Does Prius Draft Exceptionally Well?

First, let me be clear - I am NOT advocating the practice of 'drafting' or following closely to take advantage of another vehicle's aerodynamic pull. That is a dangerous practice that should never be done.

That having been said, I found myself in a situation today where following relatively closely was difficult to avoid. It's a long story and I don't want to get into it. Let the scolds consider me duly scolded - I should have found a way not to do it. Ok, dangerous behavior, duly noted, we get it. Anyway, I ended up at 40-50mph, about 2-3 car lengths back from some sort of SUV-like pickup truck thing on a road I travel often. I know the road and how the glide usually goes, so I was surprised today to find that my glides went REALLY long. I have never felt a car pulled along so much as my Prius was today. On a couple of occasions, I had to use the brake while they were on the gas, just to keep from catching up. I ordinarily stay well back to avoid rock chips and such on my new baby, so this was really the first time I experienced it... but I would swear this car drafts at least twice as much as anything I'd ever experienced - very tangible effects. Even back at safe, but marginal, following distances, the pull from the car in front was very noticeable once I become conscious of it.

So again, I'm not condoning, endorsing, sanctioning, or otherwise encouraging following closer than recommended by whatever legal bits and good samaritans say is safe and legal; but has anyone else ever noticed the Prius' amazing tendency to draft like it's being pulled by a magnet (like that other user's clever avatar)?

How much do you like your front end? Not by hitting the vehicle ahead of you but due to all the road garbage kicked up by the vehicle ahead. Even at low speed of 20-25 mph the tires of the vehicle ahead of you will still throw up crap that will ding your paint.

Over all you Prius should draft just as well as any other car.

The problem with the Prius is it is much slipperier than most cars so the MPG advantages of drafting will be much less than say a F-150 pick up. Because of the low .25 Cd this car will coast forever compared to most other cars on the road.

I would think that the lower the CD of a vehicle, the less effect drafting would have, just because there would be less drag to begin with. Motorcycles for example have horrible aerodynamics and back in my road racing days, it was really surprising how well you could draft a motorcycle, but that's on a race track.
Used to be an avid bicyclist in my younger days, and again your so much faster in the middle of a paceline that out on your own that's it's amazing.
Somebody help me, but at highway speeds isn't aerodynamic drag what eats up most of our power?

I think the three biggest issues in drafting are going to be weight, rolling resistance and drag. If you are close enough to a large enough truck, a boulder on wheels will get a benefit.

The Prius may not be puffed rice on wheels, but it is lighter than most cars and trucks. The rolling resistance also will be better, particularly if you have narrower tires at a good air pressure, and you know how to maintain a glide. And, nothing will beat the Prius for lower air resistance.

For all of these reasons, I believe the Prius may get more of a "drafting"-type of benefit when fairly far behind a large truck. I notice the effect when on the highway, even following at better than a 3 second interval.

I have the dynamic radar cruise control (DRCC) allowing me to choose one of three following distances. Initially, I set the farthest distance and followed big rigs -- not for a drafting benefit, but so that I could drive with the fuel efficiency that most truckers achieve by gaining speed on downhill runs and bleeding it off on the uphills. By having an 18-wheeler in front of me, I don't get too many cars trying to get into the cargo area of my car.

I found that even at a fairly distant range, the fuel economy seemed to kick up when I was following a truck. Yes, some of it was attributable to my intended goal of driving like a trucker. I had been doing this anyway, though, and the truck really served as more of a safety/comfort issue by keeping people from blasting away at me as they passed.

I decided that if Toyota felt the three distances all could be safe, then I would try them all. Moving closer to a truck, I found the mileage increased even further. The only down side was that if the trucker liked to brake a lot, then I would brake a lot and the mileage would suffer. Anyhow, I believe the increased fuel economy from a closer following is due, almost entirely, to a drafting effect.

Let me be clear on this point: Even at the closest point allowed in the DRCC setting, I am able to see both the left and right side view mirrors of the truck in front of me, so I know that he can see my entire car at all times. I feel this is a safe distance. It is still far enough away, though, that I've had people "cut in" and get between my car and the truck. This causes my brakes to kick in, and it doesn't make me very happy. Still, at the closest distance this doesn't happen as often as it does at the two farther-following distances.

