is the total vehicle weight.[7][8]. How can weight shift when everything is in the car bolted in and strapped down? Figure 14 can lead us to very interesting conclusions. The lateral force of the track is the sum of lateral forces obtained from each tyre. Front-back weight transfer is proportional to the change in the longitudinal location of the CoM to the vehicle's wheelbase, and side-to-side weight transfer (summed over front and rear) is proportional to the ratio of the change in the CoM's lateral location to the vehicle's track. Fitting racing tires to a tall or narrow vehicle and then driving it hard may lead to rollover. 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. Transient lateral load transfer is an important aspect of vehicle setup, but lets leave the discussion on that for another day. The second option to alter load transfer from direct lateral force component is to change roll centre heights. 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. Weight transfer is the result of acceleration, braking or cornering. 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. The amount of weight transfer is detirmined by how wide the track is (wider = less weight transfer) and how high the CG is (lower CG = less weight transfer). Liquids, such as fuel, readily flow within their containers, causing changes in the vehicle's CoM. Vertical load is the load actually seen at the tire contact patch. The inputs are essentially the loads and orientations of the tyres, and the outputs are given per unit weight on the axle, allowing for a vehicle-independent analysis. 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 "rate of weight transfer" is considered important. Weight transfer is affected by the distance between the CG Height and the roll centre. This fact can be explained at deeper levels, but such an explanation would take us too far off the subject of weight transfer. contact patch displacement relative to wheel. I have heard of many cars running well outside of these parameters and winning. This. "Right now, none. Lesser the Second: Accelerating the car will weight the rear wheels heavily, the front wheels lightly. 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. For context, we are experimenting with carbon-carbon brake discs on a non-downforce car. B. It is defined as the point at which lateral forces on the body are reacted by the suspension links. weight is transferred in proportion to static weight. In general, it is almost safe to say that the Indycar weighs less than a Formula 1 car. Weight Transfer - A Core of Vehicle Dynamics. 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. Where is the roll angle caused by the suspension compliances and K is the suspension roll stiffness. Lf is the lift force exerted by the ground on the front tire, and Lr is the lift force on the rear tire. If unsprung mass is isolated, its possible to find its own CG. 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, . 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). It must be reminded that changing this term will only change a part of the total lateral weight transfer. Go to YouTube and look up a slow-motion video of a drag race car leaving the line and watch the left rear tire. Total available grip will drop by around 6% as a result of this load transfer. With 250-lb/in front springs, the same 1000 pound weight transfer will lift the front end a total of two inches. This article explains the physics of weight transfer. Lets repeat the weight transfer equation here to make things easier: By looking at the equation, you can see that the weight transfer component from roll angle can be altered by changes in front or rear roll stiffnesses, roll moment arm or weight distribution. Friction comes from the tires on the ground and the air flowing over the car. {\displaystyle \Delta Weight_{front}} The lateral load transfer parameter. It is easy to modify through the components and is where engineers usually make more adjustments specially between sessions or before the race. {\displaystyle h} 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. 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. 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. 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). These numbers are reported in shop manuals and most journalistic reviews of cars. 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 . 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). 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. Bickel explains how the way the 4-link plays into how you adjust the 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. More wing speed means we need to keep the right rear in further to get the car tighter. 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. Weight transfer varies depending on what the car is doing. 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. 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. The weight shift component for a single axle will be: Substituting roll angle on the expression above, we have: The total moment from roll angle on a single axle will then be: The lateral load transfer from this moment is obtained by dividing this by the axle track width, t: The three components of lateral load transfer should be added in order to obtain the total lateral load transfer on an axle: The expression above can be utilized to calculate the load transfer on each axle, which can then be used to improve handling. We have established that playing with the unsprung weight component is not the smartest thing to do, so lets focus on the sprung weight components, i.e. 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. When a body rolls, the motion generates rotational torque which must be overcome every time we want to change direction. 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. We wont consider subtleties such as suspension and tire deflection yet. 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 rest of this article explains how inertia and adhesive forces give rise to weight transfer through Newtons laws. 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. 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. These numbers are just averages and are very dependent on the class of car and the tires being run. In this situation where all the tires are not being utilized load transfer can be advantageous. This analysis may even be used to prepare tyre data, in order to make the bicycle model more realistic. 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. Weight transfers occur as a result of the chassis twisting around the car's roll centre, which determined by the natural suspension setup. The driver is said to manage or control the weight transfer. 