Club 80-90 Forums

HarryMann wrote:Andrew, It would be a big dig to find it I think (plenty of other big digital and paper digs on the list too!)

But I've seen it several times, and believe it (or an early estimation of maximum I/O differential rpm) is also in the SAE paper I have by Dr Pesche, who researched the Ferguson/Rolt/GKN patents VW bought and then developed their version of it.

In retrospect, if vehicle prep hadn't been a pressing concern, I perhaps should have produced a History of the VC display for the event.

At the top of course would have been a picture of Tony Rolt, crouched in the loft of Colditz Castle (Oflag IV-c), designing the escape glider

No problems Clive. So a 'noticeable difference between front and rear rolling circumference' as VW put it is 2mm! So if your tyre pressures are out by 5psi on one wheel ......... Something here is still not right.

As I said Andrew, I ran with a lot more than 2mm for a year or so, although tried to combine them diagonally to get the diff-diff cancelling effect you speak of

So I think that figure is probably quite conservative, but that's what one would specify, aimed at long-term peace of mind when it's a bit of a guessing game. I think Dr Pesche did some calculations to show temp-rise in the longer term cruising. I'll no doubt dig at least that paper out soon, and re-read that bit... i

I have turned to the writings of Mr Derek Drew for conformation and this is what I find .....

"The VC engages when the front and rear wheels turn at different speeds, greater than 6% in relative RPMs. When the front and rear wheels turn at a greater difference in RPMs during sharp turns (above 6%), it would seem that the VC would engage and cause some binding."

So on a 185/14 tyre a 6% difference is almost 40mm in diameter or 20mm of tread depth. But standard 185/14 tyres dont have that depth of tread, they would have 8mm maybe 9mm which isn't even 3% difference.
So if you go back to what I indicated at the beginning, if you have 3 bald tyres and one brand new one there will be no effect whatsoever on the VC because it isn't engaging because the rev difference isn't enough. If it IS engaging at that small difference then its knackered anyway!

How many of us have conducted the VC test by jacking up the back of the van, engaging 'G', and letting it go on tickover with the back wheels spinning and the fronts not moving? Again Mr Drew suggests that a good VC will allow this to happen but as soon as you increase the revs the VC engages and the front wheels take up drive as its designed to do. Its also designed to have this margin of 6% to allow for the differences in tyre pressures and wear. The figure of 2mm represents a margin of 0.3% and is far too small to allow the VC to work as designed. At that level you wouldn't be able to turn a corner without the VC engaging as all the wheels are turning at different speeds and the speed of the inside front would vary too much from the outside rear and cause partial lockup of the VC

I am in total agreement with Mr Drew ....... here

toomanytoys wrote: Go on then Andrew you try it and tell us how stiff the vc goes after a few miles...

Ive looked at this Simon and the only wheels I have are 215/75/15 and 225/70/15. This should be enough of a difference so Ill fit one wheel which is 8mm (nominal) larger than the other 3. My VC is good as I had no binding at all on the blast back from Mammutpark to Dunkerque.

I will report back

My VC is tight.. even with 4 evenly worn, at correct pressure, tyres after a good run it scrubs turning... generally drop the prop if going any distance, need to fit the decoupler...

I reckon it would fail miserably in the "VC test"...

Spare wheel carrier on the front of a syncro or any T3 come to that is a no, no
On a bay yes but not a syncro
Appreciate that you are running out of places to put it but not on the front, please.

Oh and there was a pic with a vw sticker saying no more than a 2mm difference in tyre differnces on a particular tristar somebody in the uk has...

PC52 wrote: Oh and there was a pic with a vw sticker saying no more than a 2mm difference in tyre differences on a particular tristar somebody in the uk has...

Paul has one on his dash ...... underneath all the crap that is!

I still say that a 2mm difference is mathematically implausible, 6% isnt

syncropaddy wrote: The handbook speaks of a 'noticeable difference' between front and rear rolling circumference but does not quote a figure, nor does the official VW Workshop manual ........

Why are yellow stickers appearing on random dashboards when there is no mention in the technical manuals?

The Irish tech manuals were all simplified Andrew, you must know that...

HarryMann wrote:The Irish tech manuals were all simplified Andrew, you must know that...

syncropaddy wrote:

toomanytoys wrote: Go on then Andrew you try it and tell us how stiff the vc goes after a few miles...

Ive looked at this Simon and the only wheels I have are 215/75/15 and 225/70/15. This should be enough of a difference so Ill fit one wheel which is 8mm (nominal) larger than the other 3. My VC is good as I had no binding at all on the blast back from Mammutpark to Dunkerque.

I will report back

7.5mm and 8 revs per mile difference.. doubt it will be noticable.. not 22 rpm...

HarryMann wrote:The Irish tech manuals were all simplified Andrew, you must know that...

tut tut Clive

Actually, many years ago when we had our first syncro and spent all of our money on our house and not on the vehicles we run, we stuck a set of 2nd hand 14" mismatched tyres on it, ran it for years and miles without any issues at all. Never did tyre pressures or anything like that at all.

But then now, both of them are checked reguarly and we have a chart to say which tyre is going where on the next change.

toomanytoys wrote: 7.5mm and 8 revs per mile difference.. doubt it will be noticable.. not 22 rpm...

So how would 2mm be noticeable then?

HarryMann wrote:The Irish tech manuals were all simplified Andrew, you must know that...

Hmmmm ... I suppose ...... same ones as used in the UK market .......... ................. ................ .............. Im using the German ones

Is it possible that VW realised that they had omitted some important info from the manual and then produced a sticker for the cockpit area to make sure people knew whats what?

What does the manual have to say about Difflock operation out of interest?

v-lux wrote:Is it possible that VW realised that they had omitted some important info from the manual and then produced a sticker for the cockpit area to make sure people knew whats what?

What does the manual have to say about Difflock operation out of interest?

I reckon it was more like a yellow sticker day was April 1st!!

There are two pages on difflock operation. Ill scan them and PM you