VC Advice

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VC Advice

Postby ninja.turtle007 » 03 Aug 2018, 14:28

Hi,

I need some expert advice please.

Today we got stuck in sand and this is the first time we have recorded the wheels in motion other than in soft mud.

Looking back at the video, with both diff-locks engaged I was expecting to see the rear and front wheels turning at the same speed.

Is what you can see normal or do I have a VC issue?

TIA

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Re: VC Advice

Postby silverbullet » 03 Aug 2018, 17:11

Replied on fb but for the benefit of those who don't fb I wrote the following:

VC is hardly transmitting any torque to the front diff i.e. almost "failed open"
Try not to do that again, the high differential rpm between the propshaft and the front diff will damage the VC centre spindle and casing.

PS Have I supplied a refurb VC for your van?

PPS I could probably sort you a VC outright, subject to where it needs to be shipped (I have the choice of several agencies) but I am pretty busy with work.

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Re: VC Advice

Postby silverbullet » 03 Aug 2018, 17:30

Having said that, Paul's beach rescue video at 11.25 looks like the rear wheels are spinning faster than the front.
Might be the infamous "syncros dont work on sand/shingle" scenario?

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Re: VC Advice

Postby ninja.turtle007 » 03 Aug 2018, 18:06

Thank you for your reply Ian.

This VC was a refurb about 7 years ago.

The driving conditions here are extreme. We are in need of a new set of tyres, not because the tread is worn too much, but they are exhausted.

I'd rather keep the conversation on here rather than FB.

Moving my posts.

This was after a hot 300km drive. I could feel us getting stuck but had nowhere to go. We do our best to avoid any obstacles.


Another question. I've always run about 6psi more in the rear tyres. Recently I read that the Bentley recommends same pressure all around. For the last 1500km, I have had equal pressures.

Now I'm confused. Have the equal pressures cooked the VC?
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Re: VC Advice

Postby silverbullet » 03 Aug 2018, 18:11

Ok lets keep it here.

Different pressures are necessary due to different axle loads (more on the rear) and that has an affect on tyre temperatures across the tread width.
The VC is designed to accomodate small axle speed differences, thats the whole point of it.
If the VC has done more than 100,000km and never been serviced, its probably on the way out by now.
Mine had done 102k miles (before I pulled it) and was pretty tight when hot after a motorway run.

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Re: VC Advice

Postby ninja.turtle007 » 03 Aug 2018, 18:14

We are currently near Astana but expect to be in Semey in the next few days.

I have a care package leaving Hastings on Monday for us to collect in Russia.
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Re: VC Advice

Postby silverbullet » 03 Aug 2018, 18:15

Was the VC an old stock one? Some of those have been known to fail prematurely, quite why I do not know.

This still might be an example of the rear axle digging in before the VC can fully reach "hump" temperature and transmit full torque to the front axle.

Those who have lots of dry sand driving experience may be able to shed more light on that scenario.

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Re: VC Advice

Postby silverbullet » 03 Aug 2018, 18:19

Its now the weekend so nothing is going to happen until Monday anyway.

Probably best to not jump to conclusions, this may just be why VW specced the non-VC/solid shaft and decoupler for desert region markets.

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Re: VC Advice

Postby ninja.turtle007 » 03 Aug 2018, 18:21

I would say the current VC has always been a little tight. Tyre scrub at low speed turns.

Since running equal pressures the tyre scrub has stopped.

The problem we are facing is we expect to be in Mongolia in the next 4 weeks. As you probably know, the weather there is extremely unpredictable.

Snow, rain and serious subzero temps are common in as early as September.

This is why we are trying to source more aggressive tyres. But now I can't be confident in our VC.
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Re: VC Advice

Postby jes*b » 03 Aug 2018, 18:29

Run a few of the standard VC tests.....
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Re: VC Advice

Postby ninja.turtle007 » 04 Aug 2018, 04:33

I have just read back through your posts Ian, 6-hour time zone difference meant I may not have picked in your every detail.

