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. Conversely, if you hold roll centre heights at about 254 mm and vary rear roll rate distribution, lateral load distribution wont suffer relevant differences. The lateral force of the track is the sum of lateral forces obtained from each tyre. Load transfer causes the available traction at all four wheels to vary as the car brakes, accelerates, or turns. 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. Go to YouTube and look up a slow-motion video of a drag race car leaving the line and watch the left rear tire. 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. 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. If unsprung mass is isolated, its possible to find its own CG. B. However, these approaches are limited, ride height being affected by the possibility of bottoming out and track width by regulations that place a cap on vehicle width. 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. In figure 3 the effect is repeated, but from a different perspective. What happened? The CG is the middle, then you split 50/50; the CG is more toward one side than the other, then more weight transfer goes on that side and less on the other. This results in a reduced load on the vehicle rear axle and an increase on the front. Total lateral weight transfer is a combination of 3 distinct effects: Lateral force generated by the unsprung mass of the suspension and lateral acceleration is reacted directly by the tires, giving rise to a vertical component defined as Fz1. For example, if our car had a center of gravity 1 foot above the ground and the tires were 4 feet apart, we would divide 1 foot . The total lateral load transfer on the car can be calculated from its free body diagram, as shown in figure 1. 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 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. If the tyres of the car are lightly loaded, there might not be enough load sensitivity in the tyres, so that even if one end of the car takes all the lateral load transfer, the lateral force performance isnt degraded significantly. The more F and the less m you have, the more a you can get.The third law: Every force on a car by another object, such as the ground, is matched by an equal and opposite force on the object by the car. 20 - 25,000 (15 - 18,500) Formula SAE. {\displaystyle g} The reason is that the magnitude of these forces determines the ability of a tire to stick, and imbalances between the front and rear lift forces account for understeer and over-steer. In the image, the car is looked from the rear in a right hand turn. In a dirt race car, our setups determine where the weight that has transferred goes. 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. Refer again to figure 1. . 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. Liquids, such as fuel, readily flow within their containers, causing changes in the vehicle's CoM. Referring back to the total load transfer equation, we see that the total weight transfer will be caused by inertial forces acting upon the entire mass of the car. As long as the tires stay on the car, the ground pushing on them slows the car down. When cornering, the sprung mass of the car will roll by an amount , the roll angle. "The ride height is meant to be in one spot you should look to move weight, adjust the shocks . 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. This makes changes in roll moment arm to control roll angle component useless. This being a pretty typical "clubmans" type car it sits properly between the road going sports car and the sports prototype figures given in the table. Acceleration causes the sprung mass to rotate about a geometric axis resulting in relocation of the CoM. At the same time, the CoM of the vehicle will typically move laterally and vertically, relative to the contact patch by no more than 30mm, leading to a weight transfer of less than 2%, and a corresponding reduction in grip of 0.01%. {\displaystyle a} Imagine pulling a table cloth out from under some glasses and candelabra. The only forces that can counteract that tendency are the lift forces, and the only way they can do so is for Lf to become greater than Lr. The sprung mass used was 675 kg, which gives a weight of 6621.75 N. With a CG height of 254 mm and the minimum roll centres specified in 3 mm, which is very low, the moment arm will be 251 mm. If , and will have the term inside brackets resulting in . Learning to do it consistently and automatically is one essential part of becoming a truly good driver. Understanding weight transfer is a fundamental skill that racecar drivers need to know. The RF tire is. But these forces are acting at ground level, not at the level of the CG. No motion of the center of mass relative to the wheels is necessary, and so load transfer may be experienced by vehicles with no suspension at all. 