Distance from Center of Gravity to Ground.

The CG Height of your car might be available from the Web, you could estimate it (based on weight and location of all major components), or you can use one of the CG Height calculators below.

The numbers already entered are just examples and can be overwritten. If you enter decimals, please use a decimal point. Metric System users will have to use the decimal point too, as the "comma" will not work.

For raising rear wheels, see tips and warnings below (raising front wheels instead, see also last paragraph).

Standard B  = in WF  = lb WR  = lb h  = in WFR  = lb RTF  = in CG Height =  inch

B = wheelBase * WF = Weight Front (car horizontal) WR = Weight Rear (car horizontal) h = height, rear axle raised WFR = Weight Front, rear Raised RTF = roll. Radius of Tires Front (= wheel-center to ground)**

Metric B  = mm WF  = kg WR  = kg h  = mm WFR  = kg RTF  = mm CG Height =  mm

* Wheel Base: On vehicles with different wheel diameters front vs. rear, the wheel base is measured between the projections of the wheel centers on the ground. Not between the physical wheel centers.

** Rolling Radius: The distance between wheel center and ground. When calculating CG-Height, the rolling radius of the raised wheels is not part of the calculation. This holds true for a car with different wheel diameters front to rear. A lot of confusion exists because of the misconception that the wheelbase is a line that connects front and rear wheel centers. However, the wheelbase is the shortest distance between the lines that connect the centers of both front and rear tire contact patches (on vehicles with equal front and rear tire diameters, this distance simply happens to be the same as the length of the line that connects front and rear wheel centers, spindles, or axles).

 

TIPS  on  HOW TO MEASURE  "h"  and  "W_FR"

First, with the car on four scales on a level floor, determine the front weight and rear weight of the car.

Next, place the rear wheels of the car on a level that is about 15 to 20 inches (40 to 50 cm) above the level of the front wheels, with scales underneath the front wheels. The car should be able to roll freely on the scales, so chock the rear wheels or use the parking brake. WARNING: Be careful as this is a potentially dangerous situation.

Suggestion: Place the rear wheels on a four post lift. On an average vehicle the front wheels roll backwards roughly two inches (± 5 cm) when raising the rear wheels 20 inches (50 cm). So no worries about the car rolling off the scales.

Note: The higher you raise the rear wheels, the more accurate your calculations (scales indicate a small change in weight less accurate than a bigger change). But keep in mind that for safety and practical reasons you can't go much higher than 25 inches (approx. 65 cm), depending on the vehicle's wheelbase and front overhang.

Now, read both scales under the front wheels and measure the exact difference in height between the front wheels and rear wheels. Make sure you have tire pressures set right, and weight of driver + helmet, on-board fire extinguisher, fuel, etc. are all accounted for. If you are the driver and the technician, a couple of bags of blast media, or anything else resembling your weight in the driver seat will do.

 

IMPORTANT:

When "CG-Height" is used for (re)designing a brake system for a vehicle that is meant to carry more than just a driver, his/her helmet and the other necessities that are usually found in race cars, always load it up with the maximum weight it is allowed to carry! (look up its GVWR in the owners manual, if available).
Spread the load the way you would most likely do when loading the vehicle up to its maximum operating weight.

At its maximum weight, a vehicle still has to be able to decelerate as fast as its tires allow (emergency stop, for example), without creating a situation for the driver in which it is hard to apply the required pressure to the brake pedal.

 

NOTE:

If you lift the rear end of a car, weight transfers from the rear to the front, theoretically¹ resulting in a lower ride height in the front and extended rear springs. So, for more precise results, specifically on high centered/soft suspension vehicles, some people consider locking the ride height. You can achieve this with shock replacements made from flat bar, tubing, etc., or by welding up a set of old shock absorbers (see WARNING below!). Make your welding marks while the car is in horizontal position, on a smooth surface (a four post lift would be ideal) and with all other conditions (tire pressure, driver + helmet, on-board fire extinguisher, fuel, etc.) right. If you work on a car with stiff springs (in relation to its weight!), you will not notice any difference (front/rear) if you compare the raised ride height with the horizontal ride height.

¹ The weight shift will most likely not be enough to overcome friction between the suspension parts and therefore will not change ride height. A simple measurement between tire and fender before and after will show you whether this is the case or not.
Even in the theoretical case of a friction free suspension, the change in ride height front and rear, caused by the weight transfer, will not be enough to have a measurable effect on the calculated CG-Height, when measured on a sports car or race car (with a possible exception for Baja racers and other high centered/soft suspension vehicles).

 

WARNING:

Do not weld on any shock absorber before depressurizing and draining it in accordance to the instructions of its manufacturer (see documentation that came with the shocks, or contact manufacturer). Heating up a gas or liquid in a closed container can cause a potentially fatal explosion.

 

THOUGHTS:

The CG of an object is the point where all its mass seems to be concentrated.

One way to give a vehicle more stability is to lower the position of its CG.

 

You can calculate CG Height like this: Y_CG = (B • (W_FR – W_F) / (W_T • Tan α)) + R_T
Or enter your numbers in the "CG Height Calculator" above, and let your computer do the work.

YCG	= Distance from Center of Gravity to ground (= "CG Height")
B	= Wheelbase
WFR	= Weight measured under both front tires while rear of car is raised
WF	= Weight measured under both front tires while car is horizontally level
WT	= Total weight of car (= WF + WR)
Tan α	= h / √(B² – h²)       h = vertical distance that rear wheels have been raised
RT	= Rolling radius of front tires or rear tires when RTF = RTR
	If RTF ≠ RTR then RT = RTF when rear is raised (RT = RTR if front is raised)

 

TIP:

Sometimes it's more practical to raise the front wheels instead of the rear wheels (cars with a long front overhang and short(er) overhang in the rear). If you do that, you can still use these calculators, as long as you interchange W_F and W_R, and use W_RF instead of W_FR (and R_TR instead of R_TF in case front and rear tires have different outside diameters).