I read this pretty much the same as a_priori above. I think the very low rolling resistance of the Prius(LRR tires at 50psi, and generally very well designed in that respect) is what made it so noticeable. In most cars, the rolling resistance drag, in combination with the trailing vortices at the back of the car, override most of the 'pull' from the draft so you really don't feel it. But the Prius has such great aerodynamics, and in combination with the very low rolling losses, the effects of the draft seem to be very much more noticeable. Was to me anyway... I got about 5mpg higher on that run than I usually do and I was going 5-10mph faster besides.

As a funny side note, I can't help but notice that since I put in disclaimers about the unsafe nature of drafting, I just got scolded about treating my front end paint badly instead... have to chuckle Glad I got 3M paint protection film though

i have been in drafting situations before albeit briefly, and to draft effectively, u really have to be way closer than i will travel unless following something much bigger than u like a semi.

i do notice as the following distance increases, i will hit an area of buffeting turbulence that is very noticeable and somewhat extreme. granted this only happens in a very small window

problem is that all vehicles create turbulence that worsens as distance behind vehicle increases. with the Pri's low Cd thru normal air, i would think that unless drafting painfully close, the turbulence might cause an insignificant gain.

all this does assume certain wind conditions.

That 3M film isn't 100% effective when is comes to high speed projectiles coming off the tires from the vehicle ahead of you.

One of my employee's just replaced the shattered rear window in his 2000 Focus yesterday. Guess how it shattered? A truck kicked up a stone while going the other direction on a two lane road. His wife was stopped at the time. $325 to replace the rear window.

That film will stop probably 98% of the material hitting your front end. Which is what makes that 3M film worth every penny you paid for it. What about that 2% it can't stop? You know that 2% the stuff that can cost you a windshield, headlight or a dent in your hood.

Like what was said you can get the benefit of a draft as far back as 2.5 to 3.0 seconds behind a larger vehicle.

And I did make it clear that I was not advocating the practice, that it is not something I generally do, and that other people shouldn't emulate it. In fact, I specifically mentioned it was the first time it ever happened. But thanks for educating us all on the dangers of flying rocks. Very good advice.

Yes, the Prius does "draft" extremely well. Typically, I travel at no
more than the PSL in the right hand lane. A lot of vehicles pass me,
cars and 18-wheelers alike, and then pull into my lane and then pull
away. When it's an 18-wheeler, the FE slips up ~5 MPH for a while.

I like the +MPGs, but driving behind or even alongside the cab of a
truck gives me the heebee geebees -- you can also draft, or more
properly "surf" on the bow wave of an 18-wheeler.
* First off, when behind a truck, I cant see as far down the road as
I want, say 200 yds or more.
* Secondly, should one of the truck's 18 rims overheat -- typically
from a faulty air brake --and the tire explodes, the slab of tread and
hunks of side wall go flying and bouncing in totally random directions,
but biased rearwards by the air mass streaming in that direction
relative to the truck. The Prius' front end body work is really
susceptible to FOD, Flying Object Damage.

FWIW, I've seen this on two occasions, and in one of them had windows
blown in. It scared the living sh!t out of me and the passenger whose
window exploded inward, showering us both with shards of glass.
In the other, we were far enough behind to dodge the debris flying
and bouncing through the air, I'd say about 100 yds back.

This info is not meant to be a scold, just a few more data points for
consideration.

I appreciate the feedback. I was in a not dissimilar situation. This vehicle pulled out in front of me on a two lane road and not wanting to lose my glide, I let the following distance close as the other vehicle accelerated, figuring they would pull away in a few moments like they always do. But the glide never seemed to end and when I finally started looking at the gauges wondering why (like I said, I knew the stretch of road and noticed the glide was way long), I saw that I had gained an unexpected 5mpg (or more) and was going 10-15mph over the PSL besides (very out of character for me) - Gidgy (my Prius) just never slowed down. Very interesting experience. And yes, very good points about exploding truck tires - they suck! Does surfing the frontal wave really produce a tangible effect like the trailing draft? If so, that's certainly a slightly safer way of doing it, assuming you don't piss of the truck driver. How close does the truck have to be for you to feel the push?

While drafting an 18-wheeler is dangerous, I've wondered if it's possible to "surf" their wake. Do 18-wheelers even leave a wake, like a boat? It's no doubt a much smaller force than the vacuum behind the truck, but the Prius is so aerodynamic that the effect might be noticeable?

EDIT: Just read Rokeby and I guess it is possible to ride the bow wave. But now that you mention tire blowouts, I've seen the treads on roads and they're quite substantial, so perhaps it's not a great idea.

