thanks for the answers.
yesterday i read some old physic-schoolbooks and came to the same formula as flotch posted, but than i saw yr answer to atledreier and was wondering that 3 Hz are around 103 kN/m, which did not match my calculations (I calculated with total weight of the car). So it seems I have to split the weight for front and rear tyres. So I will check colcobs analyser to find out how to calculate the mass on rear and front.
btw, the guide I was looking into, was posted from you here on 15.08. (direct link).