Before starting on Eta, the core Aerovelo team members had built a series of speedbikes as part of the University of Toronto Human-Powered Vehicle Design Team. These bikes were primarily designed for the urban utility challenge of the ASME student competition, but they were also designed with top speed in mind. In 2013, one of these bikes (Bluenose) was used as a research testbed to prepare for the development of Eta. The tests involved the addition of a tail fin to improve handling qualities, on-road aerodynamic testing to correlate our design models with reality, and conversion to a camera-based vision system instead of a large windscreen.
Eta was the first bike built under Aerovelo, and the first of the bike's designed purely for speed. The goal was to combine all of our experience into one bike, and at the same time try to reduce risk by minimizing the number of new ideas that we had never previously tried. Even with this design methodology our first attempt in 2014 was plagued with broken spokes and a variety of other mechanical issues. 2015 was rededicated to small improvements and getting as much road testing as we possibly could.
There are several particular features that make Eta fast, but it is truly the way they all fit together that take Eta to the next level. In the end, everything fits into an incredibly small package, with good ergonomics for rider output, excellent handling qualities, large low-rolling-resistance tires and an advanced aerodynamic shape. A few of the particulars are outlined below:
Human Power & Ergonomics: Every bike design starts with a good fitting rig: a seat and pedals that can be used to adjust the position of the rider, test out the ergonomics and then measure the rider envelope with millimetre accuracy. For aerodynamic reasons we want the rider as recumbent as possible, but leaning too far back can reduce the rider's ability to produce power. The effect of small changes in foot height, hip angle and seat angle are measured in lab tests and gained from experience on previous bikes.
Vision System: One of the most surprising features of Eta is that it has no see-though canopy for the rider to see. Instead, the rider looks at two video screens inside the bike, projecting the image from the two cameras above. The two cameras and just in case one of them stops working at 140+ km/hr. There is also a small onboard micro-computer that shows the rider critical information with an on-screen display. This includes power, cadence, gps speed, distance down course, target power/speed. The computer is also running an onboard simulator (based on the measured power from the pedals) and displaying the difference between the actual speed and the simulated speed on the screen. This allows the rider to make an informed decision during the run as to whether or not they should go for an all-out sprint, or save their energy for another day when the weather / bike conditions are more favourable.
Aerodynamics: When it comes to aerodynamic design, the first thing we need to do is eliminate large-scale flow separation, turning a blunt object (like a golf ball) into a streamlined object (like a fish). The next strategy is to try to minimize the small-scale turbulence, that which exist in the thin layer of air next to the bike called the boundary layer. We design the bike so that any given particle of air follows a very specific pressure profile as it passes by the bike. We do this by iteratively changing the shape and simulating the airflow with a computer model. This pressure profile mimics that of the NACA 6-series airfoils that were developed at NACA (before it became NASA), to achieve extended laminar flow. Even with the right pressure profile, however, it can be extremely difficult to convince the flow to stay laminar, and any disturbances from road vibration, seams in the shell or bugs splattered on the surface will turn the laminar boundary layer turbulent. If we can manage to keep it laminar however, we can cut the drag by more than 50% over a shell that from the outside looks very similar.
Rolling Resistance & Wheels: With the aerodynamic drag greatly reduced, the rolling resistance of the tires becomes a major source of power loss. Rolling resistance goes down with wheel diameter and tire width, and so, as with almost everything on Eta, it becomes and incredibly tight packaging problem to try to fit the biggest tire in the smallest space. Rolling resistance also goes down when there is less thickness to the rubber that being deformed by ground contact, so Eta rides on ultra thin (< 2mm) tires.
Drivetrain: The drivetrain is done in two stages: first from a 93 tooth chainring to a custom 7-speed cassette, and then from a 39 tooth chainring to an 18 tooth cog at the front wheel hub. To keep the chain lines as short as possible, Eta is front wheel drive. This innovative system, refined in the speedbike community more than 20 years ago, means that the second chain is connected to a wheel that is also steering. As long as the second chain is parallel to the head tube angle, this system works incredibly well.