Mounting E9 Unibody to Rotisserie/Frame Rack

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Dowst

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Would appreciate any thoughts/insight regarding appropriate mounting of the E9 unibody to a rotisserie/frame rack for the purposes of sill replacement and general metal/body/paint work.

Given the sensitive/flexible nature of the pillarless coupe design, it is obvious that appropriate support of the unibody throughout the deconstruction/reconstruction process of both structural elements (ie sills) and less structural elements/body panels is crucially essential to maintaining overall dimensional conformity and retaining the panel gaps established during metalwork/bodywork in the final, road-going product.

I have seen a variety of body dolly/rotisserie approaches on this site, however I notice that many of these support the body at the front and rear subframe attachment points, which I am suspect of. When the car is fully assembled and on wheels/tires, the weight of the completed car is not supported by the subframe attachment points, but rather by the springs via forward strut tower/mount locations and the rear upper spring perch.

With this in mind, it seems logical that supporting the unibody from these same load bearing structures (strut mounts and rear spring perches) should be the most reliable way to ensure/preserve dimensional conformity and panel gaps when the sill structure is removed and replaced and other body work is conducted. I interpret as further evidence of this the change of door closure/gaps often exhibited by E9s when placed on a lift via the “typical” locations (forward unibody rails and rear sf mounting points). Additionally, does not selecting rear subframe mounting points for support obstruct access to this area (rear of sill/lower wheelhouse area) for reconstruction/metalwork?

Has anyone here constructed a frame rack/rotisserie configuration that attaches to the unibody at the strut tower/rear spring perch locations where it will naturally be supported by the springs when fully assembled? My objective is to ensure that chassis dimensions and panel alignment are preserved precisely as established in body work, and there are no unfortunate surprises when the car is reunited with the pavement later on.

Again, appreciate any thoughts/insight that the community may be able to offer on this topic!
 
Very interesting questions - You've given this some serious thought - which I will soon be facing on a new project.
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Celette offers rental options. Surely there are others. They may have had dedicated benches for particular models "back then" but the present universal systems combined with 3D capabilities and laser measuring should provide accurate multi-point attachment to the level that temporarily removing one point for work in the area won't affect overall alignment. I wouldn't think that frame measurement specifications were provided in suspension loaded format so sprung loading might not be the preferred solution. How were the bodies built/held together at the factory? Sub-assembly jigs? Maybe a deep dive into BMW historical archives would reveal their production line jigs for the e9. Perhaps some of the restoration specialists have that information.

Off-topic but I was fascinated years ago on a visit to the Smithsonian's Paul E Garber restoration facility in MD by the continuing use of factory BMW engine jigs, benches, etc. Was told they "were the best" (acquired as WWII reparations?)
 
Here is my rotisserie solution. A very solid beam with cross members bolted to the subframe attachment points. In addition to this the door openings and coupe was reinforced with steel tubing, almost like a roll cage. You really can't exaggerate the need for support. Especially when you replace the inner and outer sills. If the strut towers are replaced you also need to weld in some measuring points before you cut out the old steel, to get the new steel in correct position.
 

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My rotisserie holds the car at the front and rear subframe attachment points. I believe this works great, since most of the weight (engine, suspensions, subframes,...) is removed from the car when you put it on the rotisserie. Ofcourse you'l have to reinforce the body at the dooropenings etc.
If the attachment points for the subframes wouldn't be sufficient, then I would be afraid to drive an E9, and certainly take a corner or a bump in the road. The forces during driving a car or taking a turn are much bigger.

I'm working now for 8 months on the body of my E9 DIY rotisserie and it works great! If you're interested, I can deliver the drawings for this rotisserie. It's easy adaptable so you can put other cars into it too (Porsches, Ferraris, Detomasos,... ;)), and you can use it as a dolly as well.


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DIY Rotisserie

I'm exited about the rotisserie I made for the uncoming bodywork. Here's why: - it took me a just a couple of days to assemble - it's a rotisserie and a dolly - it supports the car at the right mounting points, not at the ends (bumper-points) of the car like many other rotisseries - it's about...
e9coupe.com e9coupe.com
 
to answer your questions:

- yes the front strut points are often used for a jig, but the rear spring perch I've never seen being used. I think it is because it's not dimensioned in the BMW 'floorplan' drawing, so you only can fabricate it to fit YOUR car, not to what is should be : you will not know if the rear spring perch mounting points you've created are actually in the correct position. (EDIT: I was wrong. the rear strut mounts actually ARE dimensioned in the BMW floor plan drawing, so yes, you could use them. Still, I've never seen anyone use them in a dolly. )
- Yes, whenever you support the body in certain locations, you can't repair it there where you've just bolted it down. You can overcome this by using multiple bracing points, and when you chop out a corner / sill side, you can unscrewed the mounting point there, and the remaining body+internal frame will be still strong enough.

Two risks when removing sills:
1. a relative twist of the front and rear axles; you need prevent torsion to prevent this.
2. The other risk is that the distance of the front to back axle increases (the roof sags downward- door gaps will be off, all around the glass won't fit) .

As with most bodywork, you need to start with figuring out IF the body is straight.
- If you measured it as being straight, you could build a light frame just to keep the shape. You could weld in an internal frame (with triangles in all three planes) Or you could make up an external frame, doubling as a dolly; 10 feet* 1*2 inch tubes would be sufficient. Perhaps you can go thinner if you combine such an external frame with some light internal braces.
- If you do not know if it is straight, or if you DO know it is crocked, then you need to correct it by building you car back to the factory dimensions. Thus you need something sturdier. A ladder-shaped frame supporting it from below with the 6 mounting points (+ optionally the diff mount) is a good option. I'd choose 2*4 inch tubes for length, and 1,5*1,5 inch tubes for the cross tubes. The red Celette jig in the image in the posts above is designed to PULL a body straight, not just assemble. So those big red beams are overdimenionsed if you want to rebuild it 'Lego brick style" (drill all spotwelds untill you have loose panels, and then reassemble while mounted on the fixed refence points.)
If you space the external frame about 1,5 foot away from the body, then you'll have enough room to work on the underside of the floor with an angle grinder.

I have welded in cross bracing in the interior to replace the sills, while supported on it's wheels, with engine still installed. I made triangles from A pillar to B pillar and also from A pillar into into the C-Pillar. In addition 2 cross braces spanning the width of the car. Then I chopped out one sill at a time.
After completing the sill replacements~2 years ago, I'm now building an external frame to mount it on the 6 points so I can chop of the front end, and LEGO back the inner fenders, outer fenders, hood and front clip correctly.



If you have the space, one can build a torsion resistant jig with beams below the car and support it's framerails, front struts and rear strut towers. Optionally, IF you need to cut out exactly those points other points can be added to your jig like rear diff mounting points, sill edges or logic points like the 2+2 drain holes in the frame rails under the passenger compartment
 
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I'm very impressed with the ease of accessibility with Johan's design. I'm not going to become more nimble with advancing years and the three years I've allotted may be optimistic. I am starting with a well-built dolly. Perhaps can adapt dolly to work with the hoop rotisserie. Also nice that unlike bumper mount ones the overall length is less than that of the body.
Johan, if you are willing to share drawings I would very much appreciate.
Erik - Nice reinforcing on the strut mounts
 
Thanks. (need permission to access). Great site. Now I know the very good reason why I saved all those corks! Love the idea.
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Yes, thank you. Was able to download. Now just have to download the operating files to open them.
 
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