# Aerodynamics in motorcycles

What is the approx speed in motorcycles at which chassis aerodynamics start to play a role in power output. I need a value at which say a harley body and a yzf body assuming same weight and engine will start to show considerably different characteristics.

Thank you

Bystander
Homework Helper
Gold Member
Figure drag of ~ 1- 1.5 hp per sq. ft. at 60 mph, and that increases (or decreases) with square of speed, or ratio of speed to 60 mph squared.

rcgldr
Homework Helper
If you mean ram air intake as used on some sport bikes, it adds about 5% power at around 300 kph (186 mph).

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rcgldr, honestly didn't knew that can be designed for bikes too. Not what I was after but you learn something everyday.

jack action
Gold Member
Figure drag of ~ 1- 1.5 hp per sq. ft. at 60 mph, and that increases (or decreases) with square of speed, or ratio of speed to 60 mph squared.
I would personally guess a 0.8-1.0 hp/ft² at 60 mph. The lower value being representative of a full fairing bike with a driver lying low and the higher value being representative of a bike with no fairing and a driver sitting straight. Although, I wouldn't be surprise that 1.5 hp/ft² can be achieved in an extreme case.

But these values are proportional to the cube of the speed, not the square.

Assuming similar, typical, frontal area (but the Harley would probably be larger), the aerodynamic impact would be:

$$\begin{array} \textbf{Speed (mph):} & 60 & 90 & 120 \\ \textbf{Harley drag (hp):} & 5 & 17 & 40 \\ \textbf{YZF drag (hp):} & 6.2 & 21 & 50 \\ \textbf{difference (hp):} & 1.2 & 3 & 10 \end{array}$$​

So, to refer to the OP's question, there is no particular speed at which the aerodynamic effects «begin»: The Harley requires about 25% more power at any speed than the YZF.

Bystander
Homework Helper
Gold Member
cube of the speed, not the square.
If I didn't know better, it would be embarassing to have dragged that square all the way through --- knowing better and typing it --- too many bugs in my teeth.

Doug Huffman
Gold Member
LOL "A role in power output." Try a similarly shaped HPV with much less than a horsepower and all of it at sensible physiological cost. Drag is proportional to speed squared PERIOD.

http://news.motorbiker.org/Videos.nsf/Motorcycle-Superman.jpg [Broken]

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cjl
LOL "A role in power output." Try a similarly shaped HPV with much less than a horsepower and all of it at sensible physiological cost. Drag is proportional to speed squared PERIOD.
Sure, drag force is proportional to speed squared, but drag power is proportional to speed cubed (since power is force times velocity).

rcgldr
Homework Helper
$$\begin{array} \textbf{Speed (mph):} & 60 & 90 & 120 \\ \textbf{Harley drag (hp):} & 5 & 17 & 40 \\ \textbf{YZF drag (hp):} & 6.2 & 21 & 50 \\ \textbf{difference (hp):} & 1.2 & 3 & 10 \end{array}$$​
So the YZF with a fairing has more drag than the Harley? I was thinking that the rider leaned over on a YZF would end up a bit more streamlined than a vertically oriented rider on a Harley.

jack action
Gold Member
So the YZF with a fairing has more drag than the Harley? I was thinking that the rider leaned over on a YZF would end up a bit more streamlined than a vertically oriented rider on a Harley.
Sorry, by reading the text, it is obviously a mistake on my part. It should of read as:

\begin{array}
\textbf{Speed (mph):} & 60 & 90 & 120 \\
\textbf{YZF drag (hp):} & 5 & 17 & 40 \\
\textbf{Harley drag (hp):} & 6.2 & 21 & 50 \\
\textbf{difference (hp):} & 1.2 & 3 & 10
\end{array}

berkeman
RaulTheUCSCSlug
Gold Member
Did the OP just want the difference in drag, but not take into account the actual difference in hp and torque the Harley and YZF have? Also like another person said, whether or not you have a fairing is going to make a world of difference. Some Harley Sportser flat trackers are made to be pretty aerodynamic.

Some Harley Sportser flat trackers are made to be pretty aerodynamic.
I seriously beg to differ.. I've built every type of HD Sportster racer there is, from unfaired dragsters, to half faired Sporty road racers @ Daytona, to 883 series road racers w/ nothing but a flat # plate for aerodynamic's on the front, and the flat tracker's share the same flat # plate, in front. If you talking about the little semi "boat tail" on HD flat trackers (XR 750), that was designed more fore aesthetics, than drag reduction.

The easiest thing to do is conduct coast down tests. For example, get the Harley up to 80 MPH on a straight, level road on a low or no wind day and record the time-to-speed (video camera works just fine). Do the same with the faired bike. Now plot and extrapolate a bit.

Aerodynamics play a role almost right away. Bicyclists will attest to this. There's a lot more drag at 15 MPH than at 10 MPH on a bicycle.