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Page 30
DETERMINING ROLL CENTER HEIGHTS
AND
ROLL STIFFNESS DISTRIBUTION

 
The following is for the oval and road course racers:
 
The NASCAR teams pay big bucks to have their cars set up in special fixtures so that measurements can be made of some of the basic chassis parameters. Well, I've come up with a way whereby at least some of these parameters can be determined with little more than your wheel scales and a length of chain.
 
Here's the procedure: After scaling the car, remove the right side scales. Attach a chain to any point on the chassis, extend it out horizontally to the right, and attach it to something sturdy. Next, jack up the left side of the car, shorten the chain, and ease the car back onto the scales. Note the two scale readings. Next, repeat this procedure with the chain attached at two other points, located at different heights from the shop floor and at different points along the wheelbase (the further apart the better). Plug the values into the following table, click on the "Display" button, and the results will appear below.

(May 5, 2007. The following paragraphs of explanation were somehow lost for a time. I apologize for the confusion.)

The Y pulls must be at three different points on the wheelbase and at three different chain heights.

The values input for "center of gravity height," "original RF scale," and "original RR scale" are not used in the caculations of roll center height and roll stiffness distribution.

The answers are at the bottom of the page. Click the "Display" button to refresh after changing table values.

The coefficients, near the bottom of the page, are for an expression which allows more direct calculation of the wheel loads during cornering, eliminating the need to use roll center heights or a roll stiffness distribution. The expression is of this nature:

change in front wheel loading, as a percentage of chain tension = (Ax + By + C)

where "x" and "y" are the coordinates of the chain location with an origin at the rear tire patch, "x" being positive forward and "y" being positive upward.

The lower case coordinates are for the equivalent relationship for the rear wheels. Simply substitute the CG coordinates and the total weight for the equation coordinates and chain tension as a preliminary step in calculating wheel loads in a 1G corner.

The "handling parameter," at the bottom of the page, is of very questionable significance. I'm merely tracking it for information purposes.


wheelbase =

front track =

rear track =

original LF scale =

original LR scale =

original RF scale =

original RR scale =

center of gravity height =

     

FIRST 'Y' PULL

chain height =

distance forward from rear wheels =

LF change (positive number) =

LR change (positive number) =

SECOND 'Y' PULL

chain height =

distance forward from rear wheels =

LF change (positive number) =

LR change (positive number) =

THIRD 'Y' PULL

chain height =

distance forward from rear wheels =

LF change (positive number) =

LR change (positive number) =

ANSWERS:

Ratio of Front Roll Stiffness to Total Roll Stiffness
Ratio of Rear Roll Stiffness to Total Roll Stiffness
Rear Roll Center HeightFront Roll Center Height

TIRE LOADINGS IN 1G CORNER:

Left FrontRight Front
Left RearRight Rear

CONSTANTS (SEE ABOVE):

ABC
abc

HANDLING PARAMETER (SEE ABOVE):

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