{\displaystyle a} r It is the process of shifting your body weight from one side of the kart to the other or leaning forward or back. Hence: This is the total lateral load transfer on the car. More wing speed means we need to keep the right rear in further to get the car tighter. These numbers are just averages and are very dependent on the class of car and the tires being run. is the total vehicle weight.[7][8]. Just like on asphalt, we have what is commonly referred to as Weight Transfer with dirt cars. The front and rear roll centres heights were kept equal, but varied from 3 mm to the CG height (254 mm). Assuming a 120" wb, 100lbs added 5' behind the rear axle will add 150lbs to the rear axle's scale weight, and take 50lbs off of the front axle. Under hard braking it might be clearly visible even from inside the vehicle as the nose dives toward the ground (most of this will be due to load transfer). 21 Shifting. This curve is called the cornering coefficient curve for the track. Understanding weight transfer is a fundamental skill that racecar drivers need to know. This means the driver should be in the car, all fluids topped up, and the fuel load should be such that the car makes your minimum weight rule at the designated time-usually after a race. 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, . 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. It is always the case that Lf plus Lr equals G, the weight of the car. 500 - 1500 (400 - 1,100) The suspension roll stiffness calculation for K9 was in the order of 4,500 ft-lb/degree of roll. Why? This can be done in multiple ways. When it comes to the chassis ride height, that part of the calculation is already baked into the car, and the racer should not look to the 4-link as a way to adjust this. {\displaystyle h} After that, we will see how the components of load transfer can be manipulated to tune the balance of the car. Cars will accelerate, brake, corner and transfer weight from left to right, fore to aft. Fitting racing tires to a tall or narrow vehicle and then driving it hard may lead to rollover. Notice that this conclusion doesnt necessarily hold true for different roll axis inclinations. In figure 3 the effect is repeated, but from a different perspective. Check stagger at each tire, even if using radials. Figure 1 . Weight transfer during accelerating and cornering are mere variations on the theme. 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). The input data were based on the manuals from the manufacturer of an important formula category. 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. For context, we are experimenting with carbon-carbon brake discs on a non-downforce car. Even purpose-built cars, like a contemporary Pro Stocker, have more weight on the front-end than the back. These adjustable bars generally have blade lever arms, as the one shown in figure 11. The manual of the vehicle used here specified a roll stiffness values ranging from 350,000 Nm/rad to 5,600,000 Nm/rad. The simplest component of load transfer is the one related to unsprung mass. Roll stiffnesses were input in the form of roll rate distribution, varying from 0 to 1. 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). If changes to lateral load transfer have not significant effects on the balance of the car, this might be an indication that the tyres are lightly loaded, and load sensitivity is small. 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. Before we start, its worth to give a note on units. In a brief feedback after the first outing (a set of laps in a session) of the free practice session, the driver complains about excessive oversteer in these parts of the circuit. The stiffnesses are shown in kgfm/degree, that have clearer meaning, but the data were input in Nm/rad. Load transfer causes the available traction at all four wheels to vary as the car brakes, accelerates, or turns. During cornering a lateral acceleration by the tire contact patch is created. Weight transfer of sprung mass through suspension links, The second term is the weight transfer of the body through the suspension links, Weight transfer of sprung mass through springs, dampers, anti-roll bars. Conversely, under braking, weight transfer toward the front of the car can occur. For this case, roll moment arm decrease with roll centre heights was smaller than the increase in roll centre heights themselves. 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. {\displaystyle w} An inexpensive set of shocks (such as the ones advertised as 50/50 or a three-way adjustable) should work on cars with as much as 300 to 350 . Newtons third law requires that these equal and opposite forces exist, but we are only concerned about how the ground and the Earths gravity affect the car. The first one to analyse is the kinematic or direct lateral force load transfer component. t The loads in each wheel determine the vehicles maximum cornering, braking and acceleration capability, then the lateral weight transfer is a key factor in a racing car performance. A reference steer angle, which is the average of steer angles of both wheels on the axle, is specified (but the individual slip angles are used when entering the data). The next topic that comes to mind is the physics of tire adhesion, which explains how weight transfer can lead to understeer and over-steer conditions. o What happened here? These effects are very important, but secondary. So a ride height adjustment to your race car, or a roll centre geometry . 