Now do the same, but picking a front roll centre height and imagining a vertical line instead. Here the pickup points are highlighted for better comprehension. 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. So far, we have discussed the influence of each component in lateral load transfer in isolation. Weight transfers occur as a result of the chassis twisting around the car's roll centre, which determined by the natural suspension setup. Weight transfer has two components: Unsprung Weight Transfer: This is the contribution to weight transfer from the unsprung mass of the car. {\displaystyle b} If the car were standing still or coasting, and its weight distribution were 50-50, then Lf would be the same as Lr. One important thing to notice is that its difficult to change total lateral load transfer by setup. Another example would be the effect of ride stiffness on wheel hop frequency. 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. The Trackmobile Weight Transfer System is a hydraulic system developed to implement this idea in an intuitive and easy-to-use way. Now you know why weight transfer happens. 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. Deceleration moves the center of gravity toward the front of the vehicle, taking weight out of the rear tires. 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. It must be reminded that changing this term will only change a part of the total lateral weight transfer. Join a community of over 4000 clever racing enthusiasts that want to improve their knowledge on the technical side of motorsport! is the acceleration of gravity, This article explains the physics of weight transfer. Now that we have quantified lateral load transfer on an axle, we can start to analyse how the individual components interact. 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. 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. 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. Understanding weight transfer is a fundamental skill that racecar drivers need to know. Because of this interaction with the springs, this component is also referred as the elastic weight transfer component. The car is not changing its motion in the vertical direction, at least as long as it doesnt get airborne, so the total sum of all forces in the vertical direction must be zero. 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. Figure 14 shows the contour plot. 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 . This results in a reduced load on the vehicle rear axle and an increase on the front. A. When you increase roll centre height in one axle you increase the overall lateral load transfer on that axle, while decreasing it on the opposite axle. is the center of mass height, 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. 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). 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. 35% Front 420 lbs 780 lbs 280 lbs 520 lbs LH Turn - New Stiffer Front Roll Bar 33.3% This will give: Now consider , the vertical load on the outer tyre in a corner, and , the vertical load on the inner tyre. Weight transfer is affected by the distance between the CG Height and the roll centre. Literally, the rear end gets light, as one often hears racers say. 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. Talking "weight transfer" with respect to race driving is . 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. When you apply the brakes, you cause the tires to push forward against the ground, and the ground pushes back. In the previous post about understeer and oversteer, we have addressed the vehicle as the bicycle model, with its tracks compressed to a single tyre. 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. 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). The stiffnesses are shown in kgfm/degree, that have clearer meaning, but the data were input in Nm/rad. This article uses this latter pair of definitions. For the trailer, the chain pulls down . o Varying the gravity term from 800 Nm to 11395 Nm resulted in a difference of only 0.0148 (from 0.5011 to 0.5159) or 2.96 %. The change in this arm with roll centre heights will depend on the wheelbase and weight distribution. Friction comes from the tires on the ground and the air flowing over the car. Balancing a car is controlling weight transfer using throttle, brakes, and steering. Acceleration causes the sprung mass to rotate about a geometric axis resulting in relocation of the CoM. 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. Weight transfer is generally of far less practical importance than load transfer, for cars and SUVs at least. 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. Changing the moment generated by this component requires changes in either the unsprung mass or its CG height. An outside observer might witness this as the vehicle visibly leans to the back, or squats. 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. At rest, or at a constant speed, the weight of the car could be measured by placing a set of scales under each tire. From our previous discussion on direct force weight transfer component, you know that to change roll moment arm you need to play with roll centre heights, which will ultimately affect that weight transfer component in the opposite way you want. Most autocrossers and race drivers learn early in their careers the importance of balancing a car. By rotating the lever arms, its area moment of inertia in bending is changed, hence altering its stiffness. This can be done in multiple ways. Another method of reducing load transfer is by increasing the wheel spacings. 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. These are fundamental laws that apply to all large things in the universe, such as cars. 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. 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. If you have no suspension (ex. So, as expected, the car is not wedged. The driver has hit the apex but has found the car is starting to push wide of the desired line. 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. 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. 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. 