Push and pull: The surprising physics of a too-close pass
It happens in an instant. You’re out riding, minding your own business, and before you have a chance to react, a car flies past you at great speed, and far too close for comfort.
You’re buffeted around in the squall, and for a brief moment it feels as if the car’s draft has sucked you further out into the lane. You bring the bike back under control and try to compose yourself as you continue on your way.
If you’ve been cycling on the road for any length of time this has probably happened to you. It’s terrifying, dangerous, and even when the vehicle doesn’t hit you, it has the ability to knock you off your bike, or worse. But despite such close passes being a relatively common occurrence, little research has investigated this phenomenon. Now, though, a couple of aerodynamicists out of Germany are doing just that.
What they did
To test this phenomenon, Christof Gromke and Bodo Ruck from the Karlsruhe Institute of Technology (KIT) put a life-sized adult human dummy on a city bike and attached it to a metal assembly with load cells mounted to it. They placed that assembly on the edge of a straight, 700 metre-long section of road and subjected the test dummy to a bunch of close passes in an Audi A4 Avant station wagon.
The researchers passed the dummy at a range of different distances (0.5, 1, 1.5 and 2 metres from the dummy to the car body) and at a range of different speeds (40, 60, 80 and 100 kph), testing each distance and speed combination five times. The tests took place in calm wind conditions so as to avoid biasing any of the results, and they used a distance sensor and the car’s cruise control to ensure their distances and speeds were accurate.
Looking at their data, the researchers found that in each case, the dummy cyclist experienced a “transient aerodynamic load consisting of a sequence of pressure and suction load phases.” In other words, in the brief moment a vehicle overtakes a cyclist, the rider is both pushed away by and sucked towards the vehicle, all in a fraction of a second.
In fact, it’s a little more complicated than that. As Gromke and Ruck wrote in a conference paper in late 2019, there’s actually five parts to the aerodynamic interaction:
1. A ramping up of outward pressure that pushes the rider away from the vehicle. This starts before the front of the car passes the rider’s back wheel and reaches its maximum just before the car passes the bike’s bottom bracket.
2. A quick flip from outwards pressure to a suction force. This flip happens just after the front of the vehicle passes the bike’s bottom bracket.
3. The rider is sucked towards the vehicle. Suction is the main force acting on the rider, but there may be some small pressure loads at play as well.
4. Another flip, this time from suction back to outwards pressure, albeit at a reduced magnitude compared to the previous pressure force. This flip occurs after the back of the car passes the cyclist completely.
5. The forces acting on the cyclist oscillate back to zero via small, alternating pressure and suction forces.
The image below shows these five stages (labelled p1 to p5) for a vehicle travelling at 80 km/h while overtaking the dummy at a distance of one metre.
The pink rectangular box in this image denotes the period from when the front of the car reaches the back of the bike, until the back of the car passes the front of the bike. The red horizontal line indicates the passing period with reference to the bike’s bottom bracket.
From a cyclist’s safety perspective, the most important part of the aerodynamic interaction is the second phase (p2 in the image above): the flip over from a pressure force to a suction force.
The flip-over
In most cases it’s unlikely that either the pressure or the suction force will be strong enough to push a cyclist off their bike. For example, when the car passed the dummy rider at 80 km/h and at a distance of 1 metre in the KIT experiment, the pressure force was 4.2 Newtons and the suction force measured 3.2 Newtons. By context, the average adult male is capable of pushing more than 1,000 Newtons with his back against a wall.
But while the absolute magnitudes of the pressure and suction forces mightn’t be all that likely to throw a rider off their bike, the switch from one force to the other can be more dangerous. It’s for this reason that the researchers focused their effort on quantifying the strength of this so-called “flip-over load” — the sum of the magnitudes of the outward pressure and suction force.
As you might expect, the faster the overtaking vehicle is going, the higher the flip-over load. The closer the vehicle comes to the cyclist, the higher the flip-over load will be too.
But it’s not just the magnitude of the flip-over load that’s important. So is the time it takes for that flip to happen.
Imagine you’re being pushed off the road with a certain amount of force, and then sucked back onto the road at roughly the same intensity. If that happens relatively slowly, you’ve got time to react to the different forces acting on you and your bike. If it happens significantly quicker, you might not. Here’s how the researchers put it:
“Evidently, the reaction of a cyclist to a change in load by a compensatory maneuver depends on the rapidness of the process,” Gromke and Ruck write. “In particular the cyclist’s control over the steering motion during the compensatory maneuver is significantly affected by the duration of the flip-over load.
“A slow change in load can be responded to by the cyclist by relatively smooth and small increments in steering motion, hence, by a controlled reaction. A rapid change in load, however, is prone to be responded to by relatively abrupt and large increments in steering motion and is more likely to trigger an overreaction leading to an uncontrolled driving maneuver.”
As a result, the quicker the flip-over, the greater the danger to the cyclist. And as you might intuitively expect, the closer the vehicle comes to the cyclist, and the faster that vehicle is going, the quicker the flip-over happens.
The image below provides some examples. At an overtaking speed of 40 km/h and a distance of two metres, the rider gets a relatively lengthy 0.24 s to respond (and to respond to a basically irrelevant force of 0.5 N). At 100 km/h and 0.5 metres (no thanks), the flip-over time is 0.07 seconds — less than a 10th of a second to respond to a force change that’s 60 times stronger (at 31 N).
Lessons to learn
So what can we learn from all this? A lot of what Gromke and Ruck have found so far reinforces what cyclists already know: that the faster and closer a vehicle overtakes, the scarier and more dangerous it gets. And not just because of the increased risk of contact.
But the KIT researchers are only just getting started. They’ve also run their experiment with five different types of overtaking vehicles (including a bus and a truck) and tested four different types of bikes (including a loaded touring bike, a race bike, and a child’s bike). They’re currently working through the results of those studies with a view to publishing them soon.
They’re also working towards a laboratory study to better understand the effect of pressure and suction loads on cyclists. They plan to have a cyclist ride between two fans in the lab — one pushing in each lateral direction — to mimic what happens on the road. Those tests will feature a human rider and will use an indoor positioning system to see how the rider’s motion is affected.
None of these tests are a perfect recreation of what happens in a close-passing incident. For instance, the study discussed in this article features a stationary, non-pedalling rider, so doesn’t take into account the speed a rider will be moving at when overtaken, or the aerodynamic effect of their movement, both forward and with regard to the pedals. And any lab-based experiment is always going to struggle to accurately mimic real-world conditions — while the fan experiment will allow the KIT researchers to replicate the basic and most relevant loads acting on a cyclist, it will struggle to replicate the gust from an overtaking car in the real world, in all its complexity.
Still, this work is certainly useful in illustrating what happens in those hair-raising close-pass maneuvers that so many riders are familiar with. And that’s particularly true when you consider the end goal of Gromke and Ruck’s research: to reach some evidence-based conclusions about how wide bike lanes should really be, and how much room drivers should legally have to provide when overtaking cyclists.
Most cyclists aren’t thinking in terms of suction and pressure forces when they experience a blast of turbulent air from a close pass – they just know that they don’t like it. But if Gromke and Ruck’s research helps ensure drivers give enough room, then that’ll be one less thing that sucks for cyclists.
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