what is weight transfer in a race car?

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. Weight transfer happens when a car's weight moves around its roll centre when braking, turning or accelerating. Moving weight should be used as a fine-tuning tool to get the car working as best it can for the track conditions. If you analyse figure 2, you will see that an increasing fraction load transfer will come together with a decreasing lateral force potential for the axle. A perfectly rigid vehicle, without suspension that would not exhibit pitching or rolling of the body, still undergoes load transfer. What happened here? Cars will accelerate, brake, corner and transfer weight from left to right, fore to aft. Wedge is defined as greater inside percentage at the rear than at the front. 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. A more in-depth discussion on how each of these moments are generated will now be presented. I have heard of many cars running well outside of these parameters and winning. For this case, roll moment arm decrease with roll centre heights was smaller than the increase in roll centre heights themselves. 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). W f 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. 3. 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 . 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. Substituting the values on the terms inside the brackets, we have: But if we assume that front and rear roll centers have the same height, then the moment arm will be given by: Substituting into the weight transfer equation yields: This shows that when weight distribution and roll rate distribution are equal, for a horizontal roll axis, the sprung weight load transfer component will be independent of roll centres heights. Roll is simply the effect of a suspension reacting to weight transfer. The calculations presented here were based on a vehicle with a 3125 mm wheelbase and 54% weight distribution on the rear axle, which are reasonable values for most race cars. Another reason to rule out changes in roll moment arm is that, because it directly multiplies the proportion of roll stiffnesses, it will have the same effect on both axles whether is to increase or decrease lateral load transfer. Vertical load is the load actually seen at the tire contact patch. C. Despite increasing the steering angle, the car has taken a line which is not tight enough to take the turn. What happened? When the driver gets on the brakes, the total remains the same . The only reason a car in neutral will not coast forever is that friction, an external force, gradually slows the car down. The change in this arm with roll centre heights will depend on the wheelbase and weight distribution. 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 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. Do you see where this heading? 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 weight distribution is usually quoted in terms of percentage at the front vs back. e 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. 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. Weight transfers will occur in more controllable amounts, which will result in a more efficient and stable handling race car. See you soon! This component of lateral load transfer is the least useful as a setup tool. This characteristic is also observed here. 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. This leads as to believe that the roll centre height gain is higher than the decrease in the roll moment arm . Join a community of over 4000 clever racing enthusiasts that want to improve their knowledge on the technical side of motorsport! 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. 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. Now that we have quantified lateral load transfer on an axle, we can start to analyse how the individual components interact. For setup, we look into changing the lateral load transfer in one axle relative to the other, to affect balance. A. Can you see the trend? w 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. 1. Well, a thousand changes to the car could be applied. Changing the moment generated by this component requires changes in either the unsprung mass or its CG height. Use a 1/4 to one scale. 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. Deceleration. So a ride height adjustment to your race car, or a roll centre geometry change is a very valid tuning device. t These are fundamental laws that apply to all large things in the universe, such as cars. 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. Here the pickup points are highlighted for better comprehension. 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. The added axle weight will slow the release of the stretch in the tire and help hold traction longer. h It applies for all cars, especially racing, sports and high performance road cars. As you see, when we increase front roll centre height, the lateral weight transfer decreases on the rear axle while increasing on the front. 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. When you apply the brakes, you cause the tires to push forward against the ground, and the ground pushes back. One g means that the total braking force equals the weight of the car, say, in pounds. : 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. The same is true in bikes, though only longitudinally.[4]. For context, we are experimenting with carbon-carbon brake discs on a non-downforce car. Check stagger at each tire, even if using radials. Refer again to figure 1. So far, we have discussed the influence of each component in lateral load transfer in isolation. B. 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. {\displaystyle w} This results in a reduced load on the vehicle rear axle and an increase on the front. is the center of mass height, 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. An exception is during positive acceleration when the engine power is driving two or fewer wheels. The hardest one would be to change the bar itself, though there are some antiroll bars that have adjustable stiffnesses, eliminating the need to replace bars. Steering towards the left or right moves the vehicle's center of gravity in the opposite direction, taking weight out of the left or right tires respectively. 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. Now do the same, but picking a front roll centre height and imagining a vertical line instead. Steering. What we can do is only influence which portion of the total lateral . In the post about lateral force from the tyres, we discussed tyre load sensitivity, the property that makes lateral force from a tyre to grow at a smaller rate with increasing vertical load. Here the gearbox has a removable carbon fibre structural outer sleeve, allowing changes in the design of the rear suspension without having to re-test the rear of the car for crashworthiness. Its also called the kinematic load transfer component, because the roll centres are defined by the suspension kinematics. This happens because raising the roll centre in any axle will approximate the roll axis to the sprung weight CG. The analysis procedure is as follows: The potential diagram is a benchmarking of the performance that can be achieved by a pair of tyres. 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. Queens GTO/Viper. If , and will have the term inside brackets resulting in . Ideally, this produces 0.5, or 50-percent, to show that the right front/left rear sum is equal to the left front/right rear sum. Term 2 always leads Term 3. Direct force component or kinematic component useful as a setup tool, especially when roll axis is close to the sprung CG, and the influence of roll component is reduced. Ride stiffness can be altered by either changing springs or tyre pressures (tyre pressure affects tyre stiffness, which contributes to the overall ride stiffness). 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. This curve is called the cornering coefficient curve for the track. Some race cars have push-pull cables connected to the bars that allow the driver to change roll stiffnesses from inside the car. Before we start, its worth to give a note on units. 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. When a body rolls, the motion generates rotational torque which must be overcome every time we want to change direction. 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. 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. The second option to alter load transfer from direct lateral force component is to change roll centre heights. This is characterised by the green region in the graph. That rationale comes from simple physics. 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. 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. 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. In conclusion, it was a huge effort by Tin . Its not possible to conclude directly what influence increasing roll centre heights will have. The initial lurch will sink the car. Deceleration moves the center of gravity toward the front of the vehicle, taking weight out of the rear tires. 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. 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 . The stiffnesses are shown in kgfm/degree, that have clearer meaning, but the data were input in Nm/rad. The softer the spring rate the more weight transfer you will see. 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. The fact that the problem occurs in the slowest bits of the circuit might rule out the possibility of aerodynamic changes as a solution. To obtain these, I created a MATLAB routine to calculate the total lateral weight transfer from our previous discussion, keeping the front and rear roll stiffnesses equal and constant while varying front and rear roll centre heights. A quick look at the lateral load transfer equation might lead you to think that lateral load transfer will increase with increasing roll centre heights because of the direct relation in the equation. Go to YouTube and look up a slow-motion video of a drag race car leaving the line and watch the left rear tire. Roll stiffness is defined as the resistance moment generated per unit of roll angle of the sprung mass, and it has SI units of Nm/rad. In a drag racing application, you want to narrow down the rate of the spring to the softest one you can run without having any coil bind. Literally, the rear end gets light, as one often hears racers say. 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. It must be reminded that changing this term will only change a part of the total lateral weight transfer. 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. The term is a gravity component that arises due to the sprung CG being shifted to the side when the chassis rolls. If you have no suspension (ex. At this point, tyre data is entered and lateral force for each tyre in the axle is calculated taking into account the effects described above (if the case demands it). Perfect balance would thus be 50/50, and front weight distribution would be 60/40 and so on. For weight transfer to be useful to the driver in controlling the car, the driver would need to feel the weight transfer, or something related to it. A big tire car with a lot of power is going to transfer weight much . The vehicle mass resists the acceleration with a force acting at its center of gravity. This force is then divided by the weight on the axle, This lateral acceleration is plotted against FLT, with reference steer angle as a parameter. To further expand our analysis, lets put the theory into practice. is the total vehicle weight.[7][8]. Weight distribution can be controlled through positioning of ballast in the car. For instance in a 0.9g turn, a car with a track of 1650mm and a CoM height of 550mm will see a load transfer of 30% of the vehicle weight, that is the outer wheels will see 60% more load than before, and the inners 60% less. Similarly, during changes in direction (lateral acceleration), weight transfer to the outside of the direction of the turn can occur. NOTE: This information is from an NHRA Rule Book 2019 Addendum. Figure 3 shows the plot. The rotational tendency of a car under braking is due to identical physics.The braking torque acts in such a way as to put the car up on its nose. 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). We need to recognise that not all the weight transfer goes via the springs, dampers and anti-roll bars. The front and rear roll centres heights were kept equal, but varied from 3 mm to the CG height (254 mm). The RF tire is. These numbers are reported in shop manuals and most journalistic reviews of cars. Figure 7 shows the gearbox from Mercedes W05, 2014 Formula One champion. 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. It can be varied simply by raising or lowering the roll centre relative to the ground. Figure 8 clarifies. 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. 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. Before we discuss how these moments are quantified, its interesting to derive a relation between a generic moment and the vertical load change between tyres separated by a distance . This article uses this latter pair of definitions. Turning in to a corner brings the car's momentum forward . This is generally not the first option to take because of the effect that it has on other aspects of the car. 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. i 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, . Performance Engineer, withexperience in IMSA LMP2, Porsche Cup Brazil and othercategories. is the acceleration of gravity, {\displaystyle m} The figure only shows forces on the car, not forces on the ground and the CG of the Earth. 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. is the wheelbase, Sprung weight distribution is calculated as the ratio between the distance from the sprung weight CG to the axle opposite to the one being analysed, , and the wheelbase of the vehicle , times the sprung weight . Do you see how small it is compared to the roll stiffness of the car? Conversely, if you hold roll centre heights at about 254 mm and vary rear roll rate distribution, lateral load distribution wont suffer relevant differences. 