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 :
http://www.turbominis.co.uk/uploads/...ntage-2754.JPG
'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.

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.

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?

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:
http://www.turbominis.co.uk/uploads/...ntage-2755.JPG

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.

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

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.


Jay

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 ...

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.

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

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.

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

The trick with an air cooled engine is getting air flow smoothly in and out of the fairing. A NACA inlet on either side of the dustbin ahead of the engine would work. The ideal exit point is near the rider's leg. If possible find a low pressure area behind the legs as the rear half of the fairing narrows. With the dustbin front you are looking at a long tail section to maintain aero stability. Consider an electric fan in front of the engine fins and a temperature gauge of some sort.

Here is a pic of the carboard Kammback :

http://www.turbominis.co.uk/uploads/...ntage-2762.JPG

http://www.turbominis.co.uk/uploads/...ntage-2763.JPG

the start is a little smaller than my back and little lower than my shoulders then it goes to the end along a 15° angle

Funny story : the pics were taken by an unknown colleague making fun of it and forwarded to me by one of his colleagues ...

It was the first evolution.
Since then I also put an angle on the horizontal line.

That would be a little over two month with the associated change in temperature from spring to summer !
So not really valuable on a testing POV.

The thing is I try to ride as less as possible.
After all, it is not only about MPG but also GPY : gallon per year

IT WORKS !

Today's high speed attempt did show +200 rpm and relevant speed increase on the clock !

The context :
since fitting the 16*39 sprockets (up from 15*42 IIRC) the bike was not able to hit its peak power in 5th @ 8500 rpm.

Friday I got 8000 rpm (with the raised wind shield and tucking)

Today with the cardboard kammback and not tucked to make it work properly, I got 8200 rpm on the same slightly downhill.

The start of the hi speed attempt is consistent since it is a speed camera ...

Maybe the air was hotter / more dry allowing for a better mixture (I run pretty lean being on E10) and the wind was more still (I don't recall).

Now the bike clearly shows its limits in terms of hi speed handling being very short and light but I can go round that (see my pic of the 17" rear wheel on longer swingarm)

Anyway, I am very pleased !

coast down test day
 

If weather allows, tomorrow is coast down test day to convince a colleague of mine that is rather skeptical.

I'll keep you posted !

coast down test
 

Today was coast down test day.

We rode to the "quiet back road" my colleague mentionned.
It was rather windy ...
So I went for a ride, trying to pass his parked car at 100 Km per hour, pulling the clutch and then cheking the speed at a distance, namely a sign on the side.

With the kammback : two attempt showed 62 ish Km per hour

Without the kammback, the bike was easier to maintain at speed (probably more throttle, maybe less wind) and I got a consistant 60 ish Km per hour.

Then I fitted the kammback back but we didn't do more attempts.

All in all : waste of gas ...

I don't know the distance from point A to point B either just the tiny difference in speed

Can I work out something out of it ?
:confused:

The kinetic energy at 100 km/hr is 0.5*m*v*v and that is with a bike+rider of for instance 250 kg 0.5*250*27.77*27.77=96450 in both tests
This kinetic energy at 60 km/hr is 0.5*250*16.66*16.66=34722
So without the tail enengy consummed by air and roll resistance is 96450-34722=61728

Now with the tail. Energy left at 62 is 0.5-250*17.22*17.22=37076
So energy consummed in this case is 59374. (so approx. 4% less)

Yeah! I just noticed that there is a top 10 function at ecomodder :D

(Thanks for this one :D)

I have a lead on a set of clip ons !
Looking forward to it !

Rear tire being deader than dead, I will probably convert sooner than later to the 17" setup.

Anything on your mind about the conversion ?

This morning, thrashing the bike along a curved accelerating lane (that's what they're for right : accelerating :D) in 2nd or maybe 3rd, I felt a massive wobble.
It was not fast enough for the aero to be blamed so I thought I had it right about the rear tire.

Tonight I checked and it turned out it was very low on pressure so I was half right ...
But it is shot anyway.

I just can't make my mind around big fancy swap of the rear end that is gonna see my bike off the road for a while or bite the bullet and get a tire fitted wich lacks the bling factor and should prove a problem on my quest to a massive gear reduction ratio.


