Fuel tanks and gears

I have an 85 maxim X which only has a 2.8Gal tank, everyone I ride with seems to have 4-5 gal tanks on thier bikes. Is there a tank out there that does throw the appearance of the bike way off but offers more capacity? Or am I going to have to cut stretch and weld to make a fat bob type tank while retaining most of the original profile? I would like to pick at least a gallon of capacity. Also although I am sure it is a ton of work is there a 5th gear out there that would have a taller ratio to bring down my cruising rpms on the highway 70-75 I have and plan to ride some long distances and having to stop around 95-105miles(I pull off to refill as soon as the low fuel light comes on) when everyone else can go 150 kinda bites. I turn 6000rpm's @75 and about 5500@70 I think if I could get those down to 3800-4200rpms it should also help out with the range issue. THx for the opinions guy/gals

 

HERE is a custom tank someone made for a XJ700. Over 5 gallons, but I don't know how well it would fit with the stock bars. Neat idea though... I've also heard some rumours of someone who cut up a Seca 650 tank and welded it onto the top of his X tank to add a couple more gallons of fuel. Used to have a link for it but can't seem to find it now.

 

Wow the price is up there for such a beat up tank. Obviously I am not the only one that has this on their mind. Maybe I can find a beat up stocker to slice and dice otherwise It will have to wait for the off season to cut and stretch mine out. I want to stay along the original lines and fuel cap just widen it out a bit. If you look down while sitting on the bike there is good 1-1 1/2" out the 5-valve carb covers that you can use and can easily add 2" to the top. HMMM .... well when and If I do it I will take lots of pictures of the franken tank. THe bike needs a good paint job anyway.

anyone know about the gear thing kinda like an overdrive 5th?

 

That doesn't look like a beat up tank so much as a whacked-out paint job. And the price looks cheap compared to what they're asking for a new one. Plus, it came with the front fender.

As for the range... I hear you, bro. 90 miles, and it wanted to go on reserve... NOW.

There's an article on the XJCD about someone who added the top of a 550 tank to the bottom of a 650 tank to add a gallon to create his own Frankentank.

 

As for the gears, why don't you just change sprockets?

 

maximuschop the 85 maxim X is a shaft drive. If it were a chain drive you are correct it would be pretty simple. Not sure if you can get a different set of gears for the rear end. I don't really want to loose much punch of the line and through the first 4 but 5th is mainly used for 50-55+ in my riding so if it were possible to just change 5th that is what I would prefer.

as to the tank the paint job is a moot(sp) point as it is chipped in almost every spot that somehting could ding into, and dented up in the front by the curve they put in for the bars. I have my work cut out if I tackle the tank I want to use the original cap and shape just a tad larger.

I guess I could just drive slower!! but then you get the impatient drivers in thier cars on the phone trying to run you over.

 

Changing sprockets is not an option,as this bike has a shaft final drive. Perhaps swapping out the stock 16" rear rim for an 18" rear rim from a Seca 750 or a Seca 650 turbo (along with the tallest tire possible),may help out a little bit.

 

Yeah, other than tires, you don't have much option for gearing.

On OCC they cut tanks in half down the middle and widen them, I wonder how hard that would be to do with a beat up tank? The EBay one has a harley top and maxim bottom. Yeah the paint job is a little busy for me, I think that would cause me to drift into the ditch.

 

THe cutting in half and widening is along what I want to do. just widen a bit and then narrow where the backbone goes. I have a friend who's dad owns a body shop so he has grown up around body work. I can cut and weld decent would still pressure test and line it if needed but would rather not. I have also thought about the taller wheel as I read some of these threads but I also just got new tires 400 miles ago and like the fatter tire on it. my tank is not beat up so it shouldn't be bad just shaping the metal and getting the welds right so you don't have any pinholes will be the trick.

 

There is an 18" Seca 750 rear rim on ebay (item# 160112785363) with a a buy it now price of $24.99,although adding the shipping costs is likely to triple the price.

 

sorry guys.

 

I read a how-to on a tank mod, it's a 40 hour job, a labor of love.
If you don't seal the inside, and it leaks a little, you ruin your paint.
You would need to braze every seam, and you would still end up with thick bondo in spots, which shrinks and cracks (long term).

I would suggest adapting a larger teardrop, with no surface bodywork involved.

And that tank has about 5 times the legally allowable lightning !

 

LOL that is funny, right there.

 

I have been thinking about changing my 5th gear on my 750 Seca. I had already planned to put the larger rim on the 650 maxim that I'm flying over to Europe with me this summer.

When I get back in September I'll cut myself a new fifth gear (actually I'll probably grind myself a whole new set and have them induction hardened) and see what it does for my rpm's at higher speeds. I'm really just hoping to get myself some better gas mileage.

I thought I read somewhere about a 6 speed for the XJ. Has anyone heard of that? Cuz that would be ideal now wouldn't it?

 

Would that 18" wheel fit on an '82 XJ650? Sounds like a good idea to me, if it'll fit! :wink:

Thanks,
Dean

 

is it me or....

I don't think using other sizes of sprockets, rims etc. will help to reduce gas milage....

The laws of physics tells us all that moving a particular mass with a particular speed a partucular distance always uses a paricular amount of energie.

To reduce gas milage you have to reduce the energy needed to move or give the gasoline more energie.

Given is that gasoline gives a more or less fixed amount energy while burning inside your engine.

