I have now managed to find some more of the Audi rims and have put new BFGs on two of them with the plan of doing a six wheel rotation as per the Bill Davidson article on the Wiki (this seemed a good idea because using this rotation each pair of tyres is always on the same axle). It then dawned on me that the new tyres (which are currently on the back) have a greater rolling radius than the fronts and would this damage the VC? The new tyres have a tread depth of 12mm and the old ones (on the front and the two spares) are 9mm.
I also have a decoupler and while I don't want to get involved in the controversy that they cause it does seem that leaving it open on tarmac journeys would be relevant to this issue.
I've done some searches on here and come up with the following which seems to indicate that I should be OK with this arrangement...
Re: Spare wheel on the front?
Postby syncropaddy » 10 May 2010, 18:40
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'd be very grateful for any comments that the engineers and other knowlegable people on here may have.
Thanks,
Pete