As of today I have a 16*39 on a 3,50 / 16" wheel and apparently I could adapt a RGV 250 37 tooth sprocket but it is still a long way to go for the 8500 rpm / 100 mph combo I am looking for.

Calculation is at work but on the top of my head, but it would require a minimum rear wheel size of 17" fitted with a 120 x 90 tire with the 37 sprocket to match the target.
18" would be even better since it would allow me to keep my 39 sprocket.

ABA testing the cardboard kammback, I just did the first B tank ... and broke into the 80 mpg !

I am happy althought in european terms it is meaningless : 2,85 liters to the 100 kms.

Anyway, it is not helping very much with validating the particular kammback I did on my bike (small one).
It is kind of my fault since the weather (cold and windy) and a pretty low oil level helped me to drive much slower (from 70 mph to 60 mph).
Add to this the fact I praticed my P&G more and more and we clearly can not draw any conclusion on the kammback.
What is more, with the kammed riding I did before, the proof was in the top speed ...


Anyway, I am now waiting for a set of clip-on handlebars that I want to fit really low.
After that I will need to source a busted seat to make a thinner one from.

No advice or idea about the rear wheel conversion ?

I am mostly a cyclist but follow motorcycles and the Craig Vetter site very closely as my late father-in-law was a lifelong racer and tourer.
The 16 in. wheel may be the way to go if you could find a rear sprocket in the 30 to 35 T range. A smaller wheel means less weight and drag than a larger diameter one and overall height is reduced for less air drag. If you are going for the racer's tuck position then consider adjusting the seat height so that your back is parallel to the ground. The seat padding is a personal matter but I like a bit of soft foam over a firmer foam base. Your weight should be balanced so as not to put too much pressure on the wrists or knees. The kamm back should fit snug to the hips and back without a gap to catch air.
What size is the front wheel? Could it be made 16 in. also?

Thanks for your interest Grant.

Being a cyclist, I think you know bigger (taller) wheels are best as far as rolling resistance goes.
I believe it has to do with the contact patch size or maybe the end of contact between rubber and concrete being less abrupt, the tire sticks less ?

Anyway, the 16" wheel has a preedominant advantage : it is already fitted to the bike.

But I am trying to turn the bike in a more speed oriented object where stability at speed is highly important. Hence the longer swingarm.
Also, the smaller the tires (we are talking rear tires here) the least the speed index (tire has less time to cool down) and again I have a speed target.

So I am between :
- having a rear tire fitted
OR
- buying a new chain and rear sprocket,
- converting the swingarm to twin shocks and
- bigger swingarm axis and then
- converting to rear brake disk (my current brake pedal is drum brake so basically useless).

A known money amount on one han and an unknown amount of time on the other hand.
But even if I just have the rear tire fitted, I am still gonna convert to longer swingarm sometime so it is basically a waste of money (and I don't like wasting).

Anyway I made my mind today since my current tire is so f##ked I can no longer ride so I will just have it changed and hopefully a higher spec tire might provide a little more stability and not disintegrate above 80 mph ... (as the current one seems to)

The quality of tires is always a major concern. There are good touring tires available that should be more than adequate. Since your engine is not putting out enormous torque nor are you kneedragging as in Moto 3, tire cooling should not be a issue. The larger diameter wheel has a longer tire patch but more air drag and mass. Vehicle weight, tire compound, and inflation pressure affect rolling resistance. The high speed HPV racers often use 20" wheels and triathletes typically use 26" wheels rather than the 700C on road racers.
Changing the rear suspension and wheelbase means retuning the the whole set up front and rear for best results. That's half the fun, the other half is when the bike responds like it's an extension of your mind.

Has the frame been checked for alignment since you bought it?

I am only half agree with you on this.

Since the good ol' days when from low to top notch hi speed missile, the motorbikes used pretty much the same sizes of tires (from 100 to 130 width), there has been a lot of categorization.
Categorization meaning the tire manufacturer can search for a more adequate mix in the different tires they're selling.
Cheap rubber for small bike tire sizes (like mine)
Twin compound rubber for faster bike.
Etcatera, etcaetera.

So for a rear 16" 120 width tire, tires are meant to be cheap, for a light and slow bike, hence low speed rating, and old school threads.

So my bike does not put out a lot of torque (but being a single it comes in early and abruptely) and I don't ride like a Moto GP rider but still, I want it to be capable at speed and in the twisties as I use an epic road on my way to work.

