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kk3an 03-08-2013 07:35 PM

GSA Auxiliary Tank - when 8.7 gallons just doesn't cut it
Custom 6 Gallon auxiliary fuel tank constructed over the winter of 2012 / 2013 - with dual baffles.

This tank was designed & built by me using 6061-T6 aluminum plate stock, 3/16" thick, and coated with Linex spray-on bedliner material.

Dry weight of the tank is 21 Lbs. It's secured directly to the rear frame above the stock BMW rear support (passenger / luggage area) using seven (7) 6mm stainless bolts. No modifications to the bike were made for mounting. Gravity feed to the main tank is accomplished using 5/16" ID rubber fuel hose,
a 40-micron fuel filter, and a 5/16" quick-disconnect.

A key design point was to have the ability to mount a 17" or 19" tire on top of this tank, while retaining
easy access to the fuel cap at the pumps without the need to move the tire or fuss with anything. This requirement dictated the placement of the fuel cap & vent locations.

This GSA has a combined fuel capacity of approximately 14-1/2 gallons.

This was purely a 'one-off' design, consuming in excess of 60 hours to complete.

Measuring, punching and drilling holes. General pre-assembly and cutting material to size.

The intent was to utilize as many direct-to-frame mounting locations as possible. This meant dealing with
some odd angles as fastening points to the BMW frame, and some tricky through-tank fabrication was done
to permit the use of the (3) rear-most mounting points that are available.

The two forward-most holes which secure the BMW rear support to the frame have a forward sweep of
about (7) degrees. This required the nose of the lower plate to be formed downward to match the forward
angle of these front mounting bolts. The lower mounting plate doubles as the bottom of the tank itself.

The front of the tank shown here, after a bit of grinding work to allow the nose of the lower plate to fit
flush behind the seat. This was ground even further later on.

Looking rearward at the tank, making the first tack welds.

These (3) posts are simply passageways to access the (3) rearward-most mounting points from the top
of the tank. The ID of this tubing is such that it allows a 10mm socket & extension to pass completely down
through the tank, and onto the bolt heads at the very bottom. This was the only way I could think of to
provide access to these bolts while the tank was still mounted to the bike.

Also to keep warping (heat) to a minimum, the welds are kept short and distributed around the assembly.

Mitre-cut baffles are all in place & the rest of the internal welding is complete. The fuel outlet hole is
visible here in the upper left - soon to receive a 1/2" NPT weld-in fitting.

Welds brushed clean and the inside of the tank is wiped down with brake cleaner.

With the fuel neck & another NPT fitting for the vent both welded in, the top goes on over the (3) passageways.

A good bit of grinding here to get the top plate of the tank flush with the sides.

With the tank checked for squareness, the entire top plate is welded on, and outside welds are
placed around the bottom perimeter of the tank, and up all four corners.

I wanted to use the (2) mounting points on the side of the bike, and this required some careful fitting up
of some 1/4" aluminum tabs welded at angles to match the frame. I believe these mounting points are
provided for the OEM luggage racks, which I'm not using.

Custom aluminum spacers (heavy-wall tubing) are used here between the tabs and the bike frame.

I wanted to have the top of this tank as flush as possible ahead of the fuel neck to give the tank
more of a clean look, so the welds are ground down on top as much as possible.

A bit of test fitting to the bike after letting the smoke out of the shop....

The fuel outlet is very low in the tank - about 1/8" from the bottom - and with such a forward lean-angle,
nearly all of the fuel should be able to drain out.

Still more grinding work was done on the front area to allow fitment to the stock rear support & to
allow the 90 degree ball valve to rotate 360 degrees.

The fuel neck & roll-over vent are placed here to permit a tire to lay perfectly flat on top of the tank.

I was going to weld some tie down points to the sides of the tank, but there's an ample number of places
to use for this by way of the pannier frames.

Back from being coated with Linex bedliner material.

Had to clean out some of the holes & prep for mounting.

All mounted & secured very tight to the factory threaded holes in the frame. Overflow hose also attached.

Aluminum spacers are used to take up the gap at all fastening points.
Rubber washers are used to protect the Linex.

