Hi, could anyone building an avon please do me a favour? Im wanting to make some rear braces for my roll hoops. What i need to know is where does the rear bodywork come to in relation to the chassis, ie looking downward stood behind the car, is there a gap between the tub and the rear most box section? And also is there a bigger gap from chassis to tub on the outer of the diagonals that go to the back of the car? This one would be best as it wouldnt interfere with the tank, but the braces would be on more of an angle. I cant check for myself as my tub is in storage at the mo,so any pictures of the area or measurements would be appreciated. Ive tried to do a diagram, but its not great (dont laugh) Any opinions welcome, thanks in advance.
Rescued attachment diagram.jpg

Definately the one on the right in the bottom pic.

The chassis has a small plate on the back for bars.

If you do though, you will have to butcher the top and inner of the rear panel to get them in which IMO will seriously weaken the whole back structure of the GRP.

Some have done it though.

http://locostbuilders.co.uk/viewthread.php?tid=58362

Pat...

as Pat says, there is a plate in each corner to mount the back stays on. It is a very close fit to the ends of my fuel tank. The fuel pickup hose just clears the roll bar tube.
I've bonded a tube along the inside edge of the tub in order to add stiffness to the back end.
Rescued attachment rollbar backbrace.JPG

Right i see, think my chassis' a bit different cause its an early one. It doesnt have that flat area outside the digonals. Just tried the two options for a photo, and if i go outside the diagonals, theres not much angle on the braces, and to bolt them into the triangle like in my diagram, id have to space it off a bit to stop it fouling the chassis. Other option of going behind the rear box section, now looks stronger as the braces would be squarer and angled back a bit further, but would it be strong eough with brackets welded on behind the box section? The photo might explain bettter..

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Rescued attachment roll bar small.jpg

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Rescued attachment roll bar brace 1 small.jpg

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Rescued attachment roll bar brace 2 small.jpg

I would say going behind the rear cross member is not an option since thats where the body tub locates. I would extend the box section out the side and make a flat plate to mount the back stay onto. I'm not too sure about the structural integrity of this though.

I would say going behind the rear cross member is not an option since thats where the body tub locates. I would extend the box section out the side and make a flat plate to mount the back stay onto. I'm not too sure about the structural integrity of this though.

Thats what im worried about too. How about joining onto the inside of the triangle? Would waste a lot of tank space and would there be too much shear for one bolt sideways? At least im getting better with my diagrams

I would say going behind the rear cross member is not an option since thats where the body tub locates. I would extend the box section out the side and make a flat plate to mount the back stay onto. I'm not too sure about the structural integrity of this though.



[Edited on 28/2/07 by ayoungman]

Why not weld in a couple of plates and box section where the later avon chassis has them? (Mine's an early chassis without the plates and this is what I did for my rollbar braces.)

Andy.

Mainly because like "ayoungman" says, im not sure about the strength of this, and also the braces need to be on even more of an outward angle to do this, which makes them more likely to collapse in an accident.

Thats an option I hadn't thought of.

It would need the ends of the back stays to be cut and welded. Here's a quick 3d version of what I'm thinking.

and from the side. In the event of an impact/roll, the forces would be straining on the 90 degree corner, so this is not an ideal solution. You could put a smaller diameter tube through it and use a beefy spherical joint to tie it in to the triangle.

I've compared your chassis and mine, it looks like Tiger extended the two cross members and then plated on top. So if you create a box section extention, it should be in the same position as mine.

Thats a nice easy idea, i just dont like the amount of outward angle on them, ive just been out and offered it up to the car and its worse than the photo makes it look. Ive had a rethink of the inboard idea, how about this...? only thing is theres very little rearward angle on this one because id have to get a bolt through in the triangle, and it would all rely on one bolt in shear

Or.... is there enough room at the back to weld a piece of 1" angle on either side? That would give a 3" base which i could weld a 3" square flange on to the end of the 2" tube and should be able to get 4x8mm bolts in. The brace woud also be as far back as pos (as the tub allows) and as square as it could be too. Id just have to scallop the fuel tank. What do you think to this?? PS did you use solidworks for those other diagrams? theyre great.

I think you would find that solution would interfere with the rear body panel......

BTW. your going to need a piece of plate welded to the rear lower cross member or you will have nothing to attach the bodywork to!

[Edited on 1/3/07 by DaveFJ]

oh well, looks like ill have to get the tub out then after all and have a trial fit. Thanks for your time and suggestions everyone

You might not have clearance on the fuel tank if you go straight back. Look at my original photo to see the angle.
There is approx 75mm between tub and tank at the rear. Not much room at all. Just about get your hand in there !
Let us know how you get on ?
I used Pro Desktop to model them. Took less than 5 minutes.
Flak Monkeys done a whole car on ProDesktop !
Available on the internet as a download or U2U me, if you know what I mean

[Edited on 1/3/07 by ayoungman]

[Edited on 1/3/07 by ayoungman]

I know its a bit of work cutting away the tank to clear, but ive a shiny new bulk buy tig set sat in the corner of the garage just waiting to be used to make my tank, so a bit of extra practice wont do any harm, I think id prefer it this way. Ill post up some piccys in a few weeks when ive made some progress with it. Went to get ready to try the tub on today, but found a bit more welding to do first, shouldnt take long tho.