prefactor of normal model hooke

Hi,

I'm just wondering if anyone could give me a hint, where the prefactor in the hookean standard model, 16/15*..., results from?
Following the work of Di Renzo (2004) (see. refs on the Hooke doc site), where kn is calculated from

kn = fn,max / deltan,max, i only get the prefactor of 4/3. Maybe someone could give am a clou.

Thanks,

M