However, the pitching and rolling of the body of a non-rigid vehicle adds some (small) weight transfer due to the (small) CoM horizontal displacement with respect to the wheel's axis suspension vertical travel and also due to deformation of the tires i.e. After that, we will see how the components of load transfer can be manipulated to tune the balance of the car. What happened here? W Vertical load is the load actually seen at the tire contact patch. The driver has hit the apex but has found the car is starting to push wide of the desired line. As fuel is consumed, not only does the position of the CoM change, but the total weight of the vehicle is also reduced. When this happens, the outside spring of the suspension is compressed and the inside spring is extended. This component is the easier to control. The weight of an IndyCar race car should be at least 712 kg, with an average of 1630 lbs or 739.5 kg. While a luxury town car will be supple and compliant over the bumps it will not be engineered to provide snappy turn-in, or weight transfer to optimize traction under power. But it must be considered that the Mustang at this time does not mount the carbon bottles, and there's no driver inside. Bear in mind that lateral load transfer affects the balance through tyre load sensitivity (the tendency of the tyres to generate higher lateral forces at a decreasing rate with higher vertical loads). Lets analyse the moment involved in roll. Wedge is defined as greater inside percentage at the rear than at the front. Why? 35% Front 420 lbs 780 lbs 280 lbs 520 lbs LH Turn - New Stiffer Front Roll Bar 33.3% Bear in mind that these values were obtained for a fairly heavy race car with an unreasonably high CG, and this is only one of three weight transfer components. Weight transfer is one parameter that is minimized - to aim for even loading on all four tires; resulting in maximum grip during cornering. The roll stiffness of the car is the sum of roll stiffnesses of front and rear axles: One important thing to notice is that the chassis is assumed a rigid body, and hence, the roll angle is the same for front and rear suspensions. Lifting off the gas brings the car's momentum forward. Weight transfer is affected by the distance between the CG Height and the roll centre. If we know a car needs 52.2 percent crossweight to be neutral based on the front-to-rear percentage, then running 49 or 50 percent in a neutral car means the setup is unbalanced. Consider the front and rear braking forces, Bf and Br, in the diagram. Figure 14 shows the contour plot. Then, the total lateral weight transfer is therefore a sum of the three parts: The first term is usually small in comparison, and it is also difficult to modify, and is therefore, sometimes ignored. The moment equilibrium analysis will be the same here, but we will substitute the moment from the inertial force about the CG, , by a generic moment, . the amount of body roll per unit of lateral acceleration: If we isolate the roll angle from the equation above, we can use it to calculate the moments from roll resistance moment and sprung CG side shift for a single axle. The vehicle mass resists the acceleration with a force acting at its center of gravity. For the sake of example, ride stiffness controls ride height, which has strong effects on aerodynamics of ground effect cars (almost every race car with relevant aerodynamics design). A lateral force applied on the roll axis will produce no roll; Front and rear roll rates are measured separately; Tyre stiffnesses are included in the roll rates; Vehicle CG and roll centres are located on the centreline of the car; We used steady-state pair analysis to show once again that lateral load transfer in one end of the car decreases the capability of that end to generate lateral force. In the previous post about understeer and oversteer, we have addressed the vehicle as the bicycle model, with its tracks compressed to a single tyre. Lateral load transfer or lateral weight transfer, is the amount of change on the vertical loads of the tyres due to the lateral acceleration imposed on the centre of gravity (CG) of the car. We'll assume the car's side to side weight distribution is equal. The second term can be changed modifying the suspension geometry, usually difficult or not allowed in some competitions. In wheeled vehicles, load transfer is the measurable change of load borne by different wheels during acceleration (both longitudinal and lateral). You will often hear coaches and drivers say that applying the brakes shifts weight to the front of a car and can induce over-steer. Here, is the lateral acceleration in G units, is the weight of the car, is the CG height, is the track width and and are the vertical loads on the left and right tyres, respectively. : a go-kart), the weight transfer should split between F/R axles according to the CG position, just like you instinctively done for the longitudinal acceleration. Before we start, its worth to give a note on units. Bickel explains how the way the 4-link plays into how you adjust the car. That is a lot of force from those four tire contact patches. Lets say that you are a race engineer and your driver is having trouble to go around the slowest corners on the circuit. The tendency of a car to keep moving the way it is moving is the inertia of the car, and this tendency is concentrated at the CG point. Do you see