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Physics of an automobile, suspension, and weight transfer?

  1. Aug 19, 2003 #1
    The application is drag racing in a rear wheel drive vehicle.

    The vehicle weighs approximately 3000lbs.
    Traction is not an issue as both vehicles grab the ground perfectly.
    The front and rear is suspended by a shock and spring in each corner.
    All else is equal unless otherwise stated.

    Car A launches and leaves the line completely horizontal with no suspension travel. This car has stiffer suspension setup.

    Car B launches and leaves the line where the rear end squats and the front of the car raises a few inches. This car has a softer suspension setup.

    Is energy diverted from the forward vector and lost in the suspension travel in car B? If so, can you please give me an explanation as to how this occurs?

    I am fairly physics savvy but am having a hard time understanding how power is lost in minor suspension travel.

    I understand that shocks are designed to absorbe energy (mainly from spring travel)

    The way I see it, the engine turns the transmission, which turns the driveshaft, which turns the pinion, turns the differential/axles, and turns the tires. Tires just turn, and turn along the ground. They turn as fast as the engine will power them too regardless of what the suspension may do (within reason).

    I understand that for the cars nose to lift, less rotation is seen in the tire, but is re-insert as the front comes down. I also understand that a car that may lift its nose will change its aerodynamics.

    Please help me, I am confused. :)

    This is my first post btw.

    - Keith
  2. jcsd
  3. Aug 20, 2003 #2


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    Seems to me that any torque that contributes to lifting the front of the car is not available to turn the wheels, thus the measureable amount of energy would be used in lifting the front of the car would not be available for conversion to Kinetic Energy of motion. Thus lower speeds.

    Just a gut feeling discription.
  4. Aug 20, 2003 #3
    Yes I was thinking along the same lines. Putting energy toward raising the car clearly subtracts from the amount of energy used to spin the wheels.

    Would it not also create a force in the opposite direction of his intended forward motion as the car jerked upwards?
  5. Aug 20, 2003 #4
    There's another factor that I'm thinking of: conservation of angular momentum. You know that second small blade mounted on the tail of helicopters? It's there to balance the force coming from the tendency of the helicopter body to rotate in the opposite direction to the main blade. It's the same with the car. When the wheels suddenly start to move fast the nose of the car tends to go upwards especially since its weight is mostly in the back. A part of the force needed to balance the car is covered by gravity. When you put in a very hard suspention the rest of the force needed is taken by the engine and the tension in the chasis (it can actually break). So you lose some energy there. When you put in a softer suspention the shock absorbers and the springs balance that force. So you win some energy.
    So it's all a big compromise: you want more power but not more than the tires can put to the ground, you want to keep the nose down to stay aerodynamic and not to tip over or break the chasis, etc.
  6. Aug 20, 2003 #5
    I don't see it as torque lifting the front end, but the forward vector of the rear end of the car (axle) being greater than the down vector applied by gravity on the nose, thus being the reason the car comes up.

    Sonty, are you saying you would lose more energy in stiffer suspension?

    I still don't really understand how acceleration or force is lost when the rear end of the car squats for example.

    f=ma... Mass is a constant.

    The front of the car lifts when acceleration of the axle is greater than the down force applied by gravity of the front correct? If the suspension is stiffer, it will require a greater force for the front end to come up do to the suspension components weight being applied right away. With a softer suspension the body can come up and let the suspension hang and THEN pull up the suspensions weight.

    I'm still confused.
  7. Aug 20, 2003 #6
    I'm saying the optimum choice is somewhere in the middle towards the harder suspention. When you get enough speed the aerodynamic force pushing the nose down is enough to balance the car. Keeping the nose up too long you lose because of air drag.

    f=ma right, m=constant right, the engine gives the same force right, but where do you spend that force makes the difference.
    Am talking about the back suspention, which one are you talking about?
  8. Aug 20, 2003 #7
    Aerodynamics isn't a concern at all. My real concern is just the launch... and how a car should launch for ideal energy savings.

    I'm talking about front AND back.

    The Force of the engine just rotates the tires in the end. The tires just rotate along the ground moving the vehicle forward and because the vehicle accelerates so fast in a direction, momentum kicks in and the suspended chassis wants to go backwards. Because the center of gravity is higher than the forward force vector, when the forward force vector exceeds the downard vector of the nose of the car, the front end lifts.

    Regardless if the suspension is stiff or soft, if the nose comes up, or it doesnt, is the forward force vector the same?

    I don't really know.. my entire post is basically a question.
  9. Aug 20, 2003 #8
    The forward force vector changes based upon the changes in the suspension, and the nose lifting.

    So when a dragster takes off, the torque generated by the engine, driving the wheels, lifts the front end, resulting in a minor addition to the "co-efficients of friction" at the rear end, and a slight loss of the power, due to suspension compression.

    It acts a little like a lever, hence you get a minor transfer of center of gravity, towards the rear, as the machine attempts to balance the energy lifting the front, and the traction (and slip) at the rear driving the thing forward.

    Softer suspension absorbs power, but lowers the rear end resulting in a minor addition to traction due to the lever/balancing of the center of gravity.

    It's all pretty well vector physics, Newtonian.

