Can a Bike Pass Your V8 on the Highway?
We’ve all been there, driving down a one-lane street, backed up behind a cyclist that blocks our ambition of hitting the speed limit. Seriously, bikes limp along at 10 mph and hit what—30 mph—max? Well, no. How does 90 mph sound?
That’s right. Thanks to the engineering minds of Aerovelo and its bike, Eta, cyclists could theoretically complain about passing that big bead on your high performance vehicle.
Aerovelo Cofounders Cameron Robertson and Todd Reichert lead their team of University of Toronto (U of T) engineering students and alumni to design, simulate, optimize, build and pilot this escape pod-encased bike into the history books.
After breaking its own human-powered land speed record a few times over, this little tear drop settled on an impressive 144.17 kph (89.59 mph).
“It was a culmination of years of effort,” said Robertson. “There was a lot of excitement and relief that we have taken a good path and all the choices we made showed it could be done. With Eta’s design, we showed the range of improvement. In 2000 to 2015, there wasn’t much change to the [human-powered land] speed record. It incremented 10 mph in 15 years, from 73 to 83 mph. The rate of technological change was small; it was incremental improvements. In the span of two years with Eta, however, we incremented [the record] by 6.5 mph.”
There is no wonder why the team named their bike after the Greek letter us engineers know represents efficiency. And the racing pun asking Eta’s estimated time of arrival at the finish line wasn’t lost either.
How Do You Design a Bike That Can Break a World? Use CFD Simulations!
Consistency is one of the hardest challenges when designing a vehicle to break any speed record. This is because the racing team typically only gets a few kicks at the can on official race days.
“Every day you need to execute as you will only get some days where the environment is what you want,” said Robertson. “We didn’t expect this to be a big point, but we took it based on advice from other teams that have broken records and were always on the ball. We wanted to emulate this.”
So how does one get from the snail on the road to Formula One? And better yet, how do you make the performance of this bullet consistent? The answer is computational fluid dynamics (CFD) simulations, a lot of experience, and trial and error.
“With the bike, it was important to change the design of the outer shell and then slightly modify the simulation results to get a sense of how it performed with respect to that change. Then, we would iterate again,” said Robertson. “Todd [Reichert] did 30 different iterations on the bike’s fairings, and without SOLIDWORKS we would never have been able to do that in an informed way.”
The majority of the Eta simulations were in SOLIDWORKS Flow Simulation. This simulation in-CAD package was a platform that the U of T alumni and students at Aerovelo were well versed in using. They wanted to investigate the airflow around the outer shell. To do this, the team would perform numerous pressure profile simulations.
“We use pressure profiles, which are accurately evaluated in software like SOILDWORKS Flow Simulation. As the air goes over the surface, you can assess the pressure at every point. You can then shape the pressure profile as you go down the bike and have the shape of the profile be maximally conducive to extend laminar flow,” explained Robertson. “From there, we expect, based on the pressure profile, the change to be positive or negative.”
The goal is to maintain laminar flow around the bike as long as possible due to its lower resistance compared to turbulence and transitional flows. One would think that ideally, the goal is to maintain laminar flow around the shell completely. However, if the flow doesn’t transition before the trailing edge then it can fully separate from the surface which will cause tremendous drag.
A red herring of sorts in the optimization and simulation of their bike, according to Robertson, was trying to determine the actual spot on the bike where the air transitions from laminar to turbulent. He said, “When that prediction happens, there is a large margin of error. It’s subject to small variations and it will be [hard] to implement in the real world versus simulation. We see some team point to a spot on their bike and say, ‘we have laminar flow until here and we predict it will run 150kph,’ and then it runs worse than their previous bike.”
This shows the importance of validating your simulation. No engineer should trust their model blindly. This has become a regular practice for Aerovelo. It performs simulations and then tests the bike to compare the turbulence and pressure profile. The team also relies heavily on its experiences and the experiences of the community of engineers working on breaking the human-powered land speed record.
How Does Eta Differ from Traditional Bikes?
So, after all the design changes, optimizations and simulations, what sets Eta apart from that bike in the garage?
