Dynamic weight transfer in vehicle
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The document discusses the principles of weight, mass load, and load transfer in vehicles as fundamental aspects of vehicle dynamics, which study the forces acting on wheeled vehicles in motion. It addresses how load transfer affects a vehicle's performance during acceleration, braking, and cornering, emphasizing the importance of vehicle design parameters such as the center of gravity and suspension geometry. Understanding these concepts is critical for achieving optimal vehicle configuration and performance.
Dynamic weight transfer in vehicle
- 1. { Weight, Mass Load and Load transfer in a Vehicle~ Rohan Sahdev ~Dept of Mechanical and Manufacturing Engg. ~Manipal Instt, of Technology
- 2. VEHICLE DYNAMICS • The concept of “WEIGHT, MASS LOAD AND LOAD TRANSFER IN A VEHICLE” is the basis of the field of VEHICLE DYNAMICS. • Vehicle Dynamics is simply the study of the forces which affect wheeled vehicles in motion and of the vehicle's responses, either natural or driver induced, to those forces. • In many cases it is sufficient to understand the cause and effect of the forces and responses without establishing finite values or magnitudes. • So, on considering a vehicle, we need to analyse the various aspects such as how the vehicle response is while in a corner, how the load transfers when accelerated, or when applying a sudden brake. Also, what are the parameters which govern such behaviour in a vehicle. • On understanding the very basics of load transfer, one can get an insight of the various design parameters while designing the vehicle suspension and steering.
- 3. LOAD TRANSFER LONGITUDINAL VERTICAL LOAD ON VEHICLE LATERAL LOAD ON FRONT LOAD ON REAR CENTRIFUGAL FORCE CORNERING FORCE ANTI SQUAT ANTI DIVE JACKING
- 5. A FEW CONCEPTS TO LOOK INTO
- 6. {
- 7. THE CENTER OF GRAVITY of any body is defined as that point about which, if the body were suspended from it, all parts of the body would be in equilibrium-i.e. without tendency to rotate. It is the three dimensional balance point of the race car. All accelerative forces acting on a body can be considered to act through the center of gravity of that body. We want our race car's cg to be just as low as we can get it. THE MASS CENTROID AXIS is related to the cg sort of. If we were to slice the car into a series of transverse sections-like a loaf of bread-each section would have its own center of gravity, or centroid. If, in side view, we were then to draw a line joining each of these centroids, we would have the mass centroid axis. Figure (1) applies. This axis will not be anything that resembles a straight line, even if we were to go to the considerable trouble of calculating it. However, a reasonable straight line approximation can be intuitively arrived at that will give an indication of the distribution of the vehicle's mass in the vertical plane.
- 8. THE ROLL CENTER of a suspension system is that point, in the transverse plane of the axles, about which the sprung mass of that end of the vehicle will roll under the influence of centrifugal force. It is sort of a geometric balance point. It is also the point through which the lateral forces transmitted from the tire's contact patches act upon the chassis.
- 9. THE ROLL AXIS is the straight line joining the front roll center with the rear roll center. THE ROLL MOMENT is the linear distance between the roll center at one end of the vehicle and the concentration of mass at that end of the vehicle. For the vehicle as a whole the roll moment is the linear distance between the roll axis and the vehicle center of gravity measured in the transverse plane at the center of gravity. Figure (13) applies.
- 12. DYNAMIC LOAD TRANSFER DUE TO THE FORCES GENERATED AS THE VEHICLE BRAKES, ACCLERATES AND CHANGES DIRECTION.
- 13. Such a load transfer occurs in the longitudinal plane under linear acceleration or declaration. Since the c.g.is necessarily located at some distance above the track surface, any acceleration is accompanied by a longitudinal shift of load, rearward in the case of acceleration and forward in the case of braking. The total weight of the vehicle does not change; load is merely transferred from the wheels at one end of the car to the wheels at the other end. The amount of longitudinal load transfer that will take place due to a given acceleration is directly proportional to the weight of the vehicle, the height of its center of gravity and the rate of acceleration. It is inversely proportional to the length of the wheelbase. LONGITUDINAL LOAD TRANSFER
- 14. In the case of braking the effects of load transfer are several-and all bad. 1. First off, by unloading the rear wheels, the amount of braking energy that they are capable of transmitting to vehicle is limited by the traction potential of the smaller front tires. 2. At the same time, since the load transfer increases the vertical loading on the front tires, it also compresses the front springs which cambers the tires in the negative direction (in at the top) which may help in a cornering situation but does nothing good for braking (or acceleration). 3. These same front tires, if the suspension geometry should be less than optimum, may also be caused to scrub transversely across the track as they move into the bump position due to the compression of the springs. 4. Compression of the front springs from the load transfer uses up some portion of the available suspension bump travel and brings the nose and/or chassis into perilous proximity to the race track.
