Are overflow tanks necessary on aux fuel?

[Shawn K]

I'm working on a taildragger aux fuel tank for my GL1800 that will hold the tank much closer and slightly higher than most taildragger setups (click to enlarge).



While the tank has a vent on the fill neck, I've been wondering if I should concern myself with an overflow tank. I know that gasoline expands roughly 1% for every 15 degrees of temperature increase, so with a tank capacity of 4.25 gallons that equates to just under 6 ounces in potential expansion for every 15 degrees. I'm also aware that I don't want fuel just sloshing out the vent hose onto the ground (if I had it all to do over again, I'd have opted for a fill neck several inches longer, but it is what it is).

So the way I see it, I have two options:

1) Don't fill the tank to 100% capacity, and route the vent line up several inches before routing it back down toward the ground (routing it up first would help slow down any sloshing out before it spilled onto the ground). Call it "good enough" and hit the road.

2) Weld a 2x2x3" expansion tank on the side and attach the appropriate plumbing to make it an overflow/vent tank. That would be a capacity of approximately 6 oz., so if I wanted some extra buffer I could simply make it 6" long for a total capacity of roughly 12 oz. Alternatively, I could attach the overflow/vent tank to the base of my trailer hitch and leave the tank unmolested - just attach the vent line to the tank.

I may be making much ado about nothing, so I'd like the input of people here who have experience with aux tanks.

(Bonus question: Anyone have any bright ideas for how to attach a license plate to a cylindrical tank?)

 

[Terrywerm]

The simple solution is to just not fill it completely full, leaving a little room for expansion.

My son has an auxiliary tank for his Valkyrie, and he gets around the expansion problem by running his aux tank first, then switches over to the bike's tank when the aux goes empty. He also does not fill up ahead of time, such as the night before, but if he does, he leaves a little room in the aux tank.

As for the license plate bracket, make one that attaches to the two tank bands. And to keep John Law from having an excuse to stop you, make sure you have a license plate light as well.

 

[Shawn K]

My son has an auxiliary tank for his Valkyrie, and he gets around the expansion problem by running his aux tank first

If he's running the GL1500-based Valkyrie, that bike is carbureted and makes it possible for him to run the aux tank first. Unfortunately for me, that won't work in my situation with a fuel injected bike.

 

[Dave28117]

I have a 5 gallon aux tank which is mounted to my rear seat on my 1500-. I have a vent hose running under the bike. That's the same concept as the vent on my OEM tank. I've seen no noticable expansion overflow while using it.

 

[Terrywerm]

If he's running the GL1500-based Valkyrie, that bike is carbureted and makes it possible for him to run the aux tank first. Unfortunately for me, that won't work in my situation with a fuel injected bike.

Let me guess, the fuel injected models have a return line going back to the tank (I don't know for sure, never had to work on the fuel system on my 1800, and did not take the time to look in the manual). You might want to take a quick look at the fuel fill neck on your tank. Does the fill neck extend a short distance into the tank, or does it conform entirely to the round outside surface of the tank? Most tanks have the fill neck extended part way in to create expansion space that cannot be filled.

If your tank has not yet had any fuel put in it, and it does not have a neck that extends a short distance into the tank, you could probably take it to a welding shop that does aluminum and have a small sleeve welded inside the neck that sticks into the tank a short distance. The general rule of thumb is to have 10% of the total tank volume reserved as expansion space. Come to think of it, in order to be DOT legal, the tank is required to have that space.

Just some food for thought...

 

[Shawn K]

I have a 5 gallon aux tank which is mounted to my rear seat on my 1500-. I have a vent hose running under the bike. That's the same concept as the vent on my OEM tank. I've seen no noticable expansion overflow while using it.

Do you have any kind of check valve in the vent line, or do you just run it straight down to the ground?

 

[Auburn]

I have a roll over check ball in my vent line so it will close off the vent line if the bike is dropped.

