Affect of highway barriers? Affect of side drafting large vehicles?

[Jt martin]

In some spots on the Highway, you often can drive beside concrete walls. Is it adventitious to drive near these concrete walls in order to reduce aerodynamic drag? I have to imagine that the front of your car is pushing air away from the vehicle and into the wall which may reduce overall aerodynamic efficiency. On the flipside, I know that in racing driving at the wall can increases speed. So, the question is, should you drive near the concrete wall or not?

As a second question, is it adventitious to drive beside a larger vehicle? I know that if you have a crosswind, using a larger vehicle can help reduce the negative effects of that crosswind. With this not being a factor, does driving beside a larger vehicle give you an advantage? I know larger vehicles have a tendency to suck air in near the rear of the vehicle, and push air out at the front of the vehicle. I would imagine the vacuum would be the place that you would want to be for maximum efficiency. Is that the case?

Thanks, guys.

[Frank Lee]

Is it what?

[Jt martin]

1. Is it advantageous mileage wise to drive near the retaining wall on the freeway or in a more open spot?

2. Is it advantageous to drive in the low pressure zone on the side of a truck - mileage wise. If you don't know what I mean, the next time you're on the freeway and you're passing a semi truck, notice that as you come up the side of the semi truck there's a tendency for your vehicle to be pulled in towards the truck - and as you get to the front of the truck, the air being pushed out of the way of the truck will tend to push your car away from the truck.

The lighter the car you drive will make this more apparent, as will you and the truck traveling at higher speeds.

[Gealii]

Originally Posted by Jt martin

1. Is it advantageous mileage wise to drive near the retaining wall on the freeway or in a more open spot?

2. Is it advantageous to drive in the low pressure zone on the side of a truck - mileage wise. If you don't know what I mean, the next time you're on the freeway and you're passing a semi truck, notice that as you come up the side of the semi truck there's a tendency for your vehicle to be pulled in towards the truck - and as you get to the front of the truck, the air being pushed out of the way of the truck will tend to push your car away from the truck.

The lighter the car you drive will make this more apparent, as will you and the truck traveling at higher speeds.

1) i say yes to the retaining wall if the highway is not divided as in the traffic is immediately on the other side of the wall

2) The only time i pass a semi is when they just pull out onto the same road and their traveling much slower then their speed limit which im already going so that affect is unseen but i would say yes because the air is being split off at an angle which would help

[euromodder]

Originally Posted by Jt martin

In some spots on the Highway, you often can drive beside concrete walls. Is it adventitious to drive near these concrete walls in order to reduce aerodynamic drag?

It's quite complicated and contradictory.

You get a partial tunnel effect, which is not good for fuel economy due to the negative effect of having walls in the proximity of your more-or-less streamlined shape / car - works a bit like the road.

But then, just about any cyclist will tell you pedalling in tunnels is a joy - as long as the tunnel isn't going uphill steeply - so, what's up ?

The benefit of a tunnel, is that it eliminates all sidewinds, and in my experience, even a "partial tunnel" will reduce sidewind effects if you hug the leeward side of a barrier in heavy winds.

A tunnel with 1-directional traffic will have a traffic-wise airflow, which helps your FE.
But with 2 directional traffic, you bounce into the opposing traffic's pressure wave, which is bad. Ask any biker, sometimes it almost feels as if you're slamming into something when meeting a truck.

If the barrier separates the traffic directions, outside a tunnel, you may get the benefit that it also separates or reduces the opposing pressure waves.

As a second question, is it adventitious to drive beside a larger vehicle? I know that if you have a crosswind, using a larger vehicle can help reduce the negative effects of that crosswind. With this not being a factor, does driving beside a larger vehicle give you an advantage?

In my experience, it depends on your position alongside.
Coming out of a semi's wake, my fuel economy goes down first, then improves again as I pass alongside, only to become worse again as I get near the truck's front. (all on CC of course)

Staying put alongside semis isn't going to make you many friends on the road though


These effects are only seen on the instant fuel consumption display.

[Sven7]

Originally Posted by Jt martin

1. Is it advantageous mileage wise to drive near the retaining wall on the freeway or in a more open spot?

I would say no, because forms such as planes in free airflow achieve much lower drag than those in ground effect. Could you extend that relationship to include highway barriers as a sort of double ground effect? Maybe. On a day that isn't windy, is the barrier really doing anything for you? I doubt it.

2. Is it advantageous to drive in the low pressure zone on the side of a truck - mileage wise. If you don't know what I mean, the next time you're on the freeway and you're passing a semi truck, notice that as you come up the side of the semi truck there's a tendency for your vehicle to be pulled in towards the truck - and as you get to the front of the truck, the air being pushed out of the way of the truck will tend to push your car away from the truck.

Put low pressure areas in front of your car and high pressure areas behind it as often as possible. This is why we like to drive in the wakes of other vehicles- they are lower pressure areas that help "pull" our cars along. If someone is following very, very close (inches) behind you they can actually lower your drag a very small amount as well, but this has little practical application outside NASCAR. Aerohead has a thread on this with illustrations.

Originally Posted by Frank Lee

Is it what?

You forgot affect/effect. (Affect=verb, Effect=noun)

[Frank Lee]

I didn't overlook that; I just didn't want to be mean!

[Sven7]

Haha, only moderate grammatical correction allowed?

[user removed]

Carry it to the extreme. A marble through a tube with little clearance but enough to allow passage. The air ahead of the marble has no where to go so the resistance is huge.
While increasing the clearance will greatly reduce the resistance, it will not be eliminated or even reduced to the point of no increase.

regards
Mech

[serialk11r]

Originally Posted by Sven7

I would say no, because forms such as planes in free airflow achieve much lower drag than those in ground effect. Could you extend that relationship to include highway barriers as a sort of double ground effect? Maybe. On a day that isn't windy, is the barrier really doing anything for you? I doubt it.

But cars aren't streamlined, so it's a bit different. Depending on the design of the car, I think it's possible that if you have a "tunnel effect" then forcing more air to go over the top of the car could be a good thing depending on how the rear end of the car is designed. A lot of cars have little side tapering so the tunnel effect could cut down on trailing vortices and such too.

Put low pressure areas in front of your car and high pressure areas behind it as often as possible. This is why we like to drive in the wakes of other vehicles- they are lower pressure areas that help "pull" our cars along. If someone is following very, very close (inches) behind you they can actually lower your drag a very small amount as well, but this has little practical application outside NASCAR. Aerohead has a thread on this with illustrations.

If the person behind you is following at just inches away, then their car is pretty much filling in your entire wake! You sure it's just a small effect? When driving my mom's SUV, if I get tailgated (that would be following at a bit under 1 car length, typical for California) my speed will go up a few mph while holding the throttle in the same position.