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. When the car moves in one of these directions, the car's weight moves in the opposite direction and compresses the suspension in this area. Do you see how small it is compared to the roll stiffness of the car? The tires and chassis will also make a difference in the spring selection. 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. Both of these changes will involve adding, removing or repositioning mass (and therefore parts) within the unsprung part of the car. In the automobile industry, weight transfer customarily refers to the change in load borne by different wheels during acceleration. The weight of an IndyCar race car should be at least 712 kg, with an average of 1630 lbs or 739.5 kg. Balancing a car is controlling weight transfer using throttle, brakes, and steering. This puts more load on the back tires and simultaneously increases traction. Acceleration causes the sprung mass to rotate about a geometric axis resulting in relocation of the CoM. Weight transfer has two components: Unsprung Weight Transfer: This is the contribution to weight transfer from the unsprung mass of the car. You must learn how different maneuvers . Lets now see how these components affect each other and how they affect load transfer together. The added axle weight will slow the release of the stretch in the tire and help hold traction longer. This will give: Now consider , the vertical load on the outer tyre in a corner, and , the vertical load on the inner tyre. For setup, we look into changing the lateral load transfer in one axle relative to the other, to affect balance. Because of Newtons first law. is the wheelbase, If that is the case in the front axle, the car will understeer, if it is in the rear axle, it will oversteer. Conversely, a supercar is built to approximate race geometry with few concessions to prevent spilling the drinks. Queens GTO/Viper. Here the pickup points are highlighted for better comprehension. 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 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. For a more comprehensive analysis, the effects from suspension geometry such as steer and camber variations due to ride, roll, braking, accelerating, lateral force compliance or aligning torque compliance, can be introduced before entering tyre data. 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. 35% Front 420 lbs 780 lbs 280 lbs 520 lbs LH Turn - New Stiffer Front Roll Bar 33.3% But these forces are acting at ground level, not at the level of the CG. The input data were based on the manuals from the manufacturer of an important formula category. D. Weight transfer is an advanced techniqe which can impact the cart in four directions: front, back, and then each side of the kart. Closed Wheel Race Cars How much does a NASCAR car weigh? 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. a thick swaybar is not a good idea for the front of a FWD race car. An important attribute of the suspension is the Roll-centre. Because of this interaction with the springs, this component is also referred as the elastic weight transfer component. By analysing Figure 9 you can see that lateral load transfer is very sensitive to changes in roll centre height. This basically rules out weight distribution as a way of controlling roll angle component. 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. Deceleration Weight Transfer The opposite of the acceleration weight transfer takes place during deceleration. This reduces the weight on the rear suspension causing it to extend: 'rebound'. The major forces that accelerate a vehicle occur at the tires' contact patches. 1. We see that when standing still, the front tires have 900 lbs of weight load, and the rear tires have 600 lbs each. Under heavy or sustained braking, the fronts are . [3] This includes braking, and deceleration (which is an acceleration at a negative rate). . These adjustable bars generally have blade lever arms, as the one shown in figure 11. 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. Front roll stiffness distribution only modifies Term 3 and hence increasing front roll stiffness always increases understeer. Figure 12 shows a finite element stress analysis, with colours closer to yellow and green indicating higher stresses. a 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. Some setup changes might apply, for example, CG might be lowered by reducing ride height, and track width might be increased by changing wheel offsets properly or using wheel hub spacers. is the acceleration of gravity, By way of example, when a vehicle accelerates, a weight transfer toward the rear wheels can occur. As an example, Interlagos race track, where the Brazilian Grand Prix takes place has a heavy asymmetry, with only four right-hand corners, and ten left-handers. 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. 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. Calculating the load transfer in a vehicle is fairly straightforward. Read more Insert your e-mail here to receive free updates from this blog! W Front lateral load transfer is not necessarily equal to the load transfer in the rear side, since the parameters of track, weight and height of the CG are generally different. The most reasonable option would be changes on antiroll bar stiffness. 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. The secret to answer this question is to focus not on total lateral weight transfer on the car, but instead, on how it is distributed between front and rear tracks. The same is true in bikes, though only longitudinally.[4]. The diagonal lines represent lateral force potential for constant values, whereas the curved lines show values obtained for a constant reference steer angle. 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 . Senior Vehicle Dynamics Engineer providing VD simulation support for Multinational Automakers. These are fundamental laws that apply to all large things in the universe, such as cars. G points down and counteracts the sum of Lf and Lr, which point up. Usually, I'll have 50-80 lbs," Bloomquist told RacingNews.co from Lucas Oil Speedway a few weeks back. 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 force is then divided by the weight on the axle, This lateral acceleration is plotted against FLT, with reference steer angle as a parameter. Antiroll bars are generally added to the car to make it stiffer in roll without altering the ride characteristics. As fuel is consumed, not only does the position of the CoM change, but the total weight of the vehicle is also reduced. This is given by: Here, is the sprung weight distribution to the axle being analysed and is the roll centre height for the track. 