More composite fuel tank updates

This post is really about some of the manufacturing detail I’m going into as I make the part. As I’m sure you know, the sensible route to getting a composite component is:

  1. Make the part you want out of any old stuff so you have to get the right mock-up shape. I’m using kingspan and covering it in glass. I also like kingspan because it sands really well with an orbital sander, and till take bog (body filler) nicely both as a glue and as something to put on the surface when building complicated shapes.
  2. Cover the part in glass so that it is rigid and all the foam is separated away because of course you can’t take a mould straight from foam – they will become one and we all know what happens when two become one.
  3. Where I can, I’m cladding the part in pre-made sheets of 450g chopped strand matt glass which I made on a pane of glass. The reasons are that firstly the sheet made from glass has a reasonable surface finish and secondly it’s a lot less faff to clag this on than wrap the whole thing in glass, set in resin and then flat and polish, flat and polish FOR EVERRRR until I have a sensible surface. Thirdly these sheets, being quite flat, mean I can trust the angles I have them in at so I can reduce the chances of mechanical lock.
  4. Where I can’t clag, I’m putting body filler in, which is great for filling the gaps, but must be recessed slightly back from the overall profile. The filler will then be coated in resin. This is because you can’t go body-filler straight to mould surface – when you pull the part from the mould you will leave the filler behind. So, for the want of a little deft work with an artist’s brush and some resin, I can avoid this.

As ever, this is easiest explained with pictures:

IMG_0064Here is the joint between two panels. Both panels have been cut and bogged down and while the bog is soft a bit of a push allows the bog to squeeze out into the gap, Add deft work with a lolly stick and the gap is scraped out.

You can also see a big gap between the panel and the part. This will be radiused back so I don’t have any right-angles into which I can’t get cloth.

IMG_0062Rivets are handy to stop the panel sliding down the part on the bog. whilst it’s claggy stuff, it still has a propensity to flow. I suppose this is another odd reason why kingspan is so useful – you can stick rivets into it.



IMG_0061Here is the bottom of the tank with the three panels attached. What you can’t easily see is the slight angle the tank tanks to follow the fury transmission tunnel, which is angled.

More on the fuel tank

So, work on the tank progresses. I’ve got the top part mostly clad in sheet CSM, but now am focusing on the lower part – I need to do this in order to be sure the top tank mates well to the lower tank – they’re going to be made as two tanks then bonded. I can’t make them as two totally separate but connected tanks because the fuel level sensor wasn’t bought to work like that.


IMG_0050Here is the tank on the bench – you can see where I’ve started to clad it in sheet chopped-strand-mat (CSM).




IMG_0051Here is how it looks with some of the sheet clad into place. Note that I’m not stepping it as the cut kingspan in above shows. If I were to try and follow that, there would be two consequences: Firstly, less capacity. Secondly getting cloth to follow tight right-angles won’t work and will lead to bridging and a weaker part. I will take the orbital sander to all the edges where the places meet to smooth them out. They’ll then be filled with bog and coated with resin so I can make a mould from it. Needless to say, I will have to be sure the bog is slightly recessed so after being painted with resin (to seal it) I don’t create an unwanted positive profile that gives me mechanical lock in the mould. Even half a millimeter may make it so hard to get the part out without destroying the mould. I’m not intending to reuse the mould but it would be a massive shame to destroy it in case I ever need to modify the part.


IMG_0044Here we have the trial holder for the ATL Fuel Sender Probe. The housing is a cut down mixing cup and it’s set at an angle to point the sender down into the tank. Once the angle is right and it sits just where I want it in the lower tank I will glass over this so it becomes part of the main tank (that is, part of the part of the main tank that becomes the mould.


IMG_0057Here is the tank screwed into place on temporary brackets using self tapping screws. I know the fuel-sender will sit a little proud of the old boot floor, but that doesn’t bother me. I’d sooner make a demountable hat of some description for it rather than have the tank lower and lose capacity. I had to screw the tank in early to be sure I had reliable datum points when locating and attaching the lower tank.


