Rear diff stuttering after tyre change

[MZSPD6]

My rear wheels were worn out (badly!) but my fronts are 50%+ fine. This morning I replaced the rear rubber with the same size as the front but I'm getting a feeling through the rear diff on acceleration. Im guessing because the rear tires have more rubber than the front it's the AWD system slipping to balance the uneven rotation.
Can someone let me know if this would be the case and will I need to replace the front even though they have another 10 thousand k's in them...

[Victa]

Yup... the shuddering is most likely your rear tires...

AWD demands exactly matched size tires to avoid binding of the drivetrain, the centre diff (clutch pack for us) specifically.

Chris.

[MZSPD6]

Well I've improved it somewhat, the rears were 39psi and the fronts were 32psi so I've let the rears down to match and it's a lot better! I think with a few pounds in the front will help even more :-)

[Doug_MPS6]

It would have made more sense to put your good tyres on the steering and the half worn ones on the rear. I'm surprised the dealer didn't do this as a matter of course, as that is where you need the best tread and tyres in best condition in terms of life, where you are less likely to have an age-related failure. It should reduce your wind-up issues too, as the transmission power distribution favours the front axle.

39 psi was pretty high for an unloaded rear end and it must have ridden hard. But 32 psi is a bit light for the load carrying rear end anyway (esp if you carry passengers and/or a load in the boot) and potentially increasing drag and tyre wear on your nice new tyres. If you bring your rear pressures down to match the 32 on the front front and hence run 32 all round it is too soft in my view, especially at speed and will adversely affect handling. The word barge comes to mind.

Further, while you are using tyre pressure variation to influence rolling diameter, it also affects understeer and oversteer tendencies

One problem is potentially begetting a few more that you don't need.

Swap the tyres around and stick the good ones on the front and run them all at 38 psi and you can dispense with all this messing about.

[dazza]

I had diff whine before I was lowered and when tyre pressure was uneven. I run 40psi because my tyre load is rated at 91y. 93 or 95y I used to run 38psi which I believe is our recommended pressure from dealer.

[MZSPD6]

Tonight I pumped the fronts up to 39 and it has made it almost perfect, but I can slightly still feel the drag of the AWD system. I have the diff switch installed and previously it didn't make any feeling switching it on and off, but now I can easily feel a freeing up of drag of the AWD system when I pull the switch. I'll try the new ones on the front and see how that go's!

[Doug_MPS6]

Re: Tyre swap. Good move! I hope it works out.

[MZSPD6]

Been busy as with work, darn retail!

Driving on the new rears has been very average, I've got the rear diff switch so most of the time I just set it the fronts only so the drag wasn't an issue.
Even still the wheels got fine rubber dust all over them so I wouldn't recommend only changing 2 tyres!

I finally swapped the new rears to front today and it drives way better. Feels like it used to, yay! If you are going to change only a pair make sure you put the new ones on the front ;-)