Balancing a car is controlling weight transfer using throttle, brakes, and steering. With 250-lb/in front springs, the same 1000 pound weight transfer will lift the front end a total of two inches. 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. This puts more load on the back tires and simultaneously increases traction. The reason I'm asking you is because you're one of the bigger guys in the pit area. 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. Figure 7 shows the gearbox from Mercedes W05, 2014 Formula One champion. When a car leaves the starting line, acceleration forces create load transfer from the front to the rear. The driver has hit the apex but has found the car is starting to push wide of the desired line. This component is the easier to control. Imagine pulling a table cloth out from under some glasses and candelabra. . Weight transfer involves the actual (relatively small) movement of the vehicle CoM relative to the wheel axes due to displacement of the chassis as the suspension complies, or of cargo or liquids within the vehicle, which results in a redistribution of the total vehicle load between the individual tires. 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. Let us expand that analysis by looking at the pair of tyres. n 35% Front 420 lbs 780 lbs 280 lbs 520 lbs LH Turn - New Stiffer Front Roll Bar 33.3% The overall effect will depend upon roll centre heights and roll stiffnesses, and a definitive conclusion will require a deeper analysis. Queens GTO/Viper. Its not possible to conclude directly what influence increasing roll centre heights will have. In a dirt race car, our setups determine where the weight that has transferred goes. The diagonal lines represent lateral force potential for constant values, whereas the curved lines show values obtained for a constant reference steer angle. 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. m Balance of roll damping will further modify the handling during transient part of maneuver. Here the pickup points are highlighted for better comprehension. If you represent multiple proportions, you will have multiple lines with different inclinations. The second option to alter load transfer from direct lateral force component is to change roll centre heights. 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 first point to stress again is that the overall load transfer that a car experiences, travelling on a circular path of radius R at constant velocity V (and, hence, with constant lateral acceleration Ay=V2/R) is always about the same, no matter what we do in terms of tuning. Moving weight should be used as a fine-tuning tool to get the car working as best it can for the track conditions. Weight (or Load) Transfer Explained (Actionable Tutorial) Driver61 988K subscribers Subscribe 2K Share 93K views 5 years ago Welcome to tutorial five in our Driver's University Series. 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. Perfect balance would thus be 50/50, and front weight distribution would be 60/40 and so on. See you soon! 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). 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. Figure 6 shows the CAD design of a similar gearbox, highlighting the different options for installing pickup points. Figure 14 can lead us to very interesting conclusions. The major forces that accelerate a vehicle occur at the tires' contact patches. If the car were standing still or coasting, and its weight distribution were 50-50, then Lf would be the same as Lr. Another example would be the effect of ride stiffness on wheel hop frequency. 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. It is easy to modify through the components and is where engineers usually make more adjustments specially between sessions or before the race. A big tire car with a lot of power is going to transfer weight much . We need to recognise that not all the weight transfer goes via the springs, dampers and anti-roll bars. {\displaystyle b} Literally, the rear end gets light, as one often hears racers say. Weight transfer happens when a car's weight moves around its roll centre when braking, turning or accelerating. 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. w 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. If that was the case, you should work on the roll centres heights instead, and then adjust suspension parameters accordingly. 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 . Those of you with science or engineering backgrounds may enjoy deriving these equations for yourselves. Figure 9 shows a contour plot of lateral weight transfer sensitivity (lateral weight transfer divided by lateral acceleration) on both axles of an open wheel single-seater. It is what helps us go fast! For this case, roll moment arm decrease with roll centre heights was smaller than the increase in roll centre heights themselves. Figure 4 shows the forces and moments acting on the sprung CG. "Right now, none. 1. A more in-depth discussion on how each of these moments are generated will now be presented. is the total vehicle weight.[7][8]. MichaelP. Weight transfer is the result of acceleration, braking or cornering. 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. a Weight transfer is a function of car weight, CG height, wheelbase, and acceleration. Where is the roll angle caused by the suspension compliances and K is the suspension roll stiffness. 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 views are along the roll axis. i Weight transfers will occur in more controllable amounts, which will result in a more efficient and stable handling race car. Bear in mind that the lateral acceleration obtained from a specific fraction load transfer value will not necessarily cause the correspondent load transfer on the axle. Go to YouTube and look up a slow-motion video of a drag race car leaving the line and watch the left rear tire. h 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. Roll stiffnesses were input in the form of roll rate distribution, varying from 0 to 1. As with most race car parts, you get what you pay for. Briefly, the reason is that inertia acts through the center of gravity (CG) of the car, which is above the ground, but adhesive forces act at ground level through the tire contact patches. is the acceleration of gravity, Notice that this conclusion doesnt necessarily hold true for different roll axis inclinations. Applying the small angle assumption, we have: Substituting the definition of the roll resistance moment in the equation above, we have: Solving for and dividing by we obtain the roll sensitivity to lateral acceleration of the car, i.e. So a ride height adjustment to your race car, or a roll centre geometry change is a very valid tuning device. The article begins with the elements and works up to some simple equations that you can use to calculate weight transfer in any car knowing only the wheelbase, the height of the CG, the static weight distribution, and the track, or distance between the tires across the car. 