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Post by The Urban Mythbuster on Dec 12, 2012 20:07:54 GMT
This comes from a thought I had heading off to camp last summer. The concept behind drafting is that a vehicle (V1) drives close to the rear of a vehicle (V2) that is equal or larger in size. A low pressure area is created between the vehicles and V1 is "pushing" V2 along on the road, while V1 is being "pushed" by the higher pressure behind it. Wouldn't this, in accordance, improve the fuel efficiency of V2?
Is it possible that, when pulling a trailer, the trailer is drafting the tow vehicle (assuming the trailer is smaller or equal in size to the tow vehicle)? I have a midsized SUV and tow a pop-up camping trailer. It seems like I get better MPG when I am towing; though, it seems like it should be lower due to the additional weight.
Or, is the better MPG just due to the fact that you tend to drive slower when you're pulling a trailer than when you're not?
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Post by GTCGreg on Dec 12, 2012 20:32:51 GMT
If you think of your SUV/trailer as aerodynamically one vehicle, yes, you could decrease air drag by towing the trailer. The airflow over both vehicles could produce less air eddies and less low pressure behind you, It would depend on the size and shape of each vehicle.
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Post by Cybermortis on Dec 12, 2012 20:54:49 GMT
An interesting idea Urban....
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Post by srmarti on Dec 13, 2012 0:02:43 GMT
The type of trip you take with a trailer in tow is likely different. You likely cruise along the highway more and do less stop and go traveling.
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Post by the light works on Dec 13, 2012 3:45:00 GMT
towing a trailer will be better fuel efficiency than towing the same trailer on a 100' drawbar.
adding a trailer will not necessarily make enough aerodynamic improvement to offset the extra weight and rolling resistance, though.
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Post by The Urban Mythbuster on Dec 13, 2012 4:06:24 GMT
Another angle to this would be, is there a difference in fuel efficiency between trailers towed via drawbar hitch vs 5th wheel hitch.
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Post by the light works on Dec 13, 2012 4:18:19 GMT
It is an interesting topic - I think we would want to shift it to what is the most efficient way to carry the load; just to eliminate the other variables.
5th wheel trailers tend to stick up higher than conventional trailers - which would affect the aerodynamics. an additional possibility would be a gooseneck trailer - which is essentially a 5th wheel without structure over the tow vehicle.
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Post by silverdragon on Dec 13, 2012 10:40:26 GMT
I am having trouble working this one out...
You take a vehicle and increase its size and weight and expect to get BETTER results on MPG?....
Yes driving slower than maximum will increase MPG, but driving with more weight size rolling resistance (through wheel friction) and the rest must SURELY have some effect on MPG......
My initial thoughts are, this is not going to be happening, but?.........
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Post by c64 on Dec 13, 2012 13:15:29 GMT
It depends on the aerodynamics.
The extra drag from the additional wheels isn't all that bad and the extra weight only matters when accelerating.
Whenever I pull a chunky construction site trailer without canvas and chunky machines on it with my streamlined car, I need to depress the accelerator pretty deep driving 80kph. Without trailer, this would make my car run at least 130kph easily.
On a modern semi truck with the aerodynamics perfected for its trailer, driving without trailer causes a massive extra air drag. A Mercedes "Blue Tech" Semi gives you at least 30 mpg on the highway with full payload in the trailer when you never change speed nor altitude. Without trailer you get a lot less than 20 mpg!
