Would removing the transmissiion tunnel totally cause any problems to chassis stiffness on a McSorley?

Before anyone says what about the propshaft? It's going to be a middy.

I pretty sure it would, although dont Dax run withought a proper tranny tunnel?

The tranny tunnel on a Caterham is very flimsy. I don't think it adds much in terms of stiffness in any design of Sevenesque car.

I think it might affect the torsional stiffness. to compensate, add extra diagonals in the rear bulk head (behind the seats), in the scuttle or front bulk head and in the floor.

I was going to add cross members anyway but was a bit concerned that the tunnel contributed heavily to the build.

I would say that it does contribute substantially, as the perimeter of the "book" chassis is pretty flimsy to begin with!

Most mid-engined cars (speaking from experience with Ultimas) use wide sills to get the stiffness back that is lost from having no central backbone.

If you look at a lot of the lotus chassis (europa, esprit, elan etc etc ) the backbone provides nearly all the chassis strength.

The backbone dosen't add a lot to the siffness, the original Lotus didn't even have a propshaft tunnel. But if doing away with it I would stiffen up the cockpit sides and frontn and rear bulkheads.

Interestingly the Lotus Elan backbone it didn't do much work either the Elan was a moncoque to which a backbone subframe was added as at a late stage in develpment because of cracking problems around the suspesion mounts..

Check out FRED W B and his posts in the Midi section.

i was gonna add most "midis" have no tranny tunnel, did i read on here many moons ago posted that removing it does next to nothing ?? the tranny tunnel is open at the bottom/front on most sevens, i closed mine when i converted too BEC.

Good point Jon - the tunnel is actually open on the book chassis so it can't be doing that much can it?

I will be having a tunnel of sorts but it will be lower and narrower - just enough for the sequential change rods/cables plus the electrics and brake lines.

If I use extra strengtheners in the back, front and (aready have some in the sides) paneling to make it all stronger ( i know this one is another debate!!).

IMHO, the tunnel has some effect on front back bending (note, not twisting), but a lot less resistance to twist - its so close to the axis of twist that it cant add much stiffness. The saved weight would be better spent on extra tubes around the cockpit, either for impact protection or just general bracing to add stiffness.

this is all just my thoughts obviously, no calcs or sources...

i belive the tunnel does add to the strength of the chassis cymtrix posted some mods to the standard chassis to stiffen it up this included welding the tunnel with sheet steel he also included some calcs to back it up
IIRC the caterham tunnel is one pice of curved sheet steel this may look weak but its the shape ( the curve) that gives it its streangth
good luck dave

The tunnel can and should add significant stiffness to the chassis. A book tunnel doesn't, however, because it is totally untriangulated. Panelled or triangulated it will add significant stiffness to the chassis (see cymtrik's mods).

Without a stiff tunnel (or stiff sides) the cabin area of a seven type chassis (and hence the whole chassis) will allways be 'a bit wobbly really' (up to 1500 lbft/degree maybe). Plenty of seven type chassis dont have a stiff tunnel or sides though, and are considered 'stiff enough' for most uncompetitive purposes (at least in the UK).

Liam

[Edited on 9/2/05 by Liam]

The book bulkheads aren't stiff enough to apply any significant torque to the tunnel and less in the way of bending loads.
Caterhams in original form followed the Lotus 7 the tunnel was just an alloy sheet to over the propshaft -- later it bacame part of the structure and they added progressively more stiffness.

I will be making a middy BEC so would it be fair to say that the stresses and strains will be primarily at the rear and the only real pressure onto the seat area (and as such the tunnel area) will be from turning and braking. If I triangulate the sides and add some strengthening to the footwell area shouldn't I get away with it?

Imho X brace the floor too!

Main torsional loads are due to the roll couple distribution. With a midi the cg is well to the rear so to make sure the handling is somewhere near neutral the roll stiffness at the front will need to be set fairly high by using a fairly stiff anti-roll bar -- for this to work you need a very stiff chassis.

A stiff chassis is much easier to sort the handling on because it will respond to very small changes in anti-roll bar or spring stiffness letting you tune out oversteer or understeer.

[Edited on 10/2/05 by britishtrident]

So if I cross brace EVERYTHING and then stifffen up the front some more, add an anti roll bar and stronger springs, i'll be close but VERY HEAVY