While driving to Missouri this past weekend, I fell in behind a semi or two going about 85. They drug me along in their draft, I hardly had to steer and barely needed to touch the gas. I used to do this in my van because it saved gas. In my little car, it just sorta gave me a moment to relax a little.
My question, though, is that I had always heard that truckers didn’t like it. When I asked why, I was told that it hurt their performance by towing me around. I don’t buy that. But maybe they don’t like the fact that they are getting 4 miles per gallon and I’m getting 35 and they’re towing me.
So, what do y’all think? Does it really hurt performance or do they just not like towing a car around in their draft or do they really not mind at all?
This is a WAG, but I think a trucker is less concerned with losing 1/4 mpg than the fact that if you’re tailgating close enogh to be drafted, he has no idea where you are. Most decent drivers want to know where everyone on the road is at all times so that they can plan for any strange events.
Tom~
At first, my thought was “Baloney! There’s nothing connecting the vehicles so there’s no actual ‘towing’ going on.” But then again…
What if the drafting at the back of the vehicle pushes the trailer a little? Then another vehicle might interfere with this. So…
Um, no answer. Just thinking out loud.
For a real answer, post this again and put Andrea in the title. She drives 'em. (Well, busses, lately, but she’s done rigs, as well.)
I once drafted a semi while I had a CB radio. The other truckers explained to me that if the semi had blown a tire, there was a good chance that the debris would fly right back into my windshield and could possibly break it and cause much damage to me and my car. Also, I think that they just don’t like tailgaiters - if the semi had to stop, would I notice? Or would I slam into the bumper? Either way, I don’t draft anymore.
I think Tomndebb has it right. I know how aggrevating it was driving a van when cars would disappear by getting real close. I’m sure it happens a lot more to a semi driver than to a van driver.
I had dismissed the tailgating itself because they do it with each other a lot, sometimes a half dozen or more in a line that you couldn’t squeeze a car in between. And when you get in their way when they have their momentum up for the next hill, they don’t mind how close they get to you. That bulldog drooling on my trunk is a lot more threatening than I ever would be to them.
As far as the danger of being close to a semi, that is pretty obvious. But the danger is to me not them (I hadn’t thought about a piece of tire, though). Although, I suppose they really don’t have time to mess with the accident reports if I don’t pay attention.
The question mostly had to do with whether it did change their performance and how it could.
My dad is a trucker and the issue is definitely safety, not fuel mileage.
Interestingly, though, I don’t think it has any significant impact on the truck’s fuel economy. The back of a trailer is about as poor an aerodynamic shape as it could be. The way to reduce drag is to keep the airflow attached to the moving body. That’s why airplanes taper slowly at the back. The shape of the tail of an airplane is a lot more critical in reducing drag than the shape of the nose.
When you are in the “drafting zone” you are in the area where the truck is dragging along all that air anyway. The presence of your car may even improve the overall airflow by allowing it to re-attach to your vehicle and detach with less turbulence at your rear bumper.
Also consider race cars, which routinely draft each other to save fuel. You don’t hear the guy up front complaining about getting worse fuel mileage from drafting, just the guy in back getting better. Otherwise that’d be a pretty good tactic in a close race. Stay behind him and run him out of fuel.
Let’s see. I lathered and I rinsed. But did I repeat?
Thanks Pluto, that’s about the way I thought of the aerodynamics of it. In the case of race cars you don’t want any of your competitors to get any kind of advantage, but that isn’t the case on the open road.
More or less related:
Pluto, the result is about the same in bicycle racing, but in tha case whoever is being drafted (and it is noticable) WILL get pissed off because they are doing all the work (which is why in bike pacelines, even of mixed teams, riders are polite and take turns pacing). In long track races many riders have good enough balance to just stop, dead in their tracks, and so the riders just stay at a standstill until a racer can’t stand it any more and takes the lead.
Thanks for the info M.K. I was aware that bicycle racers took turns at the lead but I thought it was simply because the front guy was doing all the work carving a hole in the atmosphere while the others were loafing. If I read your post correctly your saying that the front guy has to do more work while being drafted. I didn’t know that.
If that extrapolates up to the semi/car combination then the car does have a negative effect on the truck’s fuel economy. My guess, though, is that it is a smaller effect because of their relative sizes: a small car drafting a much bigger trailer. More importantly, for this discussion anyway, the extra work done by the draftee (bicycle or truck) is still much smaller than the work saved by the drafter.
So, when viewed as a system, the guy tailgating the truck is, overall, saving fuel and contributing less to global warming merely by risking his life and health. Sounds like a fair trade to me!
Let’s see. I lathered and I rinsed. But did I repeat?
Ask yourself a simple question. Would you like it if a semi drafted you at 85 mph? Btw, I think anyone that could “relax a little” while driving along at 80 completely ignorant of what’s going on more than 40 feet in front of them has a screw loose.