where this heading? This article uses this latter pair of definitions. These lift forces are as real as the ones that keep an airplane in the air, and they keep the car from falling through the ground to the center of the Earth. The added axle weight will slow the release of the stretch in the tire and help hold traction longer. Deceleration. Here, the lateral force acting on the sprung mass () will generate a moment on the tyres through the roll centre height that will also contribute to lateral load transfer. This force generates a lateral weight transfer in the opposite direction of the turn. This. t The net loss can be attributed to the phenomenon known as tire load sensitivity. This will tell us that lateral load transfer on a track will become less dependent on the roll rate distribution on that track as the roll axis gets close to the CG of the sprung mass. The splitting of the roll moment between front and rear axles is useful in analysing lateral load transfer and this is called roll moment distribution between front and rear axles. The reason it is relevant is that the amount of weight on a tire directly affects how much grip is available from that tire. You already know from steady-state pair analysis and from the discussion on tyre load sensitivity that lateral load transfer will decrease the lateral force capability of the axle. We dont often notice the forces that the ground exerts on objects because they are so ordinary, but they are at the essence of car dynamics. The inertial force acting on the vehicle CG will generate a moment about the roll axis. r The driver is said to manage or control the weight transfer. For this case, roll moment arm decrease with roll centre heights was smaller than the increase in roll centre heights themselves. The third term is usually split between springs, dampers and anti-roll bar, and determines the nature of body control and the level of body roll. Newtons second law explains why quick cars are powerful and lightweight. This is generally not the first option to take because of the effect that it has on other aspects of the car. For example, if the weight is shifted forward, the front tyres may be overloaded under heavy braking, while the rear tyres may lose most of their vertical load, reducing the brake capability of the car. The front end will move faster and farther because less force is required to initially extend the spring. When we corner on a circle track turning left, the lateral forces will transfer some of the weight that was resting on the left side tires over onto the right side tires. During acceleration or braking, you change the longitudinal velocity of the car, which causes load to be transferred from the front to the rear (in . Sprung Weight Transfer: This is the contribution to weight transfer from the sprung mass of the car, which itself is broken into two sub-components: It has increased importance when roll rate distribution in one track gets close to the weight distribution on that axle, as direct force component has its importance reduced (assuming horizontal roll axis). When accelerating, braking or steering, the body of the car rotates in the opposite direction, which compresses the suspension on one side of the car, while releasing the weight on the other side. Weight transfer is the result of acceleration, braking or cornering. 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. We wont consider subtleties such as suspension and tire deflection yet. There are Four Rules of Weight Transfer, Three lesser, one greater: Lesser the First: Turning the car will weight the outside wheels heavily, the inside wheels lightly. If that solution doesnt work, you could have roll centre heights that would give a roll axis too close to the sprung CG, as discussed before. The only way a suspension adjustment can affect weight transfer is to change the acceleration. Now that we have quantified lateral load transfer on an axle, we can start to analyse how the individual components interact. These data were obtained for the same open wheel car analysed in figure 9, but this time front and rear roll centres heights were held constant and equal, while roll stiffnesses varied. A. However, the suspension of a car will allow lateral load transfer to present itself in different ways and to be distributed between the axles in a controlled manner. Deceleration moves the center of gravity toward the front of the vehicle, taking weight out of the rear tires. Weight transfer has two components: Unsprung Weight Transfer: This is the contribution to weight transfer from the unsprung mass of the car. The weight distribution on the rear axle was 54 %. An exception is during positive acceleration when the engine power is driving two or fewer wheels. This is altered by moving the suspension pickups so that suspension arms will be at different position and/or orientation. These numbers are just averages and are very dependent on the class of car and the tires being run. This will decrease roll angle component, but since the roll centre height of the opposite axle will not be raised, the direct lateral force component will not increase and the overall effect will be a reduction in weight transfer on that axle. Weight transfer in a car is a function of Lateral Acceleration, Track Width, Centre of Gravity Height (CG Height) and Weight. Also, the only direct link between the front and rear tracks is