Feel the draft....FEEL the DRAFT!
 

I got on the highway as usual today, and there was an unusually large traffic quantity. Soon I entered the left lane in the large space between a large tractor towing a short shipping container and a sports car with 4 excessively bright headlights inexplicably turned on, actually managing to provide awful glare on a bright sunny day.

I found that the truck was steadily going about the speed I'd go, and was staying in the left lane instead of moving right as most truckers would do, so I decided I'd try to draft. I figured I'd need to draft closer than usual for such a short truck, and drafted at 1.5-2 seconds.

If you're familiar with containers on trucks, their bodies sit higher than a normal box trailer. The trailer is an independent frame that's pretty high to begin with, and the box sits on top of the frame. This particular one had an abnormally high frame. Looking at it I figured there wouldn't be a good draft since there was almost no truck down at my height, the bulk of it being higher than my hood.

I drafted him for what seemed like forever, until he seemed like he was getting out of my way, thinking I was tailgating pissed off. I felt bad for him, some jerk pulled out of the breakdown lane in front of him, nowhere near highway speed.

Anyway...here's what I noticed about the wind. I drive with my windows open, and usually at 70mph I have steady, comfortable wind. Behind this truck, I was getting rhythmically whacked in the head with bursts of air in a 1 second cycle. Was I in bad turbulence, or was I in a good draft?

I think there's more turbulence on those container trucks due to the ribbed sides on the containers.

I know when I would draft in my VX, which i had fairly low to the ground, behind a normal tractor trailer i could feel burst and the car would feel like it was being pushed side to side...I always assumed this was what a true draft felt like since the air seemed like it came around the sides of the truck, not straight on as if no one was in front of me...wouldn't mind to hear some input from someone who has knowledge on this...

Vortex shedding off a bluff body...

https://www.maths.manchester.ac.uk/~m...ons/vtran2.mpg

Ah, that certainly explains what I felt. Was I too far back for a decent draft?

It would be interesting to carry an accurate barometer with you to see where you got the lowest pressure.

Next time try different distances to see what effects you notice.

regards
gary

That animation is at a fairly low reynolds number I think, the wake would be 3-4x as long in the 10^5 to 10^6 range.

Von Karman strikes again!
https://www.en.wikipedia.org/wiki/Von..._vortex_street
A Karman vortex can occur after any size object. The airflow meter on my car (and many Mitsubishis) is a Karman type.

How does nobody talk about "surfing"?

Seriously it is not as dangerous, no rock chips, and still brings about a lot of gains just look at the sides right around the middle of the shape is where the air separates the most...

Since I've nearly been merged into by large vehicles over 20 times that I can remember in the last few years, and have never come close to running in the back of one, or being outbraked by one, I'll take that with a pinch of salt.

Probably half the big rigs here are carrying containers. Yesterday I watched one pull over behind another one with 40 feet between them at 70 MPH. The 6 cars behind them were less than two car lengths apart.

I have had a big rig pull over in front of me in the right lane less than 40 feet in front of me. Traffic around here is that bad.

If they don't like me following them 120 feet back, I don't need to post my feelings about that on this forum. That distance gives me a chance to avoid a chunk of big rig tire or other road debris, and believe me, at least around here it is LESS stressful.

In one case I actually had to DFOC down a hill on I95 to keep from being sucked into the rear of the trailer for at least .3 mile.

I think if you do it right you can average 65MPH with the same fuel consumption, as you would at 50 MPH without drafting. Depending on your distance it could be the same as 45MPH.

A bicyclist's speed can be doubled by drafting.

regards
gary

About the only times I can remember being "rocked" by a truck is when having to pull out close behind a flat bed or low loader to get on the highway (They have all sorts of crap blowing off the bed, stay back) or when I've been in a different lane to the truck, coming past it... oh and once or twice from trucks travelling in the opposite direction.

You really notice the buffeting on a motorcycle haha, it's like getting whacked on the side of the helmet with small pillows.

Speaking of drafting, I have noticed a good draft effect simply by driving in thick but freely moving traffic. With a large block of cars/SUV's all 1-2 car lengths apart and 2-4 lanes wide all going similar speeds my dashboard FE display definately shows higher figures than I would expect if I were going the same speed on my own. Better yet, it doesn't kill visibility like being behind a truck.

With regards to "surfing" it is definately dangerous. I recently drove a 24' moving truck nearly across the country. The cab is getting close to semi-truck size even though it was "only" a 13 ton truck. I did have a major blind spot right next to the cab on both sides, especially to the passengers side. The wide angle mirrors and looking down took care of most of my visibility on the drivers side, but several cars could easily have hidden beside me on the other side without me having any way to see them. Drivers who hung next to the truck were also the most stress-inducing. People behind me I didn't give a hoot about because I knew my bumper would eat them for breakfast without me even feeling it (actually they would have hit the car I was towing, but I still would never have noticed)

it is strange that the blind spots are a draft area but it is

I would say you were tailgating.

If you look at your SG, as you pass cars on the highway, it seems like the MPGs go up and down and back up. So even on the side of cars there seems to be draft spots, and bad wake spots.

I seem to draft best with the CR-V behind a Mazda3. The worst depends, can be boxy pickups/SUVs or some beaters.

With flowing traffic I seem to get 2~4MPGs better.
Head wind knocks 2~4MPGs.
I seem to average 25MPG with the CR-V on highway at 75MPH with cars around.

I rarely stay behind a truck. Too slow.

I always look for trucks to follow. For me the best ones are the grocery chain trucks because they only go 55. I leave with plenty of time to get to my destination so the slower the better. I just EOC behind the truck.

I know on my motorcycle I can feel a HUGE difference when I'm behind a truck. Not only that but my throttle position fo say 60mph without a truck is about 50-75%. Behind a truck it is about 10-25% for the same speed. Thats crazy awsome!