V70 AWD Transmission: 2 new transfer boxes in 2 weeks.!

[Yosser]

If you do have to 'mix and match', then the new tyres should be fitted to the front (contrary to normal advice for other cars).

My understanding is that the viscous coupling can tolerate some difference, but the fronts should not turn at a greater rate to the rear - i.e. the more worn tyres should be at the back.

Ideally you should start with 4 tyes of the same type and then rotate them to promote even wear over their service life.

[TiredGeek]

Originally Posted by ronald ray gun

I measured the RADIUS of the FRONT wheels (from the ground to the centre of the wheel) and they're both more or less bang-on 12 inches. Then I did exactly the same measurements on the REAR wheels - both more or less bang-on 12.7 inches. SO... here is a BIG difference between front and rear tyre sizes - The fronts are about 1.4 inches diameter smaller than the rears.

I am VERY surprised it's as much as that, I'd have thought 10mm DIAMETER was about what to expect (and that would cause problems) but ~35mm, certainty, that's been the problem.
I wouldn't move the car more than 10 yards with that kind of difference!

Welcome back to the world of AWD , glad we managed to help you, now swap them tyres front to rear on the same side every 6 - 10k (and if you've got a full size spare, include that in the cycle)

[david philips]

when you wrote I measured the RADIUS of the FRONT wheels (from the ground to the centre of the wheel) try measuring from the center to the top , this is the radius of the tyre remember the weight of the engine is on the front.

or could you put a rear wheel on the front and take a measurement , hate to see you buy new tyres and find the same measurement as before, i may be wrong and will gladly stand to be corrected.ttfn.

[Brendan W]

Would have said the dimension to the floor was the effective rolling radius. Have to wonder about the tyre pressures. That difference is remarkable.

[ronald ray gun]

My new Yokohamas arrived yesterday. Before they were fitted I measured the circumference of a Yoko as 85.5 inches. Then - after the Yokos had been fitted I measured the circumference of a Goodyear Eagle as 83.5 inches. This must be a far more accurate measurement than me trying to measure the radius with the tyres fitted to the car. Anyway... that's a 2 inch difference in one revolution and the smaller ones were fitted to the front.

So... now I have 4 identical Yokos fitted - 2 brand new on the front and 2 on the rear that have only done a couple of hundred miles. Immediately after they were fitted I drove a couple of miles at high speed, then straight back on the ramp. Transfer box is warm as you would expect and the viscose coupler is just slightly warm.

The difference in circumference is quite surprising and can't be the result of tread wear, as the Eagles have only covered around 1k miles. If my maths are correct then the diameter of the Goodyear Eagle is approximately 26.6 inches and a Yokohama is approximately 27.2 inches - just over half an inch difference.

I got some secondhand information from a friend of a friend. He explained the difference in tyre sizes was because the Goodyear Eagle is not a 4WD tyre but the Yoko's are. I'm not really sure that I buy this explanation but I can't find a better one.

Thanks again

John

[loopie10]

Originally Posted by TiredGeek

Right, viscous AWD 101

Early cars with the viscous coupling are very sensitive to differences.
This is how your AWD works: your viscous coupling has a fluid inside which when both ends are rotating at the same speed remains viscous, however, when there is a difference in the speeds of the input and output shafts, which means the shafts from front to rear of the car, the fluid starts to, lets say harden. Once the fluid has hardened then the two shafts are essentially locked together and rotate at the same speeds. Thus transferring power from the front wheels to the rear as now they are all locked together. (it's not quite as cut 'n dried as that, the fluid "drags" even if it's not fully locked and transfers some power to the rear if there is a very small speed differential)
So, you're in snow, the front wheels start to spin, the viscous coupling "locks up" and you get power to your rear wheels and you drive off. Perfect.

Now imagine a small difference in diameter of the tyres, even 3mm. Drive at 50mph and each end of the car the tyres are rotating at a different speed.....
So your viscous coupling starts to "think" the front wheels are slipping and starts to lock up as described above, and is designed to do. Once it has then there is mechanical wind-up in the prop shaft between the front and rear.

If you're on mud or snow then there will be a slight slippage at the tyres and the force is dissipated, but you are on tarmac so this can't happen.
The next weakest point will break to relieve the strain, which unfortunately, and expensively, is either the angle gear or if you're lucky the splined coupling between angle gear and gearbox.

If you think this isn't a known problem, just google for Range Rover mysterious crashes from the '80's. I'm sure it will bring up many hits. They changed the transfer 'box design to a viscous coupling and then found the vehicles were having crashes on motorways where they would spin for no apparent reason. Turned out to be related to tyre pressures / tyre diameters etc locking up the viscous coupling and causing mechanical wind up in the transmission. They'd made the 'box so strong though that eventually the wheels would slip to unload the stresses and spin the whole car round....

In 2003 (AFAIK) Volvo dropped the viscous and went with a Haldex unit, which works in a totally different way and is computer controlled, so doesn't suffer AS MUCH. Later than 2005 the Haldex was refined and they are much less sensitive to a small difference in tyre sizes, but if you're smart you still play safe and rotate your tyres and change as a set!
I swap mine front to rear every 6k or so, just to be sure

Your problem could very well be tyres. Different brands of the same size do actually vary in size: one makers 215x65x16 won't be exactly the same as another makers, fitting one brand to the front and another to the rear could well be the cause.
If I were you I'd jack the car up, take off a front and rear and put them side by side to compare them. Also bear in mind that if one has a softer tyre wall / different pressure then it will compress more and be a different "rolling radius".


Sorry for the long post, hope it helps

yep some wally drove my xc70 with a flat tyre for some 28 miles and the end result was a whining transfer box, now had to remove it and found output splines not rounded but leaning toward thrust side and prop side bearing spitting metal particles all over the place. this box had covered 160k and had no issues before the afore mentioned hairy nosed no nothing dirt bag drove with a flat tyre. Borrowed his car last week while xc was in hospital tried very hard to behave myself but revenge took hold redlined it all the way to mars and back, bloody thing didnt break though!!!!