- 15. So, braking has the following effects The chassis may bottom on the track- which makes a nasty noise. So, if we project ourselves down the track to the corner whose rapid approach caused the braking in the first place, we find ourselves entering the corner with the nose scraping the ground, the rear jacked up and the tire cambers all over the place.
- 16. • At some time in the corner, the driver will see his way clear to push on the throttle and start accelerating. More longitudinal transfer will now occur-but in the opposite direction. Load will now be transferred from the front wheels to the rear. • The rearward load transfer will supply this extra tire capability in the form of increased vertical load. Naturally, we don't get something for nothing. The cost, in this case is that the rearward load transfer now compresses the rear springs, uses up suspension travel, and cambers the rear tires in the negative sense. • While a bit of negative camber is, as we have seen, a good thing, the probability is that we will get too much especially if the driver jumps on the throttle instead of getting the car up on the tire. On accelerating
- 17. Anti-dive and anti-lift are tricks that can be applied to a car's front suspension geometry to control brake dive and acceleration lift. ANTI DIVE AND ANTI SQUAT GEOMETRY
- 18. • As the wheel moves, the upright tends to move with the wheel. But, since the upright is rigidly mounted, it tends to negate the Torque. As a result, when accelerating, there is a forward thrust on the lower A-arm and the upper A arm will be pulled in the reverse direction. • As the IC is in front of COG and slightly above it, the rear end squat will be reduced and the front end will rise higher due to more weight shifted to the rear. INSTANTANEOUS CENTER
- 19. • Changing the upper control arm offset negates the lifting force created. • This will result in less rise of the front end, and less weight would be transferred to the rear.
- 20. • Lateral load transfer is caused by forces very similar to those which cause longitudinal transfer-with the operating axis turned ninety degrees. • In any cornering situation, centrifugal force, acting through the vehicle's c.g. tends to throw the car out at tangent to its intended path. • This centrifugal force is resisted by the lateral forces developed by the tires. • Since the vehicle's c.g. is necessarily located above the track surface, the tendency of the c.g. to fly sideways while the tires roll on their curved path gives rise to a moment of force which transfers some of the load from the inside tires to the outside tires. • Lateral load transfer is a bad thing. The transfer of load from one tire of a pair to the other reduces the total tractive capacity of the pair. Lateral load transfer
- 23. • When the vehicle is cornering, the tire cornering forces comes into picture. • These forces can be obtained by referring to the data provided by the tire manufacturer.
- 25. • The basic relationship here is very simple: the greater the lateral acceleration the greater the centrifugal force and the greater the tire side forces we must will take place. • The tendency of a given vehicle to roll due a given lateral acceleration will vary directly with the vehicle's roll moment and the amount of mass involved. • The part of the car that is going to roll is the sprung mass. • Centrifugal force, being an acceleration, will act through the c.g. of the sprung mass. • The greater the vertical distance also between the roll center and the c.g., the greater will roll couple produced by a given lateral or centrifugal acceleration.
- 26. JACKING
- 27. • Jack forces is an effect of lateral load transfer. • For a given rate of lateral automobile tends to "jack itself up" as it goes around a corner. However, any independent system with the roll center above ground level will jack to some extent. • The effect is caused by the fact that the reaction force of the turn must act through the roll center. • If the roll center is above the ground, then the line of action between the tire contact patch and the roll center will be inclined upward toward the vehicle centerline. JACKING
- 28. • This being so the side force developed by the tire will have a vertical component which will tend to lift or "jack" the unsprung mass. • This lifting action, in addition to raising the e.g., will also move the suspension into droop with unfortunate results in the camber department. The higher the roll center (and the narrower the track), the steeper the inclination of the line of action and the greater the jacking force. • Jacking is to be avoided on any car and is the single major reason why today's projectiles feature very low roll centers.
- 29. As it is very clear from the presentation, that, Vehicle Load Transfer has got an extensive application in the design of a vehicle. Hence, the study of Vehicle Load Transfer can help us to achieve the optimum configuration of the suspension system, steering system, and most importantly, the static load distribution in our vehicle. CONCLUSION
- 30. Tune to win ---Carrol Smith Chassis engineering---Herb Adams Race Car Vehicle Dynamics--- Milliken & Milliken References