IMG_0055Here you can see the tank avoiding the diff-attaching plate and the diagonal uprights that are part of the diff-carrier. The angle isn’t great but it I have at least an 25mm between the tank and all uprights. On reflection, the clearance is a little too great and I may be losing a little capacity. However, if I’m in a sideways shunt I have a lot of room for things to deform. The layup will also be very tough – aramid and e-glass. This will have massive impact resistance, and using layers of e-glass rather than carbon will add a little flexibility. This is not load-bearing or structural.

IMG_0056I had to build in two recesses, one on either side – the assembly order is specific here. The tank goes in first and the diff carrier second. This means I need to leave enough room around the tank to get a bolt in to secure the diff carrier. And I did – I’m quite pleased with myself.



IMG_0060Here’s the top view of the top tank. Note the massive void behind it, which is where the old tank sat, well high and well outside of the axle line. You appreciate how bad this is for polar moment of inertia, right? I was tempted to add some extra bulges on the back of the tank under the roll-bar, but this will be putting mass outside/behind the axle line. What’s the point of taking such a purist view if I then compromise? If I find myself really struggling for range I can always add a tank in this void where the old tank was. If it’s empty it will only add a kilo or two. My future plan is to remove the tank-mounting frame at the back and replace it all with a single CF crush-cell. This follow’s Warren’s law of composites. The new part will be lighter, and stronger than the rotting steel it replaces.


IMG_0058Here’s the lower part of the tank in place. The wooden bits show there the original guestimate cuts of kingspan need extending out to in order to maximise volume available. This angle doesn’t show it well, but there’s an inch minimum between the tank bottom and the prop. Again, it’s an IRS so the prop isn’t moving anywhere unless it lets go. The tank also swells out at the sides at the back (where the two sticks cross). There’s about half a litre or so to be gained by swelling out the sides like this.

All these little additions to capacity may not seem a lot, but if every half-litre can be baked in then I make serious gains in capacity. Bearing in mind the old tank weighed 6KG and carried 18L of fuel in the wrong place, I feel this will be a step forward.

Started the composite fuel tank

Take a gander at the picture below – it shows the start of my new composite fuel tank. In this post I want to talk a bit about the motivations of what I’m doing and how I’m going to construct it.

tank 1 with arrowsBLUE – the three layers of kingspan that make up the back of the tank, stepped at an angle

GREEN – the tank shape in the transmission tunnel.

RED – the ATL fuel-cell sender going down the tank.


I’m making the composite tank for the following reasons:

  1. the original tank was an MG Midget tank, and sat way out back and high, behind  the cross-member that the red arrow straddles. With a full tank this made polar moment of inertia less than ideal
  2. the steel tank was rotting anyway
  3. steel is heavy, well made composites are not
  4. there’s a lot of dead space in the area where I want the new tank (i.e. where the red and blue arrows are, and above the prop-shaft where the green arrow is.
  5. i can move mass down in the car
  6. i can make the tank gated, preventing fuel from slopping around
  7. I can move the tank inboard, protecting it from impacts
  8. i can save weight, overall.
  9. when sprinting, I will only fill the car as much as the transmission tunnel tank, but when I want range, I can fill the tank to the brim, getting extra capacity over the original tank
  10. i’m going to fuel injection so needed a new tank anyway.

I’m going to make it from two layers of 300gsm aramid with a partial soric core for hard-points, with 2x 200g eglass on the other side of the core. There will be soric ribs for strength. the compartments will be three layers of e-glass, and everything will be epoxy resin infused.

It’s not a structural component, but should be very strong in itself. What you can’t see on the above photo (a teaser) is the tank going down to a point at the front in which the submerged in-tank fuel injection pump will reside. If done right this will also be the lowest point and will act as a sump to ensure the pump never starves. Gravity is my friend.