Geoff Searle is a competitive cyclist who is testing the drag1 on two different bikes. He is using the wind tunnel at the University of Canterbury to make sure he is choosing the bike with the least aerodynamic drag2. Dr Mark Jermy discusses some aspects of frame design that result in less drag.
Point of interest:
How does a standard mountain bike frame differ from the frame on a racing cycle?
Transcript
GEOFF SEARLE
I want to have a look at which ones out of the two bikes are the quickest. I want to make sure that I’m choosing the correct bike so that I can start developing my time trial speed. I’m not a time trialler by nature, I’m a bunch sprinter by nature. This year, I want to be quite a lot quicker at time trialling. A lot of this game is technique and getting yourself set up well and making sure that you are overcoming wind resistance3, which is the biggest part of where your energy is going.
DR MARK JERMY
The shape of the bike frame affects the drag. So mountain bikes and older-style road bikes have tubular frames, and that tube – that circular cross-section – produces quite a lot of drag. So the more expensive time trial bikes, where they are going for the minimum possible drag, the frame is made from almost aerofoil4 cross-sections – they look a bit like the shape of a wing, it’s a very long but narrow streamlined shape – and that reduces the drag quite a lot. The material that you make that from doesn’t actually reduce the drag, but that does effect the weight5, where you can make it stiffer for the same weight than if you used the same weight of steel.
GEOFF SEARLE
Looking at the difference between the two bikes, the issue with time trialling is that you are putting the vast majority of your work or your effort to overcoming resistance of the wind. I want to be choosing the correct bike when I go out and race. The Cervélo P3 is probably the benchmark, unless you’ve bought the brand new Cervélo P4 which costs an absolute small fortune – very expensive piece of kit. For aerodynamic bikes that you can easily buy off the shelf, that really is the quick machine.
The black bike that I was trialling is a generic carbon fibre6 one, ex China. It’s very light, but the question was, would it be slippery through the air? Something that I would say is slippery is something that cuts through the air with comparative ease, so all of my effort is going into going faster before I run out of ability to overcome friction – wind friction, drag.
Acknowledgements:
Geoff Searle
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