How beneficial is drafting/slipstreaming?

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Supersleeper

Well-known member
Joined
Aug 7, 2017
Messages
124
Location
SF Bay Area
I'm conducting a DIY hobby/project right now, that involves a custom directional anemometer I designed and 3d printed. I've currently hooked it up to an old DC motor (with minimal cogging effects) I had laying around, and attached the leads to a V/Ohm meter.

I'm experimenting with placement. So far, the front left corner of the roof line appears to be optimal, though I may play around with placement towards the middle and rear. I'm currently using velcro, but intend to move to a magnet base so I can relocate it anywhere on the roof line. My current design is closest to a turbo type impeller that should help prevent side-draft deviation. It may not be the most efficient, but I think it will suit my needs for now. It currently kicks in at about 35mph at 1v, and at 65mph, I average 12v. This works well as drafting under 40mph is quite negligible on economy anyway.

I've already made a discovery doing this in only two runs. I'm getting pretty steady results without cars around, but while drafting behind big rigs, I've noticed that any adjacent vehicles basically kills about 1/2 the drafting effectiveness. On a calm day at 65mph behind a big rig, I get roughly 4-6v. That varies by half the voltage without cars. As soon as a car or truck comes beside me, it jumps from there to 7-8v.

I've yet to equate it to a linear scale just yet, but assuming 35mph is 1v, and 65mph is 12v, 5v should be close to 50mph. Next run I'll test out my theory and it will help me determine relatively accurate voltage to wind speed ratio. If it's higher or lower I'll have a curve to base it on. I'm also looking into adjustable analog meter that I can replace the digital voltage meter with. if I can find one that has a numeric scale of 0-100 that operates between 0-20v, that would likely be ideal, and can calibrate it to actual wind speed if the scale turns out to be a linear one.

I've considered going Arduino to do a digital calculation and logging, but I'm intending on keeping this project as simple as possible. Though, with Arduino I could use a much simpler anemometer based on magnetic switch or (hall effect) to make it more weatherproof, lighter and more accurate at low speeds. As soon as I start getting that complex, I start to considering using a canned Bluetooth anemometer. Those seem to start at around $70 on Amazon, and so far this project only cost me a few cents in 3D plastic filament.

I've also started 3D printing vortex generators. Once I get a handle on my anemometer setup, I'll start implementing those and seeing just how well they actually work. I have a theory that they can be useful in the front of the car just as much in the back to reduce drag.
 
And another neat thing to know, if the big rig has one of those aero ends(a back open box type of thing that extends 4?? feet behind the rig and tapers towards the center) does that increase your drag(or decrease the benefit of tailing the truck)?
If feasible I like to ride in the tail wind of a large truck, generally staying 50 or so feet back. Closer seems better but I don't like the trucker feeling I'm cramping their space. I've also noticed much more than 50 feet and I start losing the benefit, at some point my Leaf shimmies side to side(kind of buffets) and by 100?? feet I seem to lose the benefit. Going 70mph riding in the tailwind of a large truck can cut my drag in half, or at least it feels that way :)
 
Right, those aero ends would effectively create a vortex and collapsing much of the low pressure into varying pressure points trailing the rig. if you've ever watched a stream of water come out of a hose end that isn't perfectly round, you get the idea. It does what appears to be weaving of the air back on itself in a perfect vortex. That's kind of what's nice about this anemometer is I'll know high and low pressure zones just by glancing at the meter.

I lucked out and discovered an old analog 0-15v laying around my garage. Hooked it up and works perfectly. I much prefer watching the needle move back and fourth compared to the digits. I've also discovered it works better in the lower voltage end as I've discovered it's closer to 3-4v around 35mph. I need 35mph to break the coging cycle of the motor, and once spinning, I'm able to still register spinning down to about 25mph. It's that initial bump you have to get the stator/armature past the permanent magnet. Then it caries enough momentum to work in lower speed wind conditions.
 
I noticed a definite improvement in when getting behind most vehicles especially tractor trailers as my power dots would drop from 3 to 2 at 60 on level ground typically.
 
While I appreciate the curiosity driving this, drafting strikes me as a case of penny-wise and pound foolish. If you have to do so to get to your destination you're using the wrong car, and if you're doing it to save money on energy, your medical and repair bills from an accident will far outweigh the energy savings. Assuming you and not your heirs have to pay them, that is. Until we've all got 6+ nines reliablility autonomous driving with V2V coms that enable platooning, drafting belongs on race tracks, not public roads. Aside from being illegal at the requisite following distances, it's simply unsafe. Under-run accidents almost invariably cause severe or fatal injuries, and the under-run bars on the backs of trailers can be damaged in normal usage and fail at the critical moment. Even when they work as designed, you're still looking at thousands of dollars in medical and/or repair bills. Is it really worth chancing your or your passenger's lives?
 
