Steering Wheel Not Centered on Seat - Explained

[fe1rx]

I am in the process of installing race seats in my 135i, so am looking at appropriate mounting geometry. A Google search of "steering wheel not centered on seat" will find a mountain of hits, because modern cars generally don't center their steering wheels on their seats (and once you have noticed this, it is hard to un-notice it). SAE J1100, the standard for defining motor vehicle dimensions, anticipates this missalignment. Most internet commenters consider this "feature" to be a "flaw". Installing new race seats is a good opportunity to consider whether to retain the "feature" or to fix the "flaw".

To consider the matter fully, I first determined the geometry of the seat mounting points and the steering wheel by leveling the car and then creating a datum framework and then establishing the plan view geometry with a combination of tape measure, laser and conventional plumb bobs.



The measurements are shown above. I found the following points interesting and relevant to my project:

1) the seats are not centered on their mounting bolts
2) the seats sit facing directly forward parallel to the vehicle longitudinal axis
3) the passenger and driver seat locations, and their mounting are symmetrically identical
4) the steering wheel is offset 10 mm inboard (W20 - W7)
5) the steering wheel axis is angled 2 degrees inboard (a straight edge on the steering wheel, spanning the cabin, makes this angle immediately apparent)

Keeping the 10 mm offset, at my actual race seat location, the center of my forehead is on both the seat axis and on the steering wheel axis in plan view. This would be closely true for most people's normal sitting location in OE seats.

Therefore, although the steering wheel is offset, it appears centered when looking inboard 2 degrees along its axis. The instrument cluster is also centered on this axis, which emphasizes the impression that the steering wheel axis is "straight ahead". I did a trial mount of my race seat without the 10 mm offset and the and it looked off because the relative locations of the steering wheel and instrument cluster were off.

The steering axis intersects the longitudinal axis of the vehicle approximately 10 m in front of the steering wheel. Presumably drivers generally look at the center of the car in front of them, or in the center of the lane in front of them at some middle distance, rather than strictly straight ahead, motivating manufacturers to adopt the offsets.

Therefore, the offset steering wheel position (and axis) is, in fact, a feature, not a flaw and I will be retaining it for my race seat installation.

[houtan]

thanks for posting this information. I always felt like the seat and steering wheel was offset, but just figured it was in my head. This confirms I am not crazy.

Looking forward to seeing what the new seats look like once they are installed.