FRAME ARTICLE #7


Several recent requests made it look like a post on tanks would be a good addition to the frame making articles.

Things to read:

Fournier's books will cover all the basic metal working tools/techniques, and some specifics on auto fuel and oil tank construction. The Classic Mechanics article shows craftsman Peter Kyte building a Velo Thruxton aluminum tank. All three items are highly recommended. I'm sure there are a number of other books I don't know about, probably to be found in hot rod/classic car and aviation fields. There are also some videos out on sheet metal work, the gas welding of aluminum, etc which should be advertised in the hot rod/Classic Motorbooks/classic car publications. Most of the construction/forming methods I mention will be from the three references shown above. I'm not going to go into great detail on metal work basics, since Fournier does such a good job of it (with pictures too).

Fuel tanks:

The fuel tank must, primarily, hold the vehicle's fuel. It may also serve as a structural member (Norton, Kawasaki, LCR etc monocoque frames), an ergonomic aid to the rider (something to lay on/brace against), and of course, a vital part of the esthetics of the vehicle.

I don't think in metric volumes, so I'll use the standard US gallon in this article. One gallon is 231 cubic inches, or a cube of 6.14" per side. For some reason I find it helpful when designing a tank to visualize it as needing X number of gallon-sized cubes.

The most common materials for tanks seem to be aluminum, steel, and plastic (including both high impact dirtbike style plastic and cloth/resin composites). What should you use? I think in many cases it boils down to what you can do the best job with or have access to..

There are some very strong and feather light carbon fiber tanks around, and that material may very well be the best for many race applications, but I for one don't like messing with fiberglass stuff. It does have some advantages in not needing a torch, and probably is much quieter than panel bashing (smellier though). If you build the tank on a male plug and then disolve it you can have all kinds of odd bulges/hollows that would be difficult to form in metal. I've seen aluminum/steel tanks with 5" dents in them that don't leak, and I've seen fiberglass tanks that had gaping holes in them from a similar impact. Rotationally-molded dirtbike tanks are very rugged, but I'm not sure how amenable to the home workshop that technology is. I'll confine most of my comments to metal tanks, as that is the only kind I've successfully made.

Most custom metal tanks will probably be built in aluminum, because it is easy to work, light weight, and looks so cool. You are most likely to make the tank of 1100 aluminum, which is basically pure aluminum, or 3003-H14, which is a stonger weldable alloy in 1/2 hard condition. 1100 is very soft; easy to work and weld. It is great for non-structural tanks, but since it is soft it will dent easily. The car folks make a lot of non-structural stuff from 3003-H14, as it is pretty much as easy to form and weld as 1100, is stronger, and in the half-hard condition will be more dent resistant. You might find it easier to get 1100, as I believe it sees use in some industrial ducting. If you are going to be forming all of the sheet you might go for a non-hardened alloy. Aluminum work-hardens, and if you are shaping it a lot you will periodically have to anneal it to soften it back into a formable condition. To do this, put a layer of soot from the torch on the sheet, and warm the sheet with the torch until the soot goes away. After it cools, the metal will be soft and ready to work again. If you don't anneal the metal, you run the risk of it cracking when you try to form it. Many tanks are made from 16g aluminum. You may want to make your first tank from something slightly thicker to allow more latitude in sanding surface imperfections out. Trying to hammer all the bumps out is not an easy task for the novice (that's me).

When I built the tank on the Laverda I made it from a top, two knee panels, and assorted bottom panels. As I mentioned in a previous post the tank might have been easier to make if I had used more panels in the top, as it is easier to work small amounts of curve into small panels and then weld them together than it is to put all the shape in a larger panel. Welding takes time, but it can be much more staight forward than trying to form huge amounts of shape into a panel. Peter Kyte makes the Velo tank using a top panel down the middle of the tank, and two side panels. The technique you use will vary depending on the shape of the tank. Rounded corners seem to be the most difficult due to the multi-plane curves needed. Don't forget to put baffles in the tank to prevent the fuel from sloshing back and forth in a completely uncontrolled manner. It is also worthwhile to make sure the petcocks are at the lowest point of the tank, so you can drain the tank completely dry.

You can shape the tank with body hammers and dollies and a shot bag, but an English wheel can be very nice for putting mild curvatures into larger panels. It is by no means a requirement though. Don't forget to anneal the metal when you notice it getting harder to work.