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 is the wheelbase, See you soon! Often this is interpreted by the casual observer as a pitching or rolling motion of the vehicles body. . For you to get meaningful results from the equation above, you need to use consistent units. Understanding the physics of driving not only helps one be a better driver, but increases ones enjoyment of driving as well. On independent suspension vehicles, roll stiffness is a function of the vertical stiffness of the suspension (ride rate, which includes tyre stiffness) and track width. While the skills for balancing a car are commonly taught in drivers schools, the rationale behind them is not usually adequately explained. The total lateral load transfer on the car can be calculated from its free body diagram, as shown in figure 1. Weight transfer has two components: Unsprung Weight Transfer: This is the contribution to weight transfer from the unsprung mass of the car. A larger force causes quicker changes in motion, and a heavier car reacts more slowly to forces. Figure 7 shows the gearbox from Mercedes W05, 2014 Formula One champion. In a pair analysis, steady-state lateral force is obtained for the tyres on a track (front or rear pair), through data from a single tyre. Postby BillyShope Wed Aug 22, 2007 5:48 am. When you apply the brakes, you cause the tires to push forward against the ground, and the ground pushes back. Deceleration moves the center of gravity toward the front of the vehicle, taking weight out of the rear tires. This is balanced by the stiffness of the elastic elements and anti-roll bars of the suspension. Most people remember Newtons laws from school physics. Referring to the figures, we have illustrated a street car weighing 3000 lbs, and with a typical FWD street car's weight distribution of 60% front and 40% rear. t The following weight transfers apply only to the sprung mass of the race car:-Sprung weight transfer via the roll centres (WTRC): Again, weight transfer is seperate for front and rear. Balancing a car is controlling weight transfer using throttle, brakes, and steering. In this paper, that issue is discussed with a focus on ride rates, roll rates and simple tire data analysis for a Formula SAE race car. The thing is, roll is only one part of the equation, and as the discussion on this post will show, increasing roll centre height might either increase or decrease the lateral load transfer, depending on other parameters. For the tow vehicle, the chain pulls up on the weight distribution bar. It is a fact of Nature, only fully explained by Albert Einstein, that gravitational forces act through the CG of an object, just like inertia. So far, we have discussed the influence of each component in lateral load transfer in isolation. Keep in mind, the example we used is more typical for a circle track setup; in a road race vehicle, you'll likely be shooting for a more balanced left-weight percentage of 50 percent (although that is not always . The driver has hit the apex but has found the car is starting to push wide of the desired line. Put the driver weight in the car, preferably the driver. Because of this interaction with the springs, this component is also referred as the elastic weight transfer component. In other words, it is the amount by which vertical load is increased on the outer tyres and reduced from the inner tyres when the car is cornering. Its also called the kinematic load transfer component, because the roll centres are defined by the suspension kinematics. The car should be at minimum weight, using ballast as needed to make the proper weight. An outside observer might witness this as the vehicle visibly leans to the back, or squats. When a body rolls, the motion generates rotational torque which must be overcome every time we want to change direction. 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. 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 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. Under heavy or sustained braking, the fronts are . You might not be convinced of the insignificance of this term by arguing that those values were obtained for a very light car with a very low CG. NOTE: This information is from an NHRA Rule Book 2019 Addendum. We define the Fraction Load Transfer, FLT, as the ratio between the difference to the weight on the axle: The parameter represents the total moment in the track about a point on the ground. Lets now see how these components affect each other and how they affect load transfer together. Now that we have quantified lateral load transfer on an axle, we can start to analyse how the individual components interact. Braking causes Lf to be greater than Lr. It applies for all cars, especially racing, sports and high performance road cars. If your driver complies about oversteer in the slowest corners, it means that the front axle is generating higher lateral force than the rear. MichaelP. When this happens, the outside spring of the suspension is compressed and the inside spring is extended. For example, imagine a vehicle racing down a straight and hitting the brakes. If the car were standing still or coasting, and its weight distribution were 50-50, then Lf would be the same as Lr. Again, if that doesnt work, then lateral load transfer will not be the right parameter to change. is the change in load borne by the front wheels, The front end will move faster and farther because less force is required to initially extend the spring. "Right now, none. Ride stiffness can be altered by either changing springs or tyre pressures (tyre pressure affects tyre stiffness, which contributes to the overall ride stiffness). Now do the same, but picking a front roll centre height and imagining a vertical line instead. Vertical load is the load actually seen at the tire contact patch. It is easy to modify through the components and is where engineers usually make more adjustments specially between sessions or before the race. As fuel is consumed, not only does the position of the CoM change, but the total weight of the vehicle is also reduced. Weight transfer is generally of far less practical importance than load transfer, for cars and SUVs at least. How can weight shift when everything is in the car bolted in and strapped down? On limit conditions, this will translate in one of the axles breaking loose and skidding before the other. If we define , the rear roll rate distribution and , the sprung weight distribution on the rear axle, then the lateral load transfer equation for that axle can be rewritten to give: First, lets analyse what happens when we hold roll rate distribution equal to the weight distribution on that axle. . Use a 1/4 to one scale. They push backwards on the tires, which push on the wheels, which push on the suspension parts, which push on the rest of the car, slowing it down.
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