Article,EPA,Hypermiling

Follow Large Vehicles (at a safe distance)

9 May , 2016  

-Tony Schaefer

First of all, I want to make it clear that this is not an article about drafting. To prove that point, let me explain what drafting is and why it’s dangerous, illegal in some places, and a really, truly, and seriously bad idea.

In auto racing, drafting is also referred to as “slipstreaming” because the goal is to ride in the slipstream of the car in front. In order to do this, the trailing car must be close enough to the leading car to stop the air from collapsing around its back. By doing this, the airflow moves around both cars as though they were one single vehicle. Since much of the effort of propelling any object goes into wind resistance, the trailing car is able to travel much more efficiently. This is shown in the following image, stolen from efluids.com.

Drafting

Drafting is most beneficial at high speeds where wind resistance is great. This is why racecars draft and some people draft behind semi trailers on interstates. Drafting is rude because the trailing car is much too close to the leading car. It is also extremely dangerous because driving can be unpredictable and drafting leaves virtually no room for sudden movements or braking. To reiterate: don’t do it.

So What’s the Difference

Driving behind a large vehicle at a safe distance is similar to drafting only in that the vehicle in front takes the brunt of the air resistance. Unlike drafting, the trailing vehicle is following at a safe distance. The benefit to the trailing vehicle is that the lead vehicle has disturbed the air in such a way that it does not represent as much resistance to the trailing vehicle.

Notice in the illustration above that the air flow lines are drawn parallel to each other in what’s called “Laminar Flow”. Air is illustrated as strata – or sheets – of air that don’t interact with each other. This does a great job of showing that air is moving around the car, but this is not how air really flows. The goal of most automobile manufacturers is to perturb the air as little possible in order to make their car more efficient.

However, there are many not-so-efficient vehicles on the road such as delivery vehicles, semi tractor-trailers, etc. These vehicles have large flat backs and leave huge eddies of air as they travel. Anyone who has ever been standing on the side of the road when a semi tractor-trailer rolled by understand just how strong their wind currents can be. Depending on the aerodynamics of the truck and their speed, their wake can extend hundreds of feet. This is illustrated in the following image.

Semi Aerodynamics

This image is probably the best to illustrate the difference between following at a safe distance and versus drafting. Someone drafting would want to be in front of the red, disturbed air, between the truck and the first marker line. Notice how the dark blue actually curls up and towards the back of the truck? This will literally pull the trailing vehicle towards the truck’s bumper. Following at a safe distance has the trailing car behind the disturbed air, in the light blue area, almost two truck-lengths away. Even at that distance, there is benefit thanks to the disrupted air.

The Other Reason to Follow Other Vehicles

Deserved or not, hybrid drivers have a stereotype of being slow drivers. Sadly, I have seen some cases in which this reputation is strongly earned. Having said that, there are times when I am on a multilane road and I just don’t feel like driving like a bat out of Hell, acting as though commuting is a competitive sport with a trophy handed out if you get to work faster. Perhaps I only want to drive a few MPH above the speed limit and have no concern with keeping up with the general flow of traffic.

Sometimes, I come upon a delivery vehicle or lawn care truck that is driving close to the speed I prefer. In this situation, I will maintain a safe following distance and stay there. There is a very good chance – like many of my phobias – that it’s all in my head, but I like to think that rather than me being the slow car now it looks as though I’m the poor car stuck behind the slow vehicle. But I don’t care because in the end I get to drive at my desired speed. This might sound like a stretch, but I was a little relieved to see a post on an online forum in which someone asked if they were the only ones to apply this technique. Multiple people admitting to it.

At a slower speed, any benefit from perturbed air is minimal if there is any at all. The primary point in following a large slow vehicle is the benefit of not feeling as though you are being pressured to driver faster than you would prefer.

Summary

Drafting is a really stupid idea and extremely dangerous. Following large vehicles at a safe distance can reap aerodynamic benefits without risking your life. Sometimes following slower vehicles gives you the opportunity to get out of the roadway racetrack and drive at a more comfortable speed.

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