Toyota all-wheel drive. V-Flex

© Toyota-Club.Net
Jan 2020

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).

bB 302000-2005
Ist 602002-2007
Porte 102004-2012
Raum 101997-2003
Raum 202003-2011
Starlet 801989-1996
Starlet 901996-1999
Tercel / Corsa / Corolla II 401990-1994
Tercel / Corsa / Corolla II 501994-1999
Vitz 101999-2005
Will Cypha2002-2005

V-Flex I
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 502002-2014
Probox / Succeed 1602014-..

V-Flex II
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

Avensis 2502003-2008
bB 20*2006-2016
Caldina 215G1997-2002
Caldina 2402002-2007
Camry / Camry Gracia / Mark II Qualis V201997-2001
Camry V302001-2006
Camry V402006-2011
Carina 21008.1998-2001
Corolla / Fielder / Runx / Allex 1202000-2006
Corolla Axio / Fielder 1402006-2012
Corolla Spacio 1202001-2007
Corona 21012.1997-2001
Matrix 1302002-2006
Passo 10*2004-2010
Passo 20*2010-2016
Passo 700*2016-..
Pixis Epoch LA310*2012-2017
Pixis Epoch LA360*2017-..
Pixis Joy*2016-..
Pixis Mega*2015-2022
Pixis Space*2011-2017
Premio / Allion 2402001-2007
Premio / Allion 2602007-2014
Ractis 1002005-2010
Sienta 802003-2015
Tank / Roomy*2016-..
Vista 501998-2003
Vitz 902005-2010
Will VS2001-2004

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
· ATC/DTC · V-Flex · DTV · i-Four · Torsen · E-Four ·
· C.Diff Auto A241H / A540H · C.Diff Lock · C.Diff Visc · FT-Visc ·