Jacking your BMW

[Cacatfish]

I did on older cars like e28 and such, but this one just feels heavier and maybe a little more valuable, so I stick to the factory points. I would never do the diff, but the front and rear cross-members probably wouldnt hurt the car.
BTW: never trust your jack only. Use stands or at least blocks....

[dave b]

I do the factory points only!

[scott540]

This is some good stuff. thanks for starting the poll Jeff

[632 Regal]

I guess people do lift them on the crossmembers, I dont think I will but at least we are learning stuff here. Thanks to Glen I think my foot will be stuck in my mouth on this one.

[DueyT]

Well Jeff, I figure I'd give you something to keep the interest factor up over lunch!

I used to use the factory points, but I wasn't getting a warm fuzzy about twisting the frame to lift a lowered car up onto those points.

I lift the front up with a wide block of wood under the cross member. IMHEO, linear lifting forces distributed over the structural member that forms the central load transmission path from front wheels to frame is not inconsistent with what that structure does 24/7, 365 days a year.

Now...the rear diff? I haven't done that yet and I'm not too sure it's a good idea. If I do a rudimentary stress-strain analysis of structural load transmission path from rear wheels to frame, the differential casing does not form a part of that load path. The rear struts and trailing links are what transfers vehicle weight to the wheels and Newton #2 says action-reaction, vice versa when lifting up. If the point is to get a single lift point to lift the entire rear (after already lifting the front and using jack stands under the fron factory lift points), I would say that you should either use some device to bridge between the two rear lift points or gingerly lift each side a bit at a time. The second option may not be any better for the frame since there might still be some twisting force if the fronts are already placed on jack stands...maybe ramps are better to allow for some twist. Is there a snubber on the diff that would contact and pass lifting forces to the frame immediately above the diff casing?

Overall, the front cross member is a point intersecting the standard load path...that's fair game as far as I'm concerned. The diff is not, however, and lifting by the diff casing I think is something that you do as a calculated risk. If there isn't a snubber assembly on the casing, I'd hate to think of the tangential forces acting on the drive shafts, trailing links, etc... by lifting an otherwise "floating" differential. As food for thought, consider that a vertical force applied to one end of a fixed-length beam (half-shaft) that casues a 5 degree deflection results in a lateral pulling force 11.4 times the original vertical force ( force(lat) = force(vert) * 1 / tan (horiz. deflection angle of beam)). This could equate to 5 tons lateral force per half-shaft on a 3800lb car with 50% weight on rear wheels. Even a 15 degree deflection angle still results in 3.7 times the applied vertical force...

Hmmmm....

What do you figure, Jeff? Front OK! Back...maybe a rethink?

Cheers,
Duey

[MikeV]

The diff isn't really floating, if I recall correctly (not that that's a given) there are rubber bushings/mounts on the rear of the diff carrier, and it is straight-bolted to the rear subframe crossmember. So the 'float' of the diff is entirely the give of the rear diff carrier bushings and the subframe bushings. I would think the live loads they are subjected to are much worse than the static load from jacking (but that is just a guess).

Now, the diff case, that's another story... but it is pretty massive.

[632 Regal]

I lift the front up with a wide block of wood under the cross member. IMHEO, linear lifting forces distributed over the structural member that forms the central load transmission path from front wheels to frame is not inconsistent with what that structure does 24/7, 365 days a year.


Duey, Doesn't the load from the front go to the top of the strut tower and get distributed into the unibody/frame? The crossmember ties the thrust arms and frame/unibody together, how do you feel that it holds all the weight in the middle of the crossmember? I know chrome moly is strong but I still dont think it was designed to withstand such vertical lift.

[DueyT]

Jeff, you're right about the loads...the cross member actually takes primarily vertical loads as transferred down from the strut towers and redirects them into lateral loads as augmented/balanced by the lower control arms. I guess I'm happy enough that with the big arse piece of wood I use under the cross-member, I'm (almost) effectively lifting concurrently at the outer portions and, for the most part, vertically up to the strut towers.

Duey

[George M]

I personally wouldn't lift on either the engine X-mbr or the diff. I use a hydraullic jack and lift each side incrementally on the jacking points, then place a jack stand under the front frame rail box section on each side and a jack stand under each rear suspension trailing arm. This one can be kicked to death because there are a lot of ways to lift these cars and no immaculate solution. The engine crossmember is a two piece clamshell deep draw stamping welded together and designed to sustain the weight of the engine and lower control arm attachment and not to sustain the entire weight of the car even if you use a generous block of wood to distribute the load. That said, people have been jacking on the engine X-mbr for years...I never have. Duey makes a good point about corner jacking and attendant stresses but overall frame torsional strength is designed to locally jack the car on each corner and hence stresses imparted are lower than the amount of stress relative to strength of loading the engine crossmember by jacking the entire car weight...approx. 2000 lbs of force versus approx 1000 lbs force at the vehicle corner attenuated over the vehicle frame designed to handle the localized point loading due to jacking. As to jacking on the diff...again something I would never do...not only taxing the compression strength of the differential case casting but the localized loading of the diff into where it is connected to the unit body of the car. Jack the car on its jacking point and place a jack stand under each trailing arm that is designed to bear 1/4 of the overall vehicle weight of the rear of the car. Vertical jacking on the diff will not impart resultant stresses to driveline or rear suspension components.

That said, people will continue to jack up their cars how they want :-).
George

[DueyT]

I heard about this years ago, but never really followed up on it...does anyone make a light-duty commercial/home lift? IIRC, the system was relatively simple to install on a flat slab of concrete, and could be relatively easily removed and moved as well?

Anyone welded up their own?

Cheers,
Duey