Center of Gravity?

[Gary Schulz]

Does anyone know where the closest point to the CG is with respect to the rota-dip holes? I am building a rotisserie and wanted to get it so the shell is reasonably balanced (rotationally). Seems like it would be closer to the bottom of the hole than the top given the weight of the floorpan etc...

IMG_2788.jpg

[Peter Laidler]

From my experience of mechanical matters and a bit of teaching school physics, I'd say with 1000% certainty that it will be impossible to get the centre of gravity of a bare mini shell using the rotodip holes Gary. The material weight distribution alone - and visibly - shows that it's bottom heavy.

I realise that you haven't mentioned it, but fitting a subframe would simply exassipate (spelling?) the CG situation. The lowest position within the front speedo hole with the floor=pan parallel with the ground is the best you could hope for.

[Dearg1275]

If you anticipate restoring a clubman shell go as high as you can at the front to clear the front panel. The mini shell is so light it really is not an issue. It’s easy enough to swing the shell over. Good locking screws and a few usefully sized props and you will have no problems. A broad base for your supports to give stability as you swing on the shell is more important.

D

[Andrew1967]

Agree with David.

My mate is using my rotisserie to do his GT and the he can’t fit the front panel with the front part of the pole in place … right PITA !

Another thing is once the subframes are fitted, as Peter says, it DRAMATICALLY alters the COG and if you are not careful and hold the shell firmly whilst rotating it, can be pretty dangerous. Really is a two person job.

[Gary Schulz]

I don't intend to the leave the front subframe in place. I just have it there temporarily to help with the alignment of the front-end panels. I assumed that as low as possible made sense since it would appear the vast majority of the weight is in the bottom. Good to know that it is relatively easy to rotate even though unbalanced.

Many thanks for the replies!

[1071bob]

On a mk1 shell the limiting factor for the pivot bar is the clearance to the dash rail, if you have the shaft parallel to the ground the rear pivot height is higher in the rear roto-dip hole. Make sure you have a decent support when the shell is at 90deg, don't rely on props of wood etc, if the prop fails you get hurt! I made some frames which bolt to the slinger brackets.

[Dearg1275]

If you keep the pole pinched up somewhat as you rotate the shell to act as a clutch you will slow any sudden swing. I keep it tight enough for me to put some effort into rotating things. It makes some distressing noises as it goes round but it will take more minis than I will even fix to wear out the scaffold pole.

D

[Gary Schulz]

I notice Bob has used "hard points" to attach the pipe to the car. I know some just use plates attached to the sheetmetal at the two bulkheads. If attaching to the bulkheads, do you just attach the plates to those small fasteners around the speedo pod and rear? Seems sketchy...

[Andrew1967]

My front plate fixes through the speedo aperture and clamps on to the engine side of the bulkhead. The rear is bolted through the rotodip speed fastener holes.

I’ll see if I can find some photos

[Andrew1967]

This might help

The front has a plate that sits behind the bulkhead with a shaped place of hardwood poking through the aperture and then clamped as shown on the front … the two nuts are there to keep the clamp plate tight

[Dearg1275]

At the front you can make a ply wood piece to closely fit the rota dip hole. This fastened to a plate secured by the binnacle captive nuts gives good support. The rota dip hole is that shape to prevent the shell spinning on the rota dip shaft, so why not use that shape?

D

[Gary Schulz]

At the front you can make a ply wood piece to closely fit the rota dip hole. This fastened to a plate secured by the binnacle captive nuts gives good support. The rota dip hole is that shape to prevent the shell spinning on the rota dip shaft, so why not use that shape?

D

So the torque (moment) load is taken by the mating surfaces between the wood and the aperture rather than the little fasteners? Makes good sense to me.

[Alex]

I've put the Hornetto and the A30 on rotisseries.
With the Hornet, I used an angle iron bar attached to the bulkhead and fixed through the subframe mountings and a bar between the rear wheelarches fixed through the shock/helper spring holes. Both bars were then attached to a loooong aluminum scaffold pole using clips I'd welded to the bars.
It worked, but wasn't 'great'. Scaffold poles are actually quite flexible over a 15' length, and there was a distinct droop. Flipping the car needed a helper bar as the CoG was far from right.
The second version for the A30 I used 'proper' steel tubing and mounted it as close to center of the holes as possible. Gertie sat at the most peculiar angle once the pole was level, but by God did it work !
Perfectly balanced, almost to the point that you could let go to grab the locking spanner once you'd positioned the car - she'd spin, but in a gradual manner.
When i get round to doing the next Elf, I'll certainly be aiming for the factory holes again - they put them there for a reason...

Word of warning though - they are only balanced when naked.
As soon as you bolt ANYTHING on, it will whip round like a rabid dog and bite an unwary knuckle or limb.

[Peter Laidler]

Re your last paragraph Alex. But it only happens once! You learn your painful lesson after that!