Some advice please.....

My tyre outlet (and they do a lot of specialist cars) recommends I change all 4 tyres on the WRX even if the rears are fine and they reckon on 4WDs it winds the diffs up and can be costly, althought the had only seen it actually happen on a quattro.

I call my Subaru dealer who reckon they only match them on the same axle....

Any views?

 

It depends on the difference of circumference between the two sets of tyres (front and back I mean). I have read that if there is enough of a difference, a few mm or so, it may cause an issue with the diffs.

I'm sure someone else could better explain than me.

 

Sound like a great scam, get one puncture replace all tyres, you only have to match the tyres on the same axle.

 

as long as the tyres on the same axle are no more than 2mm different in tread you will be fine.

Nice little earner if they can get people to change all for as not required at all

 

Same axle, but no need for fronts and rears to match.

Most scoobs have a viscous centre diff so no problem soaking up the tyre difference.

 

mines got a rear diff and i just changed one wheel, and its ok

 

What does "winding up the diff" actually mean? I kind of get it, but I cant imagine the physical picture. And how does a viscous diff work to aleviate the problem?

Sorry, maybe this is a bit OT and should be in the Drivetrain... please don't give me a strike for being a Troll and posting OT!

 

haha, what a scam. Ask them how these 'fragile diffs' manage in turns when all the wheels are going at different speeds!!!

 

Sounds like rubbish to me (unless there is a severe difference in rolling circumference)
Just change both tyres on the same axel.
I move mine about.
Suspension set up aside....
If I want the car to understear on limits of grip. I move the half worn backs onto the front and run a higher PSI and then get the two new tyres place on the backs running a lower PSI. I now know the WILL understear on the limits of grip and keep repeating this process. Reverse for an overstearing car.

 

Its where there is a difference in wheel speed and you get a difference front to back. Over time this adds a tension onto the prop shaft and puts stress on the diff. The last time I heard of this causing a problem was atleast 20 years ago on a 4 wheel drive truck.

I would tell them you will just take 2 tyres thanks!

 

Gone with 2 front T1-Rs thank you all, I will make up the other 2 soonish but not until I've had some value from them....
Eagles next time maybe.

 

Complete bollox

If I were you I'd kick that particular tyre retailer into touch, they are trying to take the p!ss out of you


It's not possible to wind a differential up full stop, the only components you would be 'winding up' would be the prop and drive shafts, and that can only occur if you have a full lockable differential, like say, a Landrover, Landcruiser, quattro, etc.



As to Molds' question about how a viscous coupling works, here's a little experiment for you to try at home:

1/ Mix together some Sodium Bicarbonate and Water in a cup, to the consistency of double cream.
2/ Take a large spoon (Table or Dessert sized), put into the the bicarb mix and slowly rotate it, you will find it rotates reasonably freely.
3/ Next take a firm grip of the cup and try rotating the spoon quickly, you will find it resists the rotation quite strongly.

In fact the more quickly you attempt to rotate it, the more resistance you will meet, this is because you have created what is known as a 'non-newtonian liquid'.
The viscous coupling in a Subaru works in exactly the same way, the front and rear axles are able to rotate at slightly different speeds without encountering any resistance at the coupling, as soon as one or both wheels on one axle start spinning, the difference between axle speed increases, causing the fluid in the VC to harden considerably, allowing more torque to be transferred to the non-spinning wheels.
What effectively happens is that in addition to pushing more torque into the ground, the non-slipping wheels will transfer a reverse braking effect back through the VC to the spinning wheels. What goes on when all four whells are spinning is a whole different ball game of course

Obviously Subaru uses a more complex fluid than baking soda in it's transmissions

 

Thanks everyone, especially CrisPDuk. Having had a puncture, I thought I'd have to replace all 4 tyres, but now I think I'll just get 2. My tyre centre today said that they were told about Audi TTs and to replace all 4 if there was less than 5mm on any other tyres, but he couldn't see why.

It does however say quite clearly in the handbook to keep all tyres the same tread depth to avoid transmission damage, but I've worked out that a half worn tyre has a 1% lower rolling radius than a new one, which is about the same as putting a couple of extra pounds of air in! I can't see how a car designed to go rallying could be that fragile!

 

Some cars over the years did struggle with a mix of new/worn tyres; the one that springs to mind most of all is the Vauxhall 4*4 as fitted to Cavalier and Calibra 4*4's and Turbos, this had a well known fairly fragile centre diff. Can't comment on the TT. The scoob is not renowned for having a problem, although I confess I tend to swap my tyres front-back once the fronts are down to about 4mm or so, in order that all 4 wear out at the same time.