Drafting

[Tdave]

Earlier this week I drafted behind a semi truck. You can tell from the change in wind noise when you get in that pocket behind the truck where you start to get some real benefit of reduced drag.

I reset the trip odometer.
Drafted for 15 miles.
Speed was consistent 60-65 mph.
Elevation declined 200 feet over 15 miles (if that matters).

At the end of 15 miles, the efficiency screen read .. (drum roll)

189 Wh/mi

 

[ChargeIt!]

Excellent !
How close behind the truck were you drafting ? Did you experiment with a longer following distance (due to the Roadster being so "low" a long safe distance may actually help) ? Was the airflow underneath the truck "blocked" more than normal (a refrigerator unit, for example; or a taller than normal safety bar?) ...
The curious want to know ...

 

[PopSmith]

That's an impressive number, or at least it seems to be to me.

I'm also curious how close you estimate you were to the back of the semi; probably a pretty safe distance considering you're in a Roadster which is both small and expensive. :tongue: Other than stopping/maneuvering in an emergency I'd be worried about a ruined windshield depending on how close you were while drafting.

 

[vfx]

The wind noise tip is great!

Thanks.

 

[Tdave]

I'm not sure exactly how far away I was. I'd estimate 70 feet, or a little less than a second behind. It was far away enough that once a car changed lanes between us. I once moved to 50 feet for a few seconds while I could see far ahead and knew the road was clear, but that was too close for my comfort. About a second behind is as close as I'd want to maintain on a longish trip. That seems typical for traffic around here. Normally, like during commuting, I notice I follow farther than most cars do on the road.

It wasn't a truck I could see under easily, from what I remember. I wish I had taken more notice of that, as I agree that could make a difference.

 

[mnx]

I love drafting. I used to do it on my motorcycle on the highway. I'd get right behind the semi, probably 15ft or less. Not only would I get excellent fuel economy, the wind was gone as well. It was like driving in a vaccum. The only drawback (other than safety of course) was that the bike would run hotter than normal, the little rad fan would have to kick on there was so little airflow across it. I remember one particular trip, I did several hundred km drafting in this manner. I managed to get 425km on that tank, or at least 20% farther than the average tank.

If I had a roadster I'd definitely do some testing on it too.

 

[TEG]

Old discussion: Benefits of drafting

 

[DrComputer]

I did it on a trip from LA to Santa Barbara. I drafted behind a truck for almost 45 minutes. My ideal mileage and actual mileage display actually flipped (first time I ever saw that). After my 80 mile tip the car still said I had 170 miles left of Actual miles and 165 of Ideal miles. I was impressed.

 

[TEG]

On my daily commute (in a non-Tesla EV), if I draft the whole way, I get back home with 40% charge left. If I don't find a big truck going my way I end up home with 20% charge left. Noticeable difference for sure. I have a 90% freeway commute on a busy trucking route, so probably half the time I get a chance to draft. I follow as close as I can without attracting too much attention. Also, I have to stay attentive... I am putting myself at greater risk of rear-ending a truck than I would if I wasn't trying to save energy. Sad when you have to trade safety for efficiency. The other day, I was dreaming of the freeway being a long split tunnel with fans blowing air in the direction of each lane. The way we do personal transportation we have built a giant machine to just re-churn the air surrounding this row of cars. If we can't get people to take trains or even busses, or even carpool, perhaps a way to link cars together in groups going the same place could be done someday. You want one in front to plow the air out of the way and not have the trailing cars run into the wake turbulence.

 

[ChargeIt!]

... perhaps a way to link cars together in groups going the same place could be done someday. You want one in front to plow the air out of the way and not have the trailing cars run into the wake turbulence.

Great idea ! ...
(Hmmm ... seems to me geese are smarter (or is that "less political" ? ) than people ! See them doing it all the time ! :biggrin

 

[doug]

Automated drafting is an old idea that pops up every now again.

Here's one of the more recent iterations:

'Road Train' Autopilot Driving System Cuts Fuel Consumption, Travel Time - treehugger.com

EU to Trial “Road Train” Tech: Sensor-Enabled Automated Driving (Whoa!) - earth2tech.com

 

[vfx]

Drafting is unsafe and good for your fuel bill but what about the trucking company?

 

[doug]

What about the trucking company?
It's not like your drafting more than negligibly affects the fuel economy of the truck in front of you. (If that's what you're talking about.)



.

 

[Tdave]

Drafting actually improves the efficiency of BOTH vehicles.

From Wikipedia ...

The low-pressure wake behind a group's leading car reduces the aerodynamic resistance on the front of the trailing car, allowing the second car to pull closer. As the second car nears the first, it pushes high-pressure air forward so less fast-moving air hits the lead car's spoiler. The result is less drag for both cars, allowing faster speeds.

Although a Tesla behind a big rig will have negligible positive effect on the truck. At at the distances we're talking about, the positive effect will be negligible even on a lead car or truck about the same size as us. But the point is that the effect is positive on the lead vehicle, not negative.

 

[vfx]

In racing, faster speeds just means passing other non drafting cars.

A Semi cutting the air for a tiny car has many things to worry about.

The Wiki page had these:

Drafting can be cooperative, in which several competitors take turns in the lead position (which requires the most effort and energy consumption).

Truckers are not fond of the extra stress this puts on them, worrying about cars on their tail.

I don't know of any trucking companies that are down with this esp in regards to insurance issues.

 

[TEG]

http://news.bbc.co.uk/2/hi/technology/8349923.stm

 

[PopSmith]

I'm not sure exactly how far away I was. I'd estimate 70 feet, or a little less than a second behind. It was far away enough that once a car changed lanes between us.

In my opinion that's far enough away if you are keep an eye on your mirrors.

I've had not-so-smart "drivers" that stay in the spot between me and the car I'm drafting, which usually are semi-trucks or large SUVs, at that distance and it freaks me out because they make it really hard to avoid an accident if the driver I was drafting hits their brakes with much force.

I don't know of any trucking companies that are down with this esp in regards to insurance issues.

It might put some undue stress on the semi driver if they know you are back there and they can't see you in their mirrors.

I'm not sure how it would work with a trucking company but if I run into the back of someone then it's my fault for driving too close there-by making any damage to either vehicle my fault and not the fault of the driver I was following.

 

[Tdave]

I took a Range Mode trip this weekend. For the longest segment of my trip, I was able to draft behind semi trucks for most of the way going 65 mph. I was able to achieve near Ideal Miles for that segment of my trip. Here are the stats from the Roadster trip computer:

131.4 miles traveled
2:26 hours elapsed
29.85 kWh used
227 Wh/mi

Looks to me that I could have gotten 233.3 miles on the full charge. Pretty good for highway driving at 65 mph for the majority of the trip.

 

[VolkerP]

Want to suggest an experiment in cooperative drafting here. Gather four Roadsters on an empty highway, go at 70mph (or 65 if it's the limit) with 30ft distance between the cars, and have the lead car fall back to the tail every once in a while. What would be the Wh/mi achieved in comparison to going standalone?
Perhaps Roadster owners leaving the Fremont Event could try this?

 

[Doug_G]

I dunno, I think you need to draft behind a bus, not a Roadster! You need that huge brick making a path through the air.