Hi everyone !! i want to find the equivalent mass of a car rotating parts and i came across this site which provides some cars formula : http://www.thecartech.com/subjects/auto_eng/car_performance_formulas.htm So far i understand almost everything until i get to the part where inertia resistance is calculated as an equivalent mass of rotating parts. Here is the quote : meq = equivalent mass of rotating parts [kg] = [ Iw (1/rw)2 + Ip hf (if /rw)2 + Ie ht (if ig / rw)2] where: Iw = polar moment of inertia of wheels and axles ≈ 2.7 [kg m2] Ip = polar moment of inertia of propeller shaft ≈ 0.05 [kg m2] Ie = polar moment of inertia of engine ≈ 0.2 [kg/m2] + polar moment of inertia of flywheel and clutch ≈ 0.5 [kg m2] hf = mechanical efficiency of final drive ht = mechanical efficiency of transmission system (hg x hf) ig = gearbox reduction ratio [ig1 or ig2 or ………….] if = final drive reduction ratio rw = tire radius [m] Now what i dont understand is the first part : Iw (1/rw)2 ??? Why do we divide 1 by the wheel radius?? is this a mistyping mistake?? Can someone explain this to me please? Thanks !!