The VC was from BW, about 7 years ago. It's now just over 80kms.

I have just increased rear pressures.

Foremost I'm concerned about my families safety. I have enough experience to (mostly) avoid situations where I know we will get stuck.

In the last 13 months we have only had one situation where we had to be towed out of the mud. This was due to a car that had crashed on our exit route so we had no choice other than to chance it.

That said, if the VC is failing, outcomes are less predictable.

In an ideal world we would carry a spare of everything.

What would you do in my situation?

My gut feeling is to exchange the VC.
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Re: VC Advice

Postby silverbullet » 04 Aug 2018, 09:04

jes*b wrote:Run a few of the standard VC tests.....
I second this, as your starter for 10.

Tight VC test: Get the drivetrain warmed up and find some hard standing, then do the 1st gear/full lock/idle test.
A healthy VC won't pull the idle down to stalling, although with a tdi this may need more careful interpretation.

Failing open 4x2 test: again on level hard standing, without a trolley jack its not possible afaik. You need to see if the van will pull itself along by the front wheels. If it does, then the VC is working.

You really do need to do both tests to get the full picture, otherwise its just guessing.



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Re: VC Advice

Postby Aidan » 04 Aug 2018, 09:12

I'd go for a refurbished unit if possible, VW originally advised 90k km as service life for a VC, experience shows that in a well maintained vehicle that has had the tyres rotated and looked after they can do over 100k miles but also that they can be cooked in 20-30k miles if running mixed tyres/poor pressures
Behaving odd in sand can be a sign that VC is on the way out
If you jack one front wheel vehicle in gear handbrake on engine off how much force is required to rotate the wheel, if it won't turn or needs a heck of a lot of force to even move it then VC is tight, if it spins easily then VC is failing open, it should rotate slowly with a reasonable constant effort

Keep tyre pressures as specified, rears harder than fronts, VC in good condition can 'dump' 6% variation in axle speeds which is the equivalent of being on full lock - on non standard tyres like 215/75/15 I use the 205R14C pressure 2.5 bar front and 3 bar rear with the BFGs seems to work well no wear issues on the tyres and VC seems happy, correct tyre pressures should mean effective rolling diameters are consistent as weight isn't 50/50 though quite close
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Re: VC Advice

Postby silverbullet » 04 Aug 2018, 10:28

Side note: "more aggressive tyres" ?

Lower temperatures require softer (winter) rubber compounds to maintain grip afaik.

Dare I say it but this is more like Expedition Portal forum territory, unless I am badly underestimating the syncronauts knowledge base?

Input from northern/central/eastern european members?

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Re: VC Advice

Postby ninja.turtle007 » 04 Aug 2018, 12:43

Aidan wrote:I'd go for a refurbished unit if possible, VW originally advised 90k km as service life for a VC, experience shows that in a well maintained vehicle that has had the tyres rotated and looked after they can do over 100k miles but also that they can be cooked in 20-30k miles if running mixed tyres/poor pressures
Behaving odd in sand can be a sign that VC is on the way out
If you jack one front wheel vehicle in gear handbrake on engine off how much force is required to rotate the wheel, if it won't turn or needs a heck of a lot of force to even move it then VC is tight, if it spins easily then VC is failing open, it should rotate slowly with a reasonable constant effort

Keep tyre pressures as specified, rears harder than fronts, VC in good condition can 'dump' 6% variation in axle speeds which is the equivalent of being on full lock - on non standard tyres like 215/75/15 I use the 205R14C pressure 2.5 bar front and 3 bar rear with the BFGs seems to work well no wear issues on the tyres and VC seems happy, correct tyre pressures should mean effective rolling diameters are consistent as weight isn't 50/50 though quite close


I'm feeling this.

My tyres always seem to wear evenly front and rear.

Would this be the same as sticking a spanner on the prop?

We keep looking for a postcard for you but no luck so far!! :ok
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