It can be very effective. When I was 18 and stupid I drafted big trucks with my bicycle and could easily keep up with 40mph traffic. If you've ever tried to pedal a bicycle 40mph on level terrain you know it takes an immense amount of energy *without* slip-streaming.

There is, however, a downside to closely tailing this way. I have arthritis in my left wrist to prove it. :roll:
 
GRA said:
Under-run accidents almost invariably cause severe or fatal injuries, and the under-run bars on the backs of trailers can be damaged in normal usage and fail at the critical moment.

Even when the trailer is brand new, the so-called "Mansfield" bar can still fail on you:

https://www.youtube.com/watch?v=C3MPKLy9qHU

(ironically I was watching a movie about Jayne Mansfield last night, on Amazon Prime Video)
 
Use common sense people! Don't tailgate less than 50 feet and even then keep a CONSTANT view of the vehicle in front of you. If doing that I have little worry following a large 18 wheeler, after all if I'm awake and ready to break, I have little fear my Leaf can stop quicker than the 18 wheeler in front of me :)
More worry to me than rear-ending the semi is road debris. The truck may be able to drive over something that my Leaf may not and following close doesn't give me much time to swerve out of the way, thats my main concern. My father used to slipstream semi's in his Vespa.....until one day the semi drove over a piece of heavy cardboard and made it airborne. It hit his windscreen and cracked it and almost made him lose control, after that he didn't slipstream semis anymore :D
 
Drafting, especially bump drafting can be real effective. Just set the lead car cruise 1 MPH lower and push away. You do need to be careful in the turns to not slide sideways across the front car's rear bumper. But just in case, it doesn't hurt to get one of those rubber bumper protector flaps to cut down on the scrapes. After about 4 laps, (~8 to 10 miles) swap spots to even out the battery usage. With this technique you'll win the electric-enduro for sure!!!


Wait.... you mean draft on a highway, in real traffic, with strangers......ARE YOU CRAZY?????


But I like the science....just play safe!
 
Nubo said:
It can be very effective. When I was 18 and stupid I drafted big trucks with my bicycle and could easily keep up with 40mph traffic. If you've ever tried to pedal a bicycle 40mph on level terrain you know it takes an immense amount of energy *without* slip-streaming.

There is, however, a downside to closely tailing this way. I have arthritis in my left wrist to prove it.
I think many people who saw the trailer drafting scene in "Breaking Away" attempted to replicate it when they were teenagers, and thus immortal ;) https://vimeo.com/50872582

From the bicycle records wiki:
Fred Rompelberg from Maastricht, Netherlands is the current holder of the motor-paced speed world record cycling with 268.831 km/h (166.9 mph) since 1995.[citation needed] He used a special bicycle behind a dragster of the Strasburg Drag Racing Team at the Bonneville Salt Flats.
https://en.wikipedia.org/wiki/List_of_cycling_records#History_of_motor-paced_records

Hopefully, most people here are old enough to realize that, as with rumors of Mark Twain's death, rumors of our immortality have been greatly exaggerated. :shock:
 
LOL, you guys are funny. I'm not talking about riding a rig's bumper. I'm talking about maintaining a safe distance. I might be a little shy of 2 seconds, barely. It might not be as effective as if I were a car and 1/2 length behind, but it still registers wind speeds at under 50mph, which means it's still effective.
 
Supersleeper said:
LOL, you guys are funny. I'm not talking about riding a rig's bumper. I'm talking about maintaining a safe distance. I might be a little shy of 2 seconds, barely. It might not be as effective as if I were a car and 1/2 length behind, but it still registers wind speeds at under 50mph, which means it's still effective.

Interesting. At that distance the slipstream is probably breaking up pretty badly. While the average for the anemometer may be lower than your ground speed, that result may not be conclusive evidence for lower energy requirements for the vehicle. The buffeting could be affecting the laminar flow over your vehicle and negatively affecting its aerodynamics.
 
Tsiah said:
They did this on myth busters. You'd have to be like 6-8 feet from the truck to have any useful effect to drafting.
https://www.autoblog.com/2007/10/28/mythbusters-drafting-10-feet-behind-a-big-rig-will-improve-mile/

There's a table for 55 mph, showing drag reduction for distances of 100, 50, 20, 10 and 2 feet. As these are all much too close at 55 mph (100 feet back only gives you 1.25 seconds to react), I provide this for info only. For more info there's this, which is talking about red lights:

1.) Driver Reaction Times (tr)
Driver reaction time includes recognizing the light has changed, deciding to continue or brake, and if stopping engaging the brake (remove foot from accelerator and apply brake). Reaction times vary greatly with situation and from person to person between about 0.7 to 3 seconds (sec or s) or more. Some accident reconstruction specialists use 1.5 seconds. A controlled study in 2000 (IEA2000_ABS51.pdf) found average driver reaction brake time to be 2.3 seconds. The study included all driver types, test were conducted on a controlled track and in a driving simulator.

