Rear Windows fully retracted

[Rek]

I have noticed some pictures of our cars with the rear windows completely below the level of the bottom sill. Mine won't go that far and there is a little hump of glass on show. Is this something which can be achieved through some changes or was it the result of a change in design on later models?

Its a minor thing but looks quite nice with a flush open look.

 

[nosmonkey]

Pop the rear card off, there's 2 stops that can be adjusted for the end points of the glass, mine was wound all the way out and the top of the glass stuck out about 1 1/2 inches

 

[Gary Knox]

I've adjusted the stop on my car so only about 1/8" of the glass shows above the sill. I did not want to put it all the way down and risk having the glass get 'off track' in some way when I wanted it up fast!

Gary

 

[deQuincey]

flush open not correct
define, quantify little hump,
say 10 to 20mm is about right

 

[teahead]

Should be at least as low as the front side windows IMHO. I don't like them sticking up any higher.

 

[RTHLSS1]

I was also a little bit disappointed when I fist manage to drop rear windows since they don’t go fully down. So much better looking from the side if fully down.

 

[Rek]

7 years later I get around to having a look at this to open the windows fully at the rear. I've rebuilt the mechanism on both sides and adjusted the stop so that the windows should open fully but the motor does not take the window onto the stopper. It seems that the spring is so compressed that it stops the window. There is a big screw inside the spring. Is this an adjuster of some kind? I hesitate to undo it as I have visions of it flying off into the middle distance never to be found again.

Separately, could it be that there is dirt within the spring that is stopping it compressing? I am running sardine tin motors but with relays to ensure the correct voltage. The windows move up and down in a spriitely manner, but just not all the way down.

Any ideas anyone?

 

[Arde]

Any ideas anyone?

Give it another 7 years perhaps?
I did not even know the rear windows can go that low. The spring is supposed to balance the weight of the glass but the problem is that the weight is constant and the spring force is not, I believe. So either a weaker spring or a heavier glass would be needed...
The only practical option is the motor replacement which has been well documented and SFDon recommends.

 

[bavbob]

Perhaps BMW wanted to see your kids grow up rather than fall out the rear window....ie I believe this is a safety issue and not a aesthetic one.

 

[jmackro]

The spring is supposed to balance the weight of the glass but the problem is that the weight is constant and the spring force is not, I believe.

When Rek refers to the "spring" I believe he means the coil spring built into some of the rubber stops that check the windows' upward and downward movement. Not the hairspring that seems like it is supposed to counter-balance the weight of the window (but as you point out, would apply an uneven force to the window, based on its position).

 

[eriknetherlands]

The large spring that is center mounted and helps to lift the rear windows is called a clock spring. They are designed to have an (almost) constant spring force excepted on their active end.
The metal cilinder in the middle indeed has a slot and i can see how one can look at it and thinks it adjusts something but it does not. It holds the spring end firmly in place being pressed into the frame, it can't rotate.
So the clockspring doesn't have any adjustment at all.

I did see clock springs with a lot of muck in them - perhaps this could prevent the clock to wind up as it squashed together, and thus not reach it downwards position ?

 

[Arde]

The large spring that is center mounted and helps to lift the rear windows is called a clock spring. They are designed to have an (almost) constant spring force excepted on their active end.

Interesting. The equation is Torque = k . Angle, per Hooke's law.
and Force = Torque . Arm length.

The arm length is constant, so the force is proportional to the spring displacement angle, which means that Force is higher at the lower window positions.

The only way to keep it somewhat constant is to have a small angle displacement for the entire range. Even then it is natural that the motor is fighting the spring at the very bottom. I guess one could pre-extend the coil so it actually pulls the window at the top and it has less force at the bottom, but I do not recall doing that when I cleaned mine. Getting the last inches at the top is really slow so I am not sure that is wise.