The Center of Gravity (CG) locates the balanced weight position. It is important for controlling the tire loading and therefore grip available while accelerating, braking, and cornering. The ideal CG position depends on the specific race conditions. Road race cars usually want all four tires equally loaded for maximum grip under their varying conditions. This leads to a CG very close to the center of the car both laterally and longitudinally and as low as possible to minimize weight transfer. Oval track cars typically want the tires loaded equally while cornering to the left. This leads to the CG biased to the left (driver side) and sometimes higher depending on the track’s bank and the car’s corning rate capability. Rear wheel drive drag racing cars position the CG to maximize the rear tire’s grip during acceleration, leading to a CG along the longitudinal centerline and adjusted forward and vertically based on the car’s acceleration capability.
This page steps through the process of finding a car’s Center of Gravity. The equipment needed is usually available in a typical race shop. Once the CG is found for the car’s baseline configuration, any future ballast weight and chassis height tuning adjustments can be accounted for. The impact of adjustments on wheel loads and therefore traction performance can be estimated.
Three calculators are provided. A Horizontal CG Calculator is used to store the measurements needed to find the longitudinal and lateral CG position. A CG Height Calculator stores the measurements needed to determine the CG height. Once the CG has been located using these calculators, the Weight Tuning Calculator can be used to find the impact of adding, removing or shifting weight in the car. Logged-in users can name and save the data for multiple cars or conditions. As with all user saved data on this site, the data is not available to other users. Visitors have a saved example and can make changes dynamically, but cannot save the changes.
Test Method
The methods and calculations I use are based on those presented in Race Car Vehicle Dynamics with some slight differences. For example, I use inches instead of feet for my distance scale and my coordinate system for measurements is:
- b for the horizontal distance forward of the rear axle centerline.
- y for the lateral distance from the car centerline. Positive toward the right side (U.S. passenger side).
- h for the height off the ground.
All of the measurements must be made with the car in “race ready” condition. The tire pressures should be at race settings. The driver should be seated with race gear and all fluids filled. Cars with large fuel usage during races can be tested with the fuel fill at several levels, or at the mid-fill. Fuel vents should be closed to avoid overflow while the car is pitched during the tests. Usually the suspension is locked with a “dummy” set of welded shocks, but cars with very stiff suspensions and tight shocks will not have much measurement error without locking the suspension travel.
Locating horizontal CG
The Horizontal CG Calculator is used to store the data and make the necessary calculations. The logged-in user saved data will carry over to the vertical CG calculations. You can enter the data as the measurements are made.
The car is rolled onto a set of individual wheel scales. The scales must be on a level surface. The required measurements are:
- each of the individual wheel weights
- the front and rear tracks ( the lateral distance between the centerline of the tires )
- the wheelbase, the longitudinal distance between the front and rear axles. (Typically the average is used when there is a difference between driver and passenger sides, but for higher powered drag cars I like to use the passenger side wheelbase.)
Most road race and drag cars will have the weight very close to longitudinal centerline ( the y value will be very close to zero ). Round track cars will usually have the weight biased to the driver’s side ( the y value will be negative ).
Locating vertical CG
After locating the horizontal CG and saving the data, use the CG Height Calculator to enter your measurements and make the necessary calculations. The CG height is found by measuring the increase in weight at ground level as the tires on the opposite end of the car are raised up.
While the car is level and in race ready trim, measure the vertical distance from the axle centerline to the ground for the front and rear wheels. This is the wheel’s “loaded radius”. Also measure the angle that the chassis makes with the ground from back to front. Pick a convenient chassis/body surface to make the measurement (maybe the rocker edge). This angle is used as the baseline for the car’s elevated angle measurement. This Raised Angle Calculator can be used to find the elevated angle from chassis height measurements.
Either the front or rear of the car can be elevated. Although the rear is usually recommended, when the front overhang is long, lifting the front is more practical. When the front is lifted, wheelie bars need to be tied up so they don’t drag. The tires that stay on the ground will need to be chocked onto the scales so they will not roll off. All brakes must be released. The raised tires must be allowed to roll freely on the platform used to raise them up so they do not pull horizontally as they are lifted. If you use an overhead hoist, keep the cables vertical so the hoist stays directly above the wheels being lifted. I have used a post lift with the wheels to be lifted on the lift. In that particular case we lifted the front wheels. I have also used the trailer lift ramp. Just level the ramp to make the measurements as you lift.
