what is weight transfer in a race car? what is weight transfer in a race car?

If (lateral) load transfer reaches the tire loading on one end of a vehicle, the inside wheel on that end will lift, causing a change in handling characteristic. 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. h When a car leaves the starting line, acceleration forces create load transfer from the front to the rear. The previous weight of the car amounted to 2,425 pounds, while now it is about 2,335 pounds. t By rotating the lever arms, its area moment of inertia in bending is changed, hence altering its stiffness. 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. Ride stiffness can be altered by either changing springs or tyre pressures (tyre pressure affects tyre stiffness, which contributes to the overall ride stiffness). Now that we have quantified lateral load transfer on an axle, we can start to analyse how the individual components interact. 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. 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. This fact can be explained at deeper levels, but such an explanation would take us too far off the subject of weight transfer. w 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. This can be done in multiple ways. Term 2 always leads Term 3. 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. Most high performance automobiles are designed to sit as low as possible and usually have an extended wheelbase and track. Conversely, if you increase rear roll centre height, lateral load transfer increases on the rear axle and decreases on the front axle. m Understanding the physics of driving not only helps one be a better driver, but increases ones enjoyment of driving as well. Bear in mind that these values were obtained for a fairly heavy race car with an unreasonably high CG, and this is only one of three weight transfer components. Here the pickup points are highlighted for better comprehension. 3. 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). See you soon! The weight transfer setup recognizes the importance of ride height and roll stiffness in determining a good balanced set up for the car. is the change in load borne by the front wheels, 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. 21 Shifting. The only reason a car in neutral will not coast forever is that friction, an external force, gradually slows the car down. In the automobile industry, weight transfer customarily refers to the change in load borne by different wheels during acceleration. Changing the moment generated by this component requires changes in either the unsprung mass or its CG height. 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. Inside percentages are the same front and rear. Cars will accelerate, brake, corner and transfer weight from left to right, fore to aft. Those of you with science or engineering backgrounds may enjoy deriving these equations for yourselves. 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. If it reaches half the weight of the vehicle it will start to roll over. 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. When cornering, the sprung mass of the car will roll by an amount , the roll angle. When this happens, the outside spring of the suspension is compressed and the inside spring is extended. r 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. Most people remember Newtons laws from school physics. Here they are the real heavyweights! Closed Wheel Race Cars How much does a NASCAR car weigh? Its not possible to conclude directly what influence increasing roll centre heights will have. Weight transfer in a car is a function of Lateral Acceleration, Track Width, Centre of Gravity Height (CG Height) and Weight. Weight . 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 rest of this article explains how inertia and adhesive forces give rise to weight transfer through Newtons laws. 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. 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 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. 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. If you have acceleration figures in gees, say from a G-analyst or other device, just multiply them by the weight of the car to get acceleration forces (Newtons second law!). n The difference in height between the roll center and center of gravity of the sprung mass gives rise to a moment. the kinematic and elastic components. On limit conditions, this will translate in one of the axles breaking loose and skidding before the other. With those values, the gravity term will be 1662.1 Nm. If that was the case, you should work on the roll centres heights instead, and then adjust suspension parameters accordingly. 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. f This means the driver should be in the car, all fluids topped up, and the fuel load should be such that the car makes your minimum weight rule at the designated time-usually after a race. Increasing front roll center height increases weight transfer at front axle through suspension links (Term 2), but reduces overall weight transfer through suspension (Term 3). Naturally, you're more inclined to wheelstand with an increase in acceleration. The figure only shows forces on the car, not forces on the ground and the CG of the Earth. Weight transfer is one parameter that is minimized - to aim for even loading on all four tires; resulting in maximum grip during cornering. The vehicle mass resists the acceleration with a force acting at its center of gravity. 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 . Typically a tensioned chain produces the rotational forces or torque. In figure 3 the effect is repeated, but from a different perspective. 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. 20 - 25,000 (15 - 18,500) Formula SAE. Both of these changes will involve adding, removing or repositioning mass (and therefore parts) within the unsprung part of the car. By analysing Figure 9 you can see that lateral load transfer is very sensitive to changes in roll centre height. 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. 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: If that is the case in the front axle, the car will understeer, if it is in the rear axle, it will oversteer. Lowering the CoM towards the ground is one method of reducing load transfer. The fact is that weight transfer is an unavoidable phenomenon that occurs whether or not a vehicle rolls. Then the expansion of the tire will begin to lift the car. [6] Now lets analyse what happens when roll centre heights get close to the CG height. 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? Balancing a car is controlling weight transfer using throttle, brakes, and steering. 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. is the total vehicle weight.[7][8]. The stiffnesses are shown in kgfm/degree, that have clearer meaning, but the data were input in Nm/rad. This component will, however, be altered by changes in other components (e.g. If the tyres of the car are lightly loaded, there might not be enough load sensitivity in the tyres, so that even if one end of the car takes all the lateral load transfer, the lateral force performance isnt degraded significantly. 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 . Now that we know the best ways to change roll stiffness, lets see how it affects lateral load transfer. For this analysis, only the rear axle was considered. G cannot be doing it since it passes right through the center of gravity. 