Life's a drag

[blew]

Surely top speed will depend on horsepower,drag/friction and area? Two objects with identical power,drag,weight etc but different in frontal area ought to have different top speeds???

[guest564]

The overall drag is a combination of the frontal area and the drag coefficient (cd). The main problem with a faired motorbike and rider is that, its not a smooth shape along the sides, and the air at the rear is very turbulent which gives a poor drag coefficient. An unfaired bike is very poor.
Incidently the Powerbronze fairing on my XBR gave around an 8MPH increase in top speed with the right gearing.

[Propellor]

Thanks for input chaps.

It seems, from using stats online as proof, that a bike, in spite of its poor drag co efficient, still trumps the car in the overall stakes, because what it looses in poor drag coefficient it seems to more than make up for by having a very slim frontal area. That fact should be heralded. (Assuming you agree!).

The influence of weight is very much under discussion.

The stats I looked at were based on power consumed to achieve max speed, I looked at vehicles who's top speed was around 130mph. Now, if we drop to the national speed limit, logically the motorbike's case should strengthen further, because at that speed the emphasis swings a lot more towards rolling losses and less towards aero losses. Does it not? Theoretically, the motorbike should really trounce a car? Is that the case?

[Oldtimer]

Streamlining a vehicle does not have much effect below 70 MPH

[SteveC#222]

Streamlining a vehicle does not have much effect below 70 MPH

Unless it's a bicycle. This one hit 83mph.  Reduced frontal area and very thin very hard tyres to decrease rolling resistance.

[Propellor]

Streamlining a vehicle does not have much effect below 70 MPH

Unless it's a bicycle. This one hit 83mph.  Reduced frontal area and very thin very hard tyres to decrease rolling resistance.

Chuff me! That's incredible.

Don't suppose you know if they recorded power input? Or even gearing etc?

[SteveC#222]

http://www.popsci.com/diy/article/2005-10/worlds-fastest-bike

Gearing is going to be pretty huge - probably well over 100" - difficult to power directly so the system uses an intermidiate drive gear between the chainwheel and the final drive sprocket powering the rear wheel.  Guy Martin used a similar system on a ( more or less) conventional bike when he set the bicycle speed record.



There is a formula here for working out power needed to propel a hard shelled recumbent - I'm not even going to try to work it out as it makes my head hurt but you need over 250watt to hit around 50mph.

http://www.recumbents.com/wisil/demma/downhill_physics.htm

You'll probably also need legs like these!

[Propellor]

.........

There is a formula here for working out power needed to propel a hard shelled recumbent - I'm not even going to try to work it out as it makes my head hurt but you need over 250watt to hit around 50mph.

.......

Thanks Steve. Not looked yet, but at 250 watts it's making my theories on weight look a bit shaky. Gulp.

Ps. If you could crop the second photo I'd be obliged. 

[Propellor]

Trawling for some back up evidence.

Not digested this yet as I'm busy filing lumps of aluminium, but quick read seems to back my case?

http://lofi.forum.physorg.com/Weight-And-Top-Speed_14909.html

[Moto63]

What I want to know is how did you manage to get hold of the photo of my legs, that was supposed to have been destroyed along with my first marriage certificate.

[Propellor]

What I want to know is how did you manage to get hold of the photo of my legs, that was supposed to have been destroyed along with my first marriage certificate.

Yes, your technique of kickstarting a high comp xbr with both legs really is something to behold.

[timbo]

Fantastic thread guys. Really interesting stuff, which is why I love this forum. Only wish I had the knowledge to make further comment. Now if I could only get my brother in law on here. He's been an aerdynamacist for F1 for over twenty years in Arrows/Jaguar/Red Bull 

[Propellor]

Fantastic thread guys. Really interesting stuff, which is why I love this forum. Only wish I had the knowledge to make further comment. Now if I could only get my brother in law on here. He's been an aerdynamacist for F1 for over twenty years in Arrows/Jaguar/Red Bull 

That would be something.

[timbo]

I will give it a try. You never know till you ask 

[Propellor]

Looks like this discussion has run its course, for the moment anyway.

Can I just add that in spite of me pushing the case for weight not being a factor at theoretical top speed (assuming that to be true), I believe that in the real world we will only ever be accelerating or decelerating.

Imagine you kept the bike on full throttle from the moment you left the drive. The load on the bike from wind and gradients would constantly alter. Going around a corner would add a different force vector as well as changing the gearing slightly. So the times that we would be in true equilibrium or steady state would be very short indeed.

So in that case weight constantly comes into the equation!