And since I happen to use it on the upper hand of its speed potential, I have seen my rear tire fade away in say half a thousands miles !

I don't know what to do with these information.
One thing I know is I can source down to 37 rear wheel sprocket but nothing smaller easily.
The other thing I know is 16" rear tires in the width I need have a small speed index and with my previous experience, I have been led to believe this could be an issue on my quest to 100 mph.

I am not properly minded for bike set up, I don't understand handling and my guts are telling me when to back off (so far way before the bikes limits) and given the DNA of the GN, I don't think it is smart to set it high goals (other than a higher top speed, obviously)

No.
That being said, the tire issue might be me not aligning the rear wheel properly (in the same slot but that might be not accurate enough ?) :eek:

Well I've been doing some homework. First I reviewed "High Tech Cycling" 2nd Ed. by Edmund Burke about rolling resistance. The advantage of a larger diameter tire is in the reduced amount of deformation that occurs rather than tire patch size.
Second I can suggest "Sport Rider" Tech Editor, Andrew Trevitt's book on chassis set up and tuning. See Amazon.com and the price is about $25 USD. Charly Perethian modified a 1989 Honda NX250 6 sp. for the 2011 Mid Ohio Vetter Challenge by switching the 19" front wheel to a 16" set up, changed the sprockets to 15/33, and added a slick front fairing to get 157 mpg. The power band is similar to the GN250. See the description on craigvetter.com.
Third, I looked for tires rated S or H speed that would fit the GN250. Check the Kenda K671 front and K657 rear for price and availability there. There are some sites that riders can rate tires for wear and traction. The OEM tire for the older Ninja 250s (16" rear) and others was a Dunlop. From the Maxxis website a 130/90 touring tire has a diameter of 25.1 in. Knowing the tire diameter, 5th gear ratio, and sprocket teeth you should be able to calculate the rear wheel speed for a given rpm. The only info on the transmission ratios for the GN250 I found was for a version sold in Portugal that has 5th listed as an 0.818 overdrive. I hope this gives you someting to go on.

Thank you Grant.

Since this, I had to change the tire anyway and I got a proper deal on a Pirelli City Demon in 120 90 R16 wich is the quoted size for Germany.

It is a much taller tire so now the bike pulls a very high ratio (from 15x41 to 16x39 and the big tire on top)

I, now, have to work on aerodynamics since it is now very slow in 5th.

Clip-ons bars and tiny indicator I have, no it's just about fitting these.

Then I have to think about a new healamp bracketfitted to the frame rather than fork.

The Perethian fairing is described as a extented race fairing for a H-D 750.
My bicycle fairings are similar low&slow's coroplast design and mounted to the frame.

Did some digging for the fan club :
2011 Mid Ohio Craig Vetter Fuel Economy Challenge results

Due to the taller new rear tire, the top speed is now down from 125 km/h to 120 km/h without any aero mods.

I am now working on the much smaller indicators. Needs to cut some brackets.
End weight of the indicator is gonna be a third of the original equipement ...

comments on my last fill
 

Last week I found the bike dropped on the side.
Luckily only issue is a broken mirror and an unknown amount of gas spilled !

Also around half tank, I changed the rear tire for a taller one (120 90 R16).

Only trouble is I didn't foresee the change would be this big (it is standard size for Germany) so I didn't make any notes of the size of the older tire (3.70 x 16, no idea what the wall size is).

I seem to remember I dropped 200 rpm @ 90 km/h

The bike has been on hold for 2 weeks due to funky engine noises when hot and erratic power drops.
I am guessing either :
- head gasket (there is an oil leak) + chain tensionner guide
- shot clutch (was reportedly worn but stopped slipping as soon as I changed the oil) and crap in carb and tank

My only issue is I have no room to open up the engine.

Anyway, there is opportunities even in drawbacks so the plan is to ugrade the engine as far as FE is concerned

I am thinking :
polished combustion chamber
polished exhaust ports
higher CR (from 8,9 to 10 maybe)
flowed carb (if needed)

Keep in mind the GN is air cooled with no oil cooler.

Any advice on the spec ?

