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Why Does BMW M3 Steer So Well? … Part 2

29th September 2007

Why Does BMW M3 Steer So Well? … Part 2

 

This is the second part of the "Why Does BMW M3 Steer So Well?" series. For those of you who’ve missed the first part you can read it here Why Does BMW M3 Steer So Well? … Part 1. BTW, this article was taken from Motor Trend Magazine (November 1998). I hope you enjoy the article.

 

By Kim Reynolds

To check on the M3’s steering linearity, we ventured to the skid-pad where we painted two extra circles, at 160- and 120-ft. diameters, concentric within our standard 200-footer. Circling all three, I progressively built up speed, carefully recording the steering angle all along the way. The resulting curves of lateral-g versus steering angle appear herewith.

They’re not actually linear, are they? They smoothly bend, with more and more steering angle being needed to achieve additional increments of cornering force. Why? The biggest reason is that the BMW is a “front-steer” automobile (no, not as in “front-wheel drive”). The term refers to the steering rack’s location ahead of the front wheels’ centerlines. This placement matters because of the loaded outside front wheel’s tendency to move inward under cornering (due to suspension bushing compression). Assuming you’re holding the steering wheel still (and we know the rack’s bushings don’t flex much), the front-mounted steering rack forces the wheels to reduce their steering angle, turning a little less sharply into the corner. It’s a benign understeer sensation that’s plenty preferable to the spooky-feeling alternative; i.e., having the tie rods behind the wheels’ centers, causing the wheels to twist further into the corner under lateral load as if they have a mind of their own.

Around the skidpad, my hands also noticed that the M3’s steering effort suddenly starts to diminish as the BMW’s tires proceed to squeal. Welcome to the phenomenon called “pneumatic trail.” Dr. Post mentioned it, but for additional illumination I turned to R&T pal Doug Milliken who co-authored (along with his father, Bill) the extraordinary textbook Race Car Vehicle Dynamics (available from the Society of Automotive Engineers, [412] 776-4970).

You’re probably already aware of what common caster does. Like castered wheels on shopping carts, it’s the weathervane effect where a wheel pivots around an axis that’s ahead of the actual contact point with the ground, stabilizing the wheel’s direction. The physical distance between the pivot axis (on cars it’s called the “kingpin axis”) and the contact patch is a “lever-arm” called trail (of course, it’s a distance, not an actual, physical arm). While cornering, this trail causes a goodly torque to arise around the wheel’s kingpin axis. Fed through the steering links, it’s the creature that causes most of what you feel at the steering wheel.

Pneumatic trail adds an extra, and terribly important, wrinkle to the story. While a tire corners, it seems that the lateral forces it creates are not uniformly spread around the tire’s contact patch (as you’d first think); they’re actually concentrated somewhere behind the patch’s center. In other words, the lever arm that gives rise to steering feel actually consists of two ingredients, the geometrically obvious mechanical trail I mentioned – plus this new added element, pneumatic trail.

Now it happens that as the limit of adhesion is approached, the tire’s lateral forces gradually migrate back to the center of its contact patch. If you’re following along, this zeros out the pneumatic-trail factor entirely (leaving only mechanical trail), consequently reducing steering effort. A communicative steering system like the M3’s possesses an ample percentage of pneumatic trail; think of it as the language tires use to speak to us about their limits. And it’s easily drowned out by the background noise of too much steering boost or excessive mechanical trail.

As an aside, remember those video games Paul, Patrick and I were playing? The most sophisticated of them – better called simulators – mathematically employ all of these effects. According to Carey Kriger of Digital Vehicles (maker of Formula One simulators) and Rick Moncrief, whose company, Silicon Entertainment, builds NASCAR simulators, the most convincing steering feel arises from strictly mimicking things like trail. No phony baloney stuff.

But exceptional steering feel like the M3’s also relies upon a soup of other psychologically synergistic ingredients. For example, the BMW’s large windows provide a panoramic view of the road’s texture that your mind can easily, and subconsciously, integrate with what your hands are feeling. Likewise, add in the tires’ whispers, messages that could easily have been masked by too much sound-deadening material. And the little vibrations that jiggle the steering wheel rim over tiny bumps, oscillations on the order of an eighth of an inch – not so large as to annoyingly shake your hands, but enough to be subconsciously noted. Reduce the windshield’s view, quiet the tires, or dampen the steering gear, and I’ll guarantee you’ll be a lot less impressed by the M3’s steering feel.

