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Syncro wheel rotation
Posted: 16 Sep 2011, 09:02
by PetenAli
When we bought our Syncro it had 215/75/15 BFG ATs on Audi alloy rims. The vendor kindly provided me with a 205 15" Colway spare on a steel rim - good job that I've not had a puncture!
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 ยป 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
Re: Syncro wheel rotation
Posted: 16 Sep 2011, 21:09
by slobbo
I would like to understand what "transmission wind up" is exactly. I believe it is the transmission oil cooks because the differential in tyre size causes the 4wd to engage more often than it should on the tarmac. Is this right?
Re: Syncro wheel rotation
Posted: 16 Sep 2011, 21:12
by slobbo
Just found this on Wikipedia
4WD vehicles without a centre differential should not be driven on dry, paved roads in four-wheel drive mode, as small differences in rotational speed between the front and rear wheels cause a torque to be applied across the transmission. This phenomenon is known as "wind-up", and can cause considerable damage to the transmission or drive train. On loose surfaces these differences are absorbed by the tire slippage on the road surface.
A transfer case may also incorporate a centre differential, allowing the drive shafts to spin at different speeds. This permits the four-wheel drive vehicle to drive on paved surfaces without experiencing "wind-up".
Re: Syncro wheel rotation
Posted: 16 Sep 2011, 21:20
by syncropaddy
slobbo wrote:I would like to understand what "transmission wind up" is exactly. I believe it is the transmission oil cooks because the differential in tyre size causes the 4wd to engage more often than it should on the tarmac. Is this right?
Nope .... Transmission wind up occurs when the 4 wheels are not allowed to run at their individual speeds whilst turning a bend. On a syncro this wind up occurs when the VC has engaged, causing one wheel to turn at the same speed as one other wheel thereby causing mechanical stress on the system. It can be seen sometimes when that wheel 'jumps' and is felt through the steering wheel.
Difference in tyre sizes and transmission oils have little to do with this occurrence and it is usually caused by a tight VC which itself is due to the fact that its becoming knackered or its just a tight one!
Re: Syncro wheel rotation
Posted: 16 Sep 2011, 21:28
by slobbo
So why does tyre size make a difference?
Re: Syncro wheel rotation
Posted: 16 Sep 2011, 21:33
by syncropaddy
slobbo wrote:So why does tyre size make a difference?
It doesnt unless the difference in diameter between the tyres on one axle and the other is 6% +. There is a train of thought that 2mm + will knacker your VC but this is nonsense and seems to apply only on Swedish sourced vans. Im told that the Swedish importer added this sticker on the dashboard to keep his VC warranty cost down. The man who knows is no longer with us to answer that Im afraid
Re: Syncro wheel rotation
Posted: 17 Sep 2011, 13:10
by PetenAli
Thanks for getting back Syncropaddy. Thats been very reassuring - my 3mm difference in tread (6mm difference in diameter) is well under 6% of the difference between the fronts a rears. I will keep up the tyre rotation now on a 6 wheel pattern as per Bill Davidson and that should keep things OK for the future - hopefully!!
Thanks again,
Pete
Re: Syncro wheel rotation
Posted: 17 Sep 2011, 15:32
by PetenAli
Re: Syncro wheel rotation
Posted: 17 Sep 2011, 15:58
by syncropaddy
I would put the new ones on the front .....
Re: Syncro wheel rotation
Posted: 17 Sep 2011, 16:11
by ninja.turtle007
syncropaddy wrote:I would put the new ones on the front .....
Why?
I would have put them on the rear. My thinking is the rear will wear faster so will catch up with the fronts
Re: Syncro wheel rotation
Posted: 17 Sep 2011, 16:32
by Mickyfin
Some good advice in this here thread. I used to specialise in 4x4 systems due to restoring the old Cavalier Gsi2000 4x4 models back in the UK, mostly ex BTCC models, and can confirm on a lot which has been posted here. 6% different on the same axel is pretty much your maximum difference allowed, however, its a two edge sword, and one which I would rather take the air of caution even with a spare wheel being used. I got through a transfer box in next to no time having the wrong tread on a spare I used after collecting a Gsi2000.
Re: Syncro wheel rotation
Posted: 17 Sep 2011, 18:57
by syncropaddy
ninja.turtle007 wrote:syncropaddy wrote:I would put the new ones on the front .....
Why?
I would have put them on the rear. My thinking is the rear will wear faster so will catch up with the fronts
It be the other way round on a Syncro ........ !!
Re: Syncro wheel rotation
Posted: 17 Sep 2011, 19:33
by ninja.turtle007
syncropaddy wrote:ninja.turtle007 wrote:syncropaddy wrote:I would put the new ones on the front .....
Why?
I would have put them on the rear. My thinking is the rear will wear faster so will catch up with the fronts
It be the other way round on a Syncro ........ !!
Sorry, I don't understand. I thought 95% power went to the rears unless off road/slippery surface.
Re: Syncro wheel rotation
Posted: 17 Sep 2011, 19:35
by slobbo
Your fronts wear faster cause you turning with the fronts.
Re: Syncro wheel rotation
Posted: 17 Sep 2011, 19:39
by ninja.turtle007
slobbo wrote:Your fronts wear faster cause you turning with the fronts.
But you use the rears for drive.
I've ridden motorbikes for years and the rear tyre always wear before the front. Sometimes 2 rears to one front.
