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Any downside to having a fuel tank under the seat?

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Many street bikes do and the gas tank before the seat is just a fascade. Moving all that weight gives a lower center of gravity which should translate to better and more stable handling. 

 

I removed my stock airbox and shock which had a separate reservoir. Looks like a good amount of freed up space and my calculation says I could gain at least a gallon more capacity than the stock tank. Of course could keep both tanks and just have a lot of auxiliary capacity.

 

Any negatives?

 

Would you design an under seat tank with baffles inside to keep it from sloushing around?

 

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The new YZ 450F has this sort of design.  The Husaberg in 2009 tried it out with the air filter where the tank used to be, but the bike was still dog heavy (handled heavy AND scaled heavy) despite the 70° slanted motor and the centrilization of mass.  Yamaha has improved the designs though with the tank under the seat, seems great so far.

 

I think the future of dirt bikes will be designs like Yamaha is doing now, but Yamaha got 3rd of the Big 5 in the lastest shootout I read.  That proves that a manufacturer cannot focus on only one thing, it's a whole package you gotta sell to us dirty dirt bikers!

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Many street bikes do and the gas tank before the seat is just a fascade. Moving all that weight gives a lower center of gravity which should translate to better and more stable handling. 

 

I removed my stock airbox and shock which had a separate reservoir. Looks like a good amount of freed up space and my calculation says I could gain at least a gallon more capacity than the stock tank. Of course could keep both tanks and just have a lot of auxiliary capacity.

 

Any negatives?

 

Would you design an under seat tank with baffles inside to keep it from sloushing around?

 

 

So where are you planning on putting the air box and shock after you do this?

 

It's not as simple as relocating a few things. The entire design has to be a cohesive system. Both Yamaha and OSSA have designs that locate the tank under the seat, and both designs start with the reversed cylinder layout that allows the intake to be vertical and at the front of the bike.

 

Of course the OSSA is not in production, but cool design nonetheless. But the engine is in use in their TR280i trials bike, including the EFI system, a first for production two-strokes.

 

2013Ossa.jpg

 

 

 

ossa_008.jpg

 

 

yz450f.jpg

 

 

YZ450f_2gallerymotor.jpg

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Remove some seat foam and add an extension to the stock tank.  That's what they did on the Euro MX bikes for those looooong tracks.

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I am putting on a pod air filter. The bike is fuel injected so I ought to be able to tune for the difference in air flow. The shock I am putting on has to reservoir built in the top of it. The stock shock had the reservoir on a tube and the can mounted on the frame. 

 

To my thinking a gas tank is simply a box with an inlet, a petcock, a fume/overflow vent and maybe some design to keep the fluid from shifting too radical. It has to be reasonably tough, I will probably use 16 gauge stainless. As long as gas flows to my fuel pump (which will be in the tank) and it squirts out at the right pressure then it is a no brainer. Don't see all the advanced design implications at all. 

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Eliminating the air box will reduce performance by destroying the intake resonance.

 

Maybe so but what I saw with the stock airbox was a big open space with a filter jammed up to a snorkel that led to the throttle body. Whatever resonance you are talking about didn't make it through the filter. I am not putting the pod directly on the throttle body. That is the more common problem with pod filters not performing well. It has a 2 inch diameter pipe maybe even a bit longer than the stock airbox snorkel had from filter to throttle body.

 

Nice thing about all this is if it don't work well back to stock and maybe a plan B.

Edited by geaux

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Maybe so but what I saw with the stock airbox was a big open space with a filter jammed up to a snorkel that led to the throttle body. Whatever resonance you are talking about didn't make it through the filter. I am not putting the pod directly on the throttle body. That is the more common problem with pod filters not performing well. It has a 2 inch diameter pipe maybe even a bit longer than the stock airbox snorkel had from filter to throttle body.

 

Nice thing about all this is if it don't work well back to stock and maybe a plan B.

 

 

The air box is part of a complete design that generates positive and negative pressure waves to help stuff more air into the engine during every intake cycle. By removing the air box from the equation and placing the filter out in the open, you've effectively eliminated that.

