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Designing simple car with suspension and brake system

  1. Jan 14, 2013 #1
    1. The problem statement, all variables and given/known data

    This is just for a formal engineering report, no product will be made.

    for my ME class, one of the very first courses. pre-requisite for this class is only vector static.

    This is the first week of class, and the lectures covered the role of engineers...

    I am being asked to design a small 4 wheels vehicle (can be transport on the bed of a pickup truck) which can be operate at human walking speed (~5mph), able to steer and brake, and have a reliable suspension system. This vehicle shall be able to climb a 40° slope, go over 12 in dia log, and go across a 30° incline.

    This vehicle will be used on hot ground (~120F) and used by [STRIKE]fighter [/STRIKE] fire fighter, should be able to transport equipment as well as an injure body.

    2. Relevant equations

    This is all I have from static
    F = ma
    Ff = μN
    F = -kx

    3. The attempt at a solution
    This seems overwhelmingly difficult consider the fact that I am being ask to make a complete car from scratch, including choosing the right material, choosing the right engine or motor.

    we have 3 ppl, so I was assigned to work with the suspension and brake.

    for going over the 12 in log, this has to do with the suspension system right? I will use the coil spring system. But how do I determine the diameter of the coil, the dia of the steel string, the number of loops, etc?

    For going over and across slopes, is it the same type of problem as moving a box up an incline? what is the essential thing for this to work?

    For the braking system, for this slow moving car, how do i determine the force that is needed to stop the tire? and what type of device that is usually used for making simple brake?

    Please help me with some pointers and or resource. All I have found from the internet are just explanation of the mechanism, not much calculation or the actual characteristic of these thing.

    Thanks for your time.
    Last edited by a moderator: Jan 14, 2013
  2. jcsd
  3. Jan 14, 2013 #2


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    Staff: Mentor

    The 12" log part is going to be tough, especially if you are trying to transport a patient safely. BTW, there was a typo in your post that took me a while to figure out. You meant to say "fire fighter", I'm pretty sure. I've corrected it in your post.

    I'd recommend starting at Google Images, and search on dune buggy. Some of those photos (especially the open-frame ones) will start to give you ideas. To get smoothly over a 12" obstacle, though, you may require something other than just wheels. It may take something more like tracks or something.

    You can also use Google Images for ideas about the wheels/tracks. Just enter some other search terms to try to see what the various tracked vehicles look like. Some sort of articulated track system with suspension may be the right idea.

    You might also look for images of motarized litters and other patient transport systems.
  4. Jan 14, 2013 #3


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    Staff: Mentor

    BTW, if it is just "operated" by the fire fighter, rather than them having to ride in it with the patient, that simplifies things some. You might be able to use large oversize articulated wheels of some sort...
  5. Jan 14, 2013 #4


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    Science Advisor
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    Gold Member

    The specification says it must be capable of going up a 40 degree slope at say 5mph carrying a certain load. You could make an estimate of the total weight and use that to work out the power required. Perhaps add some spare capacity? Then pick a suitable motor. If it turns out you need something like the engine from a monster truck and you've only allowed a few lbs for the weight of the whole vehicle then revise the weight estimate and repeat the calculation until it all makes sense. Perhaps look up the weight of a similar spec vehicles to start the process?

    With motor data you can work out the gear ratio to provide the required speed and torque.

    The log climb also has do with the wheel diameter and torque. Draw a force diagram of a wheel up against a log. Knowing the weight on the axle you can calculate the torque required to lift the wheel off the ground and up over the log. I suggest making the wheels quite large diameter (edit: or using tracks as others have suggested). Ever tried riding a bike over a 12" log :-)

    As for going along a slope... that's probably to do with a roll over test. eg making sure the wheels are far enough apart and the centre of gravity low enough that it doesn't roll over. Imagine you are making a sharp turn on the slope...the centripetal force might also help tip the thing over. A few force diagrams showing that won't occur at max speed with a min radius turn on a slope might be the way to go.

    There are obviously other practical issues that you hinted at - such as the need for ground clearance when going over the log. Working out details of the springs can come later. For example when you know how much suspension travel you can have.

    That would be my approach but other people might have different ideas.
  6. Jan 14, 2013 #5
    oh actually, i left this part out
    the vehicle is controlled by a hard wired tether from the vehicle to the operator walking along the side of it. The working mechanism of that purpose is not important since we have no knowledge about electricity.
    So it is like a toy car with a wired remote control.

    This is the original assignment.
    https://sphotos-a.xx.fbcdn.net/hphotos-ash3/r270/72569_541446955874824_580570462_n.jpg [Broken]

    I really dont know where to start. I spent hours searching google and youtube, but all I have found are just explanation of how each part of a typical car work.

    Course that I have taken are just simply static, dynamics, mechanics of material, and fluid mechanics. whereas the only requirement for this class is vector static and it seems to me that it isn't enough.
    Last edited by a moderator: May 6, 2017
  7. Jan 14, 2013 #6


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    Staff: Mentor

    BTW -- for extra credit, your team should consider the fact that it is usually a bad idea to be tilting a patient who is on a litter or gurney. You should be able to incorporate some things into your design to keep the patient position constant as they are moved over the 12" log obstacle.

    Have fun!
  8. Jan 14, 2013 #7


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    Staff: Mentor

    Ah, reading the assignment it is mainly aimed at transporting equipment, and only occasionally for transporting an injured FF. In that case, my comment about keeping the patient horizonal doesn't really apply much.

    And what do you mean you don't know where to start. You have been given good starting points. Now do some work on your project! :smile:
  9. Jan 14, 2013 #8
    Hi Walter, thanks for that reply.
    The thing is there isn't any specified requirements such as weight and size or speed.
    since the person is walking along the side of the car so I assume that it is 5mph.
    As for the weight, I am still not sure if that should be calculated given certain condition, or we can assume a weight and work out the force needed from the motor.
    You mentioned about centripetal force, I think it wont be needed since the problem is simplified to the case of just moving in a straight path across the slope.
  10. Jan 14, 2013 #9
    Thanks Berkeman, I am still kinda lost, but I guess I 'll try to find some sort of data to work with.
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