Caught in the wake and surfing the wave

[DC Fat Cat]

Hope everyone had a safe and fun holiday! I drove back to DC from NJ yesterday.

I had my first real chance to use the cruise control. The first half of the trip is through NJ and the terrain is pretty flat with gentle rolling hills. I set the cruise at 70mph and was getting about 48mpg during that portion of the trip.

Traffic was suprisingly light and at one point I found myself alone with a 18 wheeler who was going about 65. Although, as usual for a large truck he lost speed on the hills and gained a little on the backside, this was only a variation of about 1-2mph since the terrain was pretty flat.

Anyway, I got about 20-25ft from his rear and found myself being buffetted by his wake. On my five minute intervals on the consumption screen (or is it the other one?) it showed an increase of about 3-4mpg. Obviously I could have been a lot closer but I felt safe where I was. After a little bit I slide around him and got up near the cab until I could feel that I was just on the front edge of the preasure wave from his nose, that created about at 5-6mpg gain.

Eventually some traffic caught up with us and I had to get out of the way but it was an interesting experiment b/c I've never had a car that displayed any MPG info.

[bruceha_2000]

Anyway, I got about 20-25ft from his rear and found myself being buffetted by his wake. On my five minute intervals on the consumption screen (or is it the other one?) it showed an increase of about 3-4mpg. Obviously I could have been a lot closer but I felt safe where I was. After a little bit I slide around him and got up near the cab until I could feel that I was just on the front edge of the preasure wave from his nose, that created about at 5-6mpg gain.

If you are 20 to 25 feet behind anything going 65 mph you are way too close, you are travellling at 95+ feet per second. If that 'anything' is a semi, you might as well play Russian Roulette. On the other hand, if you can get in his side mirror and stay inside the wave off the front of the cab, it is safe and improves mpg as you found.

[cnetter]

I've played in the wake of semis along I-80 going thru Nebraska (can you say flat?). The MPG readout on my van updates every 60 seconds so the gratification is much quicker.

I heard a discussion on 'Car Talk' a few months back where the claim was that drafting another car improves the mileage of not only the car in the rear, but the lead car as well. Something about disrupting the eddies that form behind the lead vehicle. I wonder if the same is true when a large truck (i.e. a tracktor trailer) is drafted by a small car (like a Prius).

Chris

[KathyY]

We drove back from NJ to the Baltimore area yesterday. No more traffic than normal. Did see one Black '04 on the way.

[jfschultz]

If you are 20 to 25 feet behind anything going 65 mph you are way too close, you are travellling at 95+ feet per second. If that 'anything' is a semi, you might as well play Russian Roulette. On the other hand, if you can get in his side mirror and stay inside the wave off the front of the cab, it is safe and improves mpg as you found.

A good rule is 2 seconds. This is about the time it takes the car to cover one car length for each 10 MPH.

[DC Fat Cat]

Yeah, thanks for pointing that out Bruce.

Obviously I was too close in any situation where other traffic is around. Maybe 2+ car lengths away. But there was no other traffic in front of us for some time and I kept peaking around the corner to be sure. So for most instances drafting is not safe.

That being said I would highly recommend surfing the pressure wave. You can feel where the "sweet spot" is pretty easily after just a minute or two of messing around b/c the Prius is so sensitive to crosswinds. The only thing I would do is keep an eye on the truck to make sure he doesn't want to switch lanes, say to avoid a slow car in his lane or make a left exit. In general be considerate.

[DC Fat Cat]

Kathy,

I must have missed you. I have a white 2004. I saw a blue one going North on the NJTP.

[mdacmeis]

I heard a discussion on 'Car Talk' a few months back where the claim was that drafting another car improves the mileage of not only the car in the rear, but the lead car as well. Something about disrupting the eddies that form behind the lead vehicle. I wonder if the same is true when a large truck (i.e. a tracktor trailer) is drafted by a small car (like a Prius).

Chris

You are dividing the total wind resistance of both vehicles between the two vehicles, although not proportionally. The lead vehicle still takes the hit on frontal restriction, but benefits from less trailing restriction (drag). This is why the trailing vehicle in a race is able to accelerate past the lead vehicle - lower total drag via a reduction in frontal drag yields higher engine RPM thus greater speed to get by. The engine has limited torque to accelerate at these speeds, but once there as a result of the lowered drag has the horsepower to maintain it.

[bruceha_2000]

The only thing I would do is keep an eye on the truck to make sure he doesn't want to switch lanes, say to avoid a slow car in his lane or make a left exit. In general be considerate.

This works very well :-) When I moved from CA to VT 25 years ago my car had no A/C. It gets pretty darned hot mile after mile through the prairies in July even with the windows open. I would find truck drivers and hide in their shadow, making sure the driver's face was totally visible to me in their side mirror. Any time we caught up to someone in their lane, I would back off and flash the brights so they knew I was clear, then I'd jump back in after the pass. Felt sort of like a remora :-) I also backed out if traffic was approaching in my lane and couldn't pass on the left. Besides cooling shade, I also got better mileage with them pushing the wave. And it wasn't totally one sided. If there was a lot of traffic, I would run 'blocker' and make space for them to pass. All in all, it was a nice symbiotic relationship and they always waved when one of us broke it off at an exit.

[hdrygas]

I've played in the wake of semis along I-80 going thru Nebraska (can you say flat?). The MPG readout on my van updates every 60 seconds so the gratification is much quicker.

I heard a discussion on 'Car Talk' a few months back where the claim was that drafting another car improves the mileage of not only the car in the rear, but the lead car as well. Something about disrupting the eddies that form behind the lead vehicle. I wonder if the same is true when a large truck (i.e. a tracktor trailer) is drafted by a small car (like a Prius).

Chris

I heard that as well they had a MIT Physics Proff on and were discussing the Physics. The final conclusion was that with an infinite number of cars/trucks you would use minimal energy once you had all accelerated! Car Guys gotta love them :mrgreen: