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Cw x F

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Cw x F

PostAuthor: faffi » Sat Apr 14, 2018 4:33 pm

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.
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Bike year & model: 1982 XV750SE with XV1100 engine, XT600Z shock absorber, 18" Seca rear wheel
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Re: Cw x F

PostAuthor: drainplug » Sun Apr 15, 2018 8:59 pm

I call the Luftmiester (the German version of a Vetter) on my R90/6 "The old Barn Door"
I guess you just proved it.
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1983 XV920 Midnight
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