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# Spring rate in roll?

## Spring rate in roll?

(OP)
HI, I have a question that concerns the spring rate. I know my spring rate in bump. However, the program I have (Susprog3D) also asks for the spring rate in roll at the wheel. I am not quite sure what they are asking for. And secondly how would you go about calculating it? Thanks.

### RE: Spring rate in roll?

Imagine rolling the car by one degree. You'll get a certain force at the wheel due to spring compression, plus another force due to the antiroll bar. The total restoring torque is (FL-FR)*track per degree.

Typical modern suspensions get about half their total roll rate from the sta bar. ish.

Cheers

Greg Locock

### RE: Spring rate in roll?

Is there a separate entry for the roll rate due to a sta-bar?

It's not clear to me if the spring rate that you have available is the effective rate of the spring out at the wheel or just the rate of the spring itself as provided by its manufacturer, though I suspect the latter (particularly if that number is several hundred lb/in).

If what you have is only the rate of the spring itself, you'll first need to determine the wheel rate in bump by multiplying the spring rate by the square of the overall motion ratio.  This addresses the relative position of the spring on the loaded control arm, the sine of the angle that the spring axis makes with the arm, and there's another generally small effect that's based on the front view geometry.

Then, if you're working directly from the rates rather than from the actual wheel forces at a given roll angle, the formula is

[Roll Rate] = { [Wheel Rate in Bump]*[Track]^2 } / 1375

where:
[Roll Rate] is given in units of ft-lb/deg
[Wheel Rate in Bump] is in (lb/in)
[Track] is in (in)

Norm

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