I couldn't find the original thread on this subject, so forgive me for starting over. (Maybe it was the other forum.) I finally got around to looking under the seat and this is the letter I wrote to BMW. I'll let you know if and when I here back.
There's a second issue I didn't mention, but I didn't want to muddy the waters. I also found that the thigh support can move a little if it's positioned anywhere other than full in.
I would like to bring to your attention an issue with the 6 Series front seats that seems to be common amongst many owners based on the forums that I read.
Simply put the front seats have a looseness that allows movement under load when the car is cornering. It does not take much lateral input to generate movement.
I have investigated this problem to isolate the cause. I am an engineer with 30 years automotive experience, so please take my analysis seriously and forward it to the appropriate personnel.
There is a cross-car steel support tube that passes through two (I assume urethane) bushings under the forward edge of the seat frame. This assembly is part of the mechanism used to raise and lower the front edge of the seat cushion. The bushings, attached to the seat cushion pan, are oblong in relation to the diameter of the tube to allow longitudinal movement as the mechanism is raised and lowered. Lateral movement is controlled by flared "stop" features on the steel tube on either side of the outside (driver) bushing. It is the relationship between the sides of this bushing and these stop features that is problematic.
If the front seat edge is adjusted to any position other than full stop (high or low), which places the tube firmly against the bushing, lateral movement can occur between the sides of the bushing and the flares on the tube. This motion is easily felt by the seat occupant as the bushing (and seat) bounce back and forth between the two stops on the tube. Hardly acceptable on this class of automobile.
Many 6 Series owners have taken their cars to the dealership with no resolution. A permanent solution needs to be driven from the top down. It seems a simple solution would be a wider, harder lubricious bushing to prevent the unwanted lateral movement without binding the adjustment mechanism. But I will leave that analysis the the BMW engineering staff.
Thank you for your attention to this important matter. I look forward to a solution and positive response.
Feel free to contact me at any time to discuss this further.