Centre Diff Binding

Posts specific to the 3.0 litre NA H6 engine

Re: Centre Diff Binding

Postby HardwareBoB » Wed Sep 02, 2015 4:35 pm

I'm replacing it before I fit the gearbox, because it's a hell of a lot easier!
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Re: Centre Diff Binding

Postby tom_kauf » Sun Dec 06, 2015 3:03 pm

I've posted this elsewhere, but this was one of the longest Diff Binding threads, so I'll repeat it here.
Thanks to Dave, who suggested spraying the shocks with Wet PTFE Lubricant (WD40 brand) with the car jacked up, to confirm the source of the noise is the shocks.

It's CONFIRMED - NOISE IS GONE! 8). At least till it wears off, but the source of my noise is now indisputable. I got the PTFE Lubricant from bunnings (link of what it looks like below), jacked up each front wheel and sprayed some in there. Even after a lengthy drive incl driving continual circles in a carpark, there's no noise at all.

http://forum.liberty.asn.au/viewtopic.php?f=18&t=31454
cycleofabuse wrote:Same/similar noise and feel in my car (same car as yours) and it's actually the front shocks. WD40 Wet PTFE provides relief for about a week, but ultimately they need to be reconditioned. I would try that first and if the noise goes away, then at least you've diagnosed the problem without spending a fortune. Just spray some into each shock and go for a spin.

This needs to be saved somewhere for all those with the same issue. No one has suggested this in *ALL* the threads about Clunking noises while turning on this forum.

I've spent years trying to pinpoint my clunking noise. Even spent a small fortune getting my Centre Diff replaced (which did improve things, but I still had this other noise). Several mechanics incl dealer couldn’t find the source - they suspected Shocks but weren't sure.

And I'd recommend to anyone to get the actual Lubricant that Dave suggested, not just using normal WD40. Maybe normal WD40 will work too, but for the sake of $17, it's worth a try.
http://www.bunnings.com.au/wd-40-specialist-300g-high-performance-wet-ptfe-lubricant_p6100411
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Re: Centre Diff Binding

Postby Pylon 500 » Tue Aug 16, 2016 2:32 pm

G'Day All,
Dropped in here some time back, when I realised I was starting to get the binding problem.
All the usual signs, no problems when cold but after a drive, a thumping feeling when doing tight turns (like into my garage)
Cause put down to too much work by the VC centre diff, usually from improper tire inflation/tire diameter/miss-matched tires.
In my previous cars (two 'L' series and an SVX), I would often vary the tire pressures around the car to 'trim' the car for road camber.
I was also doing this with my latest car, 3.0R-Bspec 6MT, untill reading these threads.
I would then religiously check even pressures all round, AND IT MADE NO DIFFERENCE.
So I got to thinking, pressure will dictate tire diameter, so I decided to make sure my diameters were the same all round.
I made a small jig to check hub centre height (weighted rolling radius), and pumped up the tires accordingly.
Because there is more weight at the front wheels, they needed more pressure,
I was running 30psi all round, and found I needed 50psi to get the same hight at the front!
I've dropped the backs to 25psi, which brings the front down a bit 'cause the ride was pretty hard, BUT since doing this, I can do more than an hours driving, and hardly notice any binding!
Going to try and upload picture of 'height jig'...
Notice about picture, I live in Taree, home of Australia's pottholes, I'm sure we export them Australia wide, but they start HERE!
Because of that, I've removed the stock rims and fitted 16" rims with higher profile tires (same outside diameter as original) so survive the ride.
As an interesting side note, when I raised the tire pressure, my speedo is now much closer to the speeds shown on my GPS, was usually around 2~3% faster, ie; 60km on speedo was only 58km GPS.
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Re: Centre Diff Binding

Postby tom_kauf » Tue Aug 16, 2016 2:55 pm

Interesting!! Is the binding still not noticeable when turning at low speed? Because I thought that'd stress the centre-diff no matter what.
But I guess if the centre-diff doesn't heat up during normal diving, then the binding is like it would be when cold (hardly at all).
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Re: Centre Diff Binding

Postby bigBADbenny » Wed Aug 17, 2016 5:07 am

http://www.carbibles.com/tyre_bible_pg3.html
Have a read of this FYI ^

You're taking an unusual approach for sure!
Have you tried verifying your adjusted diameters with eg a rolling paint spot test?
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Re: Centre Diff Binding

Postby BillyCorgi » Wed Aug 17, 2016 8:30 am

Fit a refurbished Centre Diff?