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. Then if the car is still loose on entry we start moving the weight, at the new height, to the right. Then, a series of steer angles in the range of interest is selected. The initial lurch will sink the car. usually, production based race cars will not have any front bar at all, and rely stricly on proper spring rates . Understanding the physics of driving not only helps one be a better driver, but increases ones enjoyment of driving as well. Typically a tensioned chain produces the rotational forces or torque. https://www.allenbergracingschools.com/expert-advice/road-atlanta-track-guide/ #Sportscar #racing #motorsport #racingschool #F1 #BeARacer #MichelinRaceway #roadatlanta, Michelin Raceway Road Atlantas multi-purpose racing facility has been a fixture in the motorsport community since its opening in 1970. https://www.allenbergracingschools.com/expert-advice/road-atlanta-track-guide/ #Sportscar #racing #motorsport #racingschool #F1 #BeARacer #MichelinRaceway #roadatlanta, Allen Berg Racing Schools 1835A Centre City Parkway #408 Escondido, California 92025, (888) 722-3220 (831) 272-2844 robin@allenbergracingschools.com Hours Mo - Fr: 8:30 am - 4:30 pm, WeatherTech Raceway Laguna Seca 1021 Monterey Salinas Hwy, Salinas, CA 93908, USA, Michelin Raceway Road Atlanta 5300 Winder Highway, Braselton, GA 30517, USA, Virginia International Raceway 1245 Pine Tree Road, Alton, VA 24520, USA. n Your shock absorbers are considered after your ride and roll stiffness have been selected. When the car corners, lateral acceleration is applied at this CG, generating a centrifugal force. These effects are good for tightening up the car when winged down, but opposite for roll right. 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. The only way a suspension adjustment can affect weight transfer is to change the acceleration. 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). Similarly, during changes in direction (lateral acceleration), weight transfer to the outside of the direction of the turn can occur. Notice the smaller cornering potential for higher values of the lateral load transfer parameter. This conclusion is somehow trivial, as we know that roll moment arm decreases as roll axis gets closer to the sprung mass CG and roll rate distribution only affects the roll angle lateral load transfer component. This leads as to believe that the roll centre height gain is higher than the decrease in the roll moment arm . 20 - 25,000 (15 - 18,500) Formula SAE. 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. 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. By simply raising or lowering the couplers, our machines can gain thousands of pounds for traction. 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. 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. Now lets analyse what happens when roll centre heights get close to the CG height. B. In some categories, the rear suspension is mounted on the gearbox, for example, Formula 3, shown in figure 5. 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. The rear wheels don't steer, or don't steer as . a thick swaybar is not a good idea for the front of a FWD race car. This is multiplied by the cosine of the reference steer angle, to obtain a lateral force in the direction of the turning centre. 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. 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. Also, if you liked this post, please share it on Twitter or Facebook, and among your friends. W Total available grip will drop by around 6% as a result of this load transfer. Both of these changes will involve adding, removing or repositioning mass (and therefore parts) within the unsprung part of the car. Let us expand that analysis by looking at the pair of tyres. Its not possible to conclude directly what influence increasing roll centre heights will have. Deceleration. Naturally, you're more inclined to wheelstand with an increase in acceleration. 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. The rest of this article explains how inertia and adhesive forces give rise to weight transfer through Newtons laws. When expanded it provides a list of search options that will switch the search inputs to match the current selection. In a single axle, the roll resistance moment will be the roll angle multiplied by the roll stiffness of the axle analysed, . It is always the case that Lf plus Lr equals G, the weight of the car. 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. 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. The more the body rolls and the faster the body rolls, the more rotational . The total lateral load transfer on the car can be calculated from its free body diagram, as shown in figure 1. Before I explain this, let me talk about a good thing to understand the subject the steady-state analysis of a pair of tyres. Now lets stop for a moment to analyse the influence of the gravity term on the lateral load transfer component. It is defined as the point at which lateral forces on the body are reacted by the suspension links. Inside percentages are the same front and rear. 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 front wheels must steer, and possibly also drive. Now that we know the best ways to change roll stiffness, lets see how it affects lateral load transfer. What weight the front tires lose, the rear tires gain. So a ride height adjustment to your race car, or a roll centre geometry change is a very valid tuning device. Transient lateral load transfer is an important aspect of vehicle setup, but lets leave the discussion on that for another day. 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. Figure 12 shows a finite element stress analysis, with colours closer to yellow and green indicating higher stresses. 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. Increasing the vehicle's wheelbase (length) reduces longitudinal load transfer while increasing the vehicle's track (width) reduces lateral load transfer. 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. As a result load transfer is reduced in both the longitudinal and lateral directions. But these forces are acting at ground level, not at the level of the CG. As fuel is consumed, not only does the position of the CoM change, but the total weight of the vehicle is also reduced. 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 . 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. 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. But if total lateral load transfer is difficult to change once the car has been designed and built, then how can it be used to improve handling? The lateral load transfer parameter. Closed Wheel Race Cars How much does a NASCAR car weigh? Weight transfer during cornering can be analysed in a similar way, where the track of the car replaces the wheelbase and d is always 50% (unless you account for the weight of the driver). 