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. Put the driver weight in the car, preferably the driver. Well, a thousand changes to the car could be applied. 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. 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. 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. Weight transfer happens when a car's weight moves around its roll centre when braking, turning or accelerating. Lets now analyse roll stiffnesses. 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. No motion of the center of mass relative to the wheels is necessary, and so load transfer may be experienced by vehicles with no suspension at all. Let us expand that analysis by looking at the pair of tyres. Another method of reducing load transfer is by increasing the wheel spacings. {\displaystyle \Delta Weight_{front}} 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. 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). Roll is simply the effect of a suspension reacting to weight transfer. It must be reminded that changing this term will only change a part of the total lateral weight transfer. . Wedge is defined as greater inside percentage at the rear than at the front. Performance Engineer, withexperience in IMSA LMP2, Porsche Cup Brazil and othercategories. 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). 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 . Figure 8 clarifies. The lighter 250-lb/in rate benefits a drag car in two ways. o This force will result in a moment, whose arm is the unsprung CG height, . 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. {\displaystyle g} Now lets use the knowledge discussed here applied in the example presented at the beginning of this article, with a little more detail in it. . Literally, the rear end gets light, as one often hears racers say. 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. This bias to one pair of tires doing more "work" than the other pair results in a net loss of total available traction. This analysis may even be used to prepare tyre data, in order to make the bicycle model more realistic. Acceleration causes the sprung mass to rotate about a geometric axis resulting in relocation of the CoM. You have less lead to work with. More wing speed means we need to keep the right rear in further to get the car tighter. The braking forces are indirectly slowing down the car by pushing at ground level, while the inertia of the car is trying to keep it moving forward as a unit at the CG level. 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. Put an R-compound DOT tire on the same car and raise that force to 1.05 g of cornering force. The second option to alter load transfer from direct lateral force component is to change roll centre heights. 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. What we can do is only influence which portion of the total lateral . 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. Location: Orlando, FL. The overall effect will depend upon roll centre heights and roll stiffnesses, and a definitive conclusion will require a deeper analysis. 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. 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. This results in a reduced load on the vehicle rear axle and an increase on the front. This is multiplied by the cosine of the reference steer angle, to obtain a lateral force in the direction of the turning centre. As fuel is consumed, not only does the position of the CoM change, but the total weight of the vehicle is also reduced. While the skills for balancing a car are commonly taught in drivers schools, the rationale behind them is not usually adequately explained. 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. The total lateral load transfer on the car can be calculated from its free body diagram, as shown in figure 1. 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. [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. 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. Move that 100lbs to directly over the rear axle, and you add 100lbs to the rear axle's scale weight, and take nothing off the front axle. 2. draw the ground line ,vehicle center line and center of the left and right tire contact patches. Steering. Similarly, during changes in direction (lateral acceleration), weight transfer to the outside of the direction of the turn can occur. {\displaystyle b} After that, we will see how the components of load transfer can be manipulated to tune the balance of the car. The analysis procedure is as follows: The potential diagram is a benchmarking of the performance that can be achieved by a pair of tyres. Weight transfers occur as a result of the chassis twisting around the car's roll centre, which determined by the natural suspension setup. Conversely, if you hold roll centre heights at about 254 mm and vary rear roll rate distribution, lateral load distribution wont suffer relevant differences. In wheeled vehicles, load transfer is the measurable change of load borne by different wheels during acceleration (both longitudinal and lateral). 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. 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. Moving weight should be used as a fine-tuning tool to get the car working as best it can for the track conditions. How can weight shift when everything is in the car bolted in and strapped down? 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. Weight transfers will occur in more controllable amounts, which will result in a more efficient and stable handling race car. In this figure, the black and white pie plate in the center is the CG. Just as taking Claritin or Benadryl reduces your symptoms without curing your allergies, reducing roll reduces the symptoms but does not appreciably cure weight transfer. By simply raising or lowering the couplers, our machines can gain thousands of pounds for traction. This leads as to believe that the roll centre height gain is higher than the decrease in the roll moment arm . Turning in to a corner brings the car's momentum forward . The net loss can be attributed to the phenomenon known as tire load sensitivity. Figure 13 shows the contour plots of lateral weight transfer sensitivity as a function of front and rear roll stiffnesses. Bickel explains how the way the 4-link plays into how you adjust the car. Hence, springs and tyre pressures should only be changed when other aspects need modification, but not only roll stiffness itself (unless the vehicle has no antiroll bar). Learning to do it consistently and automatically is one essential part of becoming a truly good driver. Do you see how small it is compared to the roll stiffness of the car? 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. 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, . a thick swaybar is not a good idea for the front of a FWD race 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. e The weight of an IndyCar race car should be at least 712 kg, with an average of 1630 lbs or 739.5 kg. What would you do, in order to solve the problem? 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 rear wheels don't steer, or don't steer as . The results were the same. MichaelP. Use a 1/4 to one scale. Most autocrossers and race drivers learn early in their careers the importance of balancing a car. G points down and counteracts the sum of Lf and Lr, which point up. 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. 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. Check stagger at each tire, even if using radials. 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.