“Well, it’s different from a normal bike in almost every way,” said Robertson. “First, Eta is very recumbent. The pilot is almost completely lying down. The bike is fully enclosed for aerodynamics except for controlled intakes for ventilations. Next, it’s steered using cameras on top of the bike connected to screens in front of the pilot’s face, and steering is limited to three degrees to each side. Finally, the tires are not good for turning. So, it’s clearly designed to go straight.”
Robertson also explains that the gearing for the bike is also very different. Eta uses a two-stage drive train to accommodate the bike’s top speed. Due to the reduction created by the two-stage drive train, the wheels of Eta are able to spin many times faster than anyone can spin on a traditional bike.
With the vast differences between Eta and traditional racing bikes, don’t expect to see it on the Tour de France anytime soon (unless there’s a total relaxation of racing rules). However, Robertson is interested in a biking version of Formula1 (F1), similar to Australia’s Pedal Prix, where the engineering behind the equipment is perhaps more important than the driving of said equipment. He said, “F1 is more about the engineering and could be interesting in a bike format.”
But what really interests Robertson is how this technology could affect transportation. He imagines going to work at highway speeds on a vehicle that is human powered and 300 times more fuel efficient than your average car.
“One thing we thought about is how you could use these in the future. Imagine if you were not contending with several thousand-pound cars on the road,” wondered Robertson. “It’s interesting when comparing a small power of the human engine at about two-thirds horsepower. To ride an hour and achieve 60 mph, the bike needs to be very efficient—about 9500 mpg to an average car that is about 30 mpg.”
The Future of Aerovelo’s Record-Breaking Bike Designs
So, what is next for Aerovelo and the human-powered land speed record?
Well, Robertson believes that there are still many potential areas of improvement for Eta and similar record-chasing bikes.
Unfortunately for Eta, many of these improvements will require a completely new design and even more thinking outside the box from engineers. Two examples involve heat capture and active boundary layers.
The rule books says that the bike must be powered by humans, but does that mean it has to be powered by the legs alone? Remember, that pilot in an enclosed spot will produce a lot of heat. Capturing this human body heat could theoretically help power a bike. This might seem a little like cheating, but remember that it is still human power, and it worked well enough for The Matrix.
“You can’t use energy storage devices on the bike, but you could still drive a bike on an electric motor,” said Robertson. “So, imagine if the driver provides power to the drive train. Humans are 20 to 30 percent efficient on converting energy and the rest goes to heat. Capturing that with some efficiency using heat recapturing tiles or heat pumps could theoretically increase the energy output.”
To test out this theory, engineers at Aerovelo could create a heat transfer simulation of Eta’s replacement and use that data to crunch the electromechanical numbers.
“Simulations would also play a role in active boundary control, which is used to extend the laminar flow around a vehicle,” explained Robertson. “Active boundary control senses and manipulates the boundary layer in order to allow longer runs of laminar flow than would be possible otherwise.”
Active boundary control can be done in two ways: using either wave theory or clever ventilation.
“When air transitions from laminar to turbulent, energy and unstable waves oscillations grow to a point where the stable smooth conditions turn into the chaotic movements,” noted Robertson.“The technology senses the oscillations in the air and then introduces more waves to cancel out those waves that are promoting the turbulence transition.”
Another active boundary layer method would add a system that sucks in the nearly turbulent boundary layer of air. The air is sucked in can then be used for ventilation. However, more importantly, the surrounding flow would fill the gap made by the pre-existing boundary layer. This would effectively give the bike a new laminar boundary layer.
Unfortunately, this method would need precise knowledge of the transitional point. As previously mentioned, this would require simulations with questionable accuracy in this particular application.
Similar to the heat capture option, active boundary layer does use up energy to work the sensors, oscillators and/or pumps and any other equipment. The question is will you get more energy out of the rider than you waste on this added equipment?
“Everything we do is about system design,” noted Robertson. “We look at how we can do more with less or get more from what we have. Reduce the weight and increase strength—that is an important mind-set.”