got the R90 out in the sun

[lmo1131]

ummm ... just wonderin' ... 8n8, when you get done dinkin' around with your gas tank could you know out one of the these for me?







nice project!

[lmo1131]

I couldn't figure out how to mold the inside though. Really interesting in watching your project.

One approach would be to pull a tooling plaster cast (reinforced with hemp bats) off of the interior of a donor tank; you would have to remove the front and rear cross bars. UltraCal B30 is a suitable low-shrink tooling grade gypsum.

From that you would have a "positive" form to measure from to build a wooden buck, if in fact it might just survive as a one-off forming tool.

[8ninety8]

Yes, after much clicking on the old computer, saw many bucks for pounding out fenders and such, stunning stuff is out there as to individual's skills. Saw most tanks formed over segmented/station forms, like building a stripper canoe.

However, making a solid buck gives the chance of fitment and positioning on the bike with stock equipment such as the ergo-original seat. Gonna try to expand tank capacity a little, if possible, by fudging dimentions out a little. The original profile of these old Beemers is hard to improve, maybe a little deeper knee dimples?

Here's a pic of a prospector pulled off a segmented/station form. Made the mistake of pulling it out of warm garage onto hard snow, put a little more rocker in the bottom, damn fast turnin now, no baiting intended. Back to the tank.

[8ninety8]

A little more done today, knee dimples, and trying to figure how to use tools, it's getting lighter.

[8ninety8]

A little hard to see the curves but here's a comparo to original. Trying to keep original profile, but with a couple twists that a three piece factory stamping could never have done. i.e., compounded curves of knee dimples. The underside will try to simply make up of mostly straight plate, no curves needed. Needs a little more shaving and then back to bank to get moola for shot bag, plastic mallets, sheet stock, planisher, and tig rig.

[8ninety8]

And initial size check.

[8ninety8]

Another shot from backside. This ain't gonna be easy. There's gonna be more than three pieces to this tank. Getting that sinking feeling.

[8ninety8]

Haven't done a thing since last month, got back to it this afternoon, finally fitting the seat by shaving off backside, also trimmed front leading edge to fit frame angle. May have to raise the backend a bit more for balance. Thinking all the while about best way to split this thing up. Might leave all welds showing, buff it up, frankentank. All we're lookin for is a solid leak proof rust free tank, with more range.

[lmo1131]

There's gonna be more than three pieces to this tank.

Who cares?! It's looking pretty damned good to me so far!!

Just remember, you don't have to weld the thing up solid; tack it together as you go. That way you can always break a panel loose if you want to change something. One other piece of equipment to consider is a big enough bucket, or trough, or (?) that you can quench your panels in after to anneal them. Most metals "work harden" as they are formed. To keep forming it deeper, it's necessary to anneal the metal with a torch and quench it.

i.e. (not my work)

[8ninety8]

Getting the usual advice like, "aluminum welding is really difficult, with right tig unit, 3 grand, it'll probably take six months practice to pick up the puddle."

to, "just git a harborfreight unit, they're all the same, and you'll save big bucks."

to, "just job it out after it's formed, a pro can weld it up better, quicker, and you'll save big bucks."

Meanwhile, just keep shaping the buck, thinking about how many seams, and pulling form off the original bottom. Long project, but something that has always been in the bucket. If it doesn't work out, won't be the first time. Most who have any skills at all can do most anything with hands. As cool as most aluminim tanks seen, the usual cafe types, with bumps and dimples, usually flat looking like a turd shape. This one can stick somewhat to the original grab the eye profile BMW designed in '74. Bauhaus, form respects function, respects craft and material, simplicity of the aesthetic. Hoske and Heinrich always looked somewhat weerd.

[8ninety8]

Just pulled plug off tank bottom. Used old plaster cast stuff and whatever else was laying around. After it drys up gonna give it a good coating of glass and resin and used to make sure get good clearance underneath. Went around the mounting points, will glass over holes and add mounts later. Don't fergit the vasoline.

[8ninety8]

And outside drying up. To make it solid used some wood braces and plaster of paris/joint compound. CAMe out pretty good plug of the bottom side.

[lmo1131]

Are you going beef it up and to try and use it as a tool, or are you going to take measurements off of it and build a wooden buck?

If you need plaster in a 50# quantity (for example) you can usually get what you need from a local "pottery supply" guy (check yellow pages). A fifty pound bag is probably around $20-25 range. Cheaper than buying it by the pound.

[8ninety8]

Forms will be used to measure/hold maybe bend a little the aluminum panels against. It's all new to me, but, shape the aluminum, pound it, hold it against the form, maybe give it a good bend, pound it some more, grind on the edges, make'm line up. The pounding will be on the shot bag. Got a planisher to smooth out hammer marks. Tomorrow will add missing front end to plaster plug, add a little more around edges, and fiberglass it. Important to get the bottom right to clear all the stuff/coils/brake res./etc. It's something that has always looked near impossible, shaping metal with a hammer into something functional. CAn't imagine paying somebody to do this. Tedious already. Maybe the whole attempt is a bust, but what the hay, ya never know until you try. Just pulling the plug was way cool. The bottom of my tank, who'd a thunk it. All forms not stock.

[lmo1131]

Maybe the whole attempt is a bust, but what the hay

Well, if you're going to bite off a chunk, it might as well be a big one. There's no honor in crashin' with the brakes on.