Small aerodynamic improvements

Hey guys

While driving on the motorway, I got thinking on the easy ways to improve the aerodynamics of the motorcycle for better fuel efficientcy since I'm loosing 4-6 km/l by having side bags and my girlfriend as a passenger. With the top speed of 185 km/h (if I remember correctly) and a legally required restriction to 48 kW, I calculate that the aerodynamic drag of the whole vehicle with passenger and side bags is around 934 Newton (N) if rolling resistance is neglected. Assuming that the coefficient of drag is constant, the drag at 130 km/h is 461 N (less than half that at top speed because drag increases with the velocity squared).

For easily calculatable improvements, I set my mind on the front shock absorbers as they are essentially a tube that can almost be assumed to be in the free air stream.

The drag coefficient of a tube is pretty constant at 1,1 at motorway speeds so if we assume that they are tubes with a diameter of 47mm exposed to the free airstream for a length of 450mm each, the aerodynamic drag at 130 km/h will be about 37N corresponding to a power of 1,35 kW or 1,8 hp lost to drag on those tubes alone. This comes to 8% of the total drag under these assumptions.

Now, if we put a triangle with a length of 72,5mm behind the shock absorber so the shape kind of resembles the aerofoil NACA 0040, then the drag force is lowered to only 4,24N corresponding to 0,153 kW or 0,2 hp so if these assumptions are not too harsh, it is an addtional 1,5 hp available with such a modification. 

Going back to the fuel usage, if we assume that throttling a bit less doesn't affect efficientcy of the engine, then it should come out to an additional 1,2 km/l or a saving of 5 dkk (0,6 british pounds) for that particular trip that got me thinking of fuel spent on aerodynamic losses.

When I'm done with my exams for the summer, I might be going for a longer trip across europe to visit my brother in the Netherlands so before that, I will make some "triangles" to get the shape of the shock absorbers closer to that of an aerofoil. 



These calculations are very rough and are based on a lot of assumptions with the worst probably being that the flow around the shock absorber is assumed to be unaffected by the spinning wheel just next to the lower part of it, but I still think it highlights that the drag can be easily reduced. Since it matters to the aerodynamics, it should also be mentioned that my bike has a full fairing from TCP as far as I can tell.

Yeah, I'm not looking to completely modify the motorcycle fairing or seating position as would be needed to have a big impact on aerodynamics but to try and see what can be done without ruining everything. Having two people crouch like that would not be a good seating position for when we have to drive for hours anyway so that's out of the question.
I am however considering integrating the blinkers into the fairing so they don't stick out as much as it is again something in the free air stream.

If you recalculate with the fork tube diameter as 37mm you will get a more accurate percentage.
The 'S' model is much more aerodynamic and much easier to ride on motorways.

ANDYC wrote:If you recalculate with the fork tube diameter as 37mm you will get a more accurate percentage.
The 'S' model is much more aerodynamic and much easier to ride on motorways.

Howcome you think using 37mm will be more accurate?

I measured on my bike the lower part of the shock absorber to be 47mm and the upper part to be 37mm but you can't really add much to the upper part as it would interfere with the radiator in full steering.

I see, I was basing it on the upper section.
Surely the aerodynamic deflection from the tyre and mudguard would reduce a lot of the drag on the lower section at speed.

ANDYC wrote:I see, I was basing it on the upper section.
Surely the aerodynamic deflection from the tyre and mudguard would reduce a lot of the drag on the lower section at speed.

I agree that the tire will have some impact but the air that clings to the tire is shielded from the shock absorber by the mudguard so I can't really say the impact of it. I do however know that the mudguard decreases the drag a lot by shielding the part of the tire that is going in the opposite direction as the airflow so i guess it also limits how much air is thrown forwards onto the rear side of the shock absorbers.

A wrap around mudguard could help.

ANDYC wrote:A wrap around mudguard could help.

You mean like one that fully covers the wheel but moves along with it? 
That would definitely improve aerodynamics as rotating wheels are terrible for aero but from the link that Beresford shared and from searching on dustbin fairings, it seems that adding too much fairing at the front gives stability problems as a side wind adds a force much in front of the center of gravity and thus tends to steer/rotate the motorcycle. That, and then i may be ugly, but that's a thing we can take when we are done speculating.