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### How beneficial is drafting/slipstreaming?

Posted: Wed Oct 11, 2017 2:37 pm
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.

### Re: How beneficial is drafting/slipstreaming?

Posted: Thu Oct 12, 2017 5:11 am
Brilliant.

How close are you to the rig for those readings?

### Re: How beneficial is drafting/slipstreaming?

Posted: Thu Oct 12, 2017 5:40 am
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

### Re: How beneficial is drafting/slipstreaming?

Posted: Thu Oct 12, 2017 12:41 pm
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.

### Re: How beneficial is drafting/slipstreaming?

Posted: Mon Oct 16, 2017 10:19 am
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.

### Re: How beneficial is drafting/slipstreaming?

Posted: Mon Oct 16, 2017 5:28 pm
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?

### Re: How beneficial is drafting/slipstreaming?

Posted: Mon Oct 16, 2017 10:51 pm
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.

### Re: How beneficial is drafting/slipstreaming?

Posted: Tue Oct 17, 2017 1:55 am
GRA wrote: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:

(ironically I was watching a movie about Jayne Mansfield last night, on Amazon Prime Video)

### Re: How beneficial is drafting/slipstreaming?

Posted: Tue Oct 17, 2017 2:39 pm
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

### Re: How beneficial is drafting/slipstreaming?

Posted: Tue Oct 17, 2017 3:55 pm
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!