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Would it be easier to pull a train with flat wheels?

  1. Apr 23, 2013 #1
    A train applies steel wheels to a steel track.
    Therefore, without any distortion of the wheel or track, the contact patch would be extremely small.

    Now, if you slice a chord from the wheel, creating a flat spot contacting the track, the result is a larger contact patch.

    With a larger contact patch, more friction will occur between the wheel and track, but the weight will be spread over a larger surface area.

    So, here is my question: Will the train be more difficult to pull/slide along the track with standard round wheels or with flats cut on the wheels????
  2. jcsd
  3. Apr 23, 2013 #2


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    Flat cuts will not roll, which is the whole point of wheels. You would be dealing with a purely frictional sliding force as opposed to whatever rolling resistance you would encounter from a decent wheel.
    I should mention that railroad wheels and trucks are some of the most efficient mobile devices ever designed. A single man (albeit in good physical shape) can pull a railway flatcar with tonnes of load on it. The acceleration won't scare a Top Fuel Eliminator driver, but any movement is good and it continues to accelerate until a walking pace is reached. After that, it's just yawn city as he walks along with almost no resistance. Given the mass of the parts, I find it hard to believe myself, but it happens on the show circuit. My best advice to find this stuff is to check out the Daily Planet website and go to the archives for a copy of the episode that contains this.
  4. Apr 23, 2013 #3
    In both cases the wheels will not be allowed to rotate. ( I forgot to add this in the first post)

    The wheels will be fixed; therefore, no rotation will occur.

    With the wheels not able to rotate, will the train require more force to move with flat spots contacting the track or the small contact of round wheels?
  5. Apr 23, 2013 #4
    student715 hello well please don't feel offended but can I ask you a question ? Which scenario and outcomes seem logic to you?

    Sliding would put much more friction on the metal to metal contact area , especially under heavy load, dust or rust conditions , the only way i see a sliding "wheel" could work is if it was lubricated by oil constantly the way a bearing is.But then the train wheels would be called bearings now wheels.

    A train wheel is and makes the train a extremely efficient way of transport because the wheel touches the rail at a very small area + the resistance is also very small (ofcourse as long as the wheel is rolling) and this is all possible thanks to the properties of metal.

    But as to sliding wheels with flat surfaces well I think that is a bad bad idea as it would have alot more friction + how do you imagine the train would move forward? Electromagnets? well then why the sliding track in the first place. :)
  6. Apr 23, 2013 #5
    I am not offended :smile:

    Let me try this again.

    The train was just an analogy to describe an object readers would be familiar with.

    Here is the situation:

    I designed a very large weldment which rides on 6 steel wheels sitting on a steel track. (much like a train)
    I have pneumatic cylinders attached to the weldment that will be pushing a fixed object and applying force against the weldment.
    The steel wheels will lock by a brake on the motor used to move the weldment up and down the track.

    The engineer i am working under has completed several coefficient calculations using steel on steel and decided the weight of the weldment alone, will be enough to allow the brake to keep the weldment from being pushed my the cylinders.

    However, my mind tells me that the contact patch of the steel wheel on a steel track is so small that will be very little traction for the wheels.

    Here is where "flat wheels" come into the story. the calculations did not take into effect the surface area of the wheel. Therefore, since a flat wheel has a larger surface area it does not make a difference, the friction would stay the same, so the amount of force required to move the weldment would not change from a flat wheel to a round wheel.

    My opinion is the bigger the flat the bigger the contact patch will be. ultimately leading to more friction and less chance for the weldment to move.

    Is my opinion correct or am i way off?:confused:
  7. Apr 23, 2013 #6


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    Let's see, the dry friction force would be given by:


    where μ is the coefficient of friction and Fn is the normal force due to weight.

    Contact area doesn't factor in it.

    In other words, if you increase the contact area by x, the weight per unit area gets divided by x, so it cancels out.
  8. Apr 23, 2013 #7
    Yes! That is exactly what the engineer said.

