Does Prius Draft Exceptionally Well?

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 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.

 

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.

 

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.

 

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.

 

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! )

 

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.