Load transfer
Load transfer describes the change in load of tires on a vehicle during acceleration. Load transfer is often confused or referred to as weight transfer. This leads to unclear language usage. Weight transfer describes the physical movement of a vehicles center of mass through suspension compliance and fluid sloshing.
Load Transfer vs Weight Transfer
Load transfer is a change in load borne by tires during any acceleration. The tires of a vehicle are what provide the force. However, the contact patch of the tires is not in the same location as the center of mass of a vehicle. Thus, you must treat the vehicle as a real system rather than a point particle one. This difference in location of force and center of mass subsequently causes a moment (think of it as a torque kinda), loading a set of tires more than the other. For example, when a completely rigid vehicle brakes, it is deaccelerating by the force of the tire's contact patch, and because the vehicles CoM is vertically greater, the load of the rear tires will decrease, and the load of the fronts will increase. Load transfer also occurs laterally when turning due to centripetal force (which is acceleration).
Weight transfer is the change in the center of mass of a vehicle through suspension compliance and cargo/fluid movement. For example, when a vehicle accelerates forward, fluids and unsecured cargo will travel rearwards relative to the vehicle, thus shifting the center of mass rearward.
Load transfer matters significantly more than weight transfer. This is because load transfer determines the traction a given tire has and the traction the car as a whole has much more significantly than weight transfer. Additionally, your FSAE car should not have unsecured cargo. Any weight transfer that occurs on our car will be due to fluids and suspension compliance (which is relatively stiff anyway) and thus the center of mass barely moves.
To emphasize the minuscule effect of weight transfer compared to load transfer, here is an example I copied from Wikipedia: "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. Total available grip will drop by around 6% as a result of this load transfer. At the same time, the CoM of the vehicle will typically move laterally and vertically, relative to the contact patch by no more than 30 mm, leading to a weight transfer of less than 2%, and a corresponding reduction in grip of 0.01%."
Calculate it
An important thing to note is that FSAE cars do not have a consistent weight. This is because at speed, we produce downforce which is essentially weight.
Longitudinal
The change in load of the front wheels (longitudinal) can be determined using the following:
Where delta Load front is the change in front load, a is longitudinal acceleration, h is the height of the center of mass, b is the wheelbase length, and m is the total mass of the vehicle. Change in rear load can be found by ignoring the negative. If the center of mass is not in the middle of the car laterally, a coefficient Cx can be added to find the load change of each wheel.
Where R is the offset ratio, e is the distance the center of mass is from the center line and t is the front trackwidth. R must be calculated for both the front and rear. To calculate rear R, use rear trackwidth rather than front trackwidth for t.
Cx is the load adjustment coefficient, R is offset ratio. Cx must be calculated for each wheel. If the Cx you are generating is for a wheel laterally closer to the center of mass, add R to 1, otherwise subtract R from 1.
Multiply each front wheel's Cx by the change in front load to get the change in load of each front wheel. Multiply each rear wheel's Cx by the change in rear load to get the change in load of each rear wheel.
Laterally
The change in load laterally when cornering can be determined using:
Where LLT is lateral load transfer occurring across the car, Ay is cornering g's, h is the height of the center of mass, and t is the trackwidth. For example, a car making a right turn at 0.9g's, with a ride height of 0.3m, and a trackwidth of 0.75m will have an LLT of 0.36, or 36%. This means the lateral load transfer occurring across this car is 36% of the total mass of the vehicle. On a car with equal split mass this mean the left (outside) wheels would have 86% of the load. If the LLT is above 50%, this means the inside wheels have left the ground.
Note roll center and roll stiffness are not present in this equation. The total magnitude of load transfer cannot be changed by modifying the suspension. However, the distribution of lateral load transfer front vs rear can be changed through suspension modification. This will change the over vs understeer bias the car possesses.
Traction
Load transfer causes a change in traction in each wheel. The result of one pair of tires being loaded more than the other pair is a net loss in total traction. This can be explained by tire load sensitivity.
The amount of load each tire receives during a given motion can be changed by modifying spring rates or through the use of anti-roll bar. An anti-roll bar does not change the load of the inside tire relative to the outside tire but rather front-to-rear load distribution during lateral acceleration.