As with so many things, it looks like one of the big "secrets" of aluminum forming is to not be in a hurry. Gradually work the shape into the metal - don't grab the 8 pound sledge and put in a dent that is twice as deep as needed. It is a lot easier to keep stretching the metal to the shape needed than it is to shrink overstretched metal back to where it should be.

Welding aluminum: the real masters at alloy bodywork seem to prefer gas welding aluminum. The torch tends to anneal the metal over a wide area and makes it easier to form the welded area. I used a Tig welder on my tank because it is a lot easier to master. I've tried gas welding aluminum a time or two, but I can see that it is something that will take a good deal of practice to be able to do a good job. Besides, you can always go over the Tig welds with a torch and anneal them if more working is needed. There are also some aluminum brazing rods available that are supposed to be pretty easy to use. I would presume some of them are going to be strong enough for a non-structural tank, but you do have to remember that you will have to re-braze if repairs are needed on the joint.

Oil tanks:

Oil tanks tend to not have all the curvey bits that fuel tanks do, so 3003-H14, or even a stonger less formable metal like 6061-T6 can be used if the panels are mainly flat/single curvature. From what I can tell, oil tanks really should be circular in cross section so the return line can come into the tank at a tangent and the air can be centrifuged out of the oil as it follows the curve of the tank. Oil tanks should also probably be as tall as possible, so that the likelihood of uncovering the outlet port is reduced. This doesn't make them any easier to put on a motorbike, so you may want to have a circular tower rising from the main portion of the tank to the filler neck, with the return line coming into the tower. I think it is a really good idea for oil tanks to be designed with some sort of removeable inspection plate that makes them very easy to flush after a major engine blowup. Avoiding sharp corners in the tank would also reduce the places debris can hide.

Tank location:

Unfortunately, there is often a frame, motor, exhaust pipe, rider, etc in the way of where we would like to put the tank. Darn. Most older bikes will probably have the tank above the engine, where bikes like the FZR have their tank behind the engine/carbs.

This is a good time to go back to the 6+ inch gallon cube. If you don't have to mount the tank on top of the bike, figure out where can you fit enough cubes on the bike to accomodate the required fuel load.

Peter Williams and Paul Dunstall had a liking for pannier fuel tanks, mounted along the sides of the bike. Rod Tingate in Australia built a spar frame for a Ducati that carried the fuel in the formed spars (a lovely bike - see Cathcart's "Ducati: the untold story"). This lowers the center of gravity, and can often make it easier to move the fuel load more towards the front of the bike. Since the tanks are usually of a thin rectangular cross section they can be less complicated to construct. They may not be useable if the engine is very wide. Depending on the carburetor height vis a vis the tank, you may need to run a fuel pump and possibly a small header tank to feed the carbs. Mikuni makes some inexpensive vacuum operated pumps (snowmobile items) that can be run off a inlet manifold. I think the pannier tank is the way I will go on any tanks that don't need to be in the standard position due to rules/esthetics.

Some bikes (HD flat track/road racers, Daytona 500 Triumph) have run the oil tank under or in front of the engine. This is great if you have the room, and helps move the oil down/forward in the frame, often to a spot that is not otherwise being used. You do have to remember that most oil pumps do a better job of pushing oil than they do pulling it, so don't create a situation where the oil won't get from the tank to the oil pump.

Caps, petcocks, etc: I don't know of a source for aluminum Monza flip caps in the US, though there probably is. Bartel in Ireland can supply them, but they aren't cheap. You can get the flush fitting aircraft style fillers (some with locking cap) from Aircraft Spruce, some of them at pretty reasonable prices. Various race car parts suppliers can provide ready to weld threaded bungs for attaching to the oil tank, or you can turn them up on a lathe. The nicest petcocks I've seen are the Pingel products, but they are expensive. Since they were developed for drag racing they flow lots of gas straight from the box, so you don't have to spend time drilling out the petcock to keep the engine from fuel starvation.

Mounting: all aluminum tanks (and most other alloy parts) really like to be isolated from vibration. Use lots of dense foam, isolation mounts, etc when attaching the tank(s) to the chassis.

That's all that I can think of at the moment. I'm sure your posts will bring other items to mind. As always, I'll look forward to hearing from you.

Cheers,

Michael Moore

Euro Spares

AFM/AHRMA #364


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