21 Shifting. For the SI system, the weights should be in N, the angular stiffnesses in Nm/rad, the lengths in m, and the acceleration is nondimensional (because we are dividing lateral acceleration by the acceleration of gravity). Balancing a car is controlling weight transfer using throttle, brakes, and steering. The forces upon the springs are reacted by the tyres, and that contributes to lateral load transfer. This makes changes in roll moment arm to control roll angle component useless. Transition This is the point at which the car 'takes its set'. If unsprung mass is isolated, its possible to find its own CG. The car should be at minimum weight, using ballast as needed to make the proper weight. Most people remember Newtons laws from school physics. It is the process of shifting your body weight from one side of the kart to the other or leaning forward or back. Most high performance automobiles are designed to sit as low as possible and usually have an extended wheelbase and track. The weight transfer setup recognizes the importance of ride height and roll stiffness in determining a good balanced set up for the car. This. The input data were based on the manuals from the manufacturer of an important formula category. Because of Newtons first law. 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. 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. Likewise, accelerating shifts weight to the rear, inducing under-steer, and cornering shifts weight to the opposite side, unloading the inside tires. This basically rules out weight distribution as a way of controlling roll angle component. If it reaches half the weight of the vehicle it will start to roll over. 2. The rear wheels don't steer, or don't steer as . Bear in mind that all the analysis done here was for steady-state lateral load transfer, which is why dampers were not mentioned at all. 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. Load transfer causes the available traction at all four wheels to vary as the car brakes, accelerates, or turns. The only way a suspension adjustment can affect weight transfer is to change the acceleration. In that case, changing roll rate distribution or roll centre heights will have little effect in the balance, and other alternatives must be looked at, such as adjusting tyre pressures, tyre size and/or width or moving CG location (so that the inertial forces will be different in each axle). h An important attribute of the suspension is the Roll-centre. In the context of our racing application, they are: The first law:a car in straight-line motion at a constant speed will keep such motion until acted on by an external force. is the total vehicle mass, and 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). More wing speed means we need to keep the right rear in further to get the car tighter. Weight transfer in a car is a function of Lateral Acceleration, Track Width, Centre of Gravity Height (CG Height) and Weight. Now that we know the best ways to change roll stiffness, lets see how it affects lateral load transfer. This is altered by moving the suspension pickups so that suspension arms will be at different position and/or orientation. Hence: This is the total lateral load transfer on the car. Effect of downforce on weight transfer during braking - posted in The Technical Forum: Apologies if the answer to this is obvious, but I am trying to get a sense of whether weight transfer under braking is affected by how much downforce a car has. Even purpose-built cars, like a contemporary Pro Stocker, have more weight on the front-end than the back. The Trackmobile Weight Transfer System is a hydraulic system developed to implement this idea in an intuitive and easy-to-use way. 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: Weight Transfer - A Core of Vehicle Dynamics. Postby BillyShope Wed Aug 22, 2007 5:48 am. Lets analyse the moment involved in roll. Balancing a car is controlling weight transfer using throttle, brakes, and steering. In wheeled vehicles, load transfer is the measurable change of load borne by different wheels during acceleration (both longitudinal and lateral). 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. Senior Vehicle Dynamics Engineer providing VD simulation support for Multinational Automakers. The fact is, by increasing the roll centre height in one axle, you are increasing lateral load transfer from the direct lateral force component, while at the same time you are decreasing lateral load transfer from roll angle component. As we discussed, we should input consistent units into the equation to obtain meaningful results. Here, the load transfer is increased by means of the lateral load transfer parameter, instead of the FLT. 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. As stated before, it is very difficult to change the total lateral load transfer of a car without increasing the track width or reducing either the weight or the CG height. Weight transfer is the result of acceleration, braking or cornering. Weight transfer varies depending on what the car is doing. 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 Thus, having weight transferred onto a tire increases how much it can grip and having weight transferred off a tire decreases how much it can grip the road. If you compare figures 13 and 8, you will see that, while lateral weight transfer changes with roll centre heights along contours defined by lines that have the same inclination, the effect is different with respect to roll stiffnesses, as the lines that limit the contours have different inclinations. Understanding the physics of driving not only helps one be a better driver, but increases ones enjoyment of driving as well. This is the weight of the car; weight is just another word for the force of gravity. o a thick swaybar is not a good idea for the front of a FWD race car. is the change in load borne by the front wheels, Let's start by taking a look at four stages of understeer. The second law: When a force is applied to a car, the change in motion is proportional to the force divided by the mass of the car. When the vehicle is cornering, the centrifugal force from inertia generates a moment that makes the sprung mass roll to the outside of the corner. The braking forces create a rotating tendency, or torque, about the CG. We now have roll moment arm and roll stiffnesses to play with. As you begin to turn in (you may or may not still be on the brakes) the weight begins its transfer from inside to outside as the lateral g-loading increases. 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. Notice that this is just one possibility and other parameters might be investigated as well. Closed Wheel Race Cars How much does a NASCAR car weigh? The front end will move faster and farther because less force is required to initially extend the spring. 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. Figure 13 shows the contour plots of lateral weight transfer sensitivity as a function of front and rear roll stiffnesses. Taking the moment equilibrium about the point O, of the tyre, we can see that: Dividing the equation by t on both sides, we obtain: But assuming a symmetric weight distribution, , since the left tyre is the outside tyre. G is the force of gravity that pulls the car toward the center of the Earth.

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