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Center differential

Old 09 January 1999, 01:58 AM
  #1  
Anders
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Join Date: Mar 1998
Posts: 751
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I have never received consistent advice about the differential **** in my 22B.

The wetter it gets the more i role it forward.
1 click rain.
2 clicks heavy rain.
3 clicks monsoon.
4 clicks rain+mud/leaves.
All the way forward for snow or a carpet of hail stones.

How does it work?
Am I kidding myself?

Some people say "don't touch it unless you are doing a gravel stage!"
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Old 10 January 1999, 10:11 AM
  #2  
Charlie
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Join Date: Oct 1999
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I've heard all different advice in the centre diff too, until I went to the garage and asked...

Fully forward locks the centre diff, so this is great for snow and ice. Not so good for tarmac ;o)

Fully back gives a rear wheel bias. Apparently, it's about a 40-60 rear wheel bias.

In between is, well, in between.

To emulate a 'normal' Scooby, apparently halfway is about right, which gives a 50-50 torque split.

I believe Phil Gardner has done extensive experimentation with the handling under different settings of the centre diff (in a type R), but unfortunately, I don't believe I have his results anyore...

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Old 11 January 1999, 09:17 AM
  #3  
MarkO
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Join Date: Oct 1998
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Phil's experimentations were as follows (I'm sure he won't mind me lifting this from his SIDC post and adding it here....

The tramlining and weaving under braking (squirelling) has disappeared. Turn-in is sharper still and front end grip feels better, although I have only driven the 80 or so miles back from the garage, at night in the wet, so I can't really tell for sure.

As far as oversteer is concerned, well there's heaps to be had. As it was late and the roads wet and gooey, I considered it best (and in the interests of science) to loop round a couple of handy roundabouts connected by a curved dual carriageway, a few times. Well about 5 times actually, whilst playing with the Centre Diff control;

IN PRACTISE...
--------------

With the switch in the open position (green dash indicator at bottom of bar, coloured full green), the car drives like a rear-drive vehicle. Booting it in a bend will unstick the back, all drive is sent to the axle with the least resistance (ie: the back), the tyres light up and the a big broadside soon develops. Back off quickly and it snaps back into line (safe but a bit uncomfortable for passengers and not too cool looking), but balance the
power and the drift then continues for a while, nicely sideways and feels good into the bargain.

As you progressively wind the **** forward, more drive is sent to the front axle, to the point at which the handling feels just like my UK car (about 2/3rds to 3/4 forward I would say).

Fully forward (LOCKED on the dash bar) should not be used on the road as the axles are locked and transmission wind-up will occur.

TECHNICAL THEORY...
-------------------

The STiV (in this particuilar case) has 3 controlled differentials; an automatic viscous unit in the front (might actually be mechanical but I doubt it), an automatic mechanical LSD in the rear (you can really feel this one doing its stuff) and a manual, electrically controlled centre diff. UK cars have automatic viscous units in centre and rear positions only, the front retaining an open diff.

The centre and rear diffs on STi Type-R's are the ones that have the big effect on handling. In the OPEN position (bottom green sector of dash bar), the diff is just that - fully open. Torque will be transferred to the axle with least resistance (ie: rear normally) and handling will be rear biased accordingly. Torque split is 36:64 front:rear, so lighting up the back tyres is easy. In addition to this, the mechanical LSD in the rear axle locks both rear driveshafts together very quickly once slip is detected, spinning both rear wheels and removing nearly all rear lateral stability, ie: the car fish-tails if asked to. Compare this to a Ford Crapi that would light up one wheel only and go nowhere.

As the centre control is wound forward, more locking is applied to the diff. Slip is controlled closely ensuring that proportionally more drive is sent forwards, thus reducing the rear-drive tendancies. I have found that about 2/3 to 3/4 forward provides handling similar to the UK car.

Should the front wheels start to slip, the viscous unit between the driveshafts will progressively lock-up, ensuring that full drive is available to both wheels.

If the centre control is fully forward in the yellow 'lock' position, then the centre diff is no longer a diff as it is 100% locked. Torque split is then 50:50. This is identical to locking a centre diff in a Land Rover for example. You cannot negotiate tight bends or roundabouts as the axles require differential slip and there is no diff operation to oblige. On slow bends on the road, you will plough straight on, but on high speed, loose surfaces (rally stages, big fast sweepers at the Ring etc), stability might be aided. As you can guess, this locked position should be used only off road....

.... or in heavy snow actually, when going up hill or pulling away. The LSD's in this situation will lock each end, and the centre lock will tie both ends together - useful and actually full, permanent 4WD. Trying to negotiate a snowy corner in this guise will not work for obvious reasons.


RECOMMENDATION...
-----------------

Dry, normal driving: CENTRE OPEN

Wet, 'progressive' driving: CENTRE 2/3rds LOCKED

All other: Anywhere between the two.

But remember your mechanical sympathy when it comes to using the LOCK facility. You'll have no comeback on a dealer if you return the car with a wrecked transmission because you were playing with your new facilities! You could buy a shotgun and kill yourself, but that's only because you were stupid, not the shopkeepers fault!
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