Subaru AWD and tyres

greendream

greendream

Likes Bikes
No. 1 daughter calls me yesterday to advise she has a flat tyre, “where are you?” I ask, “out the front” comes the reply. I take a look, the tyre is farked she had driven a few K’s home after the car started feeling “funny”.
so now we have to replace the tyre but there seems to be an issue with running different diameter tyres on Subaru AWD systems, (the remaing tyres are 30-40% worn so plenty of meat left in them) . Namely that running different diameters or even types of tyres can stuff up the drivetrain.
subaru suggest 4 new tyres whilst tyre shops advice ranges from don’t worry about it just replace it with any suitable size, replace with the same, replace 2 with any suitable size/or same but keep them on the same axle, to what Subaru suggest.
Do any of you burners have experience or an opinion with these matters?
BTW one of the wheel nuts had seized so the wheel stud was snapped off in order to remove the tyre.
 
teK--

teK--

Eats Squid
There is an allowable tolerance for tyre circumference, when you are comparing left/right (same axle), or front/rear (different axle). The tolerance is really not much, it equates to the amount of maybe 2mm in tread depth.

I.e. if you trash one tyre on a fairly worn set then you should at a minimum replace the other side as well on the same axle, but preferably replace all 4 if it is a big difference.

Otherwise you risk destroying any one or several of the 3 differentials as it is working overttime.

This is why you should keep the same type of tyre as a spare, and swap that spare with the other 4 tyres regularly. THis means if you trash one tyre you would theoretically have the spare tha tyou can swap in, and it has a similar tread depth to all the other tyres.
 
The Dude

The Dude

Wasn't asking to be banned
Doesn't the centre differential compensate for different front and rear "axle" speeds?

I'm not sure what percentage of the time you are not driving in a dead-straight line, but I would have thought it was significant
Surely it will allow the 10 or 15mm difference in radius of a set of new vs. worn tyres
 
The Dude

The Dude

Wasn't asking to be banned
teK-- said:
since checked the circumference tolerance is 1/4".... so not much at all.
Click to expand...
That doesn't sound right
1/4" is about 6mm which is only 1mm in radius
There's no way you can guarantee that level of even wear
 
teK--

teK--

Eats Squid
"The Techline continues to receive calls from Technicians who have performed all types of unsuccessful repairs to transmissions with a “binding on turns” concern from the Customer. One of the first questions we will ask is whether or not the tire circumferences have been measured. All too often the answer is no. It is extremely important when diagnosing a transfer clutch-related concern, the tires must be one of the first variables you rule out. Remember, all 4 tires should be within approximately ¼" of each other in circumference when measured at the center of the tread. Otherwise, binding issues can result (or re-occur) due to the ratio imbalance caused by mis-matched tires. A quick keyword search on STIS of “Binding on Turns” will return a list of other bulletins and TIPS articles on this subject."
 
teK--

teK--

Eats Squid
ps. another option is you can get 1 new tyre, but have the tread shaved down to match the other 3....
 
Flow-Rider

Flow-Rider

Burner
You can't run different diameter tyres on them, you'll end up with a clunking centre diff in no time.
 
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Chriso_29er

Chriso_29er

Likes Bikes and Dirt
Circumference differences even in new tyres can be enough to cause trouble with some modern AWD systems.
To the point that some manufactures even require a special approved tyre from the relevant tyre brands that have been certified to be within the allowable tolerances. Even if an identical sized un-approved tyre is available from the same company.

Crap situation, but I think you would need to measure the remaining tyres to see if you can get a new tyre or two on there. Will suck if you have to do all four!
 
Minlak

Minlak

custom titis
I had a Subaru Outback AWD I knew the head mechanic where it was serviced personally- one of the reasons I no longer have an Outback is exactly this - you may get away with 1 tyre - you may not - Subaru recommended is 4 new tyres at same time and constant rotation to maintain even tyre wear.
 
link1896

link1896

Mr Greenfield
What year Subaru?

I’ve rebuilt a fair few 2001-2008 Subaru’s centre differentials. Oh the memories. (The gasket sets bearing preload on the output shaft that goes rear to front through the transaxle)

You can get away with a bit of a circumference difference for a while if the car isn’t doing tight turns to park, put the new tire on the rear, as soon as you’re in second gear with mild throttle (depending on age) the centre diff goes to 80% front wheel drive mode anyway. This isn’t a long term solution.

You can disable the centre diff.

There is a fuse position in the fuse box labelled FWD. 2003-2005 forester you put a spare fuse in to the location to disable the centre differentials duty cycle solenoid. FWD light will light on the dash. Beware, an inexperienced driver in a fwd Subaru is about to learn what a loss of traction is all about. Subaru manual tells you to do this when installing spare wheel.

Measure the circumference of existing tyres. Hopefully the three good ones are within 5% of each other, and as someone mentioned, find a tyre shop that can reduce the tread depth on the new to match.
 
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