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. Since the car does not actually go up on its nose (we hope), some other forces must be counteracting that tendency, by Newtons first law. Inside percentages are the same front and rear. is the longitudinal acceleration, This will give: Now consider , the vertical load on the outer tyre in a corner, and , the vertical load on the inner tyre. One g means that the total braking force equals the weight of the car, say, in pounds. What would you do, in order to solve the problem? Check stagger at each tire, even if using radials. The second term can be changed modifying the suspension geometry, usually difficult or not allowed in some competitions. The result will be: Now we know that the load transfer caused by a generic moment about a track will be the moment divided by the track width, and we can use that to analyse the effect of each component of load transfer. This puts more load on the back tires and simultaneously increases traction. It must be reminded that changing this term will only change a part of the total lateral weight transfer. This seems good, as more weight transfer would appear to be the goal, but less resistance is not the best way to make use of this weight transfer. By analysing Figure 9 you can see that lateral load transfer is very sensitive to changes in roll centre height. Put the driver weight in the car, preferably the driver. C. Despite increasing the steering angle, the car has taken a line which is not tight enough to take the turn. In a single axle, the roll resistance moment will be the roll angle multiplied by the roll stiffness of the axle analysed, . The forces upon the springs are reacted by the tyres, and that contributes to lateral load transfer. Then the expansion of the tire will begin to lift the car. As we move up to higher categories, the engineering gets more complex. The weight of an IndyCar race car should be at least 712 kg, with an average of 1630 lbs or 739.5 kg. Try this exercise: pick whatever value you want for rear roll centre height, and imagine an horizontal line passing through the point correspondent to that value in both graphs, and observe how weight transfer changes along this line in both graphs (remember each graph represents an axle). Weight transfer (better called "load transfer") is not a technique, it's a natural phenomenon due to the existence of inertia, that happens whenever you try to change the state of motion of the car. 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. Notice that this is just one possibility and other parameters might be investigated as well. The analysis procedure is as follows: The potential diagram is a benchmarking of the performance that can be achieved by a pair of tyres. This is reacted by the roll stiffness (or roll rate), , of the car. Senior Vehicle Dynamics Engineer providing VD simulation support for Multinational Automakers. Weight transfer is generally of far less practical importance than load transfer, for cars and SUVs at least. The first one to analyse is the kinematic or direct lateral force load transfer component. This could affect wheel hop (the ride mode that characterises oscillation of the unsprung mass between the road surface and the sprung mass) frequency and amplitude, reducing the contact of the tyres with the ground and hence, reducing grip. 35% Front 420 lbs 780 lbs 280 lbs 520 lbs LH Turn - New Stiffer Front Roll Bar 33.3% This article explains the physics of weight transfer. With those values, the gravity term will be 1662.1 Nm. Antiroll bars are generally added to the car to make it stiffer in roll without altering the ride characteristics. It is defined as the point at which lateral forces on the body are reacted by the suspension links. This can be done in multiple ways. The "rate of weight transfer" is considered important. The weight distribution is usually quoted in terms of percentage at the front vs back. When a body rolls, the motion generates rotational torque which must be overcome every time we want to change direction. In some categories, the rear suspension is mounted on the gearbox, for example, Formula 3, shown in figure 5. The initial lurch will sink the car. Do you see where this heading? The location of the components of a vehicle is essential to achieve an ideal weight distribution and it depends on the following factors: Location of Components (Engine-Transmission-Pilot-Mechanical Components, fuel tank). Roll stiffnesses were input in the form of roll rate distribution, varying from 0 to 1. Literally, the ground pushes up harder on the front tires during braking to try to keep the car from tipping forward. A big tire car with a lot of power is going to transfer weight much . This force generates a lateral weight transfer in the opposite direction of the turn. These adjustable bars generally have blade lever arms, as the one shown in figure 11. The term is a gravity component that arises due to the sprung CG being shifted to the side when the chassis rolls. Front roll stiffness distribution only modifies Term 3 and hence increasing front roll stiffness always increases understeer. The Trackmobile Weight Transfer System is a hydraulic system developed to implement this idea in an intuitive and easy-to-use way. Queens GTO/Viper. It may be a more practical way to assess vehicle handling in comparison to computer modelling, since the goal is generally to increase the lateral force on either the front or rear track. Your shock absorbers are considered after your ride and roll stiffness have been selected. You have less lead to work with. Hence, springs and tyre pressures should only be changed when other aspects need modification, but not only roll stiffness itself (unless the vehicle has no antiroll bar). By way of example, when a vehicle accelerates, a weight transfer toward the rear wheels can occur. It is these moments that cause variation in the load distributed between the tires. The figure only shows forces on the car, not forces on the ground and the CG of the Earth. It is the process of shifting your body weight from one side of the kart to the other or leaning forward or back. This graph is called the, The actual load transfer depends on the track width and the rolling moment produced by the lateral acceleration acting on the fictitious CG height. For setup, we look into changing the lateral load transfer in one axle relative to the other, to affect balance. If your driver complies about oversteer in the slowest corners, it means that the front axle is generating higher lateral force than the rear. Weight transfer is affected by the distance between the CG Height and the roll centre. 