This is not the first time this has happened. I’m riding along on a semi-rural road and a transit bus passes me. Must be electric or something, I can never hear them coming. When he passes me, also scaring the crap out of me, there is kind of suction that pulls you toward the bus. Of course since he buzzed me with about 30" clearance, the suction thing pulled me closer (with the element of surprise) so that I just barely missed being sideswiped. WHat causes the suction thing?
A large moving object like a bus creates a pressure front as it moves through the air; the front is unstable at the edges and slides alongside the object as it moves; this causes a pressure difference along the front boundary and hence the suction.
It’s very similar to the effect of an airfoil.
Bernoulli’s effect?
(Disclaimer: I’m not an engineer, just a wild guess)
The bus moves fast and because of that, it moves air faster than the ambient air. When air is moving faster, it creates a lower pressure compared to air that is standing still. You aren’t getting sucked into the bus, but you are actually getting blown into the bus. When the bus buzzes you, the air within a few feet of the bus moves quickly and therefore creates a lower pressure. The air that is further away from the bus is moving slower and therefore at a higher pressure and higher pressure beats lower pressure and blows you towards the bus. This is the same principle that causes planes to fly.
I knew I could get an answer from smart ST people. Thanks
It would be nice if bus driver training addressed this.
It would be nice if bus driver training addressed this.
… or dump truck training. I was nearly knocked off my bike earlier this year. Similar situation, except I assume that the dump truck I heard behind me would (a) slow down and (b) give me some extra space given that there was no shoulder on that section of road. Guess that they’re paid by the load and don’t give a sh*t about pushing a cyclist into the ditch. Needless to say, I haven’t taken that route since.
The effect is due to the surprise element. A bus or truck has to push a huge amount of air sideways. You experience this as a sudden very strong gust of wind from your left. Instinctively, we will steer toward the left as we are momentarily being blown further right toward the shoulder. But our reaction lags the gust, which is very quick, so in fact we end up overcorrecting and wind up closer to the vehicle.
Brian
I 2nd ergopower’s comment. There might be some turbulent air around the edge of the bus, but not strong enough to create an ‘eddy current’ type effect that will pull you towards it. You’re likely overcorrecting to the gust of air that pushes you out.
Ironically, just the other night I watched an episode of Myth Busters where they tried to prove/disprove the same theory. Despite their pseudo-dummy-influenced-for-TV engineering; there was a guy with wind gauges measuring the effect of a train as it went by at 40 or so mph. There was only an outward flow of air.
I had a very scary ‘suction’ moment a few weeks ago when a wide-load trailer pulling one of those pre-made-house-things went by hauling ass. Combined with the wind, I really thought I was going down.
How long was the train, though? The longer the object, the smaller the effect created by the pressure difference. For a large bluff object such as a bus, the apparent pressure differential would create a closer and more powerful suction force toward the rear of the vehicle. Pretty much any object moving through the air at a reasonable speed would create vortices of suction. That’s the basis of pressure drag.
How long was the train, though?
Maybe a bus & a half, maybe two (if anyone saw the show, chime in). I don’t remember exactly; I just know it was the engine and maybe a car. And it wasn’t a typical large commercial engine. Cable TV budget and a not regularly used track, it seemed small.
Pretty much any object moving through the air at a reasonable speed would create vortices of suction. That’s the basis of pressure drag.
Not questioning the science at all. The intent of my comment was that it was more likely he overcorrected than actually feeling the inward draft at the back. But that is only my opinion; I was not there. The show measured air flow at a single point. I doubt the currents are predictable; and maybe there is a pocket of negative pressure vertically above a pocket of positive.
But my guess, and using a visual just for discussions sake, is that it was more similar to the wake behind a boat, or behind a rock when whitewater rafting. The eddy’s are directly behind the object, and you’ve got to cross a positive pressure barrier first to get to the negative pressure current behind the object. I.O.W., the force of the air pushing away was much, much larger than the force pulling in at the rear. The net affect is still very positive outward, so there had to be some reaction by the rider for him to move closer to the bus as it went by.