Let the force be understood
At 400mph 65% of our present bikes resistive loads are predicted to be aerodynamic.
Simplifying things, the aerodynamic loads can be viewed as being made up of two components.
Cd value (slippery shape) : and frontal Area
Both values are then combined with others to determine resistive force at any given speed.
This relationship is a “square law”, and doubling the speed multiplies the resistive force by four.
Or put another way if the power needed to overcome the aerodynamic loads of 150mph is 55bhp
Then at double the speed 300mph we would need 220bhp
And for the same size and shape bike, to reach 400mph, we would need 400bhp.
(400/150=2.7, 2.7 x 2.7 x 55 = 400 bhp)
( this is a gross over simplification, but serves to demonstrate how these forces quickly escalate).
Frontal area
The design of Streamliners where absolute top speed is all that matters pushes both considerations of shape and area to the extremes.
There is no point in spending time and money in obtaining the slipperiest shape, you have to make the frontal area as small as possible.
Look land speed record holder John Renwick crammed into his cockpit:-
‘Photo from Classic Motorbike” and courtesy of John Renwick.
So what does all this mean for us? Well in the design process initial expected Cd Values, we were aiming for was in the region of 0.11. We were similarly aiming for a frontal area as tight as possible to our fattest component……yes the engine.
Design review
During a review of the design layout, we found we had excessive room behind the drivers head. It caused us to question our original layout. Why wasn’t the top of the engine touching the roof of the streamliner? Could the top surface of the streamliner be lowered? Yes, it would mean that the driver will have to lay flatter, and yes, less driver view, and yes, a longer bike, etc, etc but that amounted to us saving 10% of area?
And then we questioned the proposed design of the bikes sides. We repositioned the side rails and saved another 10%?
So that is 20% less area ! … How much power have we saved? Who can do the sums?
To try and share the information I’ve included a sample of our internal Bulldog calculations, and data…
Below plotted out for fun a few what ifs:
For all graphs the data highlighted in green is the simulator predicted, Cd is Cd, A is frontal area, the green mph is the predicted mph, the green hp to maintain 404mph is the predicted hp needed to achieve 404mph in the given distance. All other (non green) figures are generated by simple power law calcs. …ie the initial green figure is open to question, but the other figures are true relationships, so the ratios are correct at least!
The constant area graph shows the effect of changing Cd for a bike with a constant area.
The constant Cd graph shows the effect of changing area, for a bike with constant Cd value.
The Cd.A graph takes the worst case scenario, to find out the effect of taking ever worse values of Area and Cd
Do not worry if you can’t follow the argument I will try and translate the three graphs:
- Keeping the Area constant, and just considering Cd, a 10% change in Cd value does not have that much effect. (that is because +10% of 0.1 is small). Cd is hard to find out and control, but its a relatively small part of the overall aerodynamic equation.
- But, keeping the CD constant and just considering Area, a 10% change in area has a large effect. And its relatively easy to measure and control?
The bad news is that as the area gets larger, the Cd is harder to keep low.
- And if both Cd and Area increase the effect of each element is multiplied, you can see in the table an extra 100hp is required for a cumulative 20%.
To learn more about Aerodynamics and how we are using CFD see our How it works : Computing Power section‘.
And soon to be published for our members Aerodynamics Two. Which gives the latest update on the Aerodynamic front and more information about how we are using CFD.
And as a taste of what is inside …..
Predicted CFD results, telling us where the shape of the bike can be changed to improve both performance and high speed stability.
Our bike at 400 mph.
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