In 2020, thirty years have passed since the presentation of the most uncomplicated Toyota AWD - with the rear wheel drive connected via viscous coupling. The genuine name is フレックス- or Vフレックスフルタイム4WDシステム - but the word フルタイム (fulltime) seems superfluous here.
The first Toyota's scheme with a rear wheel drive coupling, that connecting the two parts of the intermediate propeller shaft, appeared in 1990. It is not correct to apprehend it as a traditional "viscous coupling| - Toyota themself used the designation "rotary tri-blade coupling" and in the world for similar devices the abbreviation RBC became common.
The coupling consists of a main and control chambers separated by a piston. There is a pack of wet friction discs inside the main chamber, alternately connected to splines of the housing or hub. The blade rotor is installed on the hub inside the control chamber filled with a high-viscous silicone fluid.
1 - housing, 2 - discs, 3 - piston, 4 - rotor, 5 - hub, 6 - rear cover
During straight driving the front and rear wheels rotate with equal speed, all parts of the propeller shaft also rotate synchronously, and the coupling components remain stationary relative to each other. During cornering a slight difference of front and rear wheels speed is compensated by a slow rotation of the coupling with minimal resistance. When the front wheels slip, the speed difference becomes significant, the rotor blades mix and displace aside the silicone oil in front of them, creating increased pressure behind the piston, which, in turn, compresses the friction discs - limited slipping allows to distribute torque to the rear wheels.
Toyota said the pros of this solution are different characteristics for different rotation directions (afair, in the old VW Syncro 4WD scheme an overrunning mechanism and even a vacuum drive for disconnecting the rear axle were used to eliminate unnecessary operation of viscous coupling during motor braking or reverse drive).
|Tercel / Corsa / Corolla II 40||1990-1994|
|Tercel / Corsa / Corolla II 50||1994-1999|
The two utilitarian models obtained the same layout with the coupling in the middle of propeller shaft, however, the traditional viscous coupling (similar to V-Flex II) is used, not RBC.
1 - front shaft, 2/3 - support bearing, 4 - viscous coupling
|Probox / Succeed 50||2002-2014|
|Probox / Succeed 160||2014-..|
In 1997 the most widespread of the V-Flex variants appeared - with a coupling between the propeller shaft and rear differential.
1 - viscous coupling, 2 - drive gear, 3 - pinion gear, 4 - differential case, 5 - planet gear, 6 - ring gear
The coupling transmits torque due to viscous friction in an organosilicon fluid, which fills about 80% of the free internal volume (unlike canonical descriptions, the Toyota documentation emphasizes that the viscosity of this oil is weakly dependent on temperature - it doesn't "hardens from heating").
There is a pack of perforated discs inside, alternately connected to splines of the housing or hub. A lot of cuts are made in the discs, creating maximum resistance when mixing the fluid. The viscous coupling is hermetic, mounted on two bearings and closed with an aluminum front cover (ribbed for better cooling).
1 - front cover, 2 - viscous coupling, 3 - hub, 4 - "outer" disc, 5 - "inner" disc
If the outer and inner discs start to rotate at different speeds, then the shear effect occurs (the chains of silicone oil molecules are stretched), creating a resistance force F2 applied to the disc N2 (which rotates faster) in the opposite direction to the rotation. At the same time, a force F1 equal to F2 acts on the disc N1 (which rotates slowly) in the direction of rotation. These forces counteract the difference of discs speed (the amount of viscous friction depends nonlinearly on the speed difference).
If the discs continue to rotate with a large speed difference intensively mixing the fluid, then the silicon oil temperature rises, oil expands and the pressure inside the coupling increases. The inner discs moved along the splines in the direction where the pressure is less, and are tightly pressed against the outer discs, creating the highest possible rotation resistance - the so-called "humping". Since the speed difference disappears, the temperature and pressure in the clutch gradually decrease, and the compressed air bubbles in the fluid expand, pushing the discs apart - the coupling returns to viscous friction. Under normal conditions, without long-time slipping of the front wheels, the hump effect does not occur.
As in other schemes with non-fulltime rear drive, the transfer box is simplified to an angular gearbox.
1 - gear case, 2 - drive gear, 3 - ring gear
|Camry / Camry Gracia / Mark II Qualis V20||1997-2001|
|Corolla / Fielder / Runx / Allex 120||2000-2006|
|Corolla Axio / Fielder 140||2006-2012|
|Corolla Spacio 120||2001-2007|
|Pixis Epoch LA310*||2012-2017|
|Pixis Epoch LA360*||2017-..|
|Premio / Allion 240||2001-2007|
|Premio / Allion 260||2007-2014|
|Tank / Roomy*||2016-..|
The main problem of Flex with RBC is the aging of silicone fluid, as a result of which the proper multidisc pack compression is not obtained and rear-wheel drive does not operate (however, this defect often goes unnoticed).
The V-Flex II problem is similar to ATC scheme - wear of the coupling mounting bearings, resulting in a strong roaring noise from the rear differential side. But the repair is officially prescribed here and the bearings are supplied even as genuine spare parts.
The time of V-Flex is left behind - the only Toyota model with a viscous coupling is Probox / Succeed (all the rest are Daihatsu subcompact cars with Toyota emblem). And the viscous couplings were replaced by a more complex, expensive... but also not enough effective electromechanical ATC.
Toyota all-wheel drive. Review