Weight Transfer and Body Roll Calculator

Values not highlighted are taken from the other worksheets:

Front:

Roll Couple:

35781.65

in/lbs

As more load is put on a tire, its contact patch increases and its rolling radius decreases.  This adds to body roll and, if not the same front and rear, will shift some load transfer fore or aft.

Overall:

Total Roll Stiffness:

939.83

lb/in

Vehicle's Wet Weight with Driver:

3500

lbs

WT due to Tire Deflection

-65.79

lbs

Sprung Weight Center of Gravity Height:

22

in

WT due to Body Roll:

609.57

lbs

Roll Center at Center of Gravity:

4.065

in

WT due to Roll Center Height:

129.30

lbs

WT due to Unsprung Weight:

53.41

lbs

Spring travel calculations assume a linear spring rate, meaning the suspension is not hitting its progressive jounce bumper (urethane bump stop).  Most modern cars do this, stock, and it creates a sharply progressive spring rate.  Meanwhile, some "progressive" springs may actually be linear near ride height.

Front:

WT due to CG Migration:

33.51

lbs

Weight Distribution:

58.0%

Spring Travel Req'd for Roll:

1.54

in

Unsprung Weight:

95

lbs/side

Total Weight Transfer:

760.00

lbs

Sprung Weight:

920.00

lbs/side

Unsprung Center of Gravity Height:

15

in

Rear:

Roll Center Height:

3.75

in

Roll Couple:

25968.56

in/lbs

Track Width:

58.7

in

Total Roll Stiffness:

682.08

lb/in

Spring Wheel Rate (see worksheet):

210.76

lb/in

WT due to Tire Deflection

47.75

lbs

Motion Ratio of Spring (see worksheet):

0.97

lb/in

WT due to Body Roll:

440.89

lbs

Sway Bar Wheel Rate (see worksheet):

259.15

lb/in

WT due to Roll Center Height:

108.41

lbs

WT due to Unsprung Weight:

50.42

lbs

Spring travel (and later, CG migration) calculations do not take into account roll center migration, which would slightly increase the required travel for bump travel but reduce it for droop travel.  This is an estimation only, since a roll center analysis would require much more data.

Rear:

WT due to CG Migration:

24.18

lbs

Weight Distribution:

42.0%

Spring Travel Req'd for Roll:

1.56

in

Unsprung Weight:

90.00

lbs/side

Total Weight Transfer:

671.66

lbs

Sprung Weight:

645.00

lbs/side

Unsprung Center of Gravity Height:

15

in

Overall:

Roll Center Height:

4.5

in

Total Roll Couple:

61750.21

in/lbs

Track Width:

58.9

in

Total Roll Stiffness:

1621.91

lb/in

Spring Wheel Rate (see worksheet):

186.32

lb/in

Total Weight Transfer:

1431.66

lbs

Motion Ratio of Spring (see worksheet):

0.98

lb/in

Lateral CG Migration:

0.97

in

This is a very basic calculation of how much body roll to expect.  The models for both sprung chassis roll and tire deflection are more complex, but these figures are going to be in the ballpark.

Sway Bar Wheel Rate (see worksheet):

154.72

lb/in

Sprung Body Roll:

3.10

degrees

Wheel Travel Req'd for Roll:

1.59

in

Cornering Forces:

Total Body Roll:

3.70

degrees

Cornering Force (grip):

1.1

G turn

Centrifugal Force on sprung weight:

3443.00

lbs

Weight on Wheels During Steady-State Turn:

If you see a negative number here, that tire is now up in the air and no more weight can be transferred from it.  That weight is now being unloaded from the tire next to it.  This lowers your total roll stiffness, accelerating body roll and changing the dynamics of the car and going outside the boundries of the equations here.  In short, this spreadsheet cannot handle it.  Additionally, most street cars produce aerodynamic lift on the front wheels at speed, which is not calculated here.

Centrifugal Force Front:

2024.00

lbs

Centrifugal Force Rear:

1419.00

lbs

Left Rear:

Right Rear:

63.34

1406.66

Tires:

Tire Diameter:

25.0

in

Tread Width

8.7

in

Left Front:

Right Front:

Tire Inflation Pressure:

36

PSI

255.00

1775.00

Tire Deflection (diff between LF and RF):

0.37

in, est. at:

0.36

Tire Deflection (diff between LR and RR):

0.23

in, est. at:

0.23

by Steve Edwards

See the tire deflection calculator for an understanding of what this value is.  I automatically estimate this value for you, so play with it until they're close to matching.  This number will be less than I calculate for stiff-sidewalled tires but more for a tire going off-camber.  For a stiffly sprung car, small variations here are very important.