Fuel tank as a stressed member?

[dopplershift]

Hi everyone... didnt know where to put this thread, so here goes.

I want to make a strut-tower brace for the rear shock absorber towers on my mk1 Capri.

Problem is, the fuel tank sits right between the two towers. I know I could just make the brace in a deep U shape to go around the tank, but this will reduce the stiffness of the brace. Also, I think I will want a larger tank when the 302 windsor is fitted, so it would be a very deep U bend.

So, my question is: If i get a custom fuel tank made by a "pressure vessel certified welder" , could I incorporate the the brace into the design of the tank? Can anyone think of any problems in having the fuel-tank as a stressed member in the vehicle's structure?

My thoughts were that if it was nice and rigid, so the tank didnt deform, it would be ok.

Dont know about legalities, but I will need to get an Engineer's Certificate to get it licensed anyway.

 

[michael1703]

how important is rigidity versus weight?

it would be very heavy

 

[rtbcomp]

How about two tanks, one below and one above the brace? Fabricate a tank with a hole to allow the brace to pass through it?

 

[malcolm]

Sounds like a cunning idea. Could have an ordinary straight strut brace passing through the tank and weld the holes left in the end of the tank to the strut brace. Brace and tank would need to be the same material, and the tank would need to be softly mounted to allow for flex in the struts.

Only downsides I can think of would be in a crash. Better to have the chassis bend where it wants without splitting the tank.

No need for pressure vessel quality welds as the tank is vented, but would want to be decent and fuel tight welds. Is it for road use? Not sure if there are any regs.

 

[rtbcomp]

I was thinking more in terms of a tube in the tank made from the same material, with the strut passing through the tube, with clearance all round to allow for movement.

 

[Normsthename]

Buell Motorcycles use the Frame to store the fuel, and the swinging arm to store the Oil
So I don't think that there are any problems from a Legal point of view

Andy

 

[stormin]

Is iit for competition use?
I'd consider getting a bag tank installed inside the metal skin as insurance against splitting.

 

[dopplershift]

Awesome ideas guys! (oh, and keep 'em coming!) I love this forum!

I think the "donut tank" with a seperate strut through the middle, sounds great. Or the double tanks for possible safety benefits.

Andy: Sound like awesome bikes! I love it when a design fulfills so many objectives at the same time! ...and thats why I'm studying eng. Engineering design is the most fun you can have with a 4-colour pen and a notebook!

The car is for street-use, maybe even as a daily drive (fuel $!).

Malcolm: I know there's no need for pressure-tight welds, but I'm pretty sure that here in Western Australia, any custom made fuel tank has to either be made by a cert'd welder, or pass a pressure test. ($$$).

I dont think bag-tanks are legal for street use here . But would be a great solution.

Oh, and as to weight concerns... um. maybe upto 20kg including tank and brace would be ok. The stock tank weighs 3/8ths of 2/5ths of SFA. But the weight distro is right over the rear axle... where the Capri needs it, or so I've heard, so a bit extra would be ok. (With the 302, I'm not exactly gonna be short on engine power )

 

[Hitch]

Why not make the tank in a sort of U shape, brace sits between the two higher sections, would make it easier to remove tank or brace.

 

[bigbadbugga]

Why not make the tank in a sort of U shape, brace sits between the two higher sections, would make it easier to remove tank or brace.

/\ /\ /\ WHS

make it so it sits over the brace like a saddle.

 

[Hitch]

You'd want the bottom flat(ish), if you had two portions of the tank with a higher top, youd have a problem with pickup... one of the lower portions wont be used.

 

[dopplershift]

Could have a flexible/removable hose joining the two 'sumps' I suppose? Or have both sumps gravity feed to a smaller, removable tank from which the fuel-pump picks up?