This is known as "pre-loading" before torquing the control arms. This is because the bushing in the control arms of any car is made from polyurethane (or rubber) embedded in a metal sleeve, which by itself is squeezed by the bolts, so once the metal sleeve is squeezed, it does not move (supposedly). Every time you hit a pot hole or road imperfection, the control arm goes up and down
twisting the polyurethane (or rubber) bushing up and down around the sleeve axis.
By preloading (having the car on the ground before torquing the bolts) as opposed to torquing the bolts while the car is up on the lift at the shop, the polyurethane (or rubber) is in a position similar to normal driving condition, and this "pre-loading" is supposed to make the bushing last longer.
FYI, my 1998 Volvo V70 procedure calls for torquing the control arm bolts car on the ground with full gas tank but
no need for 150 lbs in each front seat.
BMW engineers are a whole bunch of an*l-paranoid people (seriously) and they aim for perfection. They want you to torque the bolts similar to real dynamic driving condition. To make things even more complicated, they make different torquing procedures for different cars, reflecting the income status and the profile of the buyers!
For example, who'd buy E39? Maybe single people, maybe married couple in their 30's and 40's with kid(s).
So the 'smart' BMW engineers say: for E39 torque the bolts with:
- full gas tank
- 150 lbs in each front seat (but they forgot that quite a few Americans are obese and weigh more than that hehe...
) = couple in the front!
- 50 lbs in the rear center center (kid in the rear center seat)
- 50 lbs in trunk (maybe a golf bag!).
(This is again to reflect the profile of a couple with a single kid!)
Now you can guess that the 'smart' BMW engineers say: for Coupe or Z3 (just think of who would buy the Coupe?) torque the bolts with:
- full gas tank
- 150 lbs in each front seat (look it up but I think it is the case). Basically "double income no kids" = DINK!
So one day I decide to check this out, with 2 passengers in the front seats of my E39, the distance between the
top of the front tire and the
bottom of front fender goes down
exactly 1 cm (check and you will see).
Then I did a calculation by placing 150lbs in each front seat, and since the length of the thrust arm is 45 cm long, the change in the angle at the bushing is:
Arctan(1 cm/45 cm) = 0.0222 degrees!
Not much to worry about!
So if anyone asks me how I torque my thrust arms, I'd say car on the ground with full gas tank is good enough for me (
no need for 150 lbs in each front seat).
There are too many things in life to worry about than this extra an*l-paranoid crap from the BMW engineers.