    It just doesn't seem logical. I believe the information, but a car with a larger contact patch is known to have more traction. Racers always want a wider tire apposed to a skinner tire.

    That was a great answer, the engineer I am working under is genius, so you're answer, being the same answer as his, proves you may have the genius gene as well. Thank you.

  9. Apr 23, 2013 #8
    It is not an easy answer actually , as if we speak about trains they usually have enough mass so that even the small portion of the wheel that touches the rail in a given time is enough to get friction when braking so that the brakes are actually effective.

    There are two things that basically affect the amount of friction something has, the first thing would be weight or you could also say the pressure some object exerts on the pavement or rail and contact area.
    Now small pressure and small contact area surely would have less friction than small pressure and large contact area.
  10. Apr 23, 2013 #9


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    Lol, no. I'm pretty sure this was included in the secondary-school curriculum.

    Cars are a different kettle of fish, as the wheels are not rigid and hard.
  11. Apr 23, 2013 #10


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    As an uneducated guy who just likes to build things based upon common sense, may I make a suggestion? (Don't bother answering; I'm going to anyhow. :biggrin:)
    How about tapping your hydraulic supply off to one more cylinder operating a clamp that gloms onto the rail?
    By the bye, what on Earth is a "weldment"? :confused:
  12. Apr 23, 2013 #11
    Formulating friction/drag is not that simple. Weight distribution cancelation does not always hold.
    As a mental simulation of an extreme case consider:
    • A sharp nail is welded (on flat side) on the center of a smooth disk of 1 m diameter which weighs 100000 g
    • The disk is placed on soft wood with the nail on top

    • The disk can be pulled/pushed by an average person
    • The disk is tuned

    • The weight of the disk forces the nail through the soft wood

    • The assembly can not be pulled/pushed as long as the nail is not bent.
  13. Apr 23, 2013 #12


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    While it might be true in general, you'd be hard pressed to find a real-world example closer to the idealised friction than steel wheels on rails, as in OP's contraption.
  14. Apr 23, 2013 #13


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    In the case of rubber tires, the coefficient of friction decreases as the load increases, known as tire load sensitivity. In the case of steel on steel, the decrease in traction due to a smaller contact area wouldn't be as significant, unless heat become an issue.
  15. Apr 23, 2013 #14


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    Last edited: Apr 23, 2013
  16. Apr 23, 2013 #15


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    Note that train wheels do not normally slide on the rails, they roll. There is a big difference. At the point of contact the surface of the wheel is stationary with respect to the track. The force of friction that needs to be overcome to initiate motion is that of well oiled bearings and that force is less then the force of static friction between track and wheels. Your square wheels can only slide, to get it moving you need to overcome that same force of static friction. The force required to initiate rolling motion is much less then that required to start sliding between the same materials. That's why we have wheels.
    Last edited: Apr 23, 2013
  17. Apr 24, 2013 #16


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    I think what the OP is asking is if the goal is to stop a train in motion, and assuming the choices are to lock up the wheels, or to extend flat metal plates (of the same material as the wheels), on to the track so that the wheels are lifted off the track and all of the weight is on the flat metal plates, will there be a significant difference in stopping distance?
  18. Apr 24, 2013 #17


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    Thanks for the links, OCR, but that just confuses me more. Am I to understand that this entire thread is about something that doesn't exist other than in a draughting program? Even if it is a real physical joining of components, how on Earth does that qualify as an object rather than a condition and even more weirdly require it to be on 6 railroad wheels? This whole thing is squirrelly.
  19. Apr 24, 2013 #18
    This is an excellent analogy.
    Describes what I am trying to accomplish very well.

  20. Apr 24, 2013 #19
    Yes, this weldment only exists on my computer.
    However, the weldment will be built in a couple weeks.
    I am trying to give as much information as possible, but if I uploaded the file my boss would hang me.
    I will muck something similar and try to upload.

    Sorry to sound squirrel.

  21. Apr 24, 2013 #20
    Last edited: Apr 24, 2013
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