I know between you and me I don't need to say this, but for others'
benefit; Hon, you don't want to "surf" a truck's bow wave. You
never know the trucker's mind. If he gets pissed, he might, a very
small might, turn on his turn signals. He's coming into your lane and
you've got scant seconds to accelerate very quickly, or drop back. If
he's in a bad mood. you won't even get the signal. He's going to push
you into the breakdown lane, and there's nothing you can do about it.

You've already experienced what it feels like to "catch the wave," but
very briefly. Every time a truck passes you get buffeting, and then a
short period of "lift." If you have a SG, you will see a brief +MPG.
Otherwise, if you've got a sensitive "butt accelerometer" you can feel
the 'lift." It's just like missing the wave when boogie-boarding, and you
don't quite catch a wave, and it lifts then passes beneath you.

I'm not going to go more into specifics because while I acknowledge
the phenomina exists, I don't encourage pursuing it.

Absolutely right, wise and sage advice - don't ever do this, no good very bad idea. Never ever ever surf, draft or get close to anything when you're driving. Obey all traffic laws and signals, maintain safe following distances, wear clean underwear and listen to your mother, for cryin' out loud. This entire thread is merely exploring the hypothetical performance of the Prius, based only on speculation. Never ever try anything discussed here or attempt any experiments to prove or disprove these strictly hypothetical speculations. We are very very naughty people for even discussing it. Whew, ok, I think that covers it. No, seriously, I mean it, don't do this! I'll re-post this warning every few posts though so no one gets any crazy ideas.

But theoretically speaking, do you suppose the Prius would be better at this behavior in a hypothetical case of a closed course with professional drivers because of it's design? And if so, what aspect accounts for this improvement? Just low rolling resistance? Does the low CD help or hinder such efforts? I have a background in mechanical deisgn, with several courses in fluid dynamics, but its not intuitively obvious to me whether an object with a smoother flow would be pulled more or less than a brick-like object. My gut says the aerodynamics of the Prius might work against the effort, while the low rolling resistance obviously helps.

rachelseven,

I'm hardly an expert here, although my aerodynamic interest goes back
some 35 years to when I flew sailplanes, both radio controlled (up to six
channels, 12 ft wingspan at Torrey Pines and other SoCal sites) and full
size in the deserts around Tehachapi, CA.

I'm going to lay back and let some other posters offer comments.

In the meantime, for some out-in-the-real-world thinking, and
inspired seat-of-the-pants automotive aero discussions, solutions,
and attempts, go here:

Aerodynamics - Fuel Economy, Hypermiling - EcoModder.com

It's a pity we don't have an Aerodynamics Forum here. The closest we
have is threads and random postings in the Modifications Forum.

Thanks for the link!

The best "surfing" position is going to be directly in front of the tractor -- and that is a place I do NOT want to be. If anything goes wrong in front of you, your Prius is going to have to absorb an awful lot of "momentum" to prevent your injury.

Another surfing location with less danger than the front (but still more than I'm interested in) is along the side of the truck. Most of us have experienced this when passing or being passed by a truck. It is much like waterskiing on the outside edge of the boat's wake. There is very little stress on your arms and shoulders, and the water is very smooth. Problems: You can very easily fall into the trucker's blind spot, and you are VERY close to those large wheels. If anything goes wrong, you have very little time to react. It gets me nervous enough just during the passing event!

I do follow 18-wheelers fairly often when on the highway. I follow at a pretty fair distance, though I think I still benefit from having the truck break the air for me. Is it really drafting? I don't think so -- I'm just not close enough. The REAL benefit is that I don't just aimlessly accelerate into hills, burning up gasoline.

Let's just stick with the bricks.

If you have two objects trailing behind a semi, each tied by a string to a spring scale, which would place greater force on the string -- a brick, or a brick-shaped piece of balsa? How about a brick and a brick on wheels? How about the brick-shaped balsa (BSB) or a BSB on wheels? How about a BSB on wheels or a BSB on wheels with graphite-lubricated axles (GLA)? How about a BSB on wheels w/ GLA and a Prius-shaped balsa model w/ GLA? Which will spend more time following the truck if the strings are cut?

