Jalopnik recently sat down with BMW dynamics engineer Robert Rothmiller who lead much of the engineering on the all new 2020 BMW 3 Series. The resulting detailed explanation and dissection of engineering that went into the new 3 Series handling prowess is exceptional. So much that we’re going to give you a preview and then tell you to head over and read it yourself.
But before we do that lets back way up and talk about why BMW went to such extraordinary steps to re-engineer a car that has been historically seen as the benchmark in sports sedan handling. The answer is easy. Because it lost that crown with the introduction of electronically boosted steering in the F30 generation. BMW knew that electric steering wasn’t going away so they had to figure out how to make the all new G20 2020 3 Series feel like it’s older forbearers (the E46 and E90 specifically).
Here’s how they did it:
The general suspension setup, which you can see above, is the same as that of the outgoing F30 3 Series. There’s a double-joint spring strut front suspension and a five-link suspension in the rear, but it’s not literally the same, even if it looks similar. “It’s kind of a whole optimized system. There was not the possibility to carry one single part from predecessor,” the dynamics engineer told me.
With the new 3 Series, engineers have increased a dimension called Nachlaufstrecke. In english that’s mechanical trail, also called caster trail. It’s related to caster, but while caster is an angle, this trail is a linear distance.
Defined as the horizontal distance between where the vehicle’s steering axis intersects with the ground and the center of the tire’s contact patch (see yellow in the image above), trail is an important metric because it essentially quantifies the “lever arm” that the lateral force exerted by the road onto the tire (this force would push inwards at the “center of contact patch” labeled above during a turn) has to rotate the wheel about the steering axis to create that self-aligning torque.
Van As says one key lesson his team learned is that relying on the tuning latitude afforded by electric power steering—such as automatic self-centering—tends to mute feel. This time around, they worked more on the underlying kinematics, and to good effect.
Sounds promising but what about that damnvariable rate steering BMW insists is better?
BMW has been messing with variable steering for a while now, much of it hated by purists. The problem has been that the idea of steering that’s quick in corners but not darty while making little adjustments on the highway seems good, but the application of the idea feels weird.
As it turns out, there’s a mechanical part that’s responsible for the strangeness, one that BMW changed up.
Unsurprisingly, Rothmiller mentioned altering the new variable sport steering setup that comes with the M Sport and adaptive M Sport suspensions. Here’s how it works.
Rothmiller says the new steering offers 2 millimeters of additional lateral rack travel per degree of steering wheel rotation in the center, whereas the old sports steering had a similar on-center ratio as the standard rack.
But in a turn, well off center, the steering ratio changes thanks to an increased gear spacing in the rack. As Rothmiller told me, the previous car’s setup yielded a sudden change in ratio thanks to a sharp step in the spacing of the gear teeth on the steering rack.
But this is just scratching the surface. Jalopnik goes to dissect pretty much everything that is new and/or looks interesting about the 2020 BMW 3 Series. It’s an absolute must read for anyone remotely interested in BMW’s or the engineering that goes into cars like the 3 Series.