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Re: Centre Diff Binding

Postby Pylon 500 » Wed Aug 17, 2016 8:33 am

Re the high speed/low speed thing, I initially thought there must be a serious design flaw if , what I determined 'normal use' was going to have the diff over work and overheat.
But then got to thinking, 'if the diameters are even slightly different, then the centre diff is always working, and the faster you go, the more it's working'.
This feels like the case to me as I live just off a highway, so I'm high speed till fairly close to home.
Before adjusting, when I got home, there was much thumping turning into my garage.
I mean, I have a bit of slope at my garage, and I found i could leave the car in neutral without handbrake, and get out to open the rollerdoor, and the car would just sit there, held by the bind!
After the adjustment, even straight off the highway, the car rolls quietly into the garage with little effort.

Read through the carbible stuff, good info.
Haven't tried the dot check as yet, basically it's working, so I'm happy.
I remember doing this sort of thing back with my L series to figure out the different from and back diff ratio's.
Yes, they're different!, gives the vehicle the 'front wheel drive' feeling whether in FWD or 4WD, hence the big sign saying 'DO NOT 4WD ON HARD SURFACE'

When I went from the 18" rims to the 16" rims, I used this site to pick the tire size;
http://www.rimsntires.com/specspro.jsp
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Re: Centre Diff Binding

Postby BillyCorgi » Wed Aug 17, 2016 9:05 am

Power = Torque x Rotational speed.

So the energy building up in the diff fluid (or heat) is equal to the torque transmitted thought the viscous coupling multiplied by the rotational speed induced by the mismatch in wheel speeds by tyre diameters and arc paths of tyre travel, etc.

It is the heat build up in the viscous coupling that does the silicone fluid in.

As the silicon fluid begins to go "off" the viscous coupling will transmit more torque and therefore produce more heat for the same rotational speed, up until the point where tyre traction is lost.
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Re: Centre Diff Binding

Postby bigBADbenny » Wed Aug 17, 2016 11:13 am

Bc, would you consider a refurbed uprated fluid diff to have a shorter service life than stock refurbed?
Eg in the 10-15kg/mm region, not quite under steering monster eg 20kg/mm ;)
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Re: Centre Diff Binding

Postby BillyCorgi » Wed Aug 17, 2016 11:51 am

bigBADbenny wrote:Bc, would you consider a refurbed uprated fluid diff to have a shorter service life than stock refurbed?
Eg in the 10-15kg/mm region, not quite under steering monster eg 20kg/mm ;)


First principles evaluation.....
Let's say the there were a new set of tyres on the front of a Spec-B and a worn set on the rear.
Rolling diameter of a new set of 215/45/18 tyres is 650.7mm Tread depth is 8.3mm
Rear tyres are at legal minimum depth of 1.5mm.
Front rolling diameter 650.7mm rear rolling diameter 637.1mm

Vehicle is doing 110kph on freeway.
At that speed the average tyre rotational speed is 906.3 RPM
The centre diff case is rotating at 3535 RPM
The front output shaft of the Centre diff is turning slower and the rear output shaft is turning faster (as the rear tyres are worn)
The rear output shaft is turning at 3572 RPM.
The inner and outer viscous clutch plates are rotating at 38 rpm relative to each other. Or, 3.95 radians per Second

The standard diff will transmit about 40N.m of torque before slipping.
So the power (or heat) generated within the Centre Differential by the viscous coupling slipping due to differential action is
P = T x Omega
= 40 x 3.95 or about 160 watts.

(Note, having just rebuilt the Centre Differential out of Flat6Estate's car, the metal parts were showing signs of blue oxide in the thrust washer area, so things were getting warm?)

160 watts of heat needs to be dissipated by the centre differential assembly into the gearbox oil that surrounds it, remembering that the gearbox oil is also being heated up by the other rotating losses within the gearbox.

Increasing the slip stiffness of the centre differential to 10 to 15 Kg.F (100 to 15o N.m) will increase the heat dissipation of the centre differential proportionally.

Presently, I do not have knowledge of what is the critical temperature that the silicone fluid must reach to commence degradation for the standard fluid or the replacement fluid.
We can see that the driving force is really the mismatch of rotational speeds across the differential.
If a higher force Viscous Coupling Centre Differential were to be used, one should take greater maintenance of the tyres to ensure the same brand and type of tyres are used and that the tyre pressures are set correctly.
Minimising tyre mismatch would be the key to successfully using a higer specification centre differential.
Cheers!
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Re: Centre Diff Binding

Postby bigBADbenny » Wed Aug 17, 2016 5:41 pm

Fascinating response thanks!