The rage at the Honda marketing department seems to be friction reduction. This is likely a contributor to the reduced fuel useage seen of late. If you are increasing compression with an aftermarket piston, ask about a low friction coating. If you are milling the head, consider that it will retard cam timing making the engine run a little stronger at higher RPM, at the expense of lower RPM power.

I understand that friction reduction coatings can be used on other parts of the engine as well. Make experience with friction reducers a consideration when choosing engine builder.

The compression ratio chosen depends on the grade of fuel you plan to use. The 200cc Chinese derivative of this same motor that I had probably wouldn't like higher than 9.0:1 on our regular gasoline. I'm guessing your fuel is a higher grade. Consult the TTR 230 forums as this engine family is widespread and long lived in Yamahas production runs. Again your engine builder should be able to answer this question.

As for polishing, it can help with pre-detonation if there are some burrs or protrusions in the combustion chamber, as you know. Also, I've had success in smoothing any sharp transitions in the intake, like around valve seats.

A co-worker of mine has been doing the motoman.com high velocity intake service for racing quads for several years now with success. He says that the dyno has shown a good bump in bottom end and mid-range with no loss, or minimal loss of top end. I didn't get it done on my 200 before selling but do know that they have very large ports that could very well benefit from it. No idea how it affects fuel mileage though.

CR : I was thinking about having the head milled (10,5 is 1,5 mm if my calculations are correct)

So far I hope to just DIY a basic overhaul with the only outsourced operation being the head milling (I'll first investigate a thinner base gasket)

Polishing is also about running cooler, heat being reflected more efficiently through less surface exposed to the flame front.
The engine does not have an oil cooler so it is a predominant factor.

Friction work seems interesting for the bottom end wich I hope I won't need to work on.

Internal heat managment may also be partly managed with ceramic coatings. Porsche started using this with the old air cooled 964 motors in the early '80s. They only lined the exhaust port with it.

Sounds like you've already done some research, but here is a link to an article that hits the highlights in case others are interested.

A Look at High-Tech Engine Coatings and What They are Worth - Hot Rod Magazine From the results of the naturally aspirated engine it appears there is no difference in power production. The change is so little to be of significance. They didn't state temperature differences though.

So I guess I don't know what I'm talking about. Does anyone have some details on how Honda (or others) build motors to get maximum efficiency that might be used here?

Basically, it is free and can do a little good so I am going for it (polishing, not thermal barrier)

Bike has been stored in the basement,
Head has been removed,
Cam chain tensioner guide has been checked, it's likely not his fault.
I'll renew the cam chain as it is opened and a good time to do so (bike has 40K miles on the speedo), also the cam tensionner is far out.

So the head has been cleaned, the exhaust ports (one for each valve) have a funky shape and they are very much shrouded on the begining.
I did some porting on one of the ports and I will crudely check its flow and comparing it to standard clean port shape flow.

Also I did some calculations and the end pipe diameter is much smaller than the equivalent surface of the exhaust valves minus the stems.
Spells back pressure to me.
Might as well hurt 3/4 throttle P&G

Inlets are 26 mm times 2, equivalent single diameter is 36mm, carb is 34mm
Exhaust are 22 mm, pipes are 22 mm inside, tail pipe is a SINGLE 18 MMM !!!

Valves were very whittish in color indicating a rather poor mixture (I used to run on E10 and the bike was probably set up weak for regular gas to start with).
Also there are some distinct knock scars in the quench area between the exhaust valves meaning the engine was runned too hot.

I have some ideas for improving engine cooling in this area (drilling holes in the fins as close to the quench area as possible).

I'll add some pics tomorrow.

(money is on the clutch now for the funky noises)

Here is my 20 minutes homemade flowbench :
http://ecomodder.com/forum/member-re...-flowbench.jpg

The idea is to time the exit of the water through one of the exhaust valves (they have their dedicated port) and compare the result between the modded port and the stock port.
So far I have to improve the leaks between the "cylinder" and the head and to make sure the gasket (destinated to seal between a WC and its tank !) has no adverse influence on flow or at least evenly.

To make sure the result is mesurable, I chose to lift the valve only a small amount (I assume half the lift)

Nobody seems to be interested but the modded exhaust port spits out its liter in an average if 4 seconds when the unmodded and not de coked standard port is closer to 5 seconds.

It is encouraging and the modding is not fully done too, the port are very much shrouded in the first bend in spite of the valve stem already blocking off the area.