Outer Limits 4x4 Board

Thinking of making an Aluminum Fuel tank for my patrol. To fit under the car. Using 2mm for most of the construction with a 5mm base (to protect it from being pierced by rocks etc) Then rubber mount it to stop is cracking.I have noticed that all the after market companies use steel, no one else uses aluminum. The weight saving would be a bonus too. I can weld and fabriacte everything myself so dramas there. Can anyone think of any reason not to use Aluminum or any problem I may encounter.
Thanks for any input.

lots of people make custom tanks out of ally, but cost is a factor in comercially available units

i am yet to see something like that made of ally that doesn't crack. for some example piglets tray was stress cracked and was full re welded, my mate had a canopy made and all the legs cracked in a matter of weeks.

there is no room for error in design with ally in vibrating applications when mig welding and differing metal composition be it sheet or wire differences is involved. I would make an ally tank for our lotus 7 replica as it is small enough to be ok, i would not use it for a under body tank. you may get away with it but it would play on my mind every trip.

don't get me wrong i just refitted our mig to do ally and bought the gas today so i feel there are some uses

cheers mate bru

cracking is the biggest issue. I would weld in some extra crossmembers in the rear of the chassie to try and limit the flex, but then that would take away the weight advantage of the ali.

U could also strap it on instead of bolting it???

jeep97tj wrote: U could also strap it on instead of bolting it???

Bingo. That's the answer. Build the tank out of ally and a cradle out of steel with some insertion rubber between the two. I have designed heaps of ally tanks for Buses, plus most trucks these days use ally tanks. I haven't checked ally and petrol for chemical capatibility but it should be OK.

Personally I would use stainless before I would use aluminium. But aluminium is also OK - plenty of bus, truck and boat tanks are aluminium. I would use 3mm minimim though, and double weld all the bottom and side seams (inside and out - of course you cannot do this with the top).

Have a look on pirate, there are plenty of alloy tanks in buggies with good ideas for the mounting system.

Also bear in mind that alloy won't slide over rocks as easily as steel will.

Ally tanks WILL stress crack in a 4bee thats gets used.
You will need to ensure NO twisting loads are applied.
Its not just a matter of using a cradle or steel frame.

Bazzle

bazzle wrote:Ally tanks WILL stress crack in a 4bee thats gets used.
You will need to ensure NO twisting loads are applied.
Its not just a matter of using a cradle or steel frame.

Bazzle

Would three point mounting be sufficient, or would you need to do more to isolate from the chassis?

Scott

You need to prevent any movement to tank. ie, foating in a cage of foam in effect.
They can work in other environments where there is no torsional loading but mos t4bee stress the chassis to much AFAIK.
Ally will harden and crack any where stress is placed on this type of structure.
ARB tried unsucessfully a few years back to make ally tanks work with no real sucess.

Bazzle

I WOULD NOT GO ALLOY TANK !

why in new zealand a comp truck was doing an event (almost offroad racing style however it was built as a winch event)

He just refuelled and before he knew it there was fire ball behind the truck pulled over they used 5 fire extenguishers but to no avail $150,000 down the drain.

Alloy tanks crack and arent designed for offroad use so upto you if you want to go down that path.

Looks like I have some more thinking to do. Will have to have a chat with guys that provide the aluminum, see if there is a suitable grade. Will also have to mount the tank so as no chassis flex is going to affect the fuel tank. Thanks for your input. Back to the drawing board.

Have you also thought about how you might repair it if you get a leak? Ally isn't as easy to make repairs on outside a workshop and the leaks are likely to be substantial if they happen in ally.

There are 2 problems with aluminium.

1. Aluminium alloys rely on heat treatment (when rolled into plate or extruded), for their mechanical properties (tensile strength etc). The heat from welding causes the properties in the heat affected zone to become similar to the annealed state. Often because of either; poor design, unexpected loading (due to chassis twisting etc) or because it can't be avoided, the weld (the weakest part) is in an area of high stress.

Ever wondered why aircraft use mechanical fasteners, instead of welding?

2. Unlike steel, aluminium does not have a fatigue limit. Fatigue limit is the maximum allowable stress for an infinite fatigue life.

The nature of the fatigue curve means that steel can be designed for a good (even infinite) fatigue life.

However anything made from aluminium will fatigue given enough stress cycles. It behaves poorly in fatigue.

Welding greatly reduces the fatigue strength of both steel and aluminium. Poor design make this much worse.

The large reduction in the mechanical properties of aluminium in the weld zone (point 1 above) and the affect of the weld design on fatigue strength combine to lead to fatigue failure.

why would you want to make one out of ally anyway?? thick ally is heavier than the right gauge of steel and has so so many negatives why bother just childish. use a good zinc steel 1.5mm for sides 2-3mm for base and other 2 sides folded, quick lick with a tig or a good setup mig and its done. only is stuff is very well designed will it work out long term to be lighter than steel (load for load vs weight)

cheer bru

Reason for Ali. I like it, its light. As for repairing a fuel tank out bush. Going to be pretty dodgy welding a tank out side a work shop situation what ever. I've made a lot of Ali fuel tanks for Elfin sports cars, but that was not a 4x4 application. Just got to work out a way to stop the chassis flex from messing up the fuel tank.