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Drafting a truck to make it home

I was coming home from a 220 mile round trip journey and encountered a headwind on the return that pushed my wh/mile up to 380, a level that looked like i would arrive home with 0 miles. the car at 72mph seems to get about 70% of the ideal range meter. I was faced with the decision to slow down to 50mph or so or draft a semi truck. i got behind a car hauler and my wh/mile average went down to 260-270 which at times was better than ideal range. so drafting at 71mph added 30% to my range vs not drafting. I made it to the house with 20 miles still left in the battery.

Cool story, and nice to know.

How close were you to the truck?

averaged about 1.5 car lengths if i had to guess.

Yup. Drafting works great for long haul, butt haulin. Used that many times on my Vegas run. The worst is being all alone pushing all that air. They peel off once in a while and I just slow down to 65 and wait for another tasty barge to go speeding by. I pick it up, tuck in behind, and let him do the work. Even getting in a line of fast moving traffic works if you can be the 3rd or 4th car back. They whip the air and get it moving in front of you. I try to get behind a suburban or something like that.
If you follow something like an 18 wheeler, when you start to grab the draft, at 70 or so, you'll actually hear the wind noise get quieter.
I used to use the speedo to watch power usage, but the new instant mileage on the energy page really shows you what's happening in real time.

While it may work, I'd never condone it. 1.5 car lengths at 70mph is crazy. 70mph is around 110 kmh or 30.5 metres per second. The car is 5 m long, so you are 7.5 m behind the truck. You will close that gap in 250-500 milliseconds if something goes wrong.

Even being conservative, and assuming the truck could only pull 0.7g in braking force then you are still less than a second from hitting it - 1g is 9.8 metres per second.

Most humans have a reaction time between 0.5 and 0.8 seconds, and the brakes themselves have a reaction time of around 0.5 seconds (not to mention the delay in incandescent globes in the tail lights). This also assumes that you are completely attentive and ready to brake, not suffering brain fade from a long drive when you are so close that you can't see past the truck in front.

When i'm driving I can't stand tailgating and will wash my windscreen to make sure that the overspray annoys the tailgater and the drop back or go past.

Its extremely dangerous.

It can be "somewhat safer" albeit STILL NOT recommended. If the Semi Driver is aware you are there and knows what you are doing there and why, and agrees to take you in tow. A good old fashioned CB radio can come in handy for that. From their vantage point they can see much farther ahead and if in voice contact with the semi driver he could alert you of impending slow downs well in advance.

In another life "just out of college" I drove Semi trucks for a stint While I "found myself" lol!!

But that still does NOT cover an emergency for which there is little or no warning.

Yet the number of rear-end auto collisions with eighteen wheelers remains extremely low. Additional risk however is present due to the possibility of tire delamination from the truck ahead.

Efficiencies of up to 11% (mpg) can still be gained at 100 feet (30m) for the less brave.

I'd expect the number of rear end collisions to increase if more people were tailgating trucks. Most won't do it in an ICE car, with good reason.

30m is still driving less than 1 second behind the truck at 70mph. 7.5m is 250 milli-seconds behind.

I'd rather stop and charge than stop suddenly and potentially fatally.

Don't forget the added damage when the windshield is pitted from being too close to the trucks. If adequate charge is a problem, slow down to 65 mph or less and make it to your destination safely.

Mark E;
true, but that's a "difference" timing. If the truck comes to an instant dead stop, you've got troubles no matter if you're 3X as far back.

And regen slowing kicks in faster than braking.

This is not my experiance. For 7 years now, I drove a 24000 lb 5th wheeler in summer. The efficency of the brake is 3 time less than a normal car because of the big mass that you need to stop. So if you tail gate me you will have 3 time more time to stop. I am more consern with the cars that jump in front of me and try to stop! They don't realize that there is a reason why a keep a bigger distance between me and the car in front of me.

MythBusters did an efficiency test on drafting a semi a while back and found that you could get significant benefit even pretty far back - I forget the exact distance. I think it was something like 100 feet. Closer is better, of course, but only up to a point. If you get too close you're modulating the accelerator too much, not to mention it gets pretty white knuckle. This is exactly why I keep thinking all EVs should have adaptive cruise control with a configurable follow distance. That would make this MUCH safer.

I used to draft semi's when I got my Prius. Their forums are full of efficiency stories. I think I could improve from 50 to 65mgp.

After a bit, it was too "white knuckle" as Geek mentioned.

