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Old 09-04-2011, 07:47 PM   #43 (permalink)
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Join Date: Jun 2011
Location: Essex UK
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Frank – after reading your earlier post re drag calcs, I decided to look at it again.

First the Crr – I read the notes and without any other information I reasoned the low figure on the basis of m/cycles having only two tyres (half the energy consumed in deformation of contact patch) versus 4 on car – so half an average figure.
I didn’t expect to be far from the ball park, but I bow to your superior knowledge.
I have now re-run figures with .018 (middle of your estimate)

Frontal area – just completed a count (by hand) of size. I have a photoshop drawing, which I will post once I tidy it up, but the count came in at 0.673sqm – so actually less than my estimate of 0.7sqm. Again I have re-run the calc with this new input.

Cd – well this is a guess, but at least an educated one! In my initial research I found a figure of 0.17 for the ecomobile and I gave myself some “wriggle room” by using 0.19. The ecomobile is the nearest design to mine, with two exceptions (one positive ,one negative)
It runs enclosed wheels and I intend open, but it has exposed outrigger wheels at the widest part of the body which I think will cause turbulence, whereas my design is very smooth at should keep flow attached for longer.

Anyway, the net result is still only 11.5hp to hit 100mph, which is still attainable, so I’m not put off yet.

By the way, I think you are spot-on about your aerodynamic observations – I expect turbulent flow around the front wheel to quickly re-attach at the body sides, thus minimising the drag penalty. My big problem is the rear! I’m still open to suggestions about how to finish this.
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