Auxiliary Tank Creation

Article by Shaun Higginbotham, January 2010

Part One

The Grim Rider delivered his BMW R1150GS to my unit on Saturday, and as today I had some spare time, I thought I'd make a start on his auxiliary fuel tank.

The brief was for a basic aux tank to fit on the pillion seat and over the rear luggage rack.

First job was to make some cardboard templates and then transfer the pattern over on to 2mm thick steel and then cut it out with my plasma cutter...........

I then used a slitting disc on mounted on my angle grinder and then scored the metal along the fold lines, approx 1mm deep.

Then it's quite easy to bend the the 2mm thick steel to make the sides of the aux fuel tank.....

All the side bent up.....

Now its just a case of welding the aux fuel tank up.....

Although the scoring was done on the outside of the aux fuel tank, I welded up the inside.....

...and then outside and then using a flap wheel in my angle grinder, I smoothed off all the welds.....

By welding on both sides of the seams, this ensures maximum strength.

The next part of constructing the aux fuel tank was to fit some internal baffles to stop the fuel from sloshing around too much....

The first baffle divides the tank in two, length ways.....

Whilst moving the tank, I managed to give myself a decent burn on my hand - just because you weld one side, doesn't meant the otherside of the metal is cold!

Then I added a couple of baffles about half way down to stop the fuel from surging backwards and forwards during braking and accleration.....

The next step was to start making the mounting brackets....

I've now got to order a flush fitting aero fuel cap, when that arrives I'll make the top of the tank and cut out the hole for the fuel cap. That way I don't get any swarf into the tank.

The next job was to drain the fuel tank, which turned out to be a complete PITA, as the fuel tank is fitted with self sealing, quick disconnect fuel pipes

The only way to get the fuel out was to syphone out the fuel, but because of the design the fuel filler cap, that needs to be removed so you can a rubber tube down to the bottom of the tank....

Unleaded fuel sure does taste bad

After draining out as much fuel as I could, there was still a some sloshing about, so I then removed the fuel pump.....

Now I could get my hand inside the tank with some lint free rags to soak up the last of the fuel.

The fuel tank will now be left to vent for a few days and then it'll be sent off to have a fuel inlet pipe braized to the tank.

Tomorrow I'll make the top of the tank and weld on the outlet fittings and vent tube, manufacture the rear mounting brackets and route the fuel line through the bike chassis to where it needs to join the main fuel tank.

Part Two

Well the fuel cap arrived today, so I was back down the unit to do some work on Robert's aux fuel tank.

Started by cutting out the lid for the tank.....

And then cut out the holes for the fuel cap and vent.....

I then welded on the fitting for the vent pipe....

I then drilled the bottom of the aux fuel tank and welded in the outlet fitting.....

Then it was just a case of welding the lid onto to the aux fuel tank...

Then after dressing the welds, time for a trial fit....

Now its just all I have to do is fit the rear brackets and sort out the bungy hooks and it'll then be ready for pressure testing.

I pressure test all my aux fuel tanks to 20psi, so that I'm 100% confident it won't leak. In the real world, the tank will never get pressurised as it's just a gravity feed fuel system and is vented to atmosphere.

I've been looking at if it's possible to modify the fuel pump plate so that I can feed the fuel into the main tank via a bolt on bulkhead fitting. I've come to the conclusion that it'll be more trouble than it's worth trying to get a decent head of fuel from the aux tank.

So the main fuel tank has been sent off to have a fuel inlet pipe brazed to the tank.....this gives three benefits, firstly, the fitting will never vibrate loose and secondly it increases the head of fuel being supplied from the aux fuel tank by nearly 10cm.

The third benefit is that it'll enable to the tank to be fully drained, so making it easy to replace the fuel filter during servicing.

Part Three

Today I pressure tested the fuel tank, only a couple of pin prick holes, so ground out the offending areas and re-welded.

I then retested the tank to 20psi and and it's A1 okay.

I then filled the tank to the brim and then carefully mneasured the contents, including the volume of the fuel hose to the tank.

And the capacity of the aux fuel tank is.......

yep - a massive 21.7 litres. So I think Rob will have a total mileage range of just over 400 miles!

The aux fuel tank will be delivered to the powdercoaters tomorrow and the main fuel is ready for collection tomorrow morning.

Part Four

Today I delivered the aux fuel tank to the powerdcoaters and hopefully that'll be ready for collection by Friday and I got the main fuel tank back with the additional fuel inlet pipe brazed on.

Whilst the fuel pump was out of the main tank, Rob sent me a new fuel filter to fit, so out with the old one......

....and in with the new one.....

I then refitted the fuel pump and fuel cap on to the main tank and then sorted out the fuel line from the main tank to the aux tank.

Not much more I can do now until the aux fuel tank has been painted - anyone want an auxillary fuel tank making?

Part Five

Well Rob picked up his bike this afternoon and seemed to be very pleased with the auxiliary fuel tank....... so now I can reveal the final load of pictures......

And finally, the BritButtMeister - seems like he's happy, but of course making the fuel tank for him now means I've got no chance of beating him in this years Brit Butt Rally!

Hope you've all enjoyed this thread and if you need any fabrication for your bikes etc, drop me a pm.