Discussion By god they can go ....

I was out today with my wife Ann in her BMW Z3 sports car driving through the peak district in Staffordshire - Derbyshire This is a very hilly area When we were overtaken by a learner on his whatever 125 motorcycle ! Well bugger me these can really go up hill he was doing 50 plus and down hill 60 plus
What are these new bikes doing my 250 ,s and 350 can only keep up at these speeds Come on guys what am i missing out on
Trev

 

Not sure what bikes you have. An old Honda cbr250rr or any of the 250 4 cylinder sports bikes of that period are quite spritely and enjoyable to ride. If you are just interested in outright performance from a 250 then the 2 stroke pocket rockets are the thing, but they require a bit more maintenance.

 

My bikes are listed as I can see so are yours
This little thing was nothing more than a 125 single but by gum he was giving it some stick !
I really enjoyed watching him go I could tell he was a learner though as he seemed to be ballet dancing on the brake pedal That is according to his brake light
I really admire any one who gives it his all possibly getting a 100 miles to the gallon as well
Trev

 

My wife's CB125T has 17bhp and gets 100mpg! Tops out just over 110kph!
Such a fun bike to ride!

 

I have always loved the concept of getting a lot out of a little I have even fantasised of getting 100 mph out of a 50 cc machine
Fuel injection turbo and a slippy shape
I cannot say how much I enjoyed watching the guy getting the max out of the minimum
Trev

 

Yes. There is real satisfaction out of getting the most out of little bikes. Anyone can go hard on a 1000.

 

there must be some kind of formula for the speed of yer bike against ist horsepower rating
taking things like drag etc into account
any one have any tables
Trev

 

Only thing which comes close to a formula I can think of is when Barry Sheene was commentating the 500 GP races.
He'd always be lauding the speed and power of the Hondas and say that at those speeds each extra horsepower was one extra kilometre per hour by the end of the main straight

 

You're disagreeing with Barry Sheene...always loved his comment - "like a rat up a drain pipe"

 

While wind-resistance (aero-drag) goes up to square of speed, power-required to overcome that resistance goes up by cube-function of speed. So it requires eight times power to just double speed.

For example, it requires about 9bhp go push bike through air at 100kph.
To make it go 200kph, requires about 70-75bhp.

 

He's not necessarily disagreeing with Barry, who's not talking about ultimate top-speed given unlimited runway, but acceleration to end of straight. So effects of extra power given extra speed at end of straight isn't as great as would be realised with more road.

 

Hey, thanks!!! That list of bike cross-sections are very handy! Can plug into this equation to determine how much power is needed to reach any particular speed on any particular bike:

TotalPower = aWS + (rho/2)CdCaS^3 where

a = coefficient of rolling-resistance of tyres, and drivetrain (bearings & gears)
W = weight of bike+rider in Newtons
S = speed in meters/second

rho = air-density (temperature and elevation dependent, so you'll have to expand on this)
Cd = coefficient of drag, depends upon smoothness of body-shapes
Ca = frontal-area of bike+rider in square meters, mirrors add surprising amount of area

Notice that the 1st term (rolling-resistance) goes up linearly with speed while the 2nd term, power required to overcome wind-drag goes up to the cube-power of speed. When you go from 100 to 300kph, rolling resistance goes up 3x while power-required to overcome wind-resistance goes up by 3*3*3=27x !!! This is why you want to focus on aero-mods to improve top-speed. Easier than increasing gobs of power!

 

On a push bike you provide all the power. Recumbents are faster because they have less frontal area.
Around my local area I can pedal my recumbent trike up to 60kph.
Once I fitted a small nose cone that increased to 80kph.

I took a Trisled racing trike for a ride around the bike race track.
It is fully enclosed with only a few cm of the bottom of the tyres exposed.
The track has quite an uphill slope around the back of the circuit.
When I got into it my heart sank when I saw that the only front sprocket had 80 teeth.
I thought there was no way I would be able to get it up the hill.
But when I tried it we just sailed up it really fast.
The hill was nothing compared to wind resistance.

 

I can think of one way to get more power on a bike, but I am uncertain as to the conversion between horsepower and sheeppower or if it's going to go any faster with that wind resistance

 

Now.... if all those sheep were to fart at same time....
... and we ignited it.... hmmmm....

 

I think that has likely never happened, ever!

They are worse than trying to play a game of Jenga whilst juggling three pins whilst balancing a plate on a stick on your nose with a hula hoop going on one leg whilst hopping over a skipping rope