Driver Reaction Times
0.7 sec -- about as fast as it gets
1.0 sec -- old standard
1.5 sec -- common use
2.0 sec -- common use
2.3 sec -- AVERAGE
2.5 sec -- used in a few states
3.0 sec -- NSC and UK Standard

A few states, including California, have adopted a standard driver reaction time of 2.5 seconds. The United Kingdom's Highway Code and the Association of Chief Police Officers ACPO Code of Practice for Operational Use of Road Policing Enforcement Technology use 3.0 seconds for driver reaction time. The National Safety Council (NSC) recommends 3 seconds minimum spacing (3 second reaction time) between vehicles traveling in the same lane.

Test out your reaction time by playing the PassMeFast Emergency Stop game.
http://copradar.com/redlight/factors/

Here's another: http://www.visualexpert.com/Resources/reactiontime.html

Personally, while the driver's ed books and films in California said a minimum of 2 seconds spacing, I've always used at least 3 whenever possible, as my truck driver father taught me. I've been very glad to have that extra second on a couple of occasions.
 
GRA said:
t-behind-a-big-rig-will-improve-mile/[/url]

There's a table for 55 mph, showing drag reduction for distances of 100, 50, 20, 10 and 2 feet. As these are all much too close at 55 mph (100 feet back only gives you 1.25 seconds to react), I provide this for info only.

Personally, while the driver's ed books and films in California said a minimum of 2 seconds spacing, I've always used at least 3 whenever possible, as my truck driver father taught me. I've been very glad to have that extra second on a couple of occasions.
Those distances are too close at 55mph and most of the time trucks are doing 65 or 75 on the freeway here. They frequently pass me like I'm sitting still in the Leaf!
I try to stay 2-3 seconds away at the very least. If I'm that close to another car I pass them or slow down a little more.
 
As the Leaf is a local commuting vehicle any savings or economy would be very small, and therefore do not justify the risk. However, with the advent of long distance EV technology drafting could be of some help in the case of strong head-winds.

Also, I would not even dream of trying this without the Tesla or other Adaptive cruise control that will maintain the spacing.

On our cross country trip with our new Tesla in June we found ourselves in a situation that we needed to extend our range because of missing a Supercharger. In this case we slowed down from the 80mph that we were driving and tucked behind a semi travling 65mph. At the time we did have a 5-10mph head-wind. We set the follow distance at 1 car length and let auto-pilot do the rest. After a little over an hour we had gained sufficient reserve to make it to the next Supercharger. We still stayed behind the semi until it exited the freeway and continued on for a few miles to the Supercharger. Fortunately this was the only time we felt a need to do this but it worked great. just saying....
 
Graffi said:
As the Leaf is a local commuting vehicle any savings or economy would be very small, and therefore do not justify the risk. However, with the advent of long distance EV technology drafting could be of some help in the case of strong head-winds.

Also, I would not even dream of trying this without the Tesla or other Adaptive cruise control that will maintain the spacing.

On our cross country trip with our new Tesla in June we found ourselves in a situation that we needed to extend our range because of missing a Supercharger. In this case we slowed down from the 80mph that we were driving and tucked behind a semi travling 65mph. At the time we did have a 5-10mph head-wind. We set the follow distance at 1 car length and let auto-pilot do the rest. After a little over an hour we had gained sufficient reserve to make it to the next Supercharger. We still stayed behind the semi until it exited the freeway and continued on for a few miles to the Supercharger. Fortunately this was the only time we felt a need to do this but it worked great. just saying....
You've just shown why the Tesla was the wrong vehicle for the job, on that route at that time. You're being forced to trust your and your passengers' lives to some hardware/software of unproven reliability and effectiveness. Until BEVs and their charging infrastructure have improved to the point where no one will be forced to adopt such measures merely to get to their destination, they will remain an unsafe choice for any such trip.
 
Tailgating is both unsafe and illegal in a Tesla, but you will be seeing legal and safe tailgating soon:

Are those 80,000 pound trucks tailgating each other? Soon it may be perfectly normal - and safe

If you look to the next lane and see two 18-wheelers roar past at 70 mph with just 10 yards between them, you'll probably think they are dangerously close.

In this high-tech age, that may no longer be true. In fact, it might be just the opposite, and it might even end up saving you money...
 
.....what goes around, comes around......
Breaker 1 9, we got ourselves a convoy ;)
Back when gas prices were high in the 70s such long convoys of trucks were quite popular, and I never heard of too many people dying or having accidents. Oh and personally I would NOT use CC while drafting, even adaptive, eyes on the bumper of the vehicle in front you at all times and preferably the vehicle in front of that if possible(around corners etc.). Again a car will stop a heck of a lot faster than a 80,000 semi, if the driver of the car is paying attention....
 
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