Aerodynamic bicycle wheel coverings
It has been some time since I have stayed up til 1am working away to get a project done. It seems once you leave University and start working for a big company times like this are rare and I find myself missing the feeling of satisfaction when, at the end of a long session of chooking, you get to go to sleep.
Fortunately this void has recently been filled by Driven…..from my projects page, ‘since starting my graduate scheme for Jaguar Land Rover I have joined the ‘Driven’ engineering project. A team of graduates who design and build a new car every year for an electric vehicle race hosted by Greenpower.’
So far the Driven project has helped me meet many talented people all with the common goal of making an electric vehicle. This has also presented some unique challenges as the team is far bigger than those I have worked with in the past, with the idea behind this being that we function as a microcosm of the overarching company. In addition to building a car every year the team hopes to (and hopefully succeeds in) inspiring the younger engineers at the race events. I plan to use this blog to document some more of my work for the car in the hope that some principles can be applied by other teams to their cars.
Covering the wheels of our cars we can reduce the loses we have due to aerodynamic drag and hopefully this will increase our top speed and efficiency. Although I am no aerodynamics expert I am aware that a spinning wheel made of spokes presents a drag factor when those spokes are moving through the air. My housemate Shawn would be able to describe the effects of this in far more detail than I can but essentially forcing more air to move in and around the car is BAD.
As well as the spokes going through the air creating turbulence we also want to make the side of our car as flat as possible so that the air is passed down the side, rejoining neatly at the back. Since both our Driven cars have open front wheels covering these can further improve (reduce) the drag coefficient of the body. As an added bonus covered wheels can look pretty cool.
This is a quick and easy way to cover wheels we have been experimenting with. It is cheap and can be done easily at home (this was done in my kitchen). So far I haven’t done any quantitative testing on these wheels vs uncovered but as far as I know the aero benefit should outweigh the slight weight penalty.
- Stanley Knife
- Hole Saw + Drill
- Electrical Tape
- Cutting board
- 1 screw / nail
- 1x beautiful assistant, thanks Ben!
We started our build with a big sheet of thermoform plastic, around 1 – 2mm thick should be perfect. Ours is thermoform but only because it is what we could get our hands on, any plastic should work here. We did try some basic thermoforming but found that the plastic would just sag on to the spokes and create a very wavy surface.
Start by drilling a hole where the centre of the wheel will go. We then insert a screw in to this to act as a centre reference. A piece of string and a pencil can then be used as a giant compass to draw a circle of approximately right size to suit your wheel. Cut big to start with, it’s always easier to make the circle smaller than bigger if you go wrong! Once you have your circle drawn you should be able to use a stanley knife to cut around the line. Be careful here.
I apologise for the poor picture quality, it was late and my kitchen isn’t the most photogenic place anyway (though much improved by a photogenic assistant!).
Next up select a hole saw that is a bit bigger than a convenient flange on your hub. Use the hole we drilled earlier as a guide and the hole saw create a bigger hole.
Now the ‘overlap’. Our wheels (like all bike wheels) are slightly convex in that the hub sits out wider than the rim. This means that a flat piece of plastic won’t simply fit on top. To overcome this we must create a very slight cone to fit the wheel. This ‘overlap’ method is far from perfect as it does create a slight bump on the wheel which makes for a not perfectly flat surface, however, it is easy to achieve at home.
Start by cutting from the inside of the cover to the outside. Then place on your wheel. It should be noted that as the outside of the cover is pulled down to the rim a natural overlap can occur at the new cutline, simple. At this point you may have found that you have to adjust the size of your cover. Ideally you want to have around half of the rim showing and half of it covered as this will make life easier in the next step.
Next we run a line of electrical tape all the way around the outer edge of the rim. Covering the new cover and then folding some over the edge of the rim and in to where the tyre sits. Electrical tape actually seems pretty good at going around the circumference of the circle and with practice I can always do it in one long pass. A piece of tape neatly covers that overlap line and then we finish the wheels with some black hot glue just to hold the centre to the hub. It can be worth doing both sides of your wheels, just remember you still have to have access to the valve holes! I just cut a small window on the inside covering to allow access to this.