Like all other engines, a combustion engine has an efficiency at a fixed RPM. which highest point is pretty close to half the maximum RPM the engine can handle, which almost at any vehicle is geared at about 60 mph in highest gear...

the only ways to reduce gas milage i can think of is to lose mass to transport and to stay with the RPM needle around half the maximum.

 

The point is, I think, that the larger diameter wheel will have a larger circumference, too. Therefore, the larger tire will need to revolve fewer times per minute to achieve the same forward speed of the bike. Which means the engine can spins less quickly. Thereby using less energy and saving gas.

Someone will, I'm sure, correct me if I'm wrong.

Dean

 

still you are moving the same mass at the same speed over the same distance...

The option may probably work for speeds (way) above 60 mph... so reducing the RPM's of say 70 mph to the number the engine has its highest efficiency

I think there's only one way to find out...
Keep record of you milage one whole year with the standard settings.
Change the settings and ride a whole year with the new setup.

Compare the avarages of gas milage... it should be clear by then

 

I just ran across an old thread that suggests using the "middle gear set" from the 900 into a 750 to reduce cruising RPM.

These bikes weren't optimized for best fuel economy, but to be fun, and beat the competition. (the YICS is for economy) It's all a compromise.

I think a 10 to 15 % RPM reduction would improve mileage at 70 MPH,
but we won't know until someone actually tests it.
Painter has his set-up with a 16 and a 18 inch rim.

 

Internal combustion engines lose efficiency quickly at higher rpm's.

The law of physics actually don't tell us all that moving a particular mass with a particular speed a particular distance always uses a particular amount of energy. The laws of physics tell us that An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Engines run best at specific loads and rates as described by their power curve. Thermodynamics play a big part in the increase in frictions at higher rpm's. What needs to be determined is what rpm is optimal for maximum power output and then match it with your desired cruising speed. Of course there is always give and take when doing any kind of modification to your drive train but you need to decide what is important to you.

You are right about one thing though. The only way to measure success is imperically in Miles per Gallon (Or liters per 100kms like the rest of the world).

 

You should be able to put the rear wheel from a 750 Seca on a Maxim. I have both wheels for mine and swap them if I want to go on long trips. It does make a difference on the rpm's out on the highway. But I also like the looks of the shorter, fat tire on the Maxim wheel I have on it now. There is about 2" difference between the two wheels when sitting side by side.
When I get the Seca tire wore down more, I'm planning on going to a wider tire on it so I don't have to swap the wheels anymore. PD

 

Wow, a lot of misinformation here...

I think that everyone is talking about two different things, efficiency and economy. An engine's peak efficiency typically occurs at or near the peak torque and is measured in pounds of fuel per HP-hour. This occurs unthrottled (full throttle) at a very specific RPM where the tuning of the intake and exhaust actually act to draw more air into the engine by stacking pulses (a lot of physics). Since our bikes have four cylinders, four carbs, and very short intake runners, the max efficiency is at a pretty high RPM. Remember that max efficiency occures at full throttle.

Fuel economy is loosly tied to efficiency in cars and bikes. In this case, drag and friction are factored in and have a significant impact on economy. Generally speaking, going the same speed at a lower engine RPM increases economy by reducing friction in the engine as well as pumping losses since you'll need to use a little more throttle. Take my Dodge Durango which gets great (sick) fuel economy. If I leave it in cruise control going up a hill, it will downshift and go from 18 MPG to 10 or less. If I am controlling the foot feed going up a hill and prevent it from shifting down (and maintaining speed) I can get about 14 MPG going up the same hill. That is quite a difference.

Mass only factors in during acceleration so if you do a lot of stop and go driving losing weight will increase economy. The only thing that will increase economy on the open road is to reduce drag and engine speed (or install a much smaller engine). At speed your bike only needs about 25% of its peak power to maintain cruising speed. All that reserve power is for acceleration and actually reduces overal efficiency.

My final thought. I had a 1991 Ford Escort with a 5 speed. The car was gut less and required me to shift down two gears to pass anyone on a two lane road. But, since it was geared so high, it always got at least 35 MPG (~28 city and ~45 Hwy). Looking at current car offerings, really nothing compares. I had a 2002 VW Jetta with a 4 speed auto that could barely muster 32 MPG on the open road. This is because it was geared low so it felt "peppy" but was over 3500 RPM at 75 MPH.

Note: The author is a recovering mechanical engineer and is known for rambling talking above the heads of many people. Please take the above thoughts in stride and for what they're worth.

 

That is why I geared my ACH 020 transaxle with a taller gear! 3.2KRPM @ 70 was an improvement over the 4.2K I had in my 9A. I do wish they had made an even taller gear. Close ratio is nice for track work but for the highway it just sucks gas.

 

The difference in the Seca wheel and the Maxim wheel as far as performance goes isn't noticable. I don't see much difference in exceleration between the two. These bikes are geared for performance and not gas mileage. I'd like a taller geared rear differential myself. I can take off in second with the stock setup without having to slip the clutch much, so a taller rear gear would work perfect, if only they existed.

 

Since we're bench racing- - someone get out a calculator- -
I take my 750 Seca, combine the tall - gear- innards of a 750 Maxim,
and add the "middle gear" of a 900, on the Seca rear rim of course,
and what is the ratio improvement? Expressed numerically, or as a percent.

Or we can put "Elvis" in business, selling custom 5 th gears.