This study aims at developing an experimental tool useful to define accurately the friction
coefficients in bolted joints and, therefore, at relating precisely the tightening torque to the bolt preloading force
in some structural components of front motorbike suspensions. The components under investigation are
clamped joints made of aluminium alloy. The preloading force is achieved by applying a specific torque to the
fastener, by means of a click-type torque wrench. Appropriate specimens have been designed by the authors in
order to study the tribological aspects of the tightening phase. Then, experimental tests have been performed by
applying the Design of Experiment (DOE) method in order to obtain a mathematical model for the friction
coefficients. Three replicas of a full factorial DOE at two levels for each variable have been carried out. Levels
include cast versus forged aluminium alloy, anodized versus spray-painted surface, lubricated versus
unlubricated screw, and first tightening (fresh unspoiled surfaces) versus sixth tightening (spoiled surfaces). The
study considers M8x1.25 8.8 galvanized screws.