Can aerodynamic downforce from a standard car cause lateral pull in others?

[jmelinte]

I have a friend who argues that her car creates enough downforce to cause other cars to veer towards it as they pass. I can understand cars behind it being affected, but I can't imagine 4 door car creating a force similar to that felt while driving alongside a semi truck. For what it's worth, the friend's car is a Subaru WRX STI, with a lot of body work, moving at roughly 70mph.

My understanding of this phenomenon in the case of a semi is that the air being displaced by it is essentially pulled to the rear, where it has to replace the space left by the truck. As a result, air rushes along the top and sides, creating an area of lower pressure, and pulling a car traveling along side it toward this lower pressure. However, in a race car, wouldn't the downforce be created by more air being routed over the top of the car, not the sides (not to mention much less air volume being displaced)?

 

[sophiecentaur]

I have a friend who argues that her car creates enough downforce to cause other cars to veer towards it as they pass.

I wouldn't be surprised if she were right, in principle; aerodynamics is full of surprises. I would have expected the Bernouli effect would be 'measurable' as the two cars are traveling through the air, with a small gap between them. That would tend to pull them together. The low pressure at the rear of a car can affect other vehicles. Cyclists can easily slipstream a box shaped van and they don't need to be directly behind it. The effect stops as you near the front of the van, though.

 

[jmelinte]

I wouldn't be surprised if she were right, in principle; aerodynamics is full of surprises. I would have expected the Bernouli effect would be 'measurable' as the two cars are traveling through the air, with a small gap between them. That would tend to pull them together. The low pressure at the rear of a car can affect other vehicles. Cyclists can easily slipstream a box shaped van and they don't need to be directly behind it. The effect stops as you near the front of the van, though.

I can certainly understand the lower pressure behind the car having a huge effect, hence why cars draft each other in racing. The bicyclist situation makes sense too, but extrapolating that to a car gets a bit fuzzy in my head. A 200lb cyclist would get pulled easily, but would this pull have any actual effect on a 3000lb car with 4 wheels on the road?

 

[sophiecentaur]

There can be a psychological effect when a driver looks to the left and tends to steer a bit that way , too .
Measuring the effect could be difficult because the force could be small. Perhaps driving two high sided box vans close together and photographing from the rear could show a small angle of tilt between them. If the result turned out to be significant then you expect the effect between cars too, but less.

 

[jmelinte]

There can be a psychological effect when a driver looks to the left and tends to steer a bit that way , too [emoji2].
Measuring the effect could be difficult because the force could be small. Perhaps driving two high sided box vans close together and photographing from the rear could show a small angle of tilt between them. If the result turned out to be significant then you expect the effect between cars too, but less.

That was my counterpoint to her argument. I think it's more likely that people are looking toward her car and veering in that direction. I've driven past some pretty fancy cars, with far more elaborate aerodynamic engineering, and never felt any sort of lateral pull when passing. That said, I've definitely felt something when behind said cars.