Aerodynamics - Trailers?

Discussion in 'Hydrodynamics and Aerodynamics' started by Dhutch, Aug 14, 2012.

  1. Dhutch
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    Dhutch Junior Member

    Can any help with some basic aerodynamic estimates?

    Its not something I covered in my coarse but I have a large box trailer I use for all sorts of things including moving boating gear around and with the price of fuel going up almost daily I'm very aware of its effects on fuel economy on long trips, and hence if it would be possible to make some changes to reduce its drag.

    The trailer is 12' long, 6' wide, and 5' tall (four foot box with around 1 ft ground clearance) with a slight wedge at the rear to increase the threshold height.
    As the front there is a half height wedge cut off reducing the frontal area (car is about 6 inches lower, and has a downward sloping rear hatch) but otherwise its just a hard edge box.
    Currently there is a trailer board mounted over that with the lights, which may be acting as an air break, or may be acting to break up laminar flow and create turbulence reducing the drag of the rear ramp.

    I'll attach a side view below of the current set up, but what I'm wondering is, say if I moved the light board, or radiuses the front corners, or even made/found a suitable grp front valance, would I be likely to make and form of significant change, or just waste my time? Obviously I don't have the money to put it in a wind tunnel (even a model) and I'm not in need of a project. but if I can cut say 10% of my fuel bill it would be worth a weekend of time.

    Anyone worked on aerodynamics? Fag packet calcs? Gut feeling?

    [​IMG]


    Daniel
     
    Last edited: Aug 14, 2012
  2. philSweet
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    philSweet Senior Member

    Unless you are towing with a Lamborghini, the trailer will be riding in the air pocket of the car. The biggest drag will come from crosswind effects that take the trailer out of the pocket and stuff air under the trailer chassis as well. A small radius on the front edges and side upper edges, say 8" radius, is about as good as you can do. Many trailers are built that way in the US. A wedge or bevel isn't nearly as good. Trailer bearings for this size trailer typically suck in the US (but I owned a UK built trailer axle that saw 20 years of salt water and I never messed with the bearings. It was forty years old when I sold it to the next punter and I replaced it with a US made 2000# axle. I was was replacing bearings annually after that.)

    Put the weight in the car, keep the tires aired up. Run a decent diameter wheel on the trailer. And don't do like I do and carry a thousand pounds of only-god-knows-what around with you. Aero form drag is probably a minority contributor to drag unless you are going very fast or are in a strong crosswind. How fast are you going? And how far? what is the tow vehicle and engine option? What wheel dia on the trailer? Photos?

    Another simple trick is to load the trailer so the axle is the center of pecussion relative to the hitch (and vise versa). A simple test for this is to pick the tongue up and drop it a few inches onto a block the same height as your trailer ball. The suspension shouldn't bounce. This should minimize the energy absorbed by the suspension system running down the road. However, if the trailer has no shocks, it maximizes the bouncyness of the trailer (but you can't feel it in the tow vehicle). Different leaf spring arrangements have different amounts of damping. Not having any feedback from the trailer is a two edged sword. You have best control of the tow vehicle, but that isn't worth much if you don't realize something is wrong back there.
     
  3. Petros
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    Petros Senior Member

    I had work for a number of years doing aerodynamics in the aerospace industry. Anything you can do to reduce the drag will help of course, but the airflow over a trailer behind a car or truck is not quite as easy to determine with a few simple calculations.

    Putting a rounded fairing on the leading edge of the "box" will make a big difference, as well as moving any thing protruding out the top and sides inboard. You might also consider some form of flexible skirt along the front and sides, they commonly call these "air dams" on automobiles. air flows under the trailer as much as over it, keeping it out from under it will also reduce drag (and reduce aerodyanic lift as well, making more stable at hwy speeds). Basic rule is you want rounded leading edges, and sharp trailing edges so the airflow breaks clean away from the rear of the trailer (rounded edges along the rear actually will increase drag).

    benefit in terms of fuel economy is also a complex calculation since drag is only one component, speed, tire and bearing friction, weight, engine power curve, driving style will also play into it. Also consider how many miles you drive at say above about 25 or 30 mph (50 kph), because the benefit is very small at lower speeds.
     
  4. jehardiman
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    jehardiman Senior Member


    Correct, rounding all the sharp edges (even the trailing ones if you put a flow deflector on them) is the easiest way to reduce drag. Hoerner shows a 20% reduction in drag of a body like a railcar/trailer just by rounding the corners 5% of the minimum dimension.
     
  5. garydierking
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    garydierking Senior Member

  6. SamSam
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    SamSam Senior Member

    .

    And not just any damn teardrop trailer.
    Be sure to specify how many you want.

    [​IMG]
    http://www.orvis.com/store/product.aspx?pf_id=5R6G

    .
     
  7. DCockey
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    DCockey Senior Member

    The board sticking up in the airflow at the rear is a major drag producer. Getting rid of it would be my first move to reduce aero drag.

    Rounding the edges of the front would decrease drag. Once the edges are rounding sufficiently for the flow to stay attached there is very little improvement possible by further modifying the front end shape. Either the corners of the exisiting body could be rounded or a new, rounded front "valance" added.

    Rounding the horizontal longitudinal edges of the trailer described above may help a little but not enough to make any modifications worthwhile.

    Rounding the rear edges will increase drag. I've never seen added "flow deflectors" which would reduce the drag with rounded rear edges to that of sharp rear edges (assuming the added drag of the "flow deflectors" is accounted for).