 

You're also going to end up with a filter that gets dirty much faster than it did when it was enclosed and protected, and it will located right behind the engine where it's going to draw in lower density air that has been heated by the engine.

 

But by all means, carry on thinking you know more than the engineers that designed the bike. Keep us posted on the progress. 

Edited by Chokey
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I will read up on air boxes sensei Chokey. Don't hurt me :rolleyes:  

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I will read up on air boxes sensei Chokey. Don't hurt me :rolleyes:  

 

 

You have to understand, these are high performance machines, and the engineers use every tuning trick available to them to not only increase peak power, but to keep it as broad and usable as possible. Intake and exhaust resonance tuning is one of the tricks they use to spread the power out over a more usable range.

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I remember reading a great Kevin Cameron column on air box resonance tuning. I love his writing, too bad he won't publish a compilation of all his TDC columns, that would make for some fascinating and educational reading.  :thumbsup:

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I know my 690 is bigger than your typical "dirtbike" but the fuel tank is in the rear under the seat (it is the subframe, much like the Husky 449/511 or BMW Xchallenge.) I did similar to what you are talking about by removing the airbox, running a pod filter directly on the throttle body, compensated with a richer map, and put a 1.25gallon fuel tank in place of the airbox. The front tank fills the rear tank by siphoning through the rear tank's vent line (the front always goes dry first.) I've been running this for several years now and have noticed a few quirks. First is that I have to remove the seat to fill it up (though I might cut a hole in the front of the seat and extend the filler up through it), second is that it is more prone to sucking water in DEEP (over 2') crossings, and third the filter gets dirty a little faster. I'm still overall happy with the setup

 

20130111_172835_zps1c7cead4.jpg

Edited by TheLetterJ

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I run pod filters on my snowbikes - seat of the pants tells me it's slightly quicker and snappier with the pod vs. the airbox.  Have not dynoed it though, and I do have a auto-tune set up on my bike.

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LetterJ now you are talking. If the seat tank fills through the stock tank why would you have to fill it directly? Seems with that setup with the stock tank higher the seat tank would overfill and shoot gas out the vent tube unless you had some kind of bob stopper (I don't know what to call it, ball in a tube that floats up and blocks the hole). 

 

No more than a gallon and a half. I was thinking much more.

 

Thanks for sharing.

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Both tanks fill separately, both filler necks are at the same height. I do have a check valve in the vent line at the front tank(visible in front of the filler in the picture in my above post), but just to prevent overflow when I lay the bike on its side. Factory filler is at the back of the bike, underseat tank filler at the front. Fuel pump is in the bottom of the factory rear tank. Fuel line is run from the rear tank vent nipple to the bottom of the front tank. As the rear fuel level drops, it pulls a siphon through the front tank drawing fuel back into the rear tank through the former vent nipple. Rear tank is constantly being topped off until the front tank is bone dry. I now have 4.42 gallons total capacity.

 

20130111_173003_zps688f94d6.jpg

 

Here is the rear filler with an aftermarket neck that uses an Acerbis small cap and has the siphon/ vent hose exiting towards the front of the bike:

 

20121202_131907.jpg

Edited by TheLetterJ

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KTM 690? After your explanation and googling a pic of the bike I understand now I think. 4.42 gallons sounds great, especially to a guy that has 2. Awesome looking bike but I think my dr350 has a lot more room inside the frame. 

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Are you using MicroSquirt? Where is your pump? Being that your bike started life carbed, the tank situation probably will be pretty easy to work out as you could just run a line from the petcock of the stock location tank down to the lower "airbox" tank and have the lower tank fill through the upper with your fuel pump drawing from the lowest point of the "airbox" tank. Having both tanks linked at their lowest points would allow them to equalize. Depending on the exact elevation (in relation to the stock tank location) and plumbing of the lower tank, you may have to run a vent off the top of the lower tank so that it doesn't vapor lock. If so, just run that vent hose at least as high as the stock tank vent hose and it will not overflow.

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I am using a megasquirt ii and a new highflow pump for yamahas including  r6 which I made my throttle body from, Still working on getting it going though. 

 

Again thanks for all your advise. I am thinking more and more this is, if not exactly a great idea, at least a doable one.

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