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renault_megane_dci 05-10-2012 05:09 AM

250 GN, the 100 mph project
Hi all,

I have been lurking for a while and modding my bike in secret so I thought now is a good time to start my build thread.

The driver :
I got a Honda NX 125 for a year, then my bigger bike license and a '95 Suzuki GSX 750 Inazuma wich was poor on MPG (6 L / 100 km minimum) it was bare stock.
Then I went for a '97 Suzuki Freewind (revised 650 single from a newer DR in a more street oriented trim) wich was motarded and got a free flowing exhaust.
I soon fitted with a +1 front sprocket and my best was around 4l / 100.

Then I swapped it for a '81 BMW R65. It is still in the stabble since it needs some work done.
Because the R65 being on rest and having got the bug of hypermilling, I got a Honda Helix. I got it for a year, averaged 3.4 to the 100 and best 3.2. It was stock.
I thought the handling was crap, maintenance difficult and MPG potential was not there because of the CVT so I replaced it with a "proper bike" : the GN.

The bike :
'94 GN 250 with 54K Kms on the clock, a small wind screen and lower handlebars.
When I got it, I lowered the fork 1", lowered the mud guard to have it closer to the front tire as I replaced it, adjusted the windscreen to give it more of an angle then fitted a +1 front sprocket.
I also reworked the air filter case to make it more hi flowing.
Then I fitted a new chain and a -3 rear sprocket.
A few weeks back I went far a cardboard partial tail.
It's been very promising and rewarding in a sort of way ...

Planned mod are :
a bigger rear wheel (16" to 17 or 18"),
clip on handlebars,
Guzzi V8 replica dustbin fairing,
NACA hood for engine cooling,
much higher CR 10.5 to 1),
DR 350 gearbox / bottom end (longer stroke to give 282 cm3 instead of 250, oil cooling and 6 speed gearbox).

The 100mph target (up from 83) is (in my opinion) the other side of the coin of fuel efficiency.
If I can make her go that fast on stock(ish) internals, then she shall return high mpg at usual lower speeds.

I'll be updating this post with pictures pretty soon.

euromodder 05-10-2012 06:18 AM

Great taste in bikes, I've had a naked 1999 GSX750 as well :)
(AFAIK they were only made from 1998 to 200? , 2002 - 2003 maybe ? )
I never did better than 5.2 l/100km - some 45 mpg - and usually 6l/100km though I wasn't really trying back then. :rolleyes:

The better you streamline the GN, the faster it'll go on the same power. :thumbup:

But if you want the best FE, put your usual speed at the rpm where the engine feels most happy, or where its BSFC is if you can find a chart for it.
It'll still go fast for a GN250.

Taller wheels will very likely bring more spinning mass, which is counter productive to reducing fuel consumption.
If it's the overall gearing you want to extend, I'd go for an even smaller sprocket on the rear.
For tyre choice, 17" wheels would open up a lot of options, but the usual crossbrace on the swingarm might get in the way.

beatr911 05-10-2012 10:48 AM

I like the idea of a longer stroke to increase output as it provides more torque. Improving the aerodynamics to get your speed is the surest way to reducing fuel use at lower speeds though.

Got any pictures of your tail?

renault_megane_dci 05-10-2012 11:01 AM

Actually, the pictures of my tail I got from work where people made fun of my bike !
But for some reason, I can't get them to load on the server I am usually using.
You'll have to wait tomorrow ...

The toughest part to source so far were clip-ons as the fork is so tiny (33 mm).
There is always the option to butcher an upper clamp but then again, none came up on our french equivalent of craig list : Le Bon Coin.

Euromodder :
A 17" rear wheel I have already sourced but it doesnt have a rear sprocket damper and I will have to convert to rear disc.
Smaller sprocket I didn't found over the counter, I need to check how much could be a custom one or what I can adapt.

The bigger wheel comes with a longer swing arm wich I fancy fitting fitting since the bike is not too stable at speed ...
Also, it looks way hotter :thumbup:

I have a question for you guys :
Wich shall I favour :
more horizontal windshield with pretty bad air tightness with the headlamp
or more horizontal windshield more air tight

So far I have been riding with the more angled one but converted it to the more vertical one yesterday.

renault_megane_dci 05-10-2012 01:37 PM

Yeah ! I just noticed I am 6th in the motorcycle top 10 !

jkv357 05-10-2012 02:05 PM


Originally Posted by renault_megane_dci (Post 306391)
I have a question for you guys :
Wich shall I favour :
more horizontal windshield with pretty bad air tightness with the headlamp
or more horizontal windshield more air tight

I'd say you would be better-off having it laid down as much as possible to smoothly deflect the air over your head and not worry about the amount of air that will enter above the headlight. If you feel it's significant, and adding drag, just make some type of filler to block it. Or you could drill some new mounting holes so it could drop down tighter to the headlight in the new position.


renault_megane_dci 05-10-2012 02:23 PM

Actually, the issue with the windshield is it fouls the speedo if air tight with the headlamp and properly laid down.
To have the best of both world, I would need the headlamp further front.

I might as well wait for the dustbin EVO ...

beatr911 05-11-2012 11:09 AM

If you are unsure which is better, please test three tanks of fuel with each set-up. Oh, and don't change anything else during those 6 tanks. Good testing takes time and conditions that isolate the change you are testing.

My guess, and that is all it is, is that the flatter the windshield the better for fuel efficiency. Taking it off MAY be even better.

You could also install flat "drag" handlebars. These also give you the possibility to cut them narrower than your existing bars further reducing drag. Alternately, install your handlebars upside down so that the rise of the bar results in a drop. You will need to install them backwards as well so that the sweep is backward. If you don't run into clearance issues, it will put your hands in about the same position as clip-ons.

Also consider cutting the seat foam down to as low as you can go. This reduces your frontal area a little.

Make only one change at a time and test 2 or 3 tanks to see if it makes a difference. If it doesn't, you now have the option of reverting back to stock with no penalty. Be careful and patient, this is not a sprint.

euromodder 05-11-2012 12:58 PM


Originally Posted by renault_megane_dci (Post 306391)
Smaller sprocket I didn't found over the counter, I need to check how much could be a custom one or what I can adapt.

What size of sprocket do you have on it now ?
Try the UK for weird things if you can't find it locally ;)


Wich shall I favour :
more horizontal windshield with pretty bad air tightness with the headlamp
or more horizontal windshield more air tight
Try to line it up with your shoulders then see if the buffeting can be improved by putting it more vertically or horizontally.

A gap between the screen and the lamp isn't bad, it helps filling in the "vacuum" behind the screen, resulting in less buffeting for the rider.

Screens are very much trial and error / hit and miss.
Even BMW missed the mark big time with their small clear screen on the R1200R. The buffeting was debilitating.

low&slow 05-11-2012 10:11 PM

Good work on the project! I concur about lowering the angle of your windscreen and reduciing your seat height. To reach 100mph you need more power or to reduce aerodynamic drag. As hypermilers, we could recommend the drag reduction route. The dustbin fairing for the front is a good idea but don't overlook the value of a long " Vetter" Style tail. Alan Smith's , Craig Vetter's and mine have proven how effective the long,tall tails are in reducing drag and improving high speed cruising. Keeping your bike from overheating could be a problem if you do a lot of streamlining on your bike.
Good luck and best wishes. L&S

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