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. An outside observer might witness this as the vehicle visibly leans to the back, or squats. The forces upon the springs are reacted by the tyres, and that contributes to lateral load transfer. This is a complex measure because it requires changes in suspension geometry, and it has influence on all geometry-related parameters, such as camber and toe gain, anti-pitch features and so on. 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 . This basically rules out weight distribution as a way of controlling roll angle component. If we use , the remaining roll angle component will be: If we keep the roll moment arm constant, then roll angle lateral load transfer component in one track will obviously be a function of the ratio between the roll stiffness on that track and the total roll stiffness of the car. Just like on asphalt, we have what is commonly referred to as Weight Transfer with dirt cars. 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. The distribution of dynamic loads can be altered with aerodynamics, with the regulation of wings or the static/dynamic height of the vehicle. In order to determine the crossweight, calculate the sum of the right front and left rear weights, then divide this number by the total weight 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. 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. In this situation where all the tires are not being utilized load transfer can be advantageous. The Physics of Racing Part 1: Weight Transfer, 10 Tips on How to Become a Pro Racing Driver, Michelin Raceway Road Atlanta Track Guide, Allen Berg Racing Schools Announce East Coast Expansion, Allen Berg to Speak at ADAS & Autonomous Vehicle Technology Expo. Since springs are devices that generate forces upon displacements, a force on each spring arises, and these forces generate a moment that tends to resist the rotation of the body. 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. 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. Under application of a lateral force at the tire contact patch, reacting forces are transmitted from the body to the suspension, the suspension geometry determines the angle and direction of these action lines and where they intersect is defined as the roll center. 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. Use a 1/4 to one scale. Typically a tensioned chain produces the rotational forces or torque. 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. For the trailer, the chain pulls down . Performance Engineer, withexperience in IMSA LMP2, Porsche Cup Brazil and othercategories. Liquids, such as fuel, readily flow within their containers, causing changes in the vehicle's CoM. t Roll stiffness can be altered by either changing ride stiffness of the suspension (vertical stiffness) or by changing the stiffness of the antiroll bars. {\displaystyle g} This curve is called the cornering coefficient curve for the track. The input data were based on the manuals from the manufacturer of an important formula category. Senior Vehicle Dynamics Engineer providing VD simulation support for Multinational Automakers. This will give: Now consider , the vertical load on the outer tyre in a corner, and , the vertical load on the inner tyre. The equation for this component can then be expanded: Because the force coupling nature of roll centres is not as widely known as the definition of the term roll centre itself, some people are unaware of this component. The stiffnesses are shown in kgfm/degree, that have clearer meaning, but the data were input in Nm/rad. 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. The more the body rolls and the faster the body rolls, the more rotational . Steering. 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. 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. 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. Term 2 always leads Term 3. [2] This would be more properly referred to as load transfer,[1][3] and that is the expression used in the motorcycle industry,[4][5] while weight transfer on motorcycles, to a lesser extent on automobiles, and cargo movement on either is due to a change in the CoM location relative to the wheels. 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 force generates a lateral weight transfer in the opposite direction of the turn. This will have a net effect of decreasing the lateral force generated by an axle when the load transfer on it increases. If you hold rear roll rate distribution constant at 54 % and increase roll centre height, lateral load transfer will have no significant change. We now have roll moment arm and roll stiffnesses to play with. 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. 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 Consider the front and rear braking forces, Bf and Br, in the diagram. is the wheelbase, Newtons second law explains why quick cars are powerful and lightweight. In cases where the performance of a pair of tyres is being analysed without regards to a particular vehicle, the parameter is a convenient way to represent changes in lateral load transfer. 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. G is the force of gravity that pulls the car toward the center of the Earth. Lifting off the gas brings the car's momentum forward. Slamming through your gears while mashing on the gas pedal is one way to do it, and an extremely satisfying way to jump off the line just for kicks, but it isn't necessarily the best way to extract all the performance from your car as you possibly can. The following formula calculates the amount of weight transfer: Weight transfer = ( Lateral acceleration x Weight x Height of CG ) / Track width 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. 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. {\displaystyle m} Deceleration. The reason it is relevant is that the amount of weight on a tire directly affects how much grip is available from that tire. These effects are very important, but secondary. This can be confirmed by adopting the conclusions from the analysis of figure 10, where we agreed that the gravity term is negligible for roll angle lateral weight transfer component. Transient lateral load transfer is an important aspect of vehicle setup, but lets leave the discussion on that for another day. Changing the moment generated by this component requires changes in either the unsprung mass or its CG height. Weight transfer is one parameter that is minimized - to aim for even loading on all four tires; resulting in maximum grip during cornering. 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. As a result load transfer is reduced in both the longitudinal and lateral directions. Note that this component resists only roll angle, and the entire sprung mass is used here, as this is how we obtained the expression for roll angle. Your shock absorbers are considered after your ride and roll stiffness have been selected. 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. If you know the deep reasons why you ought to do certain things you will remember the things better and move faster toward complete internalization of the skills. 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. 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. 