XK120 OTS Centre of Gravity / Rotisserie construction

Before I set out and spend some hours to figure it out, may I ask if anyone has already gone through the exercise of determining the lateral centre of gravity of a steel XK120 OTS body off frame? Such as what might be helpful if one was about to put one on a rotisserie?

When I was working for International Harvester I observed the engineering staff measuring that very thing on a crawler tractor. Their method was to attach lifting slings at an angle and pick the tractor up with a crane on a single hook, so it was hanging at an acute angle, and photograph it with a camera on a stand. Then they reattached the slings at a very different angle; pick it up again and photograph. Draw axis center lines through the tractor in each photo. Superimpose the photos centered on the crane hook, and draw vertical lines through the hook. Where the lines intersect is the center of mass. The principle involved is that the center of mass will always be directly below the crane hook, no matter how you pick up the load.

You know, Rob, I think that’s going to work. The body was lifted off the frame from a single lift point positioned over its central axis and centred by a load leveller - the red line in this pic:

At this point the entire mass of the body (about 400#) and angle iron support structure (another 75#) is vertically and horizontally balanced. If I suspend the car and shift the leveller forward the horizontal balance will shift such that the rear drops and the nose lifts, and going the other way with the leveller will cause the opposite effect. Whether or not I can achieve two angles acute enough to pinpoint the exact centre of gravity of the body and bracing is a question, but I should be able to get it close enough for a reasonable balance on a rotisserie shaft.

Stay tuned.

One idea would be to balance it on two jack stands

No, for mounting on a rotisserie, you want the vertical location of the CG, and you can assume it is on the centerline of the body. You don’t care too much where it is front to rear.
Pick it at least 6 inches on the left side of center and take a picture from the front or rear, then pick it on the right side the same distance off center and take another picture from the exact same spot. Be sure to get the hook in the pictures, and something hanging vertically like the chain or even a plumb bob in line with the hook.

I’ll get the rotisserie done tomorrow. Today got one of two brackets built and welded to the cockpit bracing and the other ready for welding in. Then to calculate COG using a laser level.

A shout out to Rob for passing on this gem of knowledge! :grin:

I used a laser level instead of photographs. The skuttle being in the way I couldn’t achieve much of an angle shifting the load backward, wondered if it was going to be a deal breaker:

The tripod for the level needs to stay in one place for the entire exercise as does the shop crane. I positioned a ladder next to the car as a benchmark to keep it in line. I titled the load forward and recorded the line, then tilted the load forward some more

The lines intersect!

And after levelling the car there it is, the elusive centre of gravity

So, having that the rest was pretty simple. Shoot the laser forward to establish the position of the rotisserie shaft at the front mount

There’s just enough room for the shaft under the firewall

Next move the laser and shoot the level line back to establish the position of the rotisserie shaft at the intermediate mount

here you can just see the rear mount through the driveshaft tunnel

Tomorrow’s job is to weld in the rotisserie shaft.

Time for a glass of cab with dinner.

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Heading to the shower then the grille and winecellar. Got the rotisserie done today. Works a treat. Now have plenty of room for the E-type when I take it out of hibernation next week.

I took a quick video but my stomach is claiming dibs.

Snowed today. It is April, isn’t it?

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Same here…standard Colorado April!!!

The video I took last night - as I sipped a can of beer to demonstrate to Geoff Allam that I am an equal opportunity self lubricator during evening ruminations. See the related Liftoff! thread - didn’t turn out so I took this one just now.

The rotisserie is made from a couple of one ton engine stands, two 4’ sections of 2"x2"x1/8" square pipe and two 20" sections of 2"x4"x1/8" rectangular pipe. It’s the same one, shortened, that I used to restore my E so no added cost. The rotisserie shaft is made from 8’ of Schedule 40 2" pipe - its OD is a perfect fit in the engine stand heads.

The body bracing is mostly 2"x3/16" angle iron - I figured 1½"x1/8" angle would have been sufficient but prefer the peace of mind allowed by gross over-engineering. With the shaft, bracing and body itself the rotisserie is supporting about 500#, with no apparent strain. The bracing is bolted to the body using mostly existing holes, with a supplementary four drilled into the front bulkhead. The bracing is also welded together for maximum rigidity and ease of assembly and is single use - it will need to be cut through in a few places in order to remove it from the body once its back on the frame. Cost of the steel was about $200.

If there’s interest I’ll add design and construction details to this thread for the archive.

Particular thanks, again, to Rob Reilly for his COG suggestion.