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Post by srmarti on Dec 13, 2012 21:18:39 GMT
I am having trouble working this one out... You take a vehicle and increase its size and weight and expect to get BETTER results on MPG?.... Yes driving slower than maximum will increase MPG, but driving with more weight size rolling resistance (through wheel friction) and the rest must SURELY have some effect on MPG...... My initial thoughts are, this is not going to be happening, but?......... I'm not sure there's a universal result here. In certain setups and driving situations, it might be possible for an improvement in aerodynamics to more than compensate for the increase in weight and rolling resistance. An exceptionally better aero change with a light weight trailer might be better than just the tow vehicle. I don't think it's true for just any random tow vehicle/trailer combination. |
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Post by srmarti on Dec 13, 2012 21:24:52 GMT
5th wheel trailers tend to stick up higher than conventional trailers - which would affect the aerodynamics. an additional possibility would be a gooseneck trailer - which is essentially a 5th wheel without structure over the tow vehicle. 5th wheeler would certainly add substantial frontal area and hence drag. |
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Post by the light works on Dec 14, 2012 14:48:15 GMT
A person also has to consider the efficiency curve of the tow vehicle. my dad's pickup got 9 MPG, fully loaded and towing a trailer. it also gets 9 MPG running empty in a tailwind. that's not because towing a trailer is more efficient - it's because running empty is wasting horsepower.
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Post by The Urban Mythbuster on Dec 14, 2012 14:59:08 GMT
I've seen a lot of tractor trailers (semi + trailer) are outfitted/retrofitted with assorted cowlings, spoilers & air ramps to, theoretically, increase aerodynamics. I wonder if this really makes a measurable difference in fuel efficiency.
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Post by the light works on Dec 14, 2012 15:05:30 GMT
It's hard to say, because advances in aerodynamics are coupled with advances in drivetrain performance. the thing to do would be to put a classic body on a modern set of rolling stock and see what difference it made.
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Post by The Urban Mythbuster on Dec 14, 2012 15:12:25 GMT
There may also be something to be learned in testing a stardard (long nose) rig vs a flat nosed rig (are these even made anymore?) Flat nosed rig...  |
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Post by the light works on Dec 14, 2012 15:21:52 GMT
I think they're still fairly common in places where there isn't as much room - a cabover can be shorter wheelbase
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Post by c64 on Dec 15, 2012 16:12:36 GMT
I think they're still fairly common in places where there isn't as much room - a cabover can be shorter wheelbase Like Europe? Long nose trucks are almost unknown over here. The few still around are either special offroad versions or over 30 years old! |
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Post by The Urban Mythbuster on Dec 17, 2012 14:33:39 GMT
The question originally aimed at 'roller-skate' vehicles (car + trailer), but then it quickly became apparent that there many more options to consider surrounding the fundamental question. For the purpose of a possible test for the MBs I'm most curious about car/trailer combinations and the effects of different combinations on MPG.
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Post by GTCGreg on Dec 17, 2012 14:41:48 GMT
BTW, 30 mpg is bad for a car in the UK. Most UK cars that are not classes as "Gas Guzzler" (We use that American term as an insult) will do 30 easily... my own family car gets 35 round town, 40 on Motorway 5 up and loaded. Many cars will do 35/40, many smaller micro ones will get 40/45, and the "new" three-pot eco engines will get 60mpg+ if driven right.... However, a one-litre three pot engine struggles to get away from the lights before they change again when you fill all the seats and put a suitcase in the trunk?.... Are we talking the same gallon here? An Imperial gallon is a little over 20% larger than a U.S. gallon. To be a fair comparison, you have to take 20% off those mileage figures when comparing to U.S. mileage. |
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Post by Domovoi on Dec 17, 2012 19:39:49 GMT
I am having trouble working this one out... You take a vehicle and increase its size and weight and expect to get BETTER results on MPG?.... Yes driving slower than maximum will increase MPG, but driving with more weight size rolling resistance (through wheel friction) and the rest must SURELY have some effect on MPG...... My initial thoughts are, this is not going to be happening, but?......... Yeah I have to agree with ya. Sure wind resistance matters. But yeah. C64, have you ever driven a truck with a trailer? It definitely matters even after you have accelerated. More mass means more work maintaining speed. With my 6.7 I got between 23 and 25 on the highway empty. Loaded on the goosneck I got 19 to 20. Also, sure you can get less windresistance with a gooseneck, but that really isnt the advantage for towing that way. It places the weight more over the axles than on the bumper which increases the max load. My dodge could haul close to thirty thousand on the gooseneck but only twelve off the reese. |
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