Does Prius Draft Exceptionally Well?

[rachaelseven]

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)?

[ALS]

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.

[a64pilot]

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?

[a priori]

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.

[rachaelseven]

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

[DaveinOlyWA]

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.

[ALS]

Originally Posted by rachaelseven

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

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.

[rachaelseven]

Originally Posted by ALS

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.

[Rokeby]

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.

[rachaelseven]

Originally Posted by Rokeby

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 rims overheat -- typically from a faulty
air brake --and the tire explodes, the slab of tread and hunks of side
wall go flying in totally random directions, but biased rearwards by
the air mass streaming in that direction relative to the truck.

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

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?