GSA Auxiliary Tank - when 8.7 gallons just doesn't cut it

Discussion in 'GS Boxers' started by kk3an, Mar 8, 2013.

  1. kk3an

    kk3an Adventurer

    Joined:
    Jul 27, 2009
    Oddometer:
    84
    Location:
    East Berlin, PA
    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.

    [​IMG]

    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.


    [​IMG]

    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.


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    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.


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


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    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.


    [​IMG]

    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.


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


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


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


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    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.


    [​IMG]


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    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.


    [​IMG]

    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.


    [​IMG]

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


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    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.


    [​IMG]

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


    [​IMG]

    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.


    [​IMG]

    Back from being coated with Linex bedliner material.


    [​IMG]

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


    [​IMG]

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


    [​IMG]

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


    [​IMG]

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


    [​IMG]

    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.


    [​IMG]

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


    [​IMG]

    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.


    [​IMG]

    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.
    #1
  2. rdwalker

    rdwalker Long timer

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    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? )
    #2
  3. oldtrout

    oldtrout Been here awhile

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    Location:
    Delta/, Cariboo BC
    There is job for you in Russian space program !!

    OT

    (Nice handy work)
    #3
  4. PukaWai

    PukaWai Been here awhile

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    Big Bear City, Ca.
    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
    #4
  5. kk3an

    kk3an Adventurer

    Joined:
    Jul 27, 2009
    Oddometer:
    84
    Location:
    East Berlin, PA
    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.

    Dan
    #5
  6. oz97tj

    oz97tj Been here awhile

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    Fenton, MI
    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.
    #6
  7. going south

    going south hero & Zero...

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    Alaska, Mazatlan. sometimes seattle!
    Pretty cool, now you just have to make 100 more for everybody that is going to want one... :lol3
    #7
  8. PhiSig1071

    PhiSig1071 What's ******width?

    Joined:
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    Tucson, AZ
    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?
    #8
  9. kk3an

    kk3an Adventurer

    Joined:
    Jul 27, 2009
    Oddometer:
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    Thanks.

    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.

    Dan
    #9
  10. 2712

    2712 Been here awhile

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    Belgium
    ive to ask,

    where do you want to go?:lol3
    #10
  11. ausfahrt

    ausfahrt mach schnell

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    Apr 17, 2008
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    Location:
    Fl & Vt
    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
    #11
  12. gsweave

    gsweave Yinz, blinkers are on, JACKWAD!

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    Dan

    mad skills:clap
    #12
  13. SR1

    SR1 Back in S. Korea

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    Seoul Survivor
    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.
    #13
  14. scooteraug02

    scooteraug02 Dog Rancher

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    Atlanta, GA
    #14
  15. kubiak

    kubiak Long timer

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    madera california
    nice work and setup!
    #15
  16. Lensgrinder

    Lensgrinder Long timer

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    Very nice work, looks great. Very well thought out.

    A friend added a tank to the back of his F650GS, he had used a ball valve to transfer fuel, but I had a couple of normally closed soleoid valves, so he installed one and put the switch on his handle bars.
    It has worked well for him.
    #16
  17. WindSailor

    WindSailor Been here awhile

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    795
    Location:
    Somewhere out West
    Dan: nice setup, it's usable, you can stack gear on top of it, and without mention it's probably more than crash worthy dependable - which in my opinion really counts.

    A few years ago I went to a local seminar held by Bruce Hansen on touring the Pacific Northwest. He had a slide show of places and things (looking down a specific road, out of a window of a restaurant, at an outcropping of rocks, etc. etc.) that lasted about an hour and a half. We had a small crowd there in which THEY KNEW about 95% of those pictures that were shown. I was thoroughly impressed and kind of embarrassed I didn't know more than what I did.

    One of the guys there (I'll just call him "Pappy") was an older than dirt guy that had a GSA and told me he packs at least 5 extra gallons of fuel with him when he goes on a trip. I started laughing (couldn't help it) saying that was over kill - and ol' Pappy just raised an eyebrow and stared at me. Uh - oh.

    Well... I was wrong...

    It's the route between the gas stations that make the trip. :D Those guys I met there at that seminar literally define what Adventure touring is all about.

    Ride safe,
    Rick
    #17
  18. ROAD DAMAGE

    ROAD DAMAGE Long timer

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    Nice clean work on the tank. it looks good.

    I've got to ask ........... why add 6 gallons capacity to a bike that all my friends call "The Supertanker" anyway? :scratch

    The only time I've seen this type set up is for Iron Butt guys. Is that what you're up to?

    Even at a tad less than 9 gallons, I wish my ass could last for as long as my fuel lasts! :lol3 I've got almost 90K on my present scoot and I don't think I've ever burned a full tank of fuel in one sitting.

    Again, nice fabrication job.

    Rob
    #18
  19. Kevreif

    Kevreif Been here awhile

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    where the west and north branch susquehanna meet

    I found with my semi aggressive driving style that with my bike loaded for a three month trip that my rear tire lasted significantly longer (13,500 vs best tire mileage empty was 8,000.. both metzler tourance) with the weight and it was deff not due to lack of aggressive riding. although i had to adjust and use less front brake and more rear brake because my front tire started cupping quickly... on that note you hardly notice 50-60 pounds on the back of a GS.
    #19
  20. kk3an

    kk3an Adventurer

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    Jul 27, 2009
    Oddometer:
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    Location:
    East Berlin, PA
    Hi Rob - yep, you're spot on. This explains it: www.kk3an.com

    Rick (Windsailor) - great story for sure! I too have been in too many places where the fuel gauge dictated where I'd be sleeping that night. Not something I care to continue doing.

    Yes the weight "is what it is". In part of this winter's project, I dumped my hard luggage for soft luggage and captured about a 20 LB weight savings because of it. That alone justified the weight of this tank being 21 LBS dry (empty).

    In reality my bike is equal in weight (or perhaps lighter) with the auxiliary tank full of fuel and soft luggage, versus a typical GSA setup with hard luggage and a top box.

    The baffling is what makes or breaks something like this. With cross-baffles installed like I did, you don't even feel the fuel move. It's like having a cinder block strapped on.

    Dan

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    #20