Underseat fuel tank

[Millietant]

Mark, wouldn't it have to be a thermo-set plastic? 
I like the carbon fiber idea but instead of a fake tank inside the original shell, make a carbon fiber duplicate of the original tank.  Now that would be cool.

Joe

Hey Joe, a couple of the FJ Racers over here used underseat fuel tanks and cut up their tanks and that they just used the top of the tank shell as a cover to make the bikes look "normal". Their idea was to take all that weight from above the engine and put it lower and to let the heat escape more quickly from the engine through big holes cut in the shell  - but at the same time, they could only fit enough fuel in their tanks to cover 1 short race (about 7-8 litres) - you can't fit 22 litres under the seat without some serious fabrication and component moving !

Not a great picture but I hope you get the drift

[Millietant]

Filling the tank

[giantkiller]

I was going to make a Carbon Kevlar shell from the original tank. And an aluminum tank underneath.

But like I said all projects on hold until I get the shop/house done.

[fjman]

Filling the tank

this is very nice. do you have more pictures?

[ribbert]

...... having a tank full of gas would keep the plastic cool......

I see several references to this. What keeps the fuel cool?

Having a full tank of fuel sitting on top of that motor when it's hot is no different to a pot of water over a fire. I'm not theorising here, if wearing shorts in stinking heat and in traffic (as I often do), I can't touch the tank with my inner leg, it's so hot it stings and I suspect if I left it there it might even burn, certainly feels like it would.

Noel

[Millietant]

Filling the tank

this is very nice. do you have more pictures?

I'll see what I can find - will post some pics of the FJ's/racing on a separate thread.

[T Legg]

...... having a tank full of gas would keep the plastic cool......

I see several references to this. What keeps the fuel cool?

Having a full tank of fuel sitting on top of that motor when it's hot is no different to a pot of water over a fire. I'm not theorising here, if wearing shorts in stinking heat and in traffic (as I often so),I can't touch the tank with my inner leg, it'ts so hot it stings and I suspect if I left it there it might even burn, certainly feels like it would.

Noel.                  The thought behind the fuel keeping the tank cool is that as it heats the evaporation causes cooling .With water it limits the temperature to 100 degrees Celsius untaill it boils away.water can absorb a great deal of heat .I don't know how gasoline compares in heat absorbing capacity but the effect is the same.I looked up the temperature that gas boils at and I saw a range of between 100 and 400 degrees Fahrenheit so the tank could still get pretty hot.

[Mike Ramos]

The Midget (Mike Ramos) has significant experience with an auxiliary tank. I believe it was a soft temporary tank, however, but I bet he can tell you some things.

Gentlemen,

Another interesting discussion.

Not sure of how practical an under seat main tank would be as capacity for street use perhaps is a limiting factor.  Below are some of the issues encountered when the location was considered.

The idea of fabricating an under seat auxiliary fuel tank was entertained.  However for a variety of reasons the notion was set aside and a behind the rider location was proven more practical.
 
As Pat mentioned, 5 gallons of fuel is a considerable volume to squeeze into a limited space.  The shape of the tank would have been "unique" to say the least. 
Some of the options:  The spring from the rear shock was removed to check total wheel travel & see if the front half of rear fender could be repositioned to allow additional room – proved not very practical.
   
The sides of the tank would be to the side covers – there was concern the air flow over the engine would be hindered – perhaps (perhaps gentlemen) one of the advantages of removing the stock air box & using pods is improved air flow over the engine as more air is allowed to escape from the rear – eliminating this possible advantage and then minimizing air flow to an even greater extent by filling in more space than the stock air box seemed detrimental.

As it is, since no consideration of damping the engine noise is of concern and in an effort improve the possible cooling effect, the insulation on the inner side panels has been removed (it was in a deteriorated condition anyway).
   
As experienced many times in hot weather the main tank gets extremely hot.  An under seat would seem to have even more of a heating problem.
The lithium battery would have been mounted in the rear tail piece – but then the storage of the tool & patch kits & miscellaneous items would have been eliminated – they would have been placed in a tail pack which is not much of an advantage.

To maintain fuel level equilibrium in both the under seat tank and the main tank, either a long neck filler tube would have to be considered or the under seat tank would have had to be sealed in some way.  Some type of venting would need to be employed.  None of the options proved practical – at the very least, positive & redundant fuel shut off from both tanks would have to be assured which and such assurance is difficult to ascertain when the tank is buried & out of sight.

The solution settled on was an aluminum tank sized for a commonly available tail pack which should help protect the tank in case of a fall.  Aluminum was chosen not for the weight savings (over steel) but because it will not spark in case of an accident.  The total volume is 5.3 gallons (although 5 gallons is considered full).

There is an electric automotive two way fuel transfer valve installed – the normally open side is plumbed to the main tank so if it fails, the main tank can be used normally.  In addition to the fuel transfer valve, for safety there is a redundant electric on/off valve between the auxiliary tank and the fuel transfer valve.  Finally there is a mechanical Pingel valve on the tank itself within easy reach while under way.

Every switch is independently fused for added safety plus should the need arise, it is relatively easy to determine the cause.

The tank is directly behind the rider held in place with ROK straps.  There is a quick disconnect fuel line which sets behind the right side cover.  Installation & removal takes a minute.
 
Although the explanation seems involved, it is actually a quite simple & straight forward installation.

Ride safe,

Midget.