viscous operation

[MECCANO]

Trying to work out how the viscous operates in BW transfer boxes.

Does the transfer directly drive the rear wheels and then bring in the front when required, or act as a centre diff like an lt230.

Im assuming both rrc and p38 operate the same way.

Manual suggests the input and rear drive is linked via the viscous, aka not directly linked. Can any one confirm.

Ta muchly.

[daslandroverman]

Practical experience running my 110 front wheel drive for a couple of days (halfshaft/drive flange issues) suggests the fixed drive is to the front with the slip to the rear as it would start to creep away -roll isn't really the right word- with the hand brake on, but was fine sat in 'park'.

Seems the manual is right.

[daslandroverman]

Also,it's my understanding that the B/W acts like a solid drive between the 2 axles with the viscous allowing some slip between the two to allow for speed differential between the axles.

[Bowie69]

Fixed to the back, allows some slippage to the front, with much slippage to the front it locks up.

Jack up a front wheel, put wheel brace on a wheel nut and stand on it with transfer case in neutral, you will then get an appreciation how stiff they are.

[David Sparkes]

Trying to work out how the viscous operates in BW transfer boxes.

Does the transfer directly drive the rear wheels and then bring in the front when required, or act as a centre diff like an lt230.

Im assuming both rrc and p38 operate the same way.

Manual suggests the input and rear drive is linked via the viscous, aka not directly linked. Can any one confirm.

Ta muchly.

My reply is based on past extensive reading of the 38A manuals, and having the car in front wheel drive only when the 2 pin rear differential failed.

I note you don't say which vehicle you have.

Does the transfer directly drive the rear wheels and then bring in the front when required, No.

or act as a centre diff like an lt230? No

Manual suggests the input and rear drive is linked via the viscous, Yes

aka not directly linked. No.

On the 38A BW transfer box the rear axle is driven mechanically, via gears and shafts, including a centre differential. From this drivetrain there is a quill shaft running forwards to the viscous coupling. The handbrake operates on the output shaft to the rear propshaft.

The front axle is driven from the other output of the central differential, this is also connected to the 'other half' of the viscous coupling.

Thus the main drive forces are mechanically fed via the centre differential to both axles.

The effect of the viscous coupling is to keep the two output shafts at similar speeds.

If the rear propshaft is removed (pace myself and daslandroverman) the VC 'sees' the rear output shaft turning rapidly, as the centre differential is 'free'. The VC immediately goes into action, limiting the output shaft speed difference, and thus driving the front propshaft. The VC acts so rapidly that any delay is not apparent from the drivers seat.

The handbrake becomes less effective because the handbrake only acts on the rear output shaft. On any sort of slope there will be some VC slippage, so the car will creep down the slope. Take care in queues of traffic.

VC failure mode is to progressively reduce the amount of slippage. It's possible that running with the rear propshaft removed will accelerate failure mode, as there will be constant slippage causing the viscous fluid to heat up and thicken, so that the front axle is driven. This prolonged high temperature state may accelerate the degradation of the fluid.

I would not rely on the test advocated by Bowie69 to assess the state of a VC, as a used (in-service) VC is already some way down it's failure path.

If you want to do a mechanical test, the details are in the RRC Workshop manuals.

Regards.

[Snagger]

The BW unit does the same things in essence as the LT230 - two gear ratios and a 4wd mechanism.

The upper end (input side) has an epicyclic gear set for one ratio, with that set locked in its carrier for the other, much like in the Roverdrive. The output from that gear unit is then transmitted via a chain and sprockets to the output side. It is direct drive to the rear and through a viscous unit controlled diff to the front. Imagine a conventional pneumatic locking diff - you have an open diff with a sliding collar that can be used to lock one half shaft to the diff centre, which effectively locks up the innards of the diff. Well, this is similar in that the unit is permanently in place between the diff casing and the shaft, but it allows a certain slip rate before seizing, having the same effect as a diff lock. The resistance when the diff is "open" is still considerable, and turning a front wheel by hand with both rear wheels on the ground and the hand brake on gives a very slow rotation. If the viscous unit fails, it either fails soft, giving little resistance to that test, or fails locked, preventing any turn of the wheel.

By using the viscous unit to control the diff rather than replace it, it means the viscous unit carries less load and is subjected to half the shear rate between front and rear axle because is is only subjected to the shear between the diff unit and one of its outputs. It can do the same job with less effort. I think the Freelander viscous unit has to contend with the full load without a diff unit.

[David Sparkes]

Snagger, just to aid clarity, do you intend your operation description to apply to the LT230, or the BW box the original question was about?

Regards.

[Tim2809]

I may be wrong but the handbrake is on the output to the rear prop so if it creeps theres an issue with the handbrake not transfer box.

[David Sparkes]

I may be wrong but the handbrake is on the output to the rear prop so if it creeps theres an issue with the handbrake not transfer box.

You are both right and wrong.

When everything is normal you are right.

The business of creeping when the handbrake is applied referred to when the rear propshaft is removed (temporarily, while a problem is fixed).

You will see in my post #5 I was referencing my own experience, and that detailed by daslandroverman in post#2.

Regards.

[Snagger]

I may be wrong but the handbrake is on the output to the rear prop so if it creeps theres an issue with the handbrake not transfer box.

I was going to reply in much the same way as David did. If the hand brake is OK, the rear shaft will be well retarded. Any creep would be due tot he rear wheels not having grip but the front wheels having good traction or the rear wheels being disconnected from the hand brake (prop shaft, rear half shaft, rear diff, drive flange or other conceivable failures). The front axle wouldn't be able to be locked fast like with an LT230's centre difflock.

The twos example above is the only examples of such an issue I have heard of. I don't think it's prudent to drive these units with a rear axle disconnected, not only because of the hand brake issue, but also because it'd probably harm the viscous unit in the long run. But it can be driven that way if necessary, as they have both demonstrated. You'd just need chocks for parking.

[MECCANO]

Cheers guys, I feel educated. So if I understand correctly, the viscous in effect replaces the dog clutch (diff lock) on an lt230. Giving the variabilty with load.

I don't actually own a bw box, but wanted to understand the theory of how they worked.