As the car is lifted on one end, the other end is well chocked on the scales at ground level. The scaled end will increase in weight from the original level measurements. The change in weight is small, so multiple measurements will improve the estimate. The raised angle will need to be at least 8 to 10 degrees to get enough weight transferred to keep the CG height error less than 1/4 inch using scales with 1 lb precision. That means a 100 in wheelbase car needs to be lifted about 15 in on one end. As you lift, you should stop several times to measure the chassis angle and the weight. Once you reach your maximum reasonable lift, you should have several measurements. Use the CG calculator to find the CG height at each set of measurements and then use the average of the results as the CG height.
The calculator “Raised Angle” is the net change in angle from your baseline measurement. Cars typically have a “rake” with the chassis sloping up slightly from front to back. Be sure to take this into account when determining the “Raised Angle”. For example, if the baseline measurement is 2 degrees down to the front and the front raised measurement is 3 degrees up to the front, the “Raised Angle” is 2 + 3 = 5 degrees. If the rear is raised, and the measurement is 2 degrees down when on level ground and 8 degrees down when the rear is raised, then the “Raised Angle” is 8 – 2 = 6 degrees.
The CG height calculator “Total Front Wt” or “Total Rear Wt”, depending which end is raised, is the total weight of the end of the car remaining on the ground. So, if individual wheel scales are used, add the weight of both scales on the ground.
Locating Sprung Weight CG
The previous CG measurements and calculations were for the whole car. Wheel loads due to weight transfer occur based on this total weight. However, when the suspension travels, only the sprung mass body/chassis combo is moved. If the suspension height is adjusted to move the chassis, then the CG of the sprung mass alone is being moved. In this circumstance the unsprung weights should be accounted for. The height of the wheel center is usually a reasonable estimate for the height of the unsprung weight. The wheel center heights have already been measured for the CG height calculations. Either weigh, or get a good estimate of the weight from your car builder of the car’s components below the springs. My calculator assumes the front and rear unsprung weight is centered laterally (y = 0).
Weight Tuning
Once the car’s baseline CG location has been determined, the impact of adding, subtracting, and moving weight can be estimated. The Weight Tuning Calculator uses the data entered and stored previously by the CG calculators. Both the baseline and after adjustment values are presented in separate output tables for comparison.
The Calculator makes 4 logged-in user stored weights available. The position of each weight is identified by its horizontal distance from the rear axle centerline (b) and the vertical distance from the ground (h). Distances behind the rear axle are entered as negative b values. The weights can be the car’s ballast weight bar locations, or locations where any car components are changed. Usually the optimum ballast weight bar locations are at the extreme positions available. For example, a drag car will locate a front ballast mount as low and as far forward as possible and another rear ballast as high and as far back as possible. This gives the greatest impact on acceleration weight transfer with the minimum amount of ballast. The calculator will display weight locations in yellow on the car image when the weight position is non-zero.
When weight is added to the car enter a positive weight value in the “lbs” column. If weight is removed enter a negative value. For example, if you want to see the effect of changing the transmission to one weighing 20 lbs less than the original baseline transmission, then measure b and h to the approximate center of the transmission and enter -20 for the lbs.
The CG is sometimes weight tuned by adjusting the chassis height. The front and / or rear spring purchase is usually adjustable. The calculator uses the chassis height change at the axle centerline for its calculations. This is not necessarily the same as the spring purchase move since the there is often strut leverage involved. Use the actual chassis height change. When front or rear height changes are entered, the change is indicated on the car image with a yellow marker near the appropriate wheel center. Entering the unsprung weight values for the front and rear will improve the estimates.
When weight adjustments are made, the chassis height can change as the springs respond to the added or removed weight. If the chassis height is not returned to the baseline setting, then the chassis height change should be entered to correctly account for the move.
The reasons and methods used to tune the car’s weight distribution are covered in this slide presentation.
These static test methods can be applied to actual track data. On track measurements of the suspension travel and chassis height can be used to dynamically locate the CG and wheel loads through a run. A data acquisition analysis system can be used after runs to review the car’s weight moves after a run. Active suspension system components can be used to control the car’s weight moves during runs.