## Roll Gradient/Stiffness

## Roll Gradient/Stiffness

(OP)

I'm a little confused about the roll gradient equation given by Milliken in Race Car Vehicle Dynamics. A simplified version is at the bottom of p586, and the complete version including some derivition is found on pages 681-682.

My question came about when I was correlating some real world data to the output of the equation, and was out by a factor of 10. Milliken's equation states the result in rad/g, however I believe the acceleration units shouldn't be normalized to g's? So for example, if I'm working in metric units, the result should be rad/ms-2?

The force due to the lateral acceleration is F=MAy, with M being the unsprung mass, and Ay being the lateral acceleration (actual acceleration, not g's). This creates a moment about the roll axis which is H distance below the CG: Moment = FH = MAyH. So, if we have roll stiffness K in Nm/deg, then Moment/K = MAyH/K, so deg/Ay = MH/K. At no stage is Ay normalized to arrive at this equation.

Have I missed something obvious?

My question came about when I was correlating some real world data to the output of the equation, and was out by a factor of 10. Milliken's equation states the result in rad/g, however I believe the acceleration units shouldn't be normalized to g's? So for example, if I'm working in metric units, the result should be rad/ms-2?

The force due to the lateral acceleration is F=MAy, with M being the unsprung mass, and Ay being the lateral acceleration (actual acceleration, not g's). This creates a moment about the roll axis which is H distance below the CG: Moment = FH = MAyH. So, if we have roll stiffness K in Nm/deg, then Moment/K = MAyH/K, so deg/Ay = MH/K. At no stage is Ay normalized to arrive at this equation.

Have I missed something obvious?

## RE: Roll Gradient/Stiffness

I hope this isn't what it sounds suspiciously like . . . and I suspect that your answers aren't "out" by exactly 10.0 either.

Norm

## RE: Roll Gradient/Stiffness

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

## RE: Roll Gradient/Stiffness

When Milliken says ay, he means the acceleration due to gravity.

When Milliken says Ay, he means ay/g.

His use of symbols is listed on the inside of the front cover.

## RE: Roll Gradient/Stiffness

ay is lateral acceleration, not acceleration due to gravity.

I should never post before my first cup of coffee.

## RE: Roll Gradient/Stiffness

Perhaps you can't plug and chug like you can from a textbook. I'm not convinced that is a bad thing.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

## RE: Roll Gradient/Stiffness

## RE: Roll Gradient/Stiffness

## RE: Roll Gradient/Stiffness

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.