MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - How stiff is a shell with no tunnel?

Morning all.

As part of my on going rebuild, I'm contemplating removing the stock transmission tunnel and replacing it with a ladder frame.

This is for two reasons really.

1, I'll need to cut it about anyway to fit my MX-5 gearbox,

2, It'll be stiffer than before and given all the other work and the 2.5L 24V V6, every little helps!

However, the shell is currently up on it's side whilst I fit all the rear suspension. This gives me nice easy access to the spot welds along the lower edges of the tunnel which I could then drill out ready for fitting the lower rails of the frame. But will the shell twist if I do this? Would it be best left until the shell is back flat and being held square on stands/posts/wooden beams etc?
S Overy

You should realy fit some braces in the door openings
Otherwise saging is a big risk even when it is flat on the floor.
Just welding The doors shut should do it.

So fit the doors check that they align perfectly and then weld a piece of bar on the inside acros it
Onno K

Would that be needed if the shell was being braced on beams? That's what I did when changing the outer sills and it stayed flat.
S Overy

Then you are lucky.
The clear shell without engine etc will have helped but it is so floppy when you do some serious cutting it is always best to brace.

Adding the braces is not that mutch work.
Correcting a crooked shell is.
Onno K



The issue is more whether the main cockpit will "lozenge" if the tunnel is cut out when the car is on its side, suspended at either end. If the floors are OK and well connected with the front, middle and rear cross members then it should be Ok

If the cills and floors have already been done, or are sound, then cutting out the tunnel shouldn't be a problem. Fitted doors would be an extra precaution , that is all.
Guy

In for a penny ... In for a pound

Id take every percaution to make sure nothing moves on you, cause once it does your screwed

Try this experment....most likly you have a 5 blade cieling fan... Turn it on high and shove a stick thur the spinning blades. That will bend one of the blade connectors....now straighten the bent blade connector so the fan runs like it did before....if you can do that.... Then bracing on your car wont be needed. Cause your just to good

Prop
Prop and the Blackhole Midget

Because of the midget's narrow sills and lack of a fixed roof, the tunnel has a critical role to play in torsional stiffness, which a simple ladder frame can never replace. To achieve torsional stiffnes you need bulk both vertically and horizontally, and the Midget tunnel is the only place that there's room to do this.
The more torque you are planning on from the engine, the more torsional stiffness you will need.

If you must remove the complete tunnel, replace it with a space framed version, and make it bigger at the same time. Alternatively you might like to look at how we enlarged the tunnel to fit the Supra gearbox, a big chunky unit. One of the things we did was to shorten the effective length of the tunnel, which also increasestorsional stiffness.

It's also important to tie the tunnel in to the body structure front and rear. In original form, this is done by the double skin cavity at the rear and by the firewall link combined with the ring frame formed by the scuttle, A-posts and crossmember. Removing this crossmember for a larger gearbox also requires careful though about its replacement.

What we did for both aspects is at http://www.mgparts.co.nz/about-us/our-cars/matt-1961-midget-project/

Paul Walbran

Sorry, I hadn't realised I had put ladder frame. I did mean to say a space frame version.

The plan would be to make it as large as possible whilst still allowing the seats to fit. The front would be tied into the frame in a similar manner to yours, and would carry on through the rear bulkhead to give the diff of my De-Dion set up something to attach to.

Prop, whilst that's a dubious analogy, I get the idea! Although I don't see the difference between bracing the top of the top and stopping the bottom from moving.
S Overy

Try talking to Bill young.... He is away for a week or so.

But he shoved a chevy V-6 5 speed from an IROC camero an it looks factory...but he can certianly provide some insight.

Give me a bit... Ill see if i can find his website link

That was fast...hahaha

http://www.britishv8.org/MG/BillYoung.htm

Prop

Prop and the Blackhole Midget

A bigger space frame with the right amount of diagonal strutting should do the job well.
Paul Walbran

I'm back. As long as the sills are in good condition I doubt if removing the tunnel would hurt the body much as long as there isn't any drive train or axle in place to add weight. I removed the front half of my tunnel during the construction of my car, didn't remove it totally, left about a 2" flange around the bottom and then hand formed a new upper piece to cover the larger transmission and welded that in place. With the center crossmember modified and reinforced I've not noticed any reduction in body stiffness due to the modifications. I didn't have to alter the rear portion of the tunnel much, but did cut an access panel for the front U-joint which is screwed in place with about a dozen sheet metal screws. I recommend keeping the tunnel as narrow as possible, removing any un-necessary bits from the trans case if required. I cut off several protruding ears that I think were there to hold the transmission on fixtures during the manufacturing process and weren't needed. Made the unit almost 3" smaller side to side which allowed me to not have to cut the main frame rails, just make the new tunnel bulge outwards about 1/2" on each side above the original line.


B Young

Thanks all for your input. The sills are good as I have not long changed them, and the shell is naked at the moment bar the De-Dion bits at the back. Sounds like once it's back the right way up I should be ok to sit the shell on the beams I used for the sills and set to with the grinder! At least making a space frame tunnel should be fairly straight forward and not overly time consuming,
S Overy

This thread was discussed between 18/12/2011 and 19/12/2011

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.