 

[Russ Black]

I have a GL1800 with an Aux tank mounted across the passenger seat and have thought about this same issue. I have pondered making a slosh tank using a large tomato paste can with an inlet pipe just short from the bottom and an outlet more or less flush with the top of it. I guess any small radiator slosh tank might work just as well, but I haven't pursed it yet.

Currently at fillup, I run the transfer pump and transfer just enough fuel to give the aux tank some head space if needed. I also have put several large loops in my vent hose at the tank making a coil that catches any slop-page. It seems to be working so far. I don't have a check ball at the moment, but one is in the plan. I am also considering re-doing the whole thing as a taildragger.

 

[Shawn K]

Has anyone with an aux tank ever had issues with their garage smelling like gas fumes because of the vent line?

 

[Kendoo]

Has anyone with an aux tank ever had issues with their garage smelling like gas fumes because of the vent line?

Never. Taildragger with no puke tank.

 

[Marc11]

I have a GL1800 with an Aux tank mounted across the passenger seat and have thought about this same issue. I have pondered making a slosh tank using a large tomato paste can with an inlet pipe just short from the bottom and an outlet more or less flush with the top of it. I guess any small radiator slosh tank might work just as well, but I haven't pursed it yet.

Currently at fillup, I run the transfer pump and transfer just enough fuel to give the aux tank some head space if needed. I also have put several large loops in my vent hose at the tank making a coil that catches any slop-page. It seems to be working so far. I don't have a check ball at the moment, but one is in the plan. I am also considering re-doing the whole thing as a taildragger.

If you put loops in your vent line and if for any reason fuel gets into those loops wouldn't it then stop your tank from venting and put excess strain or worse on your fuel pump? I've always understood a vent should be clear and unobstructed, no?

Also never had fuel smell in my garage from my open vented aux tank, I've run both a puke tank and without, really never saw the need for a puke tank, only lost fuel out the vent a couple of times and in both cases it was with the puke tank set up, then again my current tank has several inches of headspace from the fuel to the vent and I don't over full the tank.

 

[EricV]

If you're going to weld on the tank, I'd just cut to the chase and have a longer neck installed that fits down into the tank as @Terrywerm describes AND w/o a vent line on the neck, but add a threaded bung in 1/4" NPT and install a roll over vent there. Then just vent to ground w/o a puke tank, and have the vent line run down the back, not the front of the tank so it's farther away from the rear tire.

I've not had gas smell in the garage from aux tanks. However, if you had very high temps in the garage, you would get some expansion and potential smell. Think tin shed in the desert rather than an attached garage that rarely sees huge temp swings.

On the side of working with what you have, it may be possible to press fit a plastic or aluminum sleeve into the neck that would protrude down into the tank a bit. I wouldn't bother to put loops in the vent line, (though I've seen it done), but I would run the vent line behind the tank, then back to the rear of the square tube to keep it farther away from the rear tire.

As for the plate, don't mount it on the straps. Put an extension into the square tubing and mount the plate to it. I'd look for class I hitch covers, which should fit inside the square tube you're using if the hitch on the bike is 2". It shouldn't be too hard to rig up a white LED license plate light, (1 12v wire from switched power and ground to the hitch cover with a disconnect back at the bike when you remove the aux tank), and bolt the hitch cover in place to avoid casual theft.

The only rally I ever did that required a catch can on the overflow/vent line was the Utah 1088. I kept a 5oz baby bottle that I cut the nipple open for the vent tube and safety wired it to the bike. One year I forgot to bring it or put it on before leaving the house in OR, so just wired on a Red Bull can. I did fill it once, running hard in aggressive twisties right after I filled the aux tank on a very hot day, but that was the only occasion I recall.

 

[Shawn K]

As for the plate, don't mount it on the straps. Put an extension into the square tubing and mount the plate to it.

