improvements under its skin still have merit. That’s my story and I’m sticking to it.

Believe it or not, my intention in this Piston Slap was not to bury BMW for being so late to the party. I wished to show how one company’s implementation of an innovation can reveal a story of advancements from across the industry. (Presumably by companies with larger R&D budgets and more willingness to share platforms with cheaper vehicles.) And one such worthy advancement is absolutely the rack and pinion steering system.

You’re right, Jay, the design is more complicated, but it’s been so thoroughly developed and iterated over the years that its additional benefits outweigh the added complexity. Did I steer Jay to the same conclusion you, the Hagerty Community, would have reached? Take it home for all of us in the comments section!