What's your take on matching f/r bias or wheel speed via tyre pressure?

I'm intrigued by the proposition of measuring by axle to ground, especially as the member in question has high profile tyres, which heuristically, I'd imagine are more affected by f/r weight distribution.

Eg if the front is heavier, the tyre will be more squashed yet still have the same rolling diameter, assuming similar pressure?

Quite the can o worms :P
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Re: Centre Diff Binding

Postby BillyCorgi » Wed Aug 17, 2016 7:48 pm

BBB you ask interesting questions.

The analysis above on the centre differential speed mismatch effects was simplified in that it only took into consideration unloaded rolling diameter.

There are two other things to consider.....
Static Loaded Radius
Dynamic Loaded Radius.

The two are different and predicting one based upon the other is fraught with, well, errors.
The very nice gauge shown above considers static load radius. Here is an article on things that affect SLR.

Image gifs upload

I would contest that the effect of Static Load Radius change will be different between tyres of different diameter and aspect ratio, but also different between the same brand and model of tyre with different diameter and/or aspect ratio.

What further complicates matters is Dynamic Load Radius.
Here not only does the weight and of the car on the tyre and flexing of the sidewall due to inflation need to be taken into consideration, also the expansion of the tyre diameter due to centripetal forces needs to be considered.
A search for "tyre dynamic load radius" turned up a whole plethora or academic articles that would take me some days to distill.
Intuitively, the transition between the static load radius and the dynamic load radius will vary with the speed of the vehicle, where higher speeds will result in higher centripetal forces on the tyre outer casing.

Setting tyre pressures to try to match the Static Load Radius may take a (small?) improvement step in minimising tyre rotational speed mismatch, however what dominates at speed; static or dynamic; is not understood (yet) by me.

As you say "Quite the can o worms"

I did find this, which is a spreadsheet calculator attempt to handle both Static Load Radius and Dynamic Load Radius
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Re: Centre Diff Binding

Postby Pylon 500 » Thu Aug 18, 2016 6:52 pm

As you say, a can of worms..
I had thought of these and a few other variables to throw into the mix.
The 'lift at speed' thing is a real variable due to the aerodynamics of various cars.
If you have a descent air dam on the front, you would be hoping to add downforce with speed, with a standard car (is a Bspec wagon standard?), you will end up with lift at high speed.
The talk of centrifugal changes to tire shape is real, but hard to calculate due to so many variables.
It can be seen on dragsters at take-off, with tires growing up to 20% in diameter, but remember, these tires are very lightly built, as is the 'rail', and probably rotating much faster than our car tires would.
Another variable is simply your loaded weight, and it's position.
Being a pilot, this comes into my way of thinking about things from a balance point of view, I mean, if you drive the car 'solo' most of the time, you can sense the difference in handling between a full tank and nearly empty tank.
This weight distribution not only affects handling, but also have an affect on the rolling diameters between front and rear wheels.
As also mentioned earlier, I may notice these affects a little more than others because of the high profile tires I'm running.
But, as I've said, since setting the tires by height instead of pressure, the binding problem has been hugely lessoned.
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Re: Centre Diff Binding

Postby bigBADbenny » Thu Aug 18, 2016 10:53 pm

In my case, I noticed alternately reversing and driving straight into my driveway seemed to improve low speed sharp turning.

However at 250k my centre diff is probably a lost cause and a good candidate for a rebuild, which luckily is now a viable option.
And thus my interest in options, upgrades and alternatives.
Given an unlimited budget, I'd get the Cusco Tarmac Diff, an open cdiff with 30/70 f/r torque split.
Next choice would be the STi DCCD diff.
But a CorgiWerks rebuild is definitely the bang for buck no brainer option.

Yet how to spec the diff, and then how to protect the investment?
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Re: Centre Diff Binding

Postby Shaheenis » Sat Aug 27, 2016 7:14 pm

Arthur, good to hear the symptom has gone away for now.

My issue (which has some similar symptoms) has been resolved by fitting new(er) front struts. No more rotational knock/NVH when executing tight turns as experienced previously. The strange thing is mine was always worse when the car was warm and had done a reasonable amount of driving which just doesn't make sense that the struts were the cause (Bilstein knock). At Christmas time when I blew a tyre I decided to replace the whole set despite having plenty of life left in them (~60%) for these same reasons discussed above. Need to look after your diffs ;).
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