BX 4x4 transmission

grgabx · Post by **grgabx** » 10 Oct 2005, 20:16

hello. i was surfing on the internet searching for information about BX 4x4
transmission. a few
weeks ago I've bought one BX 4x4 with ABS. i don't know how exactly does the
transmission works.
I've read that there is a difference between transmission with and without ABS
system. I also read
that it is permanent. well, today I went to my mechanic and we lifted the car. then
we started the
engine and put it in 1. gear-ONLY FRONT WHEELS were spinning.when I pressed the
switch for cent.
diff. blocking, all wheels started to spin. is this normal that only front wheels are
spinning when
the car is "in the air" without touching the ground? does the back wheels
automaticly turn on when
ther is ground pressure, like in normal ride? I am sorry for asking you so much
questions but I am
not shure if something is broken in transmission.please if you know about it, reply.

here is the text from the test they made:
The four-wheel-drive arrangement is deliberately simple and, unsurprisingly, closely mirrors the less sophisticated of the two systems developed by Peugeot for the 405. Torque split, assuming equal grip being available for all four tyres, is 53 per cent front, 47 per cent rear. Citroen tried other splits but found the slight front bias desirable for retaining the basic feeling of stability associated with front drive.

Unchanged is the standard five-speed gearbox but bolted on to this is a transfer box incorporating both the front and centre differentials. The centre diff can be electronically locked by means of a small rocker switch located on the centre console. For the basic 4wd, a limited slip differential is fitted to the rear but this is surpplanted by a Torsen diff if the optional anti-lock braking system is ordered. A limited slip works on the overrun as well as under power, negating the effectiveness of anti-lock. The Torsen (or torque sensing) diff works only when power is fed through it and freewheels on the overrun.
Grga-Croatia

fastandfurryous · Post by **fastandfurryous** » 10 Oct 2005, 20:29

Because the driveline to the front axle is shorter, and has less resistance, the centre differential will turn the front axle only if the wheels are off the ground. When you engaged the differential lock, the front wheels will have turned at half the speed they were at beforehand.

To confirm this, if you have the wheels off the ground, engage first gear and let the engine idle, the front wheels will turn.

Now, being very carefull, apply some resistance to both front wheels (will need two people and some thick gloves). The rear wheels will now start to turn, indicating that the center differential is fine.

Kowalski · Post by **Kowalski** » 10 Oct 2005, 21:12

fastandfurryous wrote:Because the driveline to the front axle is shorter, and has less resistance, the centre differential will turn the front axle only if the wheels are off the ground. When you engaged the differential lock, the front wheels will have turned at half the speed they were at beforehand.

Really, I'd expect the resistance to front and wheel drives to be roughly equal, (the only differences would be the oil seals into/out of the transfer case and rear diff and the joints on the driveshaft to the rear end) and I would expect both sets of wheels to turn with a normal diff BUT a Torsen diff is not a normal diff. Torsen diffs are complicated things indeed, they split torque but they need resistance on the things they're driving to work properly i.e. with no load they don't torque sense very well at all.

One of these two links will explain them better than I can (although I don't think either of these two gives a full explanation).

http://en.wikipedia.org/wiki/Torsen
http://auto.howstuffworks.com/differential10.htm

To confirm this, if you have the wheels off the ground, engage first gear and let the engine idle, the front wheels will turn.

Now, being very carefull, apply some resistance to both front wheels (will need two people and some thick gloves). The rear wheels will now start to turn, indicating that the center differential is fine.

Isn't the BX handbrake on the front wheels?

It'd be safer to use that than using your hands, you shouldn't need a lot of application of it to stop the front wheels and to get the rears going.

fastandfurryous · Post by **fastandfurryous** » 10 Oct 2005, 21:23

The driveline between the centre differential and the front differential is about 3". The drive to the rear diff is about 7' and has a number of support bearings. It's entirely possible that the rotational resistance of these bearings is enough to prevent the wheels from turning when the car is off the ground.

I wasn't aware that we were talking about a torsen centre differential.
The 405 MI4x4 setup had a torsen centre diff, but the 405 GL X4, and the Citroen BX had a standard lockable centre diff. The way of telling is that a torsen diff doesn't have a seperate diff lock. The OP has already mentioned that the Limited slip / Torsen differential is on the rear axle, not the centre.

Using the handbrake would indeed be a little safer. I forever forget that the handbrake is on the front axle!

Kowalski · Post by **Kowalski** » 10 Oct 2005, 22:04

fastandfurryous wrote: The driveline between the centre differential and the front differential is about 3". The drive to the rear diff is about 7' and has a number of support bearings. It's entirely possible that the rotational resistance of these bearings is enough to prevent the wheels from turning when the car is off the ground.

The resistances would add up, but really I would have expected all of the wheels to turn and clearly they dont. Its possible that the BX in question was dragging rear brakes or the oil in the different boxes was different viscosities, lots of potential reasons. Driveline resistances are funny things, I've had the JCB up on its jacks and had all of its wheels turning when it was out of gear!

