Preventing Fuel Loss in Rollover Event

*Inspiration for this post came from Pirate 4×4 forum and member Daless2. FATS (Fuel Air Trap System) | Pirate 4×4

I’m always amused at how Demo Derby is such a unique motorsport, and yet how often technology developed from all sorts of motorsports can be applied to it. Whether it is rally racing, circle track, drag racing, or 4×4 builds, there is at least a little something to be borrowed for the derby world. Today while pondering fuels and fuel systems I thought of an often overlooked bit of helpful info I’d ran across in the past. I remembered it as figure four vent routing, or something to that effect, but its proper name appears to be FATS. This is a method of fuel cell vent routing that prevents fuel spillage in almost all scenarios.

You may think “Wait, that’s why I have a rollover / check valve, so why should I care.” I’ll tell you why – because the check ball in a rollover valve will only completely stop the flow of fuel in the event of a complete rollover onto the roof. In addition, I have seen many more partial rollovers onto the driver’s side of a car than full ones. I highly recommend reading through the forum post linked above to fully understand FATS. The original diagram doesn’t appear to be linked anymore, but I found it on other posts, and I’ll include it here.

Credit to Daless2 | Pirate 4×4

With gas tank protectors and cage structure builds abounding these days, there shouldn’t be any problem routing vent hoses this way with lots of attachment points on a modified build car. The vent hose will always have some portion above the fuel level so fuel will not be draining while on your side or lid.

Here is where I get to put the disclaimer that I am not a professional derby car safety person, and this information in not to be considered advice and is for entertainment purposes only etc. etc. cya. That said, I typically do it a bit differently than the FATS because I am not concerned with a little bit of gas dripping out of the vent after a rollover. I just want to prevent the fuel from gushing out everywhere creating a safety hazard. A couple ounces worth dripping out after getting tipped right side up is below my worry threshold. FYI – a six foot 3/8″ hose completely full would hold about 4.4 ounces. So with that in mind, I suppose I will trademark what I remembered it called anyhow, the “Figure 4 Vent Route”.

Figure 4 Fuel Vent Routing (F4)

The critical part about this setup (as well as the FATS) is that the vent hose must run beyond the four sides of the fuel tank. FATS accomplishes this by making a big arc left and back right, up and then back down below the tank to a catch can. F4 accomplishes this by running around the tank and beyond the last side, then down outside the car. In both systems the vent hose is doing the work to trap the fuel, so the rollover valve is redundant. The effective difference is that when flipped back over, FATS will drain some of the fuel back to the tank and some will be caught in the catch can, and F4 allows the trapped fuel to drip out after getting back on four wheels. With FATS in place, there is a potential that the fuel filter catch can would not allow tank to vent properly unless taken off and drained, so keep that in mind.

Now I can rotate the F4 set up and show how the fuel would get trapped in each scenario. I believe the most common rollover event to be getting blasted in the passenger side, rolling onto drivers door, and either staying there or continuing on over to the roof. Let’s look at those two possibilities first. By the way: It helps me to visualize it by thinking I’m looking at it from the front of the car.

F4 on side after passenger door hit, least possible fuel loss (< 2 feet of hose full)
F4 on roof with a full roll over, still does not fill hose completely full

If you get rolled onto your passenger side it works then too. Notice that depending on your fuel level and vent location on tank, its possible that no fuel would be lost at all as well.

F4 after drivers side hit and rolled onto passenger door.

So save your money on a rollover valve, use a couple extra feet of fuel hose, and instead of running your vent hose straight down under the floor, go once around the tank. Your promoter should thank you! Thanks for reading and Good Luck guys!

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