Reducing capacity of fuel cell


Premier Member
IBR Finisher
Basic question: is there an acceptable method to reduce a fuel cell's capacity by a 2-3 10ths of a gallon?

My quandary is that my Triumph Trophy has a 6.6 gallon main tank. I'd like to buy a 5 gallon fuel cell but that would put be over the 11.5 gal limit by 1/10th. Allowing for another 10th in the lines, I need to reduce the capacity by 3/10ths.

This method must be acceptable to IBR inspectors as I hope to make it into the 2019 edition.

Perhaps an official statement from Mr. Austin or Mr. Wilson would be helpful (of course after the Big Dance is done this year).


Premier Member
Comments: they also consider the volume of the lines and filler neck. I've heard of people putting lengths of chain in a tank to displace some fuel. Im not advocating that but its been done.

Garry in AZ

Premier Member
IBR Finisher
I would solve the problem by structurally modifying the 5 gallon cell to reduce it's volume. Adding baffles, reducing one dimension or more slightly, etc. It's never a good idea to put something in a fuel tank that can move around. (chain, etc) My advice is to do it right the first time, modify the cell in a way that makes it fit better, mount better, or other work better for the way your bike is set up. Cutting and welding a new tank is not prohibitively expensive and once done, you never have to worry about it. While the welding is being done, you also have the opportunity to make custom mounting points, etc. Make sure you reduce it enough, you don't want to be in tech inspection two days before the start and find out it's 0.5 tenths too big. Aim for a total volume of 11.2 or 11.3 gallons maximum. I have done over 100,000 miles with an aux tank, (11.3 gallons total) and I have never put more than 10 gallons in during a rally. 10 gallons will take you 350-400 miles, the extra 1.3 is just insurance.

Other tips to ensure a smooth tech inspection, mount it solidly to the bikes frame, do not rely on a luggage rack or similar weight limited point as a mount. They will grab it and shake your bike via the fuel cell, so strong mounts are required. It's also a good idea to make sure the cell is electrically grounded to the chassis of the bike to prevent static electricity from making things quite exciting.
Make sure it's vented properly, and the vent does not exit anywhere near the end of your exhaust or any other hot surface. Ideally the vent hose would terminate at the bottom center of the rear fender.
Tether the fill cap in some manner that prevents you from losing it if you forget to put it back on.
A quick release coupler for the fuel line is a handy addition when you want to remove the cell for any reason. (Sold at marine supply houses)
Whenever possible, I greatly prefer a gravity feed aux fuel system, as it's much less complicated and there is simply less to go wrong.

Maybe reach out to two-time IBR finisher Don Duck for some ideas or input, he rides a Trophy and has a meticulously prepared bike.

Good luck!



Premier Member
IBA Member
Seems to me your plan is to build a bike for the IBR. Your making a statement and a commitment to yourself.
So the solution really is build the Auxiliary tank to make the bike fall within the rules.
Sounds simple know and its not. I know that too.


Its going to see you take the time to design and build a smarter better fitting tank that's probably deeper into the bike and makes better use of the spaces. And I'm sure would make a better addition.
Probably a design that's not all that clear to you just now but given a bit of time will become clearer and you'll end up with a tank that's great to live with and within the 11.5 gallons total.

Good luck with 2019 tilt at the big game.

Patrick Ford

Premier Member
IBR Finisher
Have you thought of modifying the filler neck to extend down enough to limit the amount of fuel when topped off? Check with Dale about this but in my old and barely working memory I think I've heard of this fix.