Drafting increases the efficiency of both leader and follower. In stock car races, the leader AND follower get better gas milage, important, since pit stops equal lost races–also I think they are limited in total fuel allowed per race?
Regarding bicycles, NO No No, the leader does not have to work harder in a drafting pair ( triplet, etc.). He gets benefit from the train as well as followers. However, he has to work harder than the followers (still less hard than if he was alone). That is why they take turns, to distribute the workload.
As miniscule as it may be, the trucker will benefit in gas mileage, but say from 4.0 mpg to 4.1mpg for example. Not enough to justify the aggravation and worry of having some nut on your tail who is tempting death.
Well, Nick, “relax a moment” didn’t mean doze off or anything. I meant, I hardly had to steer or push on the gas. I definately had to pay attention, though. I’m not ignorant of what’s going on in front of me, but you don’t have to get real close to a semi going that fast to be swept up into the draft.
If you read what I wrote before, they do it with each other and they do run up behind cars. And my little car can certainly stop a lot faster than 40 or 50 tons of semi going 80 mph. It also seems that when something that big is running 10 to 15 mph over the speed limit with small cars around, that safety isn’t their biggest concern either.
I don’t like the idea of a semi tailgating me because I can stop so much faster than it and the effect would be devastating. But I have traveled with other cars that are going at a comfortable pace and which one of us was leading and how close the other car was didn’t make much difference.
Anyway, I think that the tailgating itself isn’t so much the issue as, like Tom said, the fact that I would disappear behind them.
Argh, no, no, I didn’t mean the front man does all the work LITERALLY, he just does less work than the others are doing at the moment, as yes he is carving the hole in the atmosphere. Sorry about the confusion.
I think the real problem is that truckers have deadlines and if they have to stop because you ran into them it would slow them down. Another thing is that having accidents makes it harder to get/keep a job as a trucker, whether they were your fault or not.
I doubt you would even notice the difference in fuel economy for the truck.
I imagine a lot of truckers also believe that it hurts their mileage. The zero-sum philosophy is just too ingrained for many people. “If he’s gaining something from me, I must be losing.”
But it’s absolutely correct that drafting behind a large truck will increase the gas mileage of both vehicles, because the total drag on the truck/car combo is less than the sum of the individual drags on both vehicles.
It’s not correct to say that you are being ‘dragged’ behind the semi, or that its vaccuum pulls you in. I’ve even heard people make the ludicrous claim that they had to ride the brakes from being pulled into the back of the truck. In fact, you just hit an area of air that’s moving in the same direction you are, so your relative windspeed decreases. If you keep constant pressure on the gas pedal you’ll accelerate towards the truck, which is what some people have experienced as being ‘dragged in’. But there’s no ‘suction’ happening (or at least, so little that a car would never feel it).
I’ve drafted semis in a car, on a motorcycle and on a bicycle - and if you’re not less than 40 feet from the rear bumper, you’re not getting much of a draft at all. You need to sit in the low pressure area immediately behind the trailer, not in the turbulent air 100 feet back. Of course, this practice is very hazardous because you cannot see what’s going on in front of the truck. If he stops, you’re going to stop too. Truck drivers don’t like it when they cannot see you, and they greatly detest having to take the time to fill out paperwork or answer to various warrants and/or equipment deficiencies because the State Police had to scrape your remains off their rear bumper.
I’d like to hear a rational explanation as to how the slipstream of a vehicle traveling behind another one could affect the gas mileage of the lead vehicle. Are you saying a race car generates a shockwave that actually travels faster than the vehicle itself? Please. Or that the low pressure “draft” area behind the car acts as some sort of “unit” that the follower can “push” on? I don’t think so.
Nick how in the hell do you keep up with a semi on a bicycle?
And you are right, about 3 car lengths or so seems to be about right. It is real obvious when you get in that turbulance a little further back that it is not a good place to be, especially on a motorcycle.
I think the lack of visibility and not wanting to mess with an accident seem to be the prevailing factors for why the truckers don’t like being drafted.
As far as the aerodynamics of a vehical in the slipstream, I had thought that the lead car, or truck, got a boost because the 2nd car was pushing air in front of it into the low pressure area behind the 1st car, thereby reducing the effect of the low pressure at the back of the lead car. I have no idea how technically accurate that may be.
“I’d like to hear a rational explanation as to how the slipstream of a vehicle traveling behind another one could affect the gas mileage of the lead vehicle.”
The low pressure “draft” area causes an increase in airspeed across rear portions of the body, as the air “speeds up” to try and fill that vacuum.
(in addition to the velocity of the vehicle)
Increased air velocity increases turbulence and drag.
By “filling” that area with the front of another car, the flow at that joint becomes slower and somewhat more laminar, and the low pressure area moves to the rear of the trailing car.
The front car only has to overcome the drag induced by “parting” the air and the rear car only has to deal with the drag induced by “filling” the vacuum.
They both benefit.
At least that’s what some guy on TV said.
They wouldn’t lie on TV, would they?