A civil engineering professor modeled the effect rear spoilers have on car exhaust. What he found might have you consider what kind of cars you follow as you ride your bicycle or motorcycle.
Dr. Aonghus McNabola in the Department of Civil, Structural & Environmental Engineering at Trinity College in Dublin, Ireland investigated the influence a rear spoiler has on a car exhaust plume, using computational fluid dynamics modelling is employed to examine the relationship between concentrations of air pollutants downstream of a moving vehicle and the orientation of its rear spoiler.
He discovered that certain high spoiler angles suck car exhaust right up to head height for pedestrians and cyclists, increasing exhaust concentration as much as 57%. Spoilers with a flatter, low angle profile, however, could lower the head height exhaust concentrations.
I replicated McNabola’s findings by creating my own dynamic paper models. As you can see, a rear spoiler angled up directs poisonous car fumes straight into the unhappy cyclist’s face.
When I angle the spoiler so it’s level with the ground, though, it help keeps the fumes near the ground. The result: a happier cyclist.
The actual overall emissions don’t change. The spoilers just change the distribution of the car fumes near the rear of the car.
From Transportation Research Part D: Transport and Environment, Volume 16, Issue 4, June 2011, Pages 296-301. “Spoiling air pollution dispersion: A numerical investigation of exhaust plume dispersion from cars with rear spoilers.” For the actual fancy computer generated graphics and more discussion, see this NY Times article or the SF Chronicle Bay Bikers blog.