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Truck jumping up when it goes ober a speedbreaker

  1. Aug 23, 2013 #1
    The company where I'm interning makes machines called compactors. The collect garbage from various places, compact it and it's all dumped together. The equipment mounted on the chassis for the compactin is pretty heavy.

    What is happening in case of a specific design is, when the truck goes over a speed bump, the front tyres jump up a couple of inches and come down again.

    We calculated the CG manually as well as by software (Solid Edge). It lies between the two axles, but very close to the rear axle. So basically, the weight is concentrated towards the back part of the wheel base.

    What could be the reason of this jump? I won't topple as the CG is within the wheel base. But would the weight concentration at the back make it jump?

    Thanks in advance,
  2. jcsd
  3. Aug 23, 2013 #2


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    A higher mass at the front would certainly reduce those jumps a bit, but the velocity is more important. Just drive slower over those bumps (that's why they are in the street anyway).
  4. Aug 23, 2013 #3
    The truck *will* topple over backwards at some point, the question is at what angle. By moving the CoG very close to the rear axle you are making it very light on the front, so any bumps, especially at speed are likely to lift the front wheels off the ground (and bigger bumps or higher speed could topple it over). Also bear in mind the suspension system will have been designed for the original weight distribution, you might find some strange behaviour if you have significantly changed the weight distribution.
  5. Aug 23, 2013 #4
    The speed is pretty slow. Higher mass in the front would reduce it, but the equipment is such that there's no avoiding the weight at the back. Maybe lower density material for the back parts.

  6. Aug 23, 2013 #5
    No it won't topple. The CG is in between the front and rear axle and the back wheels act as the point of pivot. So the CG will pull it back down.

    True, the front axle experiences light weight. Drivers say they feel weightless while holding the steering wheel.

    You're right about the suspension system. It's made for a specific weight. Even the axles for that matter.

    Redesigning the rear with a lighter material could be the only option I guess.

  7. Aug 23, 2013 #6


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    That is only true if the CG is at the same elevation as the wheels, which it never is. Being above the wheels means there is a certain pitch angle beyond which it will topple over.
  8. Aug 23, 2013 #7

    jim hardy

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    I assume it's built on a rear drive truck chassis .... it single rear axle ?
    Add to your " sum of the moments" toppling calculation the driving axle torque.
    There'll be some incline where the front wheels will lift, or have so little weight as to become ineffective at steering.

    Not knowing any more than has been posted I'd be afraid to drive that thing up the steep hill where I live.

    old jim

    picture courtesy of http://metro.co.uk/2012/04/16/men-b...erloaded-with-wheat-straw-in-pakistan-390217/
  9. Aug 24, 2013 #8


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    The problem is that the CofG is too close to the back axle. The mass of equipment at the rear and the motor and cab at the front has a high inertia about the rear axle. Because the CofG is close to the rear axle most of the mass on either side of the axle cancels, so very little restoring force remains.

    After lifting the front wheels, the vehicle CofG is accelerated by gravity downwards. The cab is effectively at the end of a long lever and so accelerates faster than the CofG. The driver will feel weightless.

    There is a way to stop it jumping. Take your foot of the accelerator and brake momentarily as the front wheels hit and pass over the bump. That will load the front tyres sufficiently to remain in contact with the road. The converse is also probably true. If you rev up the engine, then drop the clutch, the front wheels will lift off the ground.

    The vehicle is of dangerous design/modification because there is insufficient effective weight on the front wheels to guarantee steering control. It would be described as “light on” and would be expected to under-steer.

    To help the balance, any mass that is unnecessarily behind the rear axle should be moved as far forward as possible. Likewise any mass such as fuel or hydraulic tanks should be moved forward. The front tyres could be filled with water rather than air.
  10. Aug 24, 2013 #9


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    That really does not say much about the load put onto your vehicle.

    Your vehicle should have a VIN plate on the driver door which lists the maximum Gross Vehicle Weight Rating ( GVWR ), the maximun Gross Axle Weight Rating ( GAWR )for both the front and rear axles. Also, the tires have a maximum load rating at recommended tire pressure.

    Exceeding the GVWR, the front GAWR, the rear GAWR, or the tire loads is definitely not a wise path to follow ( besides the apparant unsafe aspect of the front wheels lifting up ). Poor road handling and braking, and blown tires, together with premature failure of components are some aspects to look forward to while owning and driving an overloaded vehicle.

    The curb weight of the front axle includes the cab, motor, including fluids, and suspension, and a good percentage of this weight is usually right above the front axle. The rear axle curb weight is more or less distubuted over the rear axle except for the axle itself and springs. The rear curb weight for a truck could be around half the front axle curb weight, but you would have had to have had your vehicle weighed at a scale station before installation of extra components, or have obtained this information from the manufacturer. Knowing the curb weights gives an indication of how much weight of extra components can possibly be added to the vehicle and the axles and how much carrgo carrying capacity is left over.

    In your case, with the addition of the compactor, the cg of the vehicle has moved towards the rear axle, with the rear axle carrying most of the vehicle weight and the front axle very little. Your rear axle is thus carrying the rear curb weight, most of the front curb weight, and the cargo weight ( compactor ). There is a good possibility that the rear axle is overloaded due to the improper vehicle weight distribution.
  11. Aug 24, 2013 #10

    jim hardy

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    Siddhartha - Draw a free body diagram of your machine on ~20% incline and sum torques around the point where rear tires contact ground, or maybe the center of rear axle.
    Don't forget about torque from accelerating the CG forward.

    At (max engine torque X transmission 1st gear ratio X rear axle ratio) what is downward force on front wheels ?


    picture courtesy National Hot Rod Asociation
    Last edited by a moderator: May 6, 2017
  12. Aug 25, 2013 #11


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  13. Aug 26, 2013 #12
    Jim Hardy, Baluncore and 256bits, thanks a lot for your solutions. Obviously the best way towards improvement is shifting the CG towards the front axle (ps - it's a single rear axle chassis Mr. Hardy). But for that, we'll have to redesign the entire machine. For the machines that are already out there, I guess Baluncore's method of asking the driver to take his foot off the accelerator could be a way. We thought of adding weight to towards the frond end of the chassis, but then the GVW is exceeding the maximum value.
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