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# Correct handling of tyre rolling resistances: Drive / Coast and 4WD

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## Correct handling of tyre rolling resistances: Drive / Coast and 4WD

(OP)
Hi,

I'm (re)building a MATLAB simulation of longitudinal vehicle dynamics and want to sanity check my handling of tyre rolling resistance power losses:

For the Drive case (RWD), I am calculating front tyre rolling resistance losses (P = FV where F = CrrN) only and modelling rear tyre losses as Contact Patch Slip Speed x Contact Patch Longitudinal Force (P = FV).
For the Coast case, I calculate all 4 tyre rolling resistance losses.
For the Braking case, I model all 4 tyre losses as Contact Patch Slip Speed x Contact Patch Longitudinal Force (P = FV).

I am curious how this relates to coast-down tests used to derive road loads for emissions tests. These are obviously Coast condition with all 4 tyres contributing a rolling resistance loss. But if the derived road load is then used to set up a dyno, is the dyno road load not too high by the amount contributed by the driven axle? Or is this compensated for in some way.

And, for the Drive case for a 4WD, then presumably there is no contribution of rolling resistance from any tyre, just slip loss?

Thanks, Ian

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