questions while learning cdl permit information

Discussion in 'Questions From New Drivers' started by ursus, Nov 29, 2010.

  1. ursus

    ursus Light Load Member

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    Ok, I am starting to study to get my CDL permit. Now I'm not too far into it yet but I already have some questions and I imagine as I go along there will be more :biggrin_25526:

    Anyway...the first one came up while looking at the end of chapter questions on the pti. It asks for the reason you put the key in your pocket, yet I've read the section several times and it does not specifically state why this is done. There is a brief mention of taking the key after the in-cab check but nothing more.

    Second question is about weight and stopping distance. The book says that a unloaded truck will take a GREATER distance to stop then a fully loaded one. This seems wrong to me. I know for my motorcycle or car if the cargo weight is raised the stopping distance goes up. What makes a Big truck different?

    Thanks :biggrin_25514:
     
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  3. 123456

    123456 Road Train Member

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    Safety first........
     
  4. celticwolf

    celticwolf Road Train Member

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    1. You don't want someone driving off with your truck while you are under it.

    2. A loaded trailer provides more traction on the tractor tandems.
     
  5. Rerun8963

    Rerun8963 Road Train Member

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    you put the key in your pocket for safety reasons. no one will be able to start up your truck and maybe run you over if you are under the rig checking things.

    the MORE weight on a truck the better. the brakes, springs, tires etc, are made to take the weights and stopping distance IS FAR BETTER loaded than empty. on average, it takes the entire length of a football field to stop the truck when fully loaded. when empty, there is NO weight pressing down on the tires, therefore you have a smaller contact patch on the ground. the more contact patch (traction) you have the better the braking power.
     
    chompi Thanks this.
  6. chompi

    chompi Road Train Member

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    Yeah, what Rerun said! Wait until you bobtail a truck for the first time. It will be very scary! A lot of companies won't let you bobtail simply because of insurance and safety. The trucks are not meant to drive like that.
     
  7. Tankergirl80

    Tankergirl80 GangstaGirl

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    Huh? I dunno... so you don't lose it?


    A loaded semi will always take longer to stop due to it's high inertia. However what they want you to understand in that booklet is that under certain conditions i.e. slick road surfaces, is that an empty semi CAN take longer to stop. Brake friction and road friction is decreased due to less force pushing down, less road friction on a slick road that has low friction to begin with is a recipe to slide.
     
  8. Rerun8963

    Rerun8963 Road Train Member

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    uh....NO..... a loaded semi stops FASTER/shorter distance than an empty. the book says "DRY ROAD SURFACES".......longer distances for stopping on slick roads WILL TAKE/BE MORE DISTANCE to stop, for ALL trucks, loaded OR empty...

    the brakes, suspension, tires, are DESIGNED to stop a FULLY LOADED rig, in LESS distance than an empty rig or a bobtail..

    ALL vehicles WILL TAKE LONGER TO STOP ON SLICK SURFACES...not just empty.........
     
  9. Flying Finn

    Flying Finn Heavy Load Member

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    To the OP. Think about trying to stop an empty pickup. The rear tires will lock up easily as there is very little weight on them, this increases your stopping distance. Load up the bed and the rear gets lots of traction. Now you can stop in less distance.

    It is easier to lock up wheels when light or empty, that is why it takes longer to stop. Load heavy and the tires have loads more traction so the extra large brakes can do a better job at slowing the truck.
     
  10. Tankergirl80

    Tankergirl80 GangstaGirl

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    Assuming same speed, same brake friction, same road friction on a perfectly flat surface a heavier truck will always take longer to stop due to higher inertia. Brake friction and road friction is increased due to more force pushing down, but it is not enough to counter the change in inertia.

    Here is the technical breakdown for any physics junkies.


    Assuming that the brakes are applied to the maximum degree so that slippage between rubber and road (or skidding) does not occur, that's where the maximum braking can occur.

    The braking force is

    F_b = k_s F_n

    The normal force is the weight which is proportional to the mass.

    F_n = m g

    Where g is the acceleration of gravity. So the braking force is

    F_b = k_s m g

    and that braking force is proportional to the deceleration a of the vehicle

    F_b = k_s m g = m a

    so the deceleration due to braking is

    a = k_s g

    Independent of the mass of the vehicle. The increased mass increases the braking force, but it also increases the momentum by the same factor. If the vehicle was on a slippery surface, k_s would be less and the vehicle would decelerate more slowly. If the vehicle was on the moon, g would be less and the vehicle would decelerate more slowly. Also if the vehicle is pulling a trailer which does not have braking on its wheels, that increased mass from the trailer and load contributes to the momentum, but not to the braking force because only the pulling vehicle weight is pushing down on its wheels (that have brakes), so it decelerates more slowly.

    If you know the velocity when the brakes are applied, the braking distance is:

    d = v t - \frac{1}{2} a t^2

    and since the final velocity is zero

    0 = v - a t

    Then you can calculate the braking time

    t = v/a

    and

    d = v (v/a) - \frac{1}{2} a (v/a)^2 = \frac{v^2}{2 a} = \frac{v^2}{2 k_s g}



     
  11. Rerun8963

    Rerun8963 Road Train Member

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    here is the formula for all others to understand:

    a loaded truck stops in a shorter distance than an empty truck or a bobtail.

    i think i said this quite well the first few thousand times now.

    go to page 32.........(Indiana CDL manual).

    http://www.in.gov/dor/files/cdl.pdf
     
    celticwolf Thanks this.
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