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. is the center of mass height, Figure 14 shows the contour plot. If you accelerate, brake or corner harder, you transfer more weight. So far, we have discussed the influence of each component in lateral load transfer in isolation. 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. 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 . 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 . Join a community of over 4000 clever racing enthusiasts that want to improve their knowledge on the technical side of motorsport! This makes changes in roll moment arm to control roll angle component useless. For instance in a 0.9g turn, a car with a track of 1650 mm and a CoM height of 550 mm 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. But these forces are acting at ground level, not at the level of the CG. 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. What weight the front tires lose, the rear tires gain. 500 - 1500 (400 - 1,100) The suspension roll stiffness calculation for K9 was in the order of 4,500 ft-lb/degree of roll. An exception is during positive acceleration when the engine power is driving two or fewer wheels. We can split the inertial force into sprung and unsprung components and we will have the following relation: Where is the moment acting upon the sprung mass and is the moment on the unsprung mass. 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. The lateral load transfer parameter. D. What we can do is only influence which portion of the total lateral . 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. With those values, the gravity term will be 1662.1 Nm. While the skills for balancing a car are commonly taught in drivers schools, the rationale behind them is not usually adequately explained. Weight transfer varies depending on what the car is doing. Acceleration weight transfer from front to rear wheels In the acceleration process, the rearward shifting of the car mass also "Lifts" weight off the front wheels an equal amount. 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. For setup, we look into changing the lateral load transfer in one axle relative to the other, to affect balance. The car has turned in towards the apex. Can you see the trend? Lets analyse the moment involved in roll. Antiroll bars are generally added to the car to make it stiffer in roll without altering the ride characteristics. That rationale comes from simple physics. Tire Offsets. Figure 10 shows the plot of the roll angle component versus gravity term. Here, the load transfer is increased by means of the lateral load transfer parameter, instead of the FLT. Its also called the kinematic load transfer component, because the roll centres are defined by the suspension kinematics. Weight . This leads some to think that increasing roll centre heights will actually decrease weight transfer because it reduces roll. The term is a gravity component that arises due to the sprung CG being shifted to the side when the chassis rolls. This moment is called roll moment or roll couple, , because it is responsible for body roll. For this analysis, only the rear axle was considered. 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. This results in a reduced load on the vehicle rear axle and an increase on the front. You have less lead to work with. 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 total lateral load transfer on the car can be calculated from its free body diagram, as shown in figure 1. The change in this arm with roll centre heights will depend on the wheelbase and weight distribution. . This fact can be explained at deeper levels, but such an explanation would take us too far off the subject of weight transfer. When the car corners, lateral acceleration is applied at this CG, generating a centrifugal force. . 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 . If that is the case in the front axle, the car will understeer, if it is in the rear axle, it will oversteer. Any time you apply brakes, add or remove steering, and manipulate the. These numbers are just averages and are very dependent on the class of car and the tires being run. Calculating the load transfer in a vehicle is fairly straightforward. The difference in height between the roll center and center of gravity of the sprung mass gives rise to a moment. Designing suspension mounting points- ifin you do not have access to the software I mentioned and you do not yet have the car built, you can pick up the old Number 2 pencil and start drawing. What happened here? 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). 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. Front roll stiffness distribution only modifies Term 3 and hence increasing front roll stiffness always increases understeer. 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 equations for a car doing a combination of braking and cornering, as in a trail braking maneuver, are much more complicated and require some mathematical tricks to derive. 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). 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). That is a lot of force from those four tire contact patches. o is the longitudinal acceleration, The vehicle's weight is transferred forwards and the front suspension compresses: 'compression'. b It can be varied simply by raising or lowering the roll centre relative to the ground. 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. The only way a suspension adjustment can affect weight transfer is to change the acceleration. The figure shows a car and the forces on it during a one g braking maneuver. Weight transfer is the most basic foundation of vehicle dynamics, yet holds many of the keys to ultimate car control. 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. 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. or . 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. This law is expressed by the famous equation F = ma, where F is a force, m is the mass of the car, and a is the acceleration, or change in motion, of the car. A perfectly rigid vehicle, without suspension that would not exhibit pitching or rolling of the body, still undergoes load transfer. 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. Talking "weight transfer" with respect to race driving is . We'll assume the car's side to side weight distribution is equal. The same thing happens on the left . Conversely, under braking, weight transfer toward the front of the car can occur. I make no claim that this would hold true for every car in the world, but if thats the case for vehicles with wheelbases as different as the ones Ive tried, than I wouldnt be surprised if it was for other cars. The minimum weight of the car to take part in the X275 drag race is 2625 pounds. 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. Why? By simply raising or lowering the couplers, our machines can gain thousands of pounds for traction. Because of this interaction with the springs, this component is also referred as the elastic weight transfer component. An important attribute of the suspension is the Roll-centre. 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. 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.
