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towing empty trailer and trailer with a car on top use same energy,

Discussion in 'Model X' started by kiwibacoin, Apr 23, 2018.

  1. kiwibacoin

    kiwibacoin Member

    Joined:
    Apr 14, 2018
    Messages:
    10
    Location:
    New Zealand
    Hi,
    So I had to go pickup a broken a car.
    when i was an empty trailer at motorway speeds i notice that it uses same energy as with a car on top.
    Even though the weight doubles. 750 kg trailer + 800 kg car.

    Is this just due to drag?

    75D only getting 50% range with an empty trailer isnt fun.
     
  2. ecarfan

    ecarfan Well-Known Member

    Joined:
    Sep 21, 2013
    Messages:
    19,018
    Location:
    San Mateo, CA
    The increased energy usage when towing is primarily due to the increased drag, not the weight (which is only a significant factor when going uphill). Your empty trailer creates a lot of aerodynamic drag. Your trailer with a car on it likely creates somewhat more drag since it has a greater frontal area.

    Are you certain that your energy usage display shows “the same” number when the trailer is empty as it does with a car on the trailer, under the same conditions? You would need to run a test on the same road in the same weather conditions (temp, rainfall or no rainfall, winds) with the trailer empty and then with a car on the trailer. Then you would have valid energy usage data.

    The reality is that towing anything uses a lot more energy. This is true for petrol-powered cars as well. But petrol cars cannot measure their energy usage nearly as precisely as we can in our Teslas, and they can “recharge” easier and faster by stopping at a petrol station and filling up in a few minutes. They accept that as the penalty paid for towing. EV owners pay the same penalty but we notice it more because we have a digital readout of exactly how much more energy we are using and we have to plan additional charging stops because of that.
     
    • Like x 1
  3. Peteski

    Peteski Active Member

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    Location:
    UK, Milton Keynes
    You were quite possibly driving with a tailwind on the outward journey and a headwind on the inbound leg which happened to cancel out the increased drag and weight of the towed car. The weight would be much less significant than the aero drag presuming a constant speed on a level road. Basically the weight only comes into play while accelerating up to speed, although there will also be a small increase in tyre rolling resistance. But really it's all about aerodynamics at constant highway speeds unless hills are involved.
     
  4. Yinn

    Yinn Active Member

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    Nov 15, 2016
    Messages:
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    Location:
    Behind you
    I've found weight to be a minimal factor while towing; especially if the towing primarily occurs at highway speeds. It has more of an impact at low speeds. The reason for this is simple, once it's moving the amount of energy for sustaining speed based on weight is minimal. Your biggest opponent is aerodynamic drag and is where the majority of energy will be spent.

    While it's true at a base level; if you take it one step further it then depends on additional variables. Empty trailers actually create a lot of drag. This is especially true if you have an empty open trailer with a fold down ramp. Those ramps now act as parachutes/air brakes. When the car is on the trailer; it may provide more frontal; that much is true. But because of the shape of the car, it can actually create less drag. Most cars have a drag coefficient of ~0.25-0.35 nowadays. When the car is on the trailer it allows the air to flow around the car and can allow the air to go up and over the ramp. Otherwise the folding ramp itself can create over 4 times that drag coefficient.
     
    • Helpful x 1

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