    Really neat to watch the 'slo-mo' of the rear tires during the "Bleaching" process (WATER poured out onto the track, and the tires spun to smoking to heat, and clean) as you can see the displacment of the sidewalls of the tires as they are gripping at a 'tractionable surface' and then lossing that traction and 'rounding out' due to centrifugal force.
  10. Aug 20, 2003 #9
    Auto suspensions are very interesting. In the case of drag racers, taken to an extreme, some racers have the power to lift the front wheels completely off the ground. This does cost them time and to counter that, they may have "wheelie casters", small wheels that stickout from the the rear of the car that prevent the front from leaving the ground.

    Also, not all suspension types behave the same when the engines full power is applied to them. Leaf spring rear suspensions have a tendency to be twisted by the rear axle under torgue into an "S" shape. Actually causing the rear of the car to lift up on some models. This is controlled with traction bars, bars that attatch to the spring under the axle and extend forward with a rubber snubber that hits the frame and minimizs spring twist. (some designs). Independent rear suspensions have a tendency to squat under full power launches.
  11. Aug 20, 2003 #10
    friction is increased at the rear and decreased at the front.

    I don't understand how power is lost due to suspension compression though.

    The front end will lift depending on how much power you have. With a softer supsnsion your chassis will lift first then the suspension will hang and carry up the wheel assemblies with it (if the front end lifts that far)..

    So say you have a soft suspension and the car only lifts up 2". Its not enough to carry the wheels. Wouln't it be easier for the car to lift like this, the 2", then it would be if the suspension was stiff and thus the added weight of the wheel assemblies was too much for the car to lift at all?
  12. Aug 20, 2003 #11
    Sorry don't understand what you are driving at here. ("Pardon the pun" AKA PTP)
  13. Aug 20, 2003 #12
    The car will accelerate forward, that's a given.

    If the suspension is soft enough to allow weight transfer though, where and how is drive energy being lost? That's what I don't understand.

    Another example is this. You have trains of equal power on parallel tracks. Each carries a flat bed which a vehicle on it died down by the wheels. Vehicle A has solid suspension (no give) where as vehicle B has soft suspension.

    The mass of the trains are equal and so is the power output of the locomotive.

    If forward energy is lost due to weight transfer, then the train with the solid suspended vehicle on it would accelerate faster correct?
  14. Aug 20, 2003 #13
    Because in your train example the will be no 'leveraging' action. (Transfer of weight)

    Take a board, lift one end only, the weight is transfered towards the end that is still on the ground, A soft suspension will abosrb more of the weight transfer's energy (quicker and longer motion/distance travelled) then will a stiffer suspension.
  15. Aug 20, 2003 #14
    Magnus, the engine's energy used to lift the front end of the car is lost to you for purposes of accelerating. Gravity brings the front end down again and you don't regain the energy. Therefore if you can keep the car front end from lifting, you keep more of the engine's energy available for acceleration. Fairly straight forward.

    Folks who drag race production automobiles try to stiffen up the suspension as much as possible. If one looks at purpose built drag cars like top fuel dragsters, you see that they have no suspensions and very long wheelbases to control front end lift.

  16. Aug 20, 2003 #15
    In the train example though, if the trans where able to accelerate as fast as the cars did at the track, and only the rear wheels where tied down.. wouldn't there be leveraging action due to momentum? The center of gravity of the car is higher than the point of forward force thus it goes upward? And if this is correct, then would the trains still accelerate evenly?

    OldHubcap, I must have a bad mental image of the equation then. I just picture the wheels moving forward along the track. I picture them moving so fast that the front end comes up because of the rate of acceleration. I don't think of it as the engines power cranking up the front end as if the wheels where not moving.

    Example: You have an RC car. You grab its rear wheels and give it a quick JERK forward.. front end comes up. Because the front end comes up it takes more energy for you to jerk the car forward than it would if the car where to remain parallell?
  17. Aug 20, 2003 #16
    Nice, but on 'minor details' (which is what you need to finesse in competative drag racing) lifting of the front end, and the resultant weight transfers, increases the co-efficients of friction on the rear end of the car, giving it greater traction for forward propulsion.

    Balancing it out well, gets the best result. That is also why Wheely bars let it lift the front, somewhat.

    Better weight transfer means better rear end traction = greater accelerative possiblity.
  18. Aug 20, 2003 #17
    Very well put Mr. Parsons. You do need some degree of weight transfer to increase traction. Drag racing, like all motorsports, is about finding the proper balance. :smile:
  19. Aug 20, 2003 #18
    I understand the effects of weight transfer on traction...

    did you guys read my last reply? That's where I'm getting confused.
  20. Aug 20, 2003 #19
    With respect to front end lift on a drag car.
    The lift occurs when the car initially accelerates and still has little forward speed, thus allowing us to discount significant lift caused by airflow. This means that the energy to lift the front end ultimately comes from the engine. If you break it down into vectors, if the car had no lift, the only vector is pointing forward.

    <<<<<<<<<< Imagine this is the vector for a perfectly rigid suspension. All 10 vector arrows go forward

    If the front end lifts, the vector picture looks like this

    ^<<<<<<< In this case 3 vector arrows used to lift front of car and only 7 vector arrows accelerate the car.

    Its a rather crude illustration, but I hope it sheds light on what is happening.
  21. Aug 20, 2003 #20
    I can understand that.

    If your forward vector is so great though that the front end cannot come down due to the fact that the down force applied by gravity isn't enough to overcome the acceleration, what are you supposed to do to conserve energy?

    In that scenario, isn't there no enrgy lost?
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