I might sue this method as a last resort as an alternative to being stuck in the Californian or Arizonan desert but that's about it. If I knew I could make it to a supercharger or home by slowing down to 60 that would be my first choice. Drafting is an option but a damn dangerous one.

that of course was intended to read *use*

This is a cool thread. Loved reading ths innovative trick. Not sure whether its as dangerous as some folks are saying though.

Your not sure if sitting behind a 40 ton vehicle at a distance that will close to zero in less then a second if they suddenly brake is dangerous?

@Brian H: you are correct, however being a 7.5m behind also means that you are relying on the truck seeing and reacting, since that close your vision is completely obscured. I observe a 2.5 to 3 second distance as a minimum at speed and it has meant that I've avoided accidents for the last 32 years.

I've seen some accidents developing in front and had time to react, brake, swerve or whatever. In on example where I was not far back - closing to overtake - a wombat appeared from the car in front. I was on a motorcycle and only had time to stand up and attempt to lift the front wheel - and got away with it. I hit the wombat at 120 kmh and was lucky.

For all of the non Aussies here a wombat is a solid ball of muscle the size of a large dog. They can do severe suspension damage if you hit one, and have caused serious accidents. In this case I can only assume that the car hit it first, running over it but between the wheels.

My point with this - if I was my normal 2-3 seconds behind I could have avoided the obstacle easily, but as I was closing I had less than a second to react. No time at all.

I use the 4 second rule where ever possible so drafting is out for me.
I know this is silly to do on purpose but if the battery ran out of charge, could you not charge the battery while getting a tow using regen mode? This would mean that you would not need to get towed all the way. Would probably need a pretty strong tow rope.

@lph

Im pretty sure you are NOT supposed to be "pulled." Flatbeds are the recommended towing procedure.

@lph, @cloroxbb is right. The Model S cannot be towed. You'll void the warranty. The only allowable towing is to get it onto a flatbed [or out of a ditch :-)]

Assuming there was a way to attach the tow rope... what is the reason for voiding the warranty? Would it damage the car in some way?

@lph - the only way is to attach to the suspension, and it needs a 2x4 under the front of the car to take some of the strain. There is no towing eye.

A model S can do 60-0 in about 110'. A truck takes multiples of that. Unless it crashes ahead of you, there's lots of braking time before the breaking time sets in. ;)

One of our truck drivers drilled a hole in the floor of his cab, and placed a piece of 3/4 inch conduit through it. He has a bag of marbles fastened to his seat and when he can't see the car tailgating behind him, he reaches into the bag and drops the marbles down the conduit. Discourages tailgating!! So remember, if you can't see his mirrors, he can't see you.

Always thought that some truck drivers did not have all their marbles ;-)

Drafting works really great for the fuel economy but the hood of the vehicle will look like it has freckles pretty quickly. Think it's pretty severe particularly if you drive on roads that get salt on them at all. Additionally, just the dust and dirt blowing across the fields in the Midwest leave a lot of residue on the road and it hacks the crap out of the hood. Road rash is a big deal! I don't want that on my Tesla.

@Brian: that's where you are wrong. Yes, the car can stop a lot faster than the truck. No contest.

However, your reaction time eats up the space *really* fast, and if you are distracted a all before you know it its an emergency stop to avoid rear ending the truck. Plus being so close you can't see past to get advanced warning of what's coming up.

Oh, and when the truck does do an emergency brake - which you haven't seen coming - then at 0.7g that's just under 7 metres per second that you gain on it. At 7.5 m behind the truck the gap is gone and you have crashed in just on a second. Add into that the human reaction time of between 0.5 and 0.8 seconds assuming an attentive driver expecting an emergency stop. It's about 1.2 seconds in practice.

Once your reaction time is up you have either hit the truck, or are about to.

Mark E, you mentioned that brakes take 0.5 seconds to react. That can't be right, half a second for brakes to activate is eternity in accident timescale.

I agree that one to even two seconds is average action time though. Note that this is not REaction, but actual action. From time to notice that "hey something is happening" to "decision what to do" can be rather long time. Less for experienced driver, longer for inexperienced (also inexperienced driver might do wrong decision far more likely, but that's another matter completely).

Also you mention "are about to [crash]", at that point it doesn't matter much if car can decelerate faster than truck, truck started decelerating earlier, so it is already going quite a lot slower, so add your own car initial velocity before deceleration starts to that and you notice that distance required for you to reach same or slower velocity is quite a bit longer than your reaction time.

tried this for about 2 to 3 miles at 60ish MPH and wow what a difference, was running about 200 wh/m.


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