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. By rotating the lever arms, its area moment of inertia in bending is changed, hence altering its stiffness. Use a load of fuel for where you you want the car balanced, either at the start of the race, the end of the race or an average between the two. One thing we can tell without any deep analysis is that increasing the roll centre height in one axle decreases the lateral weight transfer on the opposite axle, everything else kept constant. In the automobile industry, weight transfer customarily refers to the change in load borne by different wheels during acceleration. Put an R-compound DOT tire on the same car and raise that force to 1.05 g of cornering force. 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. Consider the front and rear braking forces, Bf and Br, in the diagram. Figure 13 shows the contour plots of lateral weight transfer sensitivity as a function of front and rear roll stiffnesses. A flatter car, one with a lower CG, handles better and quicker because weight transfer is not so drastic as it is in a high car. In this analysis, we will be interested in lateral load transfer in a single axle, and I will discuss the three mechanisms by which that happens, namely, roll resistance moment from springs and antiroll bars, direct lateral force load transfer and lateral load transfer from unsprung mass. 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. We now have roll moment arm and roll stiffnesses to play with. That rationale comes from simple physics. Usually, I'll have 50-80 lbs," Bloomquist told RacingNews.co from Lucas Oil Speedway a few weeks back. The following formula calculates the amount of weight transfer: Weight transfer = ( Lateral acceleration x Weight x Height of CG ) / Track width Balancing a car is controlling weight transfer using throttle, brakes, and steering. is the center of mass height, Figure 14 can lead us to very interesting conclusions. Before we start this analysis, lets make some important definitions: Load transfer from direct force is one of the two components related to the lateral force acting upon the sprung mass. The net loss can be attributed to the phenomenon known as tire load sensitivity. Hence: This is the total lateral load transfer on the car. W A car weighs so much overall, and that is distributed - let's assume for the sake of argument, equally - between front and rear. w If that is the case in the front axle, the car will understeer, if it is in the rear axle, it will oversteer. The simplest component of load transfer is the one related to unsprung mass. Here, the load transfer is increased by means of the lateral load transfer parameter, instead of the FLT. Deceleration Weight Transfer The opposite of the acceleration weight transfer takes place during deceleration. This leads some to think that increasing roll centre heights will actually decrease weight transfer because it reduces roll. The trend in dirt racing seems to be leaning toward a left side weight percentage of around 53.5 to 55 and somewhere between 75 and 125 pounds of wedge. Perfect balance would thus be 50/50, and front weight distribution would be 60/40 and so on. Deceleration. Also, if you liked this post, please share it on Twitter or Facebook, and among your friends. How can weight shift when everything is in the car bolted in and strapped down? 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. Its also called the kinematic load transfer component, because the roll centres are defined by the suspension kinematics. This is altered by moving the suspension pickups so that suspension arms will be at different position and/or orientation. First off I would point out don't assume your tires are correct just based on there all but the same as the leaders, take a kart with 59 % left and 70 % cross he will be on a more juiced tire than a kart with a more balanced set-up like 56 % left and 57 % cross, now if you know his chassis and set-up 100 % ya you can feel little better about the Tires. Most high performance automobiles are designed to sit as low as possible and usually have an extended wheelbase and track. Now do the same, but picking a front roll centre height and imagining a vertical line instead. 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. Now that we know the best ways to change roll stiffness, lets see how it affects lateral load transfer. The rear wheels don't steer, or don't steer as . 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. The lateral load transfer parameter. 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). Performance Engineer, withexperience in IMSA LMP2, Porsche Cup Brazil and othercategories. Weight transfer during accelerating and cornering are mere variations on the theme. The softer the spring rate the more weight transfer you will see. 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