Cw x F

[faffi]

Power requirements. A result of area and slipperiness, primarily. So while a fairing may offer less air resistance than no fairing, its size may still mean you need more power for any given speed.

 

The first scientifically developed motorcycle fairing for mass production was that of the 1976 BMW R100RS. With a normal sized rider seated normally, it delivered a Cw x F value of 0.43. The comparable value for a stock naked R100 was 0.48, while the touring version, the R100RT, could only muster 0.53, the same as a GL1100 Interstate. The 1982 CB1100R could better all with a value of 0.41 and, with a prone rider, only 0.37.

 

So what does these values mean in the real life? Well, in order to travel at 200 kph (about 123 mph), the R100RS need 65 crank hp. The upright sitting, unprotected rider on the naked R100 need about 6 hp more, and the R100RT another 6 hp. Or put it differently; the RT rider would either need roughly 20% more power - or be satisfied with a top speed of about 185 instead of 200 kph, if we assume the RS had 65 hp. Speed cost power. Power cost energy. At 100 kph (62 mph), the RS need about 8 and the RT nearly 10 hp, but only 1/8 of what they need to do double that speed.

 

Why bring this up? Because I want protection from the elements. But this comes at a cost, often several; weight, price, turbulence (buffeting), looks, performance, less access for service and repair, handling, view - and stuff I probably forgot to mention. So I am weighing up whether the good can outweigh the bad, or not.

 

One more interesting observation from my own history. A Kawasaki outfitted with a Vetter Windjammer V fairing with lowers - perhaps the most popular aftermarket touring fairing in history - had a Cw x F value of 0.59, which is little to brag about. The price to pay for great protection. This value also illustrate how un-aerodynamic I am when seated on a motorcycle: I actually fitted a Windjammer V to my CB250 N (same as the 1980 CB400T Hawk, only with much less power) and experienced a speed gain of about 5 kph / 3 mph. No wonder why most of my bikes gained at least 15 kph / 10 mph with a different rider at the controls. I have no clue as to why.

 

 

[fzar]

So as much as that was interesting to read why don't you plug your numbers into a real world FZ-07 windscreen's or in comparison ask in the racing section how they feel about the fairings/time/top-speed. I understand your saying how un- aerodynamic you are,but if your being scientific about it let's see if some mathematical equations can point us all in a better direction at our choices before we purchase, e.g how tall a screen is needed for touring to help mpg/comfort and what angle suit's it best. Racing on the other hand I imagine it's beneficial. I've no idea what aftermarket fairings are available other than AP MOTOARTS kit from Andy Palmer, I know @blackout does some homemade fibreglassing for his personal fairings, and people use R6,R1's salvaged fairings. Maybe @mossrider, @twf, @scratchpad , @hordboy, could chime in as I'm almost certain they race. 

[blackout]

Lee Parks agrees.  From his book....

[fzar]

Interesting stuff, Thanks blackout for that perspective. This is why MGP are adding wings to the fairings of their bikes, objectively if anyone questions the reasoning, straight line speed and aerodynamics have little to do with a motorcycle tipping in to a corner.(Or a lot) Its balls, tyres, suspension.. etc. As a teardrop shape maybe the least resistance in a straight line you still have to account for air intake for cooling, so compromise is inevitable. How does your fairing work for you, @blackout, lap times/acceleration/top speed when you track your bike? Do you race?

[blackout]

7 minutes ago, fzar said:

Interesting stuff, Thanks blackout for that perspective. This is why MGP are adding wings to the fairings of their bikes, objectively if anyone questions the reasoning, straight line speed and aerodynamics have little to do with a motorcycle tipping in to a corner.(Or a lot) Its balls, tyres, suspension.. etc. As a teardrop shape maybe the least resistance in a straight line you still have to account for air intake for cooling, so compromise is inevitable. How does your fairing work for you, @blackout, lap times/acceleration/top speed when you track your bike? Do you race?

I do not race yet.  Still learning to ride track.  I tracked the bike 3 times last year and hope to do much more this year, but time and money are always an issue. 

 

My head fairing that I ran last year made the bike feel much more stable at speeds.  This was noticed at speeds over 60 mph.  The head fairing directed the air off my chest and helmet which eliminated any front lift from the crappy aero affects of my blunt body.  But, the head fairing did start fluttering excessively at speeds over 100 mph.  And I could not reduce the fluttering with added bracing or venting, so I am going full fairings this year.  I have no idea about lap times or top speed.

[r1limited]

And I was worried the circumference of the odometer while oscillating was causing multidimensional detonation resulting in abundance of preflatulance

[faffi]

20 hours ago, fzar said:

So as much as that was interesting to read why don't you plug your numbers into a real world FZ-07 windscreen's or in comparison ask in the racing section how they feel about the fairings/time/top-speed. I understand your saying how un- aerodynamic you are,but if your being scientific about it let's see if some mathematical equations can point us all in a better direction at our choices before we purchase, e.g how tall a screen is needed for touring to help mpg/comfort and what angle suit's it best. Racing on the other hand I imagine it's beneficial. I've no idea what aftermarket fairings are available other than AP MOTOARTS kit from Andy Palmer, I know @blackout does some homemade fibreglassing for his personal fairings, and people use R6,R1's salvaged fairings. Maybe @mossrider, @twf, @scratchpad , @hordboy, could chime in as I'm almost certain they race. 