Using the handbrake would indeed be a little safer. I forever forget that the handbrake is on the front axle!

If you got rid of that Peugeot of yours and drove a Citroen dailly I'm sure you'd remember (and maybe you'd leave embarrasing green puddles more often too)!

fastandfurryous · Post by **fastandfurryous** » 10 Oct 2005, 23:28

Kowalski wrote:If you got rid of that Peugeot of yours and drove a Citroen dailly I'm sure you'd remember (and maybe you'd leave embarrasing green puddles more often too)!

Indeed. You want to swap it for your '94? Didn't think so!

anyway... It handles well, and I need an estate car.

Maybe an XM break .... Hmm....

Silly really, as I drive the CX on a reasonably regular basis too, and it isn't until I have to think about it that I remember the handbrake is independent on the front discs.

Driveline resistances do indeed do some interesting things. I'm fairly sure that the citroen quoted torque split for a BX 4x4 of 53% front and 47% rear is bugger all to do with design and more to do with the fact that the driveline to the rear has more bearing grease and gear oil to overcome, and so you get less nett torque at the back wheels than you do at the front ones.

I'm sure I remember reading about the Range Rover limited slip centre differential "applies 50/50 load split in normal driving, but applies more torque to the rear axle under cornering, making the car feel like a conventional rear-wheel drive"

Which is true, but is also un-avoidable. As the front axle goes round a larger radius than the back axle, the LSD attempts to compensate for the slower axle, and applies more torque. Range Rover made it sound like a feature, whearas in actual fact, to make it NOT do this would be quite an achievement!

adamskibx · Post by **adamskibx** » 11 Oct 2005, 01:55

If you want to know if its al working, just rev it, let the clutch up and pull off at full speed- if it skids like mad then its not driving all 4 wheels. It will be working though. There is bound to be more resistance in the rear wheel transmision

fastandfurryous · Post by **fastandfurryous** » 11 Oct 2005, 08:42

Blimey! don't do that! You will very quickly find that you've broken the transmission, causing £££ of damage. The bx4x4 transmission (and 405 4x4) is well known for it's fragility.

In order to use the wheel-spinny method for checking, you need a very slippery surface (wet grass is ideal) and someone else outside the car to watch.

The very design of the 4x4 arrangement means that if the car even moves, then the system is working. When these fail, all you hear is the wizzing-whirring noise of broken gear-teeth.

adamskibx · Post by **adamskibx** » 13 Oct 2005, 18:25

LOL Yeah maybe that would be a bit stupid coming to think of it. Dont suppose it would work that well anyway as the front wheels would scramble for grip anyway due to the differential.

grgabx · Post by **grgabx** » 13 Oct 2005, 20:54

test was succesful. up the hill in reverse gear with handbrake ON- the back wheels started to spin. thanks guys for all helpful advices .

www.citroenklub.hr

fastandfurryous · Post by **fastandfurryous** » 13 Oct 2005, 20:56

All working properly then.

Just don't be too violent with the clutch, or indeed the accelerator in 1st or 2nd gear, and it should be OK.

Dave Bamber · Post by **Dave Bamber** » 13 Oct 2005, 22:47

grgabx wrote:test was succesful. up the hill in reverse gear with handbrake ON- the back wheels started to spin. thanks guys for all helpful advices .
www.citroenklub.hr

Would it be possible for you to post a few pictures of the transmission set up at the front and rear including the drive shaft to the rear wheels? I'd like and I'm sure a few others on here would like to see how it's put together.

Kowalski · Post by **Kowalski** » 13 Oct 2005, 23:24

The BX 4x4 is famous for its fragility, its fairly ironic but one of the ways you can kill a 4x4 or at least put it under great strain is driving off road with it. If you get a wheel spinning because its on a slippery patch and then that wheel finds some grip it can get a rather large shock load on it suddenly it slows down, and thats a killer for your transfer box, rather like dumping the clutch with the throttle open.

I suspect that with the transmision being fragile it might also be sensitive to having the correct type and level of oil in it too, something else for you to worry about / keep an eye on.

fastandfurryous · Post by **fastandfurryous** » 13 Oct 2005, 23:45

The whole point of the BX 4x4, and the other trend 4x4 vehicles at the time (Pug 405, Vauxhell chavalier, Fraud Bathtub.. sorry, Sierra) was nothing to do with off-road ability, it was to do with on-road stability in adverse conditions

The Audi Quattro was being marketed as a safe car, and with 4x4 and abs it certianly was. Other manufacturers were trying to take a slice of this market. To take any 4x4 "normal car" off-road any more than a rough track would be foolhardy. After all, none of these vehicles had anything more than standard suspension strength, and no more articulation.

grgabx · Post by **grgabx** » 14 Oct 2005, 18:53

http://home.wanadoo.nl/cwdegier/images/ ... ne-600.png
this is one of the drawings