I don't know why it never occurred to me to do that, but I think you have a banner idea there. The hitch itself (Kuryakyn) is a 1.25" receiver. The I.D. on the square tubing for the tank mount is 1", so I should be able to stop by Lowe's and get some tubing off the rack.

Funny how easy it is to get stuck on certain ideas and not see other ones. Thanks for the suggestion.

 

[EricV]

It's easy to get stuck looking at a problem from one perspective. Another set of eyes may often see something obvious to them, but not necessarily to the first person and vise versa. Thus so many "brain storming" sessions to solve problems occur.

 

[Shawn K]

It's easy to get stuck looking at a problem from one perspective. Another set of eyes may often see something obvious to them, but not necessarily to the first person and vise versa. Thus so many "brain storming" sessions to solve problems occur.

You and Brian both are really valuable to me in that regard. I've learned to listen to your experience and weigh your views heavily into my thinking.

This aux tank project has taken longer than I originally hoped (a confluence of events), but I'm determined to get this done. Range is now my limiting factor (I need gas before I need a break), so I'm determined to push on and see what adding 150 miles per fill-up can do for me.

 

[Russ Black]

If you put loops in your vent line and if for any reason fuel gets into those loops wouldn't it then stop your tank from venting and put excess strain or worse on your fuel pump? I've always understood a vent should be clear and unobstructed, no?

Actually the vent line is not clogged. Each of the loops works as a mini purge tank connected in a chain. As the fuel pump draws fuel out of the aux tank it creates a small vacuum that draws any fuel that may have collected in the loops of the hose back into the tank automatically. Think of how the radiators overflow tank works. It uses the pressure and vacuum created by the expansion and contraction of a liquid as it heats and cools to move the coolant into and out of the radiator instead of using a pump. Different, at minimal pressures and vacuums, but the principle is the same.

As far as any fuel smell, none noticed.

 

[Marc11]

Actually the vent line is not clogged. Each of the loops works as a mini purge tank connected in a chain. As the fuel pump draws fuel out of the aux tank it creates a small vacuum that draws any fuel that may have collected in the loops of the hose back into the tank automatically. Think of how the radiators overflow tank works. It uses the pressure and vacuum created by the expansion and contraction of a liquid as it heats and cools to move the coolant into and out of the radiator instead of using a pump. Different, at minimal pressures and vacuums, but the principle is the same.

As far as any fuel smell, none noticed.

Thanks, learned something today.

 

[Dave28117]

Do you have any kind of check valve in the vent line, or do you just run it straight down to the ground?

no, just straight to the ground. I do intend to install a check valve. my stock overflow has a check valve built into the the gas cap.

 

[EricV]

no, just straight to the ground. I do intend to install a check valve. my stock overflow has a check valve built into the the gas cap.

Those two are different. If you're putting a check valve in an overflow line, it needs to be right at the tank, not down stream in any way. The reason is if you get any fuel at all on top of a check valve in the vent line, you just lost the ability for the vent line to draw air into the tank. The fuel will sit there holding the check valve closed.

BTDT with a gravity feed aux tank and a check valve just off the filler neck. No air coming in the vent line means no gravity feed. It's worse for a pump system, in that you start pulling fuel and creating a vacuum in the tank, and at some point either burn out the pump, collapse the tank or if you're lucky, just stall the pump and blow a fuse.

This is why the roll over valve works so well for a vent line, it's right at the tank, so no fuel can get on top of it unless the bike is other than vertical. No sloshed fuel will sit on the ball and keep it closed in normal running, only do that, exactly what you want it to do, when the bike is on it's side.

 

[AtoZ]

One thing with AUX tank, don't put a line in cap to it lower than bike, when it heat up and start draining it will empty you tank and all tanks if it connected to rest.
Had 8 gallon with extra tanks, drained 2-3 gallon out before some called me and said FIRE HAZARD.
My new bike i got separate 2 gallon (5 gallon total) with valve and fill it almost all up no issue with overflowing leaving about 0,5 - 1 inch below cap.