Putting fuel tank behind seats in racecars?

[slacker.cam]

I've been thinking about what kind of tank setup I want to run on my other AE85 and a number of times now I've come back around to the idea of putting the fuel tank behind the seats, like where the back seat normally is. Why don't you ever see this done?

In my tiny mind it seems like a great place to put it for a few reasons:

- You're centralising a large dynamic mass so you reducing and stabilising the rotational inertia in yaw (a BIG bonus considering theres no performance decrease)

- If the other option is a fuel cell in the boot then you will lower the CoG (at least this is true in an AE85/6)

- If the other option is a standard under-boot tank (in which case the CoG will likely be similar) then you free up this space so you can run a big diffuser

- Shorter fuel lines - less weight

- It's inside of the strength of the roll cage so in a crash it is protected

The downsides would be:

- Could be harder to fill depending on the filler arrangement

- It's physically closer to the driver so if the tank was ruptured then you're more likely to get burned

I'm thinking that the tradeoff between it being in a stronger location vs. it being closer to the driver would be the main issue here.

Anyone else have any ideas on the topic?

[JoKer]

ya downsides pretty much sum it up, boot is best + slightly better grip on rear end

[Shandangles]

cant have any part of the fuel system in the "cabin" part of the car for msnz use, i dont know where the rule is but thats always been the case when iv been scrutineered.

in the boot with sheets of some material between the "cabin" and the boot is sweet as tho.

chop the spare wheel well out, weld in a flat sheet, make a tank thats the required size in any spor under there you want. sorted

[toyabusa]

I have done this with my car, there are rules for doing it regarding drains, vents and it being enclosed. In a competition car all your reasons make sense and were why we did it.

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[slacker.cam]

I should have mentioned about the venting/sealing issues.

I've had a read through Schedule A and it pretty much just says that the fuel system has to be fully sealed from the cabin so you could build a box around it - just the same as if you have a fuel cell in the boot of the car. I can't see any rules in there saying you can't have it behind the seat (well if toyabusa has done it then i guess its all good) but I may have missed something?

Toyabusa - that looks awesome. Pretty much exactly what I imagined. Except possibly centered between the box section of a equal length 4-link setup depending on if I get that far on the inital build.

[toyabusa]

Yea that was made to the MANZ rules. In the centre would be good if you could still get it low enough with the driveshaft tunnel/diff head not getting in the way. Also considering the possibility of a driveshaft letting go?

[mikuni]

There are two different Vectra chassis, the one I went with has the fuel tank in front of the rear axle, which is one of the main reasons I chose it. Seems a good idea to me and from what I can see, the only reason they moved it was a sacrifice to fit the IRS the other chassis had.

Only issue I have come across is because I want the car so low, the exhaust kind of needs to go through the tank, which can be done and complied to MSNZ, but is just too much effort. I imagine the same would apply/be slightly worse with a RWD car, as you need to get the driveshaft through here also. A fuel cell tucked to the passengers side away from the centre would be one solution, but you'd end up getting some weird dynamic corner weights going on with a driver and a full tank offset across the cars centre line.

But yeah, optionally putting a huge mass behind the rear axle just doesn't make sense.

[toyabusa]

With mine I have the surge tank on the other side to compensate and also the exhaust runs up into the car at that point and the muffler sits there too, exhaust exits infront of the rear wheel.

In saying that i dont think the diagonal thing would really make too much difference with driver and tank being so close to eachother (compared to them being out at the far corners)

[Testament]

Cam, the AE86 has the engine out the front - so a little weight in the rear is not a bad thing. traction etc. Presuming the ae86 isn't a ford pinto with a fuel tank in the rear bumper.

[westy]

Just seems a bad idea to me. Would rather not sit in a cabin with a bin full of gas/fumes.

[browndog]

The replacement for the Australian V8SC will feature a fuel tank in front of the rear axle, for many of the reasons you have stated. As mentioned, you will have to enclose it in a box. It is generally agreed to be safer in this position than in the rear of the car. In the event of an accident to the rear of the car, a rear mounted fuel tank is very likely to rupture, where a mid mounted one is not. Due to the design of rollcages the "mid mounted" fuel tank is contained within the "non crumple zone" part of the cage, so although you are sitting a bit closer to all of that fuel chances are it is protected much better. And just because a fuel tank is in the back, does not mean the fire won't come into the cockpit to say hello! It is more difficult to fit a tank in the middle of the car, but as well as the safety benefits, there are benefits to the handling of the car, as you have already alluded to. I say go for it!

[westy]

But aren't the V8SCs putting it under the floor?

Not that I have a better reason than 'it would give me the shits'.

[Roman]

A car with a good factory gas tank position is an MR2.

It has it dead centre in the car, just underneath the gear lever.

Long and narrow tank as well, so less susceptible to fuel surge.

So despite being pretty much as close to the driver as possible, I've never heard of a crash involving gas tank rupture making the driver explode.

I guess if you're in a crash bad enough to damage the tank, you're likely a pile of mush before you catch on fire anyway

I must say it gives me the shits, where a good shunt in the rear of my car would likely make my gas tank go pop.

[drtdvl]

The 205/306 has the tank mounted under the molded rear bench in front of the rear axle.

[JoKer]

as low as possible and as rear as safe to 'balance' car is always a race-car drivers desire (or where ever they think weight should be to improve handling)

[mikuni]

Theoretical 'perfect' balance on corner weights doesn't always translate to the real world though. Getting it to perfect 50/50 but with all the weight hanging right out in front of the front axle and behind the rear axle with nothing in between would surely cause a pendulum effect and make the car sluggish to turn in, but still with a tendency to over steer mid corner.

Ideally you want to keep weight inside the axles and to the centre of the car, but obviously this isn't always possible.

I'd be reluctant to put a fuel cell in the cabin, even with appropriate breathers and venting, but putting it under even a false floor could be a good solution?

[browndog]

But aren't the V8SCs putting it under the floor?

Not that I have a better reason than 'it would give me the shits'.

It will be under a panel or box, bit hard to say what the "floor" is in those cars as they are pretty much a spaceframe. I would not put it inside the cabin, even if it is in a box. But slung under the car where the back seat is, if you have room would be good. Problem is most plastic fuel tanks (like Jaz) are too tall to do this. Most race cars with mid mounted fuel tanks use bladder cells.