Submitted by Da_Dokta t3_z79t64 in explainlikeimfive
What is a slipstream and how do they work? Is there an ideal distance behind another car you should stay to get the maximum benefits? Is there a distance where it actually becomes a hinderance? If there is a crosswind does it destroy any chances of a slipstream?
It's not necessarily true that there are only benefits at distances where you're "basically tailgating". For example this paper found a noticeable benefit for two tractor trailers even at 20 meters distance.
20 meters might be a little closer than "optimal safety", but in real life on busy highways many people are following that close behind or closer. This paper found the biggest benefit at 60 feet but even at 160 feet there were significant benefits.
There's research being done to automate the process for large trucks so that they move together as a 'platoon', communicating with each other for semi-autonomous braking and accelerating. It would save a whole lot of fuel on long-haul drives.
Leave it to Americans to reinvent trains in a super complicated way.
Semi-autonomous vehicles on existing roadways is way less expensive and less complicated than building completely new rail lines everywhere.
How is it less complicated? We need new technology for that to happen. On the other hand we have trains right now. Tested and ready to go. There are all kinds of safety issues with having those trucks on public roads with other traffic which might mean they don't get approval any time soon.
Rail is complicated for different reasons. Technology isn’t the problem with rail. It’s the cost, getting the right of way, nimbyism, politics, environmental, and so on.
Have you ever seen a rail line built? They’ve been building a light rail line near my house since before I moved in 12 years ago and it’s not scheduled to finish for another 2 years. Funding and engineering went on for years before that.
Whatever new tech we need to get the trucks following each other will be done before this rail line. I think the technology is pretty close already. My wife’s 2016 Subaru has adaptive cruise control and lane assist which seems like enough to draft a semi.
Yes, that's why we have tons of already existing semi autonomous vehicles with many more under construction as opposed to rail. Nobody ever in history has ever build rail and no one in their right mind would think to buil new lines
Woah there Tex. Calm down. It sounds like you’re thinking I’m saying something that I’m not. We can have both. And I really rather like traveling on light rail.
But it’s said there wil probably never be another heavy rail line built in the us because of the cost.
Yeah for sure, I guess I drew a line between the "slipstream" and drafting.
The first vehicle is "breaking" the air, so there's less resistance to the following vehicles. This is also why cyclists and birds tend to flock.
I was thinking of this as two different things and addressed one part, but really it's all a combination of factors.
Is that the same concept as a bukkter travelling through water? It causes a void then water quickly closes the space behind it causing tiny explosions. I was watching the slow-mo guys fire bullets under water. It would cause a void behind the bullet and the water pressure caused subsequent explosions.
On public roads? The slip stream is too close for safe driving. It’s only effective at speeds where your following distance should be much higher so you have time to react in an emergency.
Spend the couple extra bucks on the gas and avoid the potential for large bucks to hospitals and insurance companies.
>> public roads ....too close for safe driving
Although not impossible, as my father used to point out by drafting right behind big trucks for 200 miles and getting almost a whole mile per gallon better gas mileage... occasionally it upset the trucker...
Just because you didn't get in an accident, doesn't mean that it was safe. Trucks may change speeds slower than cars as a general rule, but you can't see what is happening in front of that truck...
I don't think they made any claims about it being safe. To me they seemed to imply the opposite, but just that their father did it anyway.
Mythbusters did an episode on this. Final conclusion: Yeah, it could help, but only at VERY dangerously close distances. And you'll go grey trying to drive that way.
Came here to mention this as well. From what I remember it barely helped at all too. Plus with all the adjusting you'd likely have to do with the gas pedal it's probably almost equal to just set the cruise control and not tailgate.
I remember that episode. it was one of the episodes that led me to believe that those people were largely incompetent and made things dramatic to improve ratings.
Lets say your a freshman (car) in highschool. When the bell rings you've got 5 minutes to get to your class on the other side of the building. But since the bell rings for everyone the hallways are immediately packed with other students (air) and your little scrawny self has to push through everyone else (air resistance). You make it to class but your exhausted having to push through everyone to make it.
Now the next day you realize that one of the lineman (the truck) on the football team gets out of class right next-door and he's heading in the same direction. As he walks, the other students (air) move or get pushed out of his way and then eventually fill back in after a couple feet. You realize you can let him do the hard work of clearing out the other students and then your just follow him close enough that the other kids havent had a chance to fill back in behind him. Congrats! Your using his slip stream! You get to class using less energy!
Now as to why you dont want to do this. Lets say one day he stops to tie his shoe, you dont react fast enough and run into the back of him. He's big enough that he may not even realize you did, but for you? You just ran into a human wall, broke your nose, dropped all your books and split your drink.
Now picture that doing 75 MPH running into a semi. Most of the times you do it, nothing will go wrong and youll save a couple cents of gas. But the one time it does your going to spending 10s of thousands to fix the mistake. The risk verse reward is just not worth it.
Great Eli5
If the car in front of you starts getting the air moving by driving through it, it takes less energy for you to have to push the air so that little extra energy turns into a little extra speed. If you watch motorcycle racing or car racing they do it all the time. Sometime 5-15 mph difference..
Less resistance means less work you car has to do. The physics are the same on racetracks or roses and the distances between cars where a slip stream works are greater the faster you go so it usually isn’t worth the risk at high way or normal road speeds..
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Slip steam only works well at extremely fast speeds. The distance required to slip stream is too small to be safe for highway driving. You might get some effect behind a transport truck, but not much.
How it works. As the 1st vehicle moves, it is pushing air away from the vehicle. This creates a small pocket of slightly less dense air behind the vehicle. If a 2nd vehicle can follow behind, in that pocket of less dense air, then it has less wind resistance slowing it down, giving it the slip stream advantage.
ELI5: Air is heavy and it takes alot of work to push it out of the way. By having someone else push the air out of the way, you can save that energy and increase your MPG.
Most cars are aerodynamic enough that slip streaming isnt something you can do anymore as the air returns back to where it was faster than you can hope to occupy that space. If you had 2 big boxy vehicles, than you could slipstream one off of the other and decrease total fuel usage at the cost of an increased accident risk as any gap will demish the benefits.
If you are driving a regular car on a regular road for regular traffic purposes... don't even think about slipstreams
If we are talking racecars (e.g. NASCAR, Formula 1, etc), front car cuts the air/ air resistance, the car behind it is protected and can speed up without resistance. In a straight line or a long simple turn this is easy.
It gets more complicated when the car in front is going through a bunch turns back and forth at varying speeds, creating "dirty air" cuz that air immediately around that cut air is super turbulent and is going to collapse, creating fluctuating levels of resistance or downforce on the car behind. It's not like this turbulence doesn't exist on straight lines, but it's more of on obstacle on turns where you actually want a consistent weight of air across the car to push it down into the ground to keep grip. On a straight the air resistance is just going to hamper your top speed.
Mythbusters episode on drafting. Starts with don't try this at home. https://youtu.be/lttgT1XZVvE
Dayofsloths t1_iy5i0nf wrote
When something moves through air, it leaves an empty space behind it. New air moves to fill that space, which is the phenomenon you're calling a slipstream.
If it's a very large object moving quickly, like an 18 wheeler going down the highway, there can be a lot of air moving in behind it, and if you're close enough, you can be pulled along with the air.
The ideal distance will vary depending on the size and speed involved, but it's generally a bad idea because you need to be an unsafe distance from them. You're basically tailgating them.
A cross wind would have an effect, because the point of lowest air pressure behind the truck would move, but again, not really worth considering because you'd have to be right up their ass.