INTRODUCTION A wet clutch is a mechanical device which transmits power from the driver member to the driven member through frictional and viscous effects. Wet clutches play a significant role in on-road, off-road vehicles, and in industrial machinery equipped with automatic transmissions (AT's), limited slip differentials (LSD's), lock-up clutches (LC's) and wet brakes (WB's) [ 1]. During a wet clutch closing ( i.e., engagement) and opening ( i.e., disengagement) process frictional heat is generated which causes wear and deterioration of the disks. Therefore, the disks must be cooled in order to increase the service life of the clutch. For this purpose, the ATF is continuously supplied to the disks, irrespective of the state (closed or open) of the clutch. Inherently, this supply of the ATF also helps in maintaining the disks surfaces clean, and provide smoother performance. However, the use of ATF reduces the power transmission capacity of the wet clutches due to reduction in the coefficient of friction, as compared to dry clutches. Therefore, in order to reach the required power transmission capacity, the clutches are assembled with several pairs of friction disk (FD) and separator disk (SD). This arrangement of disks is termed as multi-disks wet clutch. During the operation of an AT, some of the clutches are closed to transmit power while the remaining clutches are in open state. In addition both the sets of closed and open disksare rotating surrounded by the ATF. Ideally, when the clutch is in open condition no torque should be transmitted. However, in reality, the relative motion between the FD's and SD's causes viscous shearing of the fluids film in the gap between the disks resulting in a drag torque on both the disks. Fluids collectively refer to the ATF and the mist (suspension of ATF in air). Depending upon the configuration of the clutches in an AT, the drag torque transmitted by the open clutches is considered as loss. This unwanted drag torque reduces efficiency and increases temperature of the transmission. The increased temperature in turn promotes the wear in the gears, bearings, and the aging of the ATF. Currently automotive designers are concerned about how to reduce the drag torque in open wet clutches because it is one of the potential areas where the improvement can be made to increase the efficiency, performance, and lifetime of systems incorporating wet clutches [ 2]. This requires a good understanding of the physical phenomenon and the drag torque characteristics of the open wet clutches. The articles addressing the issue of drag torque through experimental observations are limited in open literature. Some of the old published reports in this regard are as follows. In 1971, Schade [ 3] carried out several experiments to understand the effect of disk grooved pattern on the behavior of drag torque. Subsequently, Lloyd [ 4] in 1974 investigated the influence of parameters like ATF flow rate, 2013-01-9073 Published 11/15/2013 Copyright © 2013 SAE International doi:10.4271/2013-01-9073 saefuel.saejournals.org Experimental Characterization of Drag Torque in Open Multi- Disks Wet Clutches Shoaib Iqbal, Farid Al-Bender, Bert Pluymers, and Wim Desmet Katholieke Universiteit Leuven ABSTRACT The objective of the present study is to experimentally characterize the drag torque in open multi-disks wet clutches. For this purpose, two groups of experiments are performed using an advanced SAE#2 test setup. The experiments are designed under real conditions of variable automatic transmission fluid (ATF) flow rate and higher clutch speed range as experienced by wet clutches in real operation. The first test group examines the influence of parameters such as ATF flow rate, ATF temperature, and disks size whereas, the second test group investigates the effect of disks rotational conditions on the drag torque. By analyzing, the obtained experimental results, the relationship between the amount by which the parameters are v