GN250 top speed

[ducatilover]

Sorry for off topic.

Whats your spada go like? I'm looking for a bike at the moment.

Goes good for a 250 twin. Sounds like a beast too.

[Indiana_Jones]

Top speed of a GN250?

How long is a piece of string?

-Indy

[ducatilover]

Top speed of a GN250?

How long is a piece of string?

-Indy

Double half it's length

[xwhatsit]

I can never quite work out bike top speeds. Take two bikes with almost identical weight and horsepower and the speed it can attain often varies wildly. Anyway, does weight matter in top speed? I don't think weight figures into the equation at all, it's only relevant in acceleration?

I figure it must be something to do with aerodynamics. Despite what they look like and what intuition might suggest, motorcycles are actually terribly unaerodynamic. Most bikes get nowhere near the sort of drag coefficients your average Jap sedan has. Naked bikes are particularly rubbish of course, and that might have something to do with it; perhaps a GN with its tall forks has a lot of swirling and confused air around in front of the engine. Who knows.

I mean look at the original Speed Twin. 27bhp; my RS when new made 26bhp. Nonetheless the Speed Twin comes close to cracking 100mph (not quite there though). RS never got close to that. Of course different ways of measuring power figures come into play but it's roughly correct.

[ducatilover]

I figure it must be something to do with aerodynamics.

The nail just got hit there mate. The main disadvantage of a motorcycle is the c.d, your typical garden variety porsche 911 has a c.d of around .29, versus a hayabusbusbus which for a botormike is rather slippery at around .7 from memory.

[Dare]

The nail just got hit there mate. The main disadvantage of a motorcycle is the c.d, your typical garden variety porsche 911 has a c.d of around .29, versus a hayabusbusbus which for a botormike is rather slippery at around .7 from memory.

I imagine the least aerodynamic part of a motorbike is usually the guy sitting on it..
Wonder if more aerodynamic gear would help or just make riding harder?

[xwhatsit]

I imagine the least aerodynamic part of a motorbike is usually the guy sitting on it..
Wonder if more aerodynamic gear would help or just make riding harder?

There's definitely a difference on my gutless piece of shit between wearing loose flappy PVC wets and an open-face compared with zip-together leathers and a full-face -- maybe 10kph+ at the top end.

[Karl08]

130 -wind assisted

[cheesemethod]

I managed over 135 going down a hill on mine. This was on a dry day with no wind on a nice clean piece of asphelt - and I still wouldn't recommend trying this. I really don't think the frame and suspention are designed for high speed, bike feels a little numb past 120.

I find 110km/h on the flat is no problem, but as soon as you add wind or a slight incline you are at full throttle watching the speedo drop. Fun little bikes, but not good on the ego when you're on long straights. Overtaking on one forms good habits though - you have to think ahead, get in the right gear at the right speed before the overtaking lane starts and you can usually squeeze past most slowpokes.

[ready4whatever]

140km p/h tops

[cheesemethod]

140km p/h tops

Sounds about right, IIRC mine hits the bottom of the redline 130ish, so 140 would be abusing it but still achievable.

I assume GNs don't have rev limiters?

[ducatilover]

Sounds about right, IIRC mine hits the bottom of the redline 130ish, so 140 would be abusing it but still achievable.

I assume GNs don't have rev limiters?

140? Down a hill, flat on the tank, tail wind, slip stream. It's cheating. They are basically useless

[cheesemethod]

I'd love to have the time and money to blueprint and balance a GN motor and add an exhaust. Would be interesting to see if getting rid of the flaws of mass production would make much of a difference.

[ducatilover]

I'd love to have the time and money to blueprint and balance a GN motor and add an exhaust. Would be interesting to see if getting rid of the flaws of mass production would make much of a difference.

It's still air cooled though, less ability to expel heat from combustion obviously, which means for reliability [not like our race car] you have to run less aggressive cam profiles [acceleration/deceleration faces, not duration] and have smaller valves. That's the flaws with air cooling, you are relying on airflow, the oil and air fuel mixture entering the combustion chamber to cool it.
I reckon you could get a reliable 30hp from one though.
The old briggs & Stratton power junior dragster made 40hp, very peaky and running on methanol. Wouldn't use it as a engine for any other purpose

By all means, win lotto and do a gn up!

[cheesemethod]

I don't mean really trying to hot it up that much, but I'm sure there's a lot of work you could do to smooth out the ports and chamber (if the outside of the motor is anything to go off I can't imagine what the inside of the ports look like). Maybe a little skim of the head, heat wrap the pipes to keep the gases hot and moving. The primaries are a nice bend but where they join isn't particually flash.