Good points, but without a wind tunnel, it's difficult to be scientific. The only things I know is that the said Honda gained speed with the fairing, and that also was true for my Vulcan 800 when I fitted a National Cycle Heavy Duty Touring wind shield. The same shield used on the Intruder 1400, where it had to sit more upright, had no impact on top speed. 

 

Finding the best fairing is v-e-r-y difficult, especially for touring. One that protects the whole front of you as a rider sitting upright is very prone to creating turbulence behind. That could be around the helmet, in the back/neck area or at the abdomen area. Or all of the above, or someplace else. It is easier to get protection without too much disturbances if you are willing to lean down quite a bit and use a narrow bike with narrow handlebars. You can then use a low screen that comes toward you and duck behind it. The more upright a screen is fitted, the worse its aerodynamics and the worse the turbulence. 

[gregjet]

The teardrop shape is not necessarily the ideal shape for min resistance. In the late 60's early 70's the Kamm back was developed. That is where the streamline remains attached right to the rear of a cutoff back but the area behind the rear has a shape that induces a vortex that does not shed. ie it stays attached rotates so the upper area is going the same direction as the air flow. That makes the form have a much longer virtual form. Think Ford GT40. Also the theory behing the delta shaped masts of Bethwaite for sail boats and super cavitating propellers.

 

By the same token you can induce a virtual form by shaping the front stream lines and ensuring you get a standing vortex and no shedding ( the fluttering described above). My 07 has a modified design stolen from a Cagiva Raptor designed by design and aero genius Galluzzi. The original was tiny . almost unnoticeable, but combined with the rider sitting deep down behind the tank , meant that the airflow hardly interfered with the rider up to about 120kph. After than the flow would be lower and start to increasingly hit the rider.

I modded the design to increase the area to give a bit more protection by adding faired lower area ( I didn't want to screw with the upper design effectiveness.). I added a spaced screen to avoid vortex shedding and still give height. As it is now it allows me to sit upright up to 160kph without any buffetting, even with a helmet peak ( though sticking the peak into the airflow is pretty unpleasant. The is no airflow induced wobble at any speed I can get the bike to.

The biggest problem is keeping the flow attached BEHIND your body , to get a smooth reattach and stop shedding.

However ANY fairing shape, using virtual shape, will be efficient at some speeds and crap above and below.

[faffi]

I think I've mentioned elsewhere also that the shape of the rear is more important than the shape up front. A pyramid pushed bottom (flat) end first is more aerodynamic than on pushed with the point end first, even if it defy logic.

 

Pat Hennen, who used to dominate the classic motorcycle racing scene until his accident, also found that a wider fairing that would close slightly at its trailing edge reduced air resistance more than a narrow fairing continuing to flare out where it was cut.

 

Personally, since I am no engineer in the field, can only repeat what I've read or been told, so any flaws must be discussed with someone in the know

[Beemer]

Possibly, the #1 thing I like about a fairing is how it deflects the rocks that are kicked up on the road and flying at me at high speed. When I see them I still instinctively duck behind the screen because they hurt so much when taken at highway speeds. Other than rocks hitting you in the chest, big, fat, hard shelled bugs or just large bugs in general can hurt like hell and bug goo no look good on you. 😝

[faffi]

I have only been hit by rocks a couple of times since I first took to riding back in 1980, and I think all hit my fingers. Then a few years ago I began hitting birds ever so often, killed many every year. They would just fly into me, hitting me everywhere from the neck to the feet. Don't think I hit any last season, and hope not to hit anymore.

[gregjet]

Faffi,

Are you a bird magnet ( or does that joke only work in England and Australia). In Aus the bird of choice in the Galah (https://en.wikipedia.org/wiki/Galah  ) . Maybe not the stupidest bird in the world but LOVES flying into the path of cars and motorcycles. I suspect there are few motorcycles that haven't got ping and grey feathers stck in some crevice somewhere. Phillip island is famous for seagulls hits ( ask Lorenzo).

The rear drag thing I emphasize as often as it can as I never seems to get understood.

And yes the turning in of the sides will improve flow. Perfect frontal shape ( sub sonic) is a Rankine half body. Basically 2 mirrored  hyperbola joined at the low point.( can remember the formula), and chord ( ratio of length to width) is dependant on the wind speed for min. resistance. Of course it only works perfectly when the air is straight on to the long axis. Same with the tead drop shape. Sorry but this is the LEAST technical picture of the shape I could find. It relates ONLY to an inline airflow and relates to the FRONT of a body only, but it will give you and notion of ideal shapes. Remember the whole shape doesn't have to be solid. Once the streamlines are established you can use virtual body. ( https://www.revolvy.com/main/index.php?s=Rankine+half+body&uid=1575 ) .