I spent a good bit of time searching for this 90 degree ball valve with 1/2" NPT male / female threads.

Inline 40-micron fuel filter. Ideally everything would be mounted on the left side of the bike instead
of the right, but the best connection point to the main tank was on the right, and being a gravity fed
system, it made the most sense to have a straight & gradual slope down one side of the bike
versus trying to cross over to the other side somewhere.

To my delight, removal of the charcoal canister provided a perfect & protected spot for the weldless bung.

The Pingle weldless fuel bung (P/N 62136) now lives where the charcoal canister once was.
Had this been on the left instead of the right it would have been ideal, but certainly not an issue.

The new 5/16" fuel line (lower) is secured well away from the telelever, and the right side tank vent
line (upper) was re-routed and follows the fuel line. Dual wire barbs are securing the fuel line connection.

rdwalker 03-08-2013 08:23 PM

Very impressive work!
Just a question: where does the urine bag mount? I assume that you stop for nothing... :evil

(14.5 gal ~ 500 mile range ~ 10 hrs seat time? )

oldtrout 03-08-2013 08:58 PM

There is job for you in Russian space program !!


(Nice handy work)

PukaWai 03-08-2013 09:39 PM

Really nice looking results! A rather "massive" design - I would think 1/8" sheet with some extra reinforcement at mounting points would be plenty and so would a 1/4" ball valve, if you can find one.
Nice looking gas cap, but it looks like the filler pipe is smooth, so how does it attach? Friction fit?
I would wonder why you need that much gas, but your signature line says it all! :thumbup

kk3an 03-08-2013 09:46 PM

The cap is actually threaded (fine threads). Perhaps the camera just didn't pick the threads up.

I agree 100% - 1/8" plate would have been ample for this. I just have a good deal of 3/16" around from other projects, so used it instead. Plus it keeps the warping down a bit more.

I appreciate your comments.


oz97tj 03-08-2013 09:58 PM

Very cool, but as others have said holy heavy duty! And holy range too!

Seems a few RotoPax mounted in that location would have served just as easily for far easier.

going south 03-08-2013 10:26 PM

Pretty cool, now you just have to make 100 more for everybody that is going to want one... :lol3

PhiSig1071 03-08-2013 10:52 PM

Nice clean work.

That's a hell of a lot of gas. And weight, 21lbs dry, plus 6lbs per gallon, 6 gallons is 36 lbs, 57lbs total high up and back on the bike. Plus the tire.

Is this for a hack rig? Any thoughts on increased tire wear due to that extra weight on the back?

Where is the transfer filler on the main tank? Is it going to be pretty low on gas before it starts draining out of the aux tank?

kk3an 03-08-2013 11:19 PM


Sure there's some weight there versus just an empty space - but 57 LBS is a good deal less than the average passenger (who may easily be 150 LBS+).

The only real 'transfer' I've added is the 90 degree ball valve right on the auxiliary tank - that puts fuel in the right side of the main (stock) tank.

After that, the secondary transfer from the right to the left side of the main tank is done via the BMW "sucking jet pump" that pushes fuel from the right to the left (where the BMW fuel pump is).

I open the auxiliary valve once the main tank gets down to about 1/4.

There are actually a good many bikes set up like this - just more typically with off-the-shelf tanks, instead of something custom.


2712 03-09-2013 12:44 AM

ive to ask,

where do you want to go?:lol3

ausfahrt 03-09-2013 02:10 AM

Cool idea and it looks like you did a nice job on the fab work. I don't think that this is something that I would have done but I commend you for following through on your idea.

Well done.:freaky

gsweave 03-09-2013 03:57 AM


mad skills:clap

SR1 03-09-2013 05:23 AM

I have kind of a "beer keg" looking one that the PO bought...I've never used it. MUCH lighter than your 21lb dry tank, (it's about 8?) and holds maybe 5gal. but not super heavy?

Yours is more offroad capable though.

scooteraug02 03-09-2013 07:24 AM

From Summit racing 16.5x16.5x9 10 gallons. $183

Not as rewarding as making your own though. Nice job.

kubiak 03-09-2013 08:56 AM

nice work and setup!

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