In other words, there really isn’t a single, simple answer to the title of this story. It resides in a thousand little fragments, from stiff steering bushings to ample pneumatic trail, to subtle steering boost, to even simply letting the driver hear the tires and easily see the road. Add them up as scrupulously as BMW has learned to do, and you have in your hands what feels like steering magic.

 

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26th September 2007

Why Does BMW M3 Steer So Well? … Part 1

 

I’m sure that many of you guys will be interested in knowing why BMW M3 cars handle so good. This is an article that Jim has sent me couple weeks ago. I think this article is great and since it’s too long, I decided to make it on 2 parts, this is the first one. If you like this post, I will be posting the second part very soon, so stay tuned and let me know what you think.

This article was published on the Motor Trend Magazine (November 1998)

 

By Kim Reynolds

It’s 11:00 A.M. and Contributing Editor Paul Van Valkenburgh, Road Test Editor Patrick Hong and yours truly are lost in concentration. Video screens are painting our faces with the glow of computer-generated stock cars sliding through the streets of San Francisco, rock-scattering rally cars slithering precariously near mountain-edge precipices, and mighty Indy cars hurtling past blurred super speed way grandstands. Behind us, kids walking past our video game stations at the Park Palace amusement arcade are giving us odd looks, and not just because we’re the oldest gang of truants ever to set foot in the place. The odd looks are because it’s obvious that we’re not really playing these games; we’re just twisting the steering wheels back and forth, holding their rims with the ends of our fingers.

What’s going on here? Shush…this is research.

In fact, we’re in search of one of the automobile’s most elusive and abstract properties – steering feel. Quite frankly, of all the things I want to know about cars, understanding steering feel is very near the top of the list.

It’s such a mysterious, ethereal thing, steering feel. I mean, how does it happen that when I close my eyes for a few seconds while behind the wheel of a great-steering car, the wheel’s torques and tiny kickbacks manage to conjure in my mind such an accurate rendering of the road’s texture and the tire’s dynamics? It’s analogous to how our gray matter can translate the vibrations of a phonograph needle upon a record into a Bach fugue, a Benny Goodman solo, or the Beatles’ White Album.

But enough abstract philosophizing; what the heck is mechanically going on between my hands and the road surface to engender such communication? To get to the bottom of it, we borrowed a BMW M3, arguably the world’s current prime example of proper steering feel.

Hoisted in the air atop our garage lift, the M3’s underbelly yields no obvious clues. There’s a robust structure of welded steel tubes corseting the engine’s belly, no doubt greatly stabilizing the M3’s front suspension geometry. Pokes with an X-Acto knife at the synthetic bushings isolating the ZF steering box from the subframe identify them as unusually stiff. A notably stout structure, but so far we’ve uncovered no big answers.

However, wrap your hands around the M3’s steering wheel while it’s idling and you’ll notice there’s a remarkable amount of engine vibration being transmitted up through the steering column. The wheel’s rim almost hums, a characteristic we’ve come to closely associate with small BMWs. A clue perhaps? Being a suspicious sort, I immediately deduced it to be a deliberate engineering ploy to enhance the M3’s impression of low-speed road feedback by sneakily feeding-in engine vibration. Paul, more clever than I, postulated that the vibration might be a smart way to continuously break up the steering gear’s natural stiction, resulting in an impression of unusual fluidity. These BMW guys are geniuses!

Smug in our penetrating insights, I addressed the question to the Bavarian Motor Works. Answer? It’s just something they can’t get rid of. Great steering feel, they stated, requires a very solid structure with minimal flex anywhere in the steering gear. In other words, that steering rack I noticed so rigidly mounted to the engine’s subframe just can’t avoid also being awash with the inline-6’s tremblings. According to Munich’s engineers, the steering wheel’s buzz is simply part of the deal. Lesson one: Minimal compliance is the first ingredient of sensitive feel.

All right, then how about an element that’s vastly less stiff – the tires? A call to the M3’s tire supplier, Michelin, put me in contact with Dr. Bill Post, whose specialty is just this sort of thing. The good doctor’s most interesting point on the subject, and an entirely unexpected one, is that the most under-appreciated factor in facilitating linear steering behavior is a good rear suspension. Rear suspension?

Dr. Post explained that when you turn into a curve, steering linearity relies enormously on the rear tires loading up with cornering forces in a nicely progressive manner. If there’s anything unpredictable about the car’s rear end, it’ll destroy the car’s steering quality. Makes sense.

 

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