A question, potential horsepower versus required horsepower.

[PeterS]

A fuel injected model is close to A$6000 .Bikes are very expensive here, at least 40% above US prices.
Used BMW G650 Xcountry Motorcycles For Sale - bikesales.com.au

[sendler]

A$5,000 for a used CBR250R. They are $3,300 in the USA. I see the US dollar has crashed in Australia. They are nearly the same.

[PeterS]

The initial purchase price for almost everything is higher here, sometimes double. Our wages are good but the prices are higher too.

[aerohead]

Originally Posted by PeterS

I've been looking at a variety of bikes from the perspective of building an aero tourer and it seems that there is a significant advantage in fuel injection. The trouble is that here in Australia the only fuel injected bikes are quite big, usually 750 cc and up (especially at a price I can afford).

I could go to a smaller carbed bike but a question first about horsepower potential and used horse power.

With a cd of 1.0 a K75 BMW gets 43 mpg US at 70 mph as the spec sheets say. But using the calculator at the top of this page if I feed in a cd of .4 (as in a full Vetter fairing ) it says I will get 99 mpg US.

Is this believable?

Firstly this is a large engine , a 3 cylinder 750 but also a very good one that has an early fuel injection system. If the reduced drag only requires 8 hp versus 17.5 hp with a cd of 1( per the calculator ) is that all the engine will develop ? Meaning only using that much fuel .

Any input is welcome !

General Motors published a relationship between drag reduction and mpg which is valid if BSFC can be maintained.
A 10% drag reduction will net a 6% mpg improvement at 70 mph.
Going from Cd 1.0,to Cd 0.4 is a 60% drag reduction and should produce a 36% mpg increase.
1.36 X 43 = 58.48 mpg if all other factors are held constant.
If the frontal area were to be reduced as well as the Cd,you could expect better numbers.
NOTE: it is essential to do gear-matching after the streamlining such as to maintain the same engine loading as existed before the streamlining or you risk losing up to 60% of the benefit.

[PeterS]

Gear matching is a problem on a BMW, a chain drive makes things a lot simpler.

[aerohead]

Originally Posted by PeterS

Gear matching is a problem on a BMW, a chain drive makes things a lot simpler.

Yes,the shaft-drive makes for an interesting challenge.I've got a Honda 700 Shadow and face same challenge.

[Xist]

Originally Posted by PeterS

The initial purchase price for almost everything is higher here, sometimes double. Our wages are good but the prices are higher too.

I believe they refer to Hawaii's "Paradise tax" or something.

[alvaro84]

Originally Posted by aerohead

Yes,the shaft-drive makes for an interesting challenge.I've got a Honda 700 Shadow and face same challenge.

I have BMW too, but not with shaft - with a belt, and the gears are still factory default after 5 years.

But I still get decent numbers Not as good as sendler does with his CBR250.

It would be nice to try a different ratio, honed to our 56 mph speed limit - but the easiest way would be scrap the whole final drive and go to chains. (Something I'm still reluctant to do )

[PeterS]

Originally Posted by alvaro84

I have BMW too, but not with shaft - with a belt, and the gears are still factory default after 5 years.

But I still get decent numbers Not as good as sendler does with his CBR250.

It would be nice to try a different ratio, honed to our 56 mph speed limit - but the easiest way would be scrap the whole final drive and go to chains. (Something I'm still reluctant to do )

My next bike will be a BMW G650 cross country , it should be good for 67mpg US in stock form.

[jkv357]

Originally Posted by aerohead

NOTE: it is essential to do gear-matching after the streamlining such as to maintain the same engine loading as existed before the streamlining or you risk losing up to 60% of the benefit.

How do you do gear-matching?