the chassis (all-wheel drive cars are an exception), and vehicle behaviour can be evaluated by looking at the relative performance of front and rear tracks. The term between brackets in the equation above is the roll rate distribution or roll stiffness distribution for a given axle, and it will ultimately control the elastic lateral load transfer component. Lets now analyse roll stiffnesses. So, as expected, the car is not wedged. If you represent the rear roll stiffness as proportion of front roll stiffness in a line plot, the result will be a straight line, with an inclination equal to the proportion between the roll stiffnesses. Inside percentages are the same front and rear. The actual wheel loads are calculated for a series of FLT, which can go from 0 to 1.0, for the given track load. For the tow vehicle, the chain pulls up on the weight distribution bar. One way to calculate the effect of load transfer, keeping in mind that this article uses "load transfer" to mean the phenomenon commonly referred to as "weight transfer" in the automotive world, is with the so-called "weight transfer equation": where Use a 1/4 to one scale. Roll is simply the effect of a suspension reacting to weight transfer. This puts more load on the back tires and simultaneously increases traction. The same will not be true for the weight shift component, because the axle will only support the fraction of the sprung weight distributed to it. Here, the load transfer is increased by means of the lateral load transfer parameter, instead of the FLT. The analysis begins by taking the moment equilibrium about the roll axis: Where is the roll resistance moment, and is the roll moment. But these forces are acting at ground level, not at the level of the CG. Because of this interaction with the springs, this component is also referred as the elastic weight transfer component. For a 3,500-pound car cornering at 0.99 g, the traction in pounds is 3,465 pounds (3,500 x 0.99 = 3,465). o Now lets use the knowledge discussed here applied in the example presented at the beginning of this article, with a little more detail in it. This bias to one pair of tires doing more "work" than the other pair results in a net loss of total available traction. If the car were standing still or coasting, and its weight distribution were 50-50, then Lf would be the same as Lr. Weight transfer is an advanced techniqe which can impact the cart in four directions: front, back, and then each side of the kart. {\displaystyle m} First notice that there are two particular regions in the plot, where any changes to one of the components will produce no sensitive effect on weight transfer. Senior Vehicle Dynamics Engineer providing VD simulation support for Multinational Automakers. *This website is unofficial and is not associated in any way with the Formula One group of companies. This will have a net effect of decreasing the lateral force generated by an axle when the load transfer on it increases. The effects of weight transfer are proportional to the height of the CG off the ground. By way of example, when a vehicle accelerates, a weight transfer toward the rear wheels can occur. h In the automobile industry, weight transfer customarily refers to the change in load borne by different wheels during acceleration. For you to get meaningful results from the equation above, you need to use consistent units. In figure 3 the effect is repeated, but from a different perspective. . From the general lateral load transfer equation, we know that this component is changed by modifications to either the weight distribution of the car, or the roll centres height. This button displays the currently selected search type. Read more Insert your e-mail here to receive free updates from this blog! In that case, the tires on the right side of the car are going to be on the outside of the corner many more times than the left side tires. It is always the case that Lf plus Lr equals G, the weight of the car. For the analysis procedure, one can adapt the load transfer equation obtained above, using , the weight on the track analysed, instead of , and , the height of a fictitious centre of gravity for the track of interest, instead of . {\displaystyle a} Lets say the car is rear wheel drive with a rear weight distribution and large, lightly loaded tyres. Notice that this conclusion doesnt necessarily hold true for different roll axis inclinations. In my time in Baja, I have done calculations of the type for vehicles that had roughly the same weight distribution and wheelbases of approximately 1500 mm. One g means that the total braking force equals the weight of the car, say, in pounds. is the wheelbase, By the methods presented here, the simplest solution would be shifting roll rate distribution to the front, by either stiffening the front antiroll bar or softening the rear. G is the force of gravity that pulls the car toward the center of the Earth. The amount the body rolls is affected by the stiffness of the springs/bars, and the speed of the roll is affected by the stiffness of the shocks. Figure 8 clarifies. The rear wheels don't steer, or don't steer as . You divide the center of gravity height by the width of the contact patches, and then multiply that by the acceleration and weight of the vehicle. To further expand our analysis, lets put the theory into practice.

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