Motorcycle aerodynamics, a slippery and windy, slope?

[PeterS]

Originally Posted by redyaris

Please provide a referance for the Cd of 1.2 for the standard motorcycle?

It was the figure given to me by an engineer of my acquaintance but I note your reference on your other thread .I have found it very difficult up to now to get any substantive information on this subject.

''Aerodynamics of road vehicles edited by Hucho the chapter on motorcycles.''
Based on that could you estimate the cd of the old fairing in the photo above ?

and thanks for any help ,seriously !

[redyaris]

The overwelming problem is the size of the windscreen and its shape. The lower part of the bike is ok. The side view is not enough even to guess at the Cd. I would guess that the overall bike is very poor aerodynamicaly.

[PeterS]

It's certainly not ideal from an aerodynamic perspective ....I agree! I would definitely lower angle and height of the screen .


What number do you apply to a standard bike re ''Aerodynamics of road vehicles'' ?

My problem is that my state registration department will not allow any modifications to the basic motor cycle or seating position .The wonderful ''Allert'' would not be registerable here .I need something that can be bolted to a standard bike .

[redyaris]

If you click on Gray you can see what i have done, the other place to look is craigvetter.com. an other referance is an online book called without the hot air or something like that.

[PeterS]

Thank you , an excellent resource !David MacKay FRS: Sustainable Energy - without the hot air: Contents

Gray is an interesting project ,how do you find it in cross winds with the extra bodywork on the rear ? I can see something similar being very useful on the Dreamliner modification .

Any difficulties getting on and off ?

[PeterS]

Originally Posted by dcb

I'm really not sure if lower CG helps or not here. Certainly lower center of pressure helps, and having the center of pressure below the center of gravity means the side winds will have to work harder to tilt the bike.

Sort of like the c-clamp on a yardstick trick.

I'm reading this thread , there are gems everywhere .This above was particularly helpful ! I have to admit that after I saw Allert for the first time I immediately drew an Enfield sized version .... the rear is designed to slide back as Allert's slides forward .The bike plus rider would weigh around 230 kg / 500 pound .





I can see from dcb's comment above that the higher centre of pressure and the rider's weight higher too would very likely cause instability .

[dcb]

thx Pete Here is a diagram I slapped together looking at a tall streamlined bike as a "wing" of sorts from the top, don't know if it is entirely accurate and haven't figured out if the wheels make a significant gyroscopic stabilizing contribution at higher speeds or not either, so take w/salt.



re: diagram above, I sort of think it is not accurate in this case as you approach 90 degrees. If you had just a sidewind pushing on the area, you would expect some force to be created, dunno.

[Frank Lee]

I've heard about this book on motorcycle dynamics, although I haven't been able to go through it yet myself: Amazon.com: Motorcycle Design and Technology Handbook (Motorbooks Workshop) (9780760319901): Gaetano Cocco: Books

It might be fruitful to look at the reviews and the "other things people bought" too.

[PeterS]

dcb, good info but the problem is more the unexpected burst of wind from the side at any angle.I can't work out how a wind from say 15 degrees off the front would effect things but if it was a constant it would probably be OK .


There seems some small advantage in the higher ''fuselage " ,a slower roll period.The shorter the structure ,the faster it will lean but how this relates to the distribution of mass and the balance of the areas ....I have no idea !

A couple of bicycle related links which discuss gyroscopic effects and side loads .Not perfect for this discussion as bikes are pretty porous from side wind perspective .
http://en.wikipedia.org/wiki/Bicycle...xternal_forces
http://www.sheldonbrown.com/brandt/wind.html

and a review of the Hucho book
http://www.recumbents.com/wisil/demma/aero_review.htm

I'll keep reading through this excellent forum and try to learn something !

I really wish I could put a question to Allert's builder ...I'm sure he could put me right in a second !



Edited to add .In the original 1957 article on the Dreamliner there was mention that a prototype fairing that was 'slabsided ' had been rejected as bad in cross winds ,the final Dreamliner was the same as an unfaired bike , the reason given by the designer was the indented sides .The flat sides of the original had been much worse .

[PeterS]

Originally Posted by Frank Lee

I've heard about this book on motorcycle dynamics, although I haven't been able to go through it yet myself: Amazon.com: Motorcycle Design and Technology Handbook (Motorbooks Workshop) (9780760319901): Gaetano Cocco: Books

It might be fruitful to look at the reviews and the "other things people bought" too.

Thanks Frank , I looked at the reviews but there's no mention of the silly kind of stuff I have in mind !