    The comments above are based on my experience doing research and in-house consulting on automobile aerodynamics as well as the work of my colleagues.
     
  8. SamSam
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    SamSam Senior Member

    No aerodynamics or fag packet calcs (?), but a gut feeling says if the board was doing any good at all, every semi-trailer in America would have one on top and on the sides too.

    Drive slower and inflate your tires with more psi (or however you do it over there ;)). It will be a rougher ride but it is 'Hard Times' for these times.

    How is the narrowboat steering working?
     
  9. Dhutch
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    Dhutch Junior Member

    Thanks everyone for the comments. Valid point about commercial not having anything to break the flow off round the back, i guess the hard edge is enough on its own. Sounds like it might be worth round the front corners. I need to some something with the front anyway as it has a removable pannel for tiedown access which is a poorly effected after thought from the previous owner and now worn out to boot. Retrospectivly rounding the longitudinal edges without adding quite a bit of weight or remaking them using structural aluminum extrusions proberbly isnt possable or worth it, but if I ever sell it and build my own from scratch should bear it in mind.

    Making sure the wheel bearings are in good condition is proberbly relevent too. The wheels are 12inch with somethink like 145x60R12 and although I always make sure the presures right ive not looked at the bearings for a while other then to check for excessive play. As a box trailer is obviously not used for launching so the bearings are not immersed

    Cruising speed on the motorway and dual carrageway is 60mph (limited by law, and to a certain extent, stability) which consitutes the majority of the miles. Tow car is a 12yo Peugeot 306 1.8 nat asp petrol.

    Never heard of the pecussion test but will try it on a few loads. Obviously being twin axle helps here. The suspension is a rubber bush based trailing arm setup which is very common in the UK, dont know whats common in the US but there are basically no leafsprung trailers left. Indespension are a common brandname.

    I think an air-dam or skirt round the trailer wouldnt be overly feasable as although almost all the distance is on main roads the last 100yrds is often over feilds, inclinded driveways, up curbs, etc and by the time it was high enough to cope with that it might not be worth it at all. However it would be a consideration I would bear in mind for future, partiuarly if my next trailer has inboard wheels.

    Tear drop would be nice, but without restyling the whole rear end, and making a lot of it move to get things in/out not overly pratical. One of the uses for the trailer is a car transporter for a small kitcar to autotesting events which obviously needs to be rear loaded (unless the trailer is designed to be un-hitched and front loaded over the drawbar)

    Will try and get some photos up this evening when im home.

    ##

    The narrowboat steering is working well, I havnt re-tightened it as tight as it was when it was new or determined if that was down to the u-clamps slipping or the rope bedding in but the nature of the new pulleys and the slightly tight fit into the quadrent means its in no real risk of falling off. Wear on the rope round the quadrent appears ok so far. Minor squeaking in the pulley bearings (nylon on brass) which im going to try some injector-pin lubricant on (uncal is an injection moulding consultant). Total used to date is about 150hours.


    Daniel
     
  10. Erwan
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    Erwan Senior Member

  11. Petros
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    Petros Senior Member

    There is a simple test you can do to determine how much you have reduced the drag, called a "Coast down test": You time carefully how long it takes to slow down from one speed to the next. For example, you hold the car steady at say 65 mph, than put it in neutral, start your stop watch, and allow it coast down to 55 mph. this will give you the average for 60 mph. It is important there be little/no wind (or if you are doing a test you do it in both directions), and on a level road. If you know the exact weight of the vehicle you can estimate the exact amount of horsepower it takes to hold it steady speed of 60 mph. But if you are just trying to determine if you reduced the drag, the time to slow from one speed to the next is all you need. It is also important the rig weight the same for each comparison test.

    So do several coast down tests with the unmodified car and trailer, than remove the light bar and temp mount it on the back side, duck tape some cardboard fairings on the front of the trailer, perhaps a skirt or air-dam, etc and go do another coast down test.

    If you can do this accurately (best to average three or more tests), and you can give me the weight of the vehicles, I can calculate how much less hp it takes to hold the steady speed, and from that I can even estimate how much fuel you will save. Make sure all of your tires are well inflated and you have no brakes dragging, etc.

    As one of my engineering professors used to say, one simple test is worth a thousand expert opinions.
     
  12. Dhutch
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    Dhutch Junior Member

    That does sound pretty exception. I can certainly say the adding the trailer to the 306 roughly halves my mpg!

    Daniel
     
  13. philSweet
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    philSweet Senior Member

    yup, those trailing arm trailer setups are all but unknown in the US. That's what my forty year old brit trailer had. I referred to it as an axle because I didn't want to distract the thread too much then. So I am doing so now.:D
     
  14. SamSam
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    SamSam Senior Member

    I imagine on some trailers smoothing the bottom and making it more aerodynamic might help.
     

  15. DCockey
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    DCockey Senior Member

    If the bearings have enough drag to noticeably affect fuel economy then they will be in bad shape.

    A similar type of suspension with trailing arms and springing by rubber in a tube is very common on trailers in north america other than boat trailers. I've seen it advertised as available on a few brands of boat trailers. Torflex by Dexter is a popular version. http://www.dexteraxle.com/torflex_axles
    [​IMG]
    Linked from the Dexter Torkflex website.
    The primary difference from the Indespension design is a fixed tube usually runs across the trailer rather than two stub units.
     
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