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Boonie wrote:Want...to...write...paragraph...on...drag...coefficients...and...airflow...over...surfaces...

Fluid...dynamics...head...going...to...explode...

Let me, basically drag is caused by an object pushing molecules aside in order to move forward, and is usually known as your Cd number or Reynolds number (he came up with the drag equation), it is actually calculated for objects moving through fluid not air. If an object has a Cd of 1 it means that the molecules hitting the front of the object would come to a complete stop creating an area of negative, stagnent pressure behind the object, this object would basically be a flat surface and require a lot of power to move forward. This is why you don't get bikes with flat faces pointing forward.

With the shiny, matt thing, a matt surface has texture to it and everytime a molecule tries to move over this surface it will have to keep moving over the texture, shiny things however have less texture and so create less drag.

I hope this makes sense cos it's quite hard to explain with out all the equations but I can't remeber them off the top of my head.

Caz wrote:having done my best to try and forget all my fluid dynamics lets see what i can remember

think of a golf ball, they have dimples in to induce spin and lift and therefore to make the ball go further. However lift / spin are a different matter to drag. a highly polished surface will have less wind resistance. but if you were to hit two similar golf balls through the air - one highly polished and smooth, the other with the now conventional dimples - the one with dimples in would induce spin and therefore travel further.

I'll dig out my fluid dynamics books when i get home if I must

Right when I get home I'm going to dimple my front fairing and see if I can make my bike fly!!

Just thinking about this... isn't the idea to get behind the fairing and get your helmet out of the flow of air?

Caz wrote:I'll dig out my fluid dynamics books when i get home if I must

please dont my brain hurt enouth when im at work!

Boonie wrote:Just thinking about this... isn't the idea to get behind the fairing and get your helmet out of the flow of air?

Yer but even with the double bubble on my ZXR and my chin bar on the tank the top/back of my helmet still catches a bit of drag, also when cornering I'm leaning out of one side of the fairing.

Also I feel I should probably point out that I'm not really worried about the drag of my helmet (the fact I'm 6'3" means I'm nt exactly aerodynamic on a 400 anyway), I just like shiny things and I have phones Aria Europe and they said as long as there is no petoleum based ingredients in the polish it will be fine.

Caz wrote:having done my best to try and forget all my fluid dynamics lets see what i can remember

think of a golf ball, they have dimples in to induce spin and lift and therefore to make the ball go further. However lift / spin are a different matter to drag. a highly polished surface will have less wind resistance. but if you were to hit two similar golf balls through the air - one highly polished and smooth, the other with the now conventional dimples - the one with dimples in would induce spin and therefore travel further.

I'll dig out my fluid dynamics books when i get home if I must

disagree.

smooth surface will tend to encourage laminar flow. rough (or dimpled) surface will tend to encourage turbulent flow. Turbulent flow will tend to stay attached to the surface of the ball or helmet better, hence the wake will be smaller, hence less drag.

but outside motogp it probably makes sod all difference, so if you want a nice shiny helmet then go for it!

Ill sum this up i dont think you would even feel the difference TBH would you.

superman wrote:Ill sum this up i dont think you would even feel the difference TBH would you.

Of course you would you can always feel when something has just been polished it has that lovely slidy feeling and usually smells of vanilla or sweets.

LOL ill stay clear of this thread from now on

Jesus this is getting like work now! drag, lift coefficents friction. but to clear things up form what ive been tought is that polished surfaces produce less drag. Reason why certain Aero Engine parts are polished.