OK. I haven't done these experiments, but I'm trying to paint a picture of what I THINK is going to matter. It is a bit like the Cub Scout Pinewood Derby. In that event, weight is secondary to low rolling resistance (think GLA), though not by too much. Aerodynamics is not much of an issue with the Pinewood Derby, but it probably means a bit more when scaled up to real automobiles.

(Well . . . there was five minutes I could have spent doing something a bit more worthwhile! )

More worthwhile, perhaps. More fun? Only if you had a life-sized, Prius-shaped, graphite-lubricated, balsawood test dummy and a closed test track with a professional semi driver

Here's what I would guess... only a guess based on "engineer's gut".

Low Rolling Resistance - Most important positive effect on 'draftability', imo. The drafting effect basically removes a portion of the aerodynamic drag. The lower the rolling resistance, the greater proportion of the total drag that is removed by the draft effect. Therefore, the more the draft effect will tend to be felt, according to my theory. LRR would include tire loses, friction in drivetrain, etc.

Low Weight - Less important than LRR, primarily significant as it impacts rolling resistance. Lighter vehicles have lower tire friction losses naturally. Lower weight is almost assuredly a positive effect on drafting though.

Low CD/Aerodynamic Shape - This is where I get unsure. Objects get most of their drag from the 'push' on the front of the object and the 'pull' on the back of the object caused by the air rushing back into the empty space created by the object. Better shape means better control of the trailing edge vortices and less drag from the back of the object. Lower overall drag on the vehicle would once again mean that removing the headwind would remove a greater portion of the overall drag... making the effect seem larger. But at the same time, a good aerodynamic shape means there is less 'push' from the headwind in the first place, and therefore less gain from the draft.

So I guess by this logic, the low CD is probably about a wash in terms of it's effect on draftability. I would therefore posit that what is most important for this characteristic is the relative ratio between the aerodynamic drag and the rolling resistance of the vehicle. If vehicle 1 has low rolling resistance compared to it's aerodynamic drag, removing the aero drag will seem to greatly improve the coast. But if vehicle 2 has high rolling resistance, making the aero drag a proportionally smaller portion of the total drag, then removing the aero drag (or some portion thereof) by drafting will make a proportionally smaller, and therefore less noticeable, improvement. The overal improvement could actually be greater for vehicle 2, but it could FEEL like less of an improvement. And also, considering the crazy low rolling resistance of the Prius (it coasts like nothing I've ever driven or ridden), removing a chunk of the aero drag by drafting (DON'T EVER ACTUALLY DO IT) would theoretically leave the car with just about nothing left to slow it down.

So I see your 5 minutes and raise you 10

Hi All,

Hmm. I just do not feel a large drafting action in my Gen II. And when a truck comes up along side from behind, I feel a large drag. This is due to the increasesed wind speed (sidewards wind summing with travel wind). So much so, it knocks me out of SHM, and I have to apply more pedal. I actually feel the air blowing sideways inside the car as a truck passes.

I have felt a front surfing effect, when being tail-gated by trucks. But its a very small effect. I once had a motorcycle tail gate me about 1 foot off the rear of the drivers side tail light. I was in cruis control at 60 mph, and had steady MPG readings. With the bike there, the mileage was 2 mpg better.

I think the best aerodynamic orientation for the Prius is in clean air, with no vehicles alongside, in front, or close behind. The turbulence off other cars, which typically have poor aerodynamics (especially flat bed trucks!) disturbs the flow over the Prius, and as that flow design is highly tuned, one gets increased form drag, which offsets much of the reduction in pressure drag.

The day I drove my car home I had a lot of drafting action, but the air was very dense (dry and 15 F), we were heading into the wind, and all going 70 mph. In humid summer air at 53 mph, it seems there is little to no drafing effect.

The Aerocivic guy (google Aerocivic) says much the same thing about his .17 Cd Civic.

As far as tires go, I think we need something that is anti-visco-elastic. Visco-elastic materials get stiff when shear stressed. If with we had anti-visco-elastic materials, the tires would be stiff and not flex on smooth highway, but then flex when they need to for a pot-hole.

Just did a long road trip few weeks back. Typically, I have to speed up to catch a draft from other vehicles. As you get closer, there's that distance behind that has the extra buffetting; I'm not sure the turbulance at that point actually helps. If you get closer, you get the draft better, but I find another problem: to maintain that distance you engine load is not smooth; you need to go up and down in power, and I think that uses more fuel than if you didn't. Plus that's not a safe distance to do for any real amount of time; so generally, I've given up drafting and just do 55-60.