3 comments. First; glad to see that you are an equal oppprtunuty imbiber. Just need the single malt now for the trifecta. Second; I agree with the plan to over-engineer the rotisserie. It has been said that “too strong never broke”. Third; I would be very interested in a construction diagram with measurements defining the COG. My 120 is a fhc which makes access for hanging at an angle to get intersecting lines a bit more tricky. A known starting point would make adjusting for the weight of the roof a bit easier. Oh and fourth; congratulations on a well designed and executed concept.

A construction diagram isn’t required, Geoff, and would do no good anyway. Adapting the concept to your particular car is what will work. The cockpit bracing is the starting point. This one’s bolted to the body through 25 existing and 4 additional bolt holes. One side of the cockpit between firewall and rear bulkhead is a quarter inch longer than the other and similar non-symmetry above. It’s a custom build. Just like the body itself. No two arrangements will be the same or interchangeable. Measure. Cut. Dry fit. Mark bolt holes, bolt down, MIG weld into place. Proceed to next piece to be measured, cut, dry fitted etc.

Yes please. And thanks in advance.

I think you are just showing off now

Great work

That’s the idea, Nick, same principle whether a 400 lb OTS body or a 50,000 lb TD25 tractor. Although when I observed this operation at Harvester in 1980 they didn’t have laser levels and they shot from the front and back of the tractor since they were more interested in determining the rollover point if it was driven on the side of a hill.

So for a FHC you would need a lifting fixture, perhaps something like this.

This is how the factory lifted FHC bodies.

It looks to me like in front they hooked under the wing valences, and in back they had a bar under or behind the battery boxes, but it would have to slide out after the body was on the chassis.

There may be a way to simplify it. The basic cockpit bracing is installed first onto which the lifting points are welded, primary purpose is to lift the body off the frame without flexing. If the doors are removed the end of the shop crane lifting arm can be entered into the cockpit at a 45º angle from the rear and attached to the bracing a couple of inches below the roofline - this will still be several inches above the COG so a safe lift. It would take some pondering to figure out whether it’s do-able.

I’m taking some time off from the XK120 but will do up a concept video in awhile. Today’s got appointments to honour and other out-of-the-shop tasks to perform. Tomorrow I’m taking the E-type out of winter storage, then a long drive to get the cobwebs out.

(Thanks, Phil. But this isn’t work. It’s play!)

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Looks like too much work for lazy ol me. Was just thinking of piggybacking off someone else’s hard work and calculations. :smiling_imp::smiling_imp:

There’s no math involved beyond a bunch of measurements, and the CoG of any particular autobody will be affected by how the cockpit bracing is configured. I didn’t calculate the precise weight distribution of the bracing, though it is biased above the OTS body’s CoG because of the need to buttress the rear bulkhead with additional steel - the front bulkhead, unlike the rear, is a rigid box section that stands on its own. The bracing is otherwise symmetrical from side to side. As it is, when I shot the centre line with the laser level to determine the precise vertical positioning of the rotisserie shaft:


it was just under the top of the gearbox arch in the firewall and allowing zero upward positioning. While I would have preferred to bias the shaft a few mm above the CoG so that the body would have a natural tendency to right itself on the rotisserie, the shaft is actually positioned 2 mm below the CoG because there was no room to bring it upward. I could have cut a 5 mm notch in the shaft in that location to clear the firewall, welding in a patch to retain the structural integrity of the shaft, but judged the payback to be not worth the effort, and if I want to lower the CoG to just below the midline of the shaft it would be a matter of adding a counterweight to the bottom of the framework - by clamping on a length of heavy angle iron, for example. That seems a simpler solution.

Because of the added weight of the roof, the CoG of a coupe will be still higher up on the body, so to do the same schtick you will have to notch the shaft to clear the firewall unless you add additional steel below, or leave the steel floor panel assemblies in place - though the added rigidity provided by the roof is likely enough to reduce the massiveness of the rear buttress.

You probably have enough there to proceed, Geoff. Your rotisserie shaft, like mine, will be at the extreme top of the firewall opening and the simplest approach will be a matter of adding some mass below to balance out the body.

I should emphasise that this setup is for finishing the bodywork, and cleaning up the underside prior to painting. You’d have to take a different approach, with additional bracing, to do this with a body with structural rust damage.

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Thanks for that Nick. Makes the decisions pretty simple. I will go with counterweighting as that will allow for pretty easy adjustment of the balance.

Have fun and be careful. There’s always a risk working around heavy objects and your welding skills need to be up to snuff.