  We show how the sum of the two average tau polarisations in the decay chain
chi20 -> stau1 tau -> tau tau chi10 in minimal supersymmetry with conserved
R-parity can be measured at the LHC. This is accomplished by exploiting the
polarisation dependence of the visible di-tau mass spectrum. Such a measurement
provides information on the couplings of the involved SUSY particles and allows
a more precise determination of the di-tau mass endpoint. If different tau
decay modes can be distinguished, the polarisation and endpoint measurement can
be improved even further.