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 . 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. h We derived the equations of lateral load transfer in one axle of the car, showing that its composed of three components: Unsprung weight component not useful as a setup tool because of the effect that it has on ride, specifically wheel hop mode. This analysis may even be used to prepare tyre data, in order to make the bicycle model more realistic. 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). For you to get meaningful results from the equation above, you need to use consistent units. The effects of weight transfer are proportional to the height of the CG off the ground. 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. 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. It is these moments that cause variation in the load distributed between the tires. 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. 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. Weight transfer is an advanced techniqe which can impact the cart in four directions: front, back, and then each side of the kart. is the longitudinal acceleration, This. The moment can be divided by the axle track to yield a lateral load transfer component: Where is the unsprung weight on the track being analysed. The driver is said to manage or control the weight transfer. The previous weight of the car amounted to 2,425 pounds, while now it is about 2,335 pounds. 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 minimum weight of the car to take part in the X275 drag race is 2625 pounds. [3] This includes braking, and deceleration (which is an acceleration at a negative rate). During cornering a lateral acceleration by the tire contact patch is created. We now have roll moment arm and roll stiffnesses to play with. 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. The most reasonable option would be changes on antiroll bar stiffness. These numbers are just averages and are very dependent on the class of car and the tires being run. Weight transfer varies depending on what the car is doing. In the image, the car is looked from the rear in a right hand turn. 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. t The weight transfer setup recognizes the importance of ride height and roll stiffness in determining a good balanced set up for the car. Put an R-compound DOT tire on the same car and raise that force to 1.05 g of cornering force. Just like on asphalt, we have what is commonly referred to as Weight Transfer with dirt cars. On limit conditions, this will translate in one of the axles breaking loose and skidding before the other. [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. Learning to do it consistently and automatically is one essential part of becoming a truly good driver. 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. This characteristic is also observed here. The second term can be changed modifying the suspension geometry, usually difficult or not allowed in some competitions. This component will, however, be altered by changes in other components (e.g. 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. 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. e While the skills for balancing a car are commonly taught in drivers schools, the rationale behind them is not usually adequately explained. Because of Newtons first law. The same thing happens on the left . . Moving weight should be used as a fine-tuning tool to get the car working as best it can for the track conditions. 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. Here they are the real heavyweights! The term is a gravity component that arises due to the sprung CG being shifted to the side when the chassis rolls. Wedge is defined as greater inside percentage at the rear than at the front. Roll stiffnesses were input in the form of roll rate distribution, varying from 0 to 1. This article explains the physics of weight transfer. This makes changes in roll moment arm to control roll angle component useless. 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. Set tire pressures first. Lesser the Second: Accelerating the car will weight the rear wheels heavily, the front wheels lightly. The net loss can be attributed to the phenomenon known as tire load sensitivity. Conversely, if you hold roll centre heights at about 254 mm and vary rear roll rate distribution, lateral load distribution wont suffer relevant differences. A more in-depth discussion on how each of these moments are generated will now be presented. The figure only shows forces on the car, not forces on the ground and the CG of the Earth. 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. As you see, when we increase front roll centre height, the lateral weight transfer decreases on the rear axle while increasing on the front. In general, it is almost safe to say that the Indycar weighs less than a Formula 1 car. Figure 14 can lead us to very interesting conclusions. G is the force of gravity that pulls the car toward the center of the Earth. Deceleration Weight Transfer The opposite of the acceleration weight transfer takes place during deceleration. 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. 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. Weight transfer is the most basic foundation of vehicle dynamics, yet holds many of the keys to ultimate car control. The manual of the vehicle used here specified a roll stiffness values ranging from 350,000 Nm/rad to 5,600,000 Nm/rad. An additional curve might be obtained by plotting the intersections of the lateral accelerations with the lateral load transfer parameter lines, against the reference steer angle. Usually, I'll have 50-80 lbs," Bloomquist told RacingNews.co from Lucas Oil Speedway a few weeks back. I have heard of many cars running well outside of these parameters and winning. Hence: This is the total lateral load transfer on the car. Figure 4 shows the forces and moments acting on the sprung CG. Load transfer is a crucial concept in understanding vehicle dynamics. Weight transfer is a function of car weight, CG height, wheelbase, and acceleration. The same is true in bikes, though only longitudinally.[4]. The tires and chassis will also make a difference in the spring selection. This reduces the weight on the rear suspension causing it to extend: 'rebound'.
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