Motorcycle aerodynamics, a slippery and windy, slope?

[PeterS]

Originally Posted by redyaris

One other point about motorcycles aerodynamics and cross winds is the mass of the bike. If you have two bikes with the same side aera to work on and one of the bikes is twice as massive, then the more massive bike will be less affected by the cross wind F=ma. That is why; as you reduce the mass of the bike getting the aerodynamics right become ever more critical as well as the location of the center of gravity and the center of pressure... Take a look at the side area of one of the big touring bikes like the
goldwing and then ask the owner what its like in a cross wind? I ride with a friend who has a gold wing and it leans much less than my GS500 or my KLR650 both of which have less mass and less side area.

I plugged some numbers into this calculator Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com ,doubling the weight of the machine but keeping everything else the same .Remarkable !

121 mpg US at 50 mph at 330 pound .
114 mpg US at 50mph at 660 pound .

Not that I would build something at 660pound but there seems little disadvantage to the extra weight and it could aid cross wind stability if placed low ?

[Frank Lee]

Allert didn't take special measures to reduce weight and he said it was to aid stability.

[PeterS]

Frank ,is there anything on the forum about the effects of distribution of weight on stability in fully faired bikes ?

I'd look but wouldn't know how to phrase the search .

[Frank Lee]

Probably just me saying the center of pressure needs to be behind the center of gravity.

[PeterS]

Originally Posted by Frank Lee

Probably just me saying the center of pressure needs to be behind the center of gravity.

OK ,so that means the centre of the areas of the side of the fairing behind the centre of the mass ,that is pretty much like a sailboat .It tends to turn you up into the cross wind rather than down wind .On a boat that should be about 10% of the overall length .

Have I got that right ?

[euromodder]

Originally Posted by Frank Lee

Allert didn't take special measures to reduce weight and he said it was to aid stability.

On a motorcycle, putting more weight above the saddle can really be felt.

Some bikes do feel top heavy, others feel light - despite having similar weights.
Obviously on a vehicle that's inherently unstable, the lower the CG, the better.

In Allert's streamliner, his body sits lower in the bike than it'd have sat using the stock seating position.

(For a human, the CG is close to the belly button.)

So raising the bike's CG by adding some extra topweight (the fairing) didn't really harm the overall stability of the bike + its rider compared to the stock situation.

[Frank Lee]

Yes there is the CG height variable... seems to me the sportbike/racerbike guys have discovered that a higher CG is actually helpful to their cause.

As far as stability though I have mainly been thinking of fore/aft CG and it's relation to the C of P.

[dcb]

Originally Posted by Frank Lee

Yes there is the CG height variable... seems to me the sportbike/racerbike guys have discovered that a higher CG is actually helpful to their cause.

Yup, less leaning required.

just thinking out loud, perhaps for crosswind you want the fore-aft cg to align with the cp. But you want the cp to be be as low as possible with the cg as high as possible (with the least frontal area too and weight a secondary consideration).

Some discussions say to have a more forward CP, as the effect is somewhat self correcting? But at very high speeds (i.e. bonneville) this sounds unnerving to me, dunno, maybe the fast spinning wheels help.

Yahoo! Groups
"If a cross wind pushes the rear of the bike off line, it will be similar to
a dirt track power slide which could lead you to places you don't want to go
unless you can correct out of it.

Conversely, If a cross wind pushes the front of the bike off line, the
steering will be moved which will trigger an automatic correction.

This is exactly what I experienced in the ECO with 50MPH cross winds on
three different howling windy days. The ECO has more "sail" area in the
front than the back. It immeadiately causes a steep lean toward the wind
just as it would on your typical motorcycle."

[Grant-53]

I have a copy of Amazon.com: Motorcycle Design and Technology Handbook (Motorbooks Workshop) (9780760319901): Gaetano Cocco: Books
and have found it very useful. One point not commonly made is that down force aids stability. A full body fairing might start as NACA wing profile inverted with a slight negative angle of attack. The shape can be developed as composite ellipsoids using the 3D graph features of a spreadsheet. The lateral Cd and Area determine the effective side forces the wind generates. Seat height may be limited by law as in NYS at 27 inches. Getting the rider a comfortable fit with the motorcycle is important. Wheelbase, fork angle, and trail need to be considered in the setup. A proper Kamm tail will help reduce side loads and make for handy storage compartments.

[PeterS]

Grant ,is there any chance of you posting a graphic or drawing of that ? I do MUCH better with drawings .