250 GN, the 100 mph project

[renault_megane_dci]

This morning, thrashing the bike along a curved accelerating lane (that's what they're for right : accelerating ) 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.

[renault_megane_dci]

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 ?

[Grant-53]

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?

[renault_megane_dci]

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)

[Grant-53]

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?

[renault_megane_dci]

Originally Posted by Grant-53

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.

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 !

Originally Posted by Grant-53

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.

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.

Originally Posted by Grant-53

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.

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)

Originally Posted by Grant-53

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

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 ?)

[Grant-53]

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.

[renault_megane_dci]

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.

[Grant-53]

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.

[renault_megane_dci]

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