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. For the trailer, the chain pulls down . 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. Before we start, its worth to give a note on units. 2. In conclusion, it was a huge effort by Tin . Lets say that you are a race engineer and your driver is having trouble to go around the slowest corners on the circuit. Notice that this conclusion doesnt necessarily hold true for different roll axis inclinations. This bias to one pair of tires doing more "work" than the other pair results in a net loss of total available traction. 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. This is the weight of the car; weight is just another word for the force of gravity. Check stagger at each tire, even if using radials. m 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. 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 only reason a car in neutral will not coast forever is that friction, an external force, gradually slows the car down. Some large trucks will roll over before skidding, while passenger vehicles and small trucks usually roll over only when they leave the road. {\displaystyle m} This article explains the physics of weight transfer. Braking causes Lf to be greater than Lr. 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. To further expand our analysis, lets put the theory into practice. This button displays the currently selected search type. Weight transfer occurs as the vehicle's CoM shifts during automotive maneuvers. Do you see where this heading? Weight transfer in a car is a function of Lateral Acceleration, Track Width, Centre of Gravity Height (CG Height) and Weight. G cannot be doing it since it passes right through the center of gravity. 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 The change in this arm with roll centre heights will depend on the wheelbase and weight distribution. For this analysis, only the rear axle was considered. Lets now analyse roll stiffnesses. Your shock absorbers are considered after your ride and roll stiffness have been selected. Reference:Dr. Brian Beckman The Physics of Racing, Michelin Raceway Road Atlanta is 2.54 miles long, with 12 turns winding their way through the scenic Georgia countryside. The simplest component of load transfer is the one related to unsprung mass. The distribution of dynamic loads can be altered with aerodynamics, with the regulation of wings or the static/dynamic height of the vehicle. So a ride height adjustment to your race car, or a roll centre geometry . Roll angle component or elastic component the most useful component as a setup tool, since it is the easiest to change when antiroll devices are present. The softer the spring rate the more weight transfer you will see. 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. 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. When the car corners, lateral acceleration is applied at this CG, generating a centrifugal force. And as discussed in Weight Transfer Part 2, the driving coach Rob Wilson talks weight transfer almost exclusively when he describes what he is teaching to drivers. As such, the most powerful cars are almost never front wheel drive, as the acceleration itself causes the front wheels' traction to decrease. Its not possible to conclude directly what influence increasing roll centre heights will have. {\displaystyle w} 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 . The minimum weight of the car to take part in the X275 drag race is 2625 pounds. After that, we will see how the components of load transfer can be manipulated to tune the balance of the car. Understanding weight transfer is a fundamental skill that racecar drivers need to know. Transition This is the point at which the car 'takes its set'. 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. 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. is the change in load borne by the front wheels, Set tire pressures first. Now you know why weight transfer happens. Notice that this is just one possibility and other parameters might be investigated as well. 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. 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. When expanded it provides a list of search options that will switch the search inputs to match the current selection. 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. Now lets stop for a moment to analyse the influence of the gravity term on the lateral load transfer component. As a result load transfer is reduced in both the longitudinal and lateral directions. 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. 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. 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. The figure only shows forces on the car, not forces on the ground and the CG of the Earth. 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. Any time you apply brakes, add or remove steering, and manipulate the. The results were the same. 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 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). The stiffnesses are shown in kgfm/degree, that have clearer meaning, but the data were input in Nm/rad. 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. 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 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. C. Despite increasing the steering angle, the car has taken a line which is not tight enough to take the turn. Since these forces are not directed through the vehicle's CoM, one or more moments are generated whose forces are the tires' traction forces at pavement level, the other one (equal but opposed) is the mass inertia located at the CoM and the moment arm is the distance from pavement surface to CoM. 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. In order words, the goal would be to reduce lateral load transfer in the rear axle in comparison to the front axle. The car has turned in towards the apex. Let's start by taking a look at four stages of understeer. A perfectly rigid vehicle, without suspension that would not exhibit pitching or rolling of the body, still undergoes load transfer. The following information applies to NASCAR-style Stock Cars; it may also be useful to production-based sports car racers with the engine in the front and the drive wheels in the back. The first one to analyse is the kinematic or direct lateral force load transfer component.