Tread depth truth?

My understanding built from this and the OB site is that if the car is driven on an out of spec tire, the effect is the same as that of constantly driving over slippery terrain/actual off-road: the car "thinks" that there is a slipping tire. Thus premature wear of the center diff/MPT, rather than catastrophic failure is what is at stake-if true.

In any case, I do not plan to use my spare for more than the nearest reasonable place to receive a replacement (shaved to tread depth) while keeping speed under 45 in the process. That could be anywhere between a dozen and a hundred miles. I think that speed is important as it is mentioned in the manual.

The reason I really feel I need a proper AT spare is that we do go well into unpaved territory and tackle some really rough terrain here and there thus risking a lot of trouble should the spare donut be the only option.
 
Gidday MAS

Heavens to Murgatroyd, mate: You haven't got one of those awful space saver spares, have you?

When the Outback first came out here, I had a close look at it. Subaru had put one of those god-awful things in as standard! I rang up their head office here and informed them that calling a car "Outback" and supplying a spare that could only be used at less than 50 Kmh for no more than 80 Kms (or vice versa ... :iconwink: ) was an open and shut case in court.

I'm sure they got plenty of advice like that, because about 3 months later they replaced the abomination with a proper spare rim and tyre ...

It can be 1,000 Kms (or more) of dirt roads from one tyre place to the next here in Oz. True, this isn't the norm any more, but it isn't hard to find commonly used roads where the distance is over 500 Kms ...

As soon as I bought Roo2, I bought a s/h matching alloy as it came with a steel rim in the spare. At least it was a proper tyre ...

If one has to fit a tyre that is too big or too small, I suspect it is best to fit it on the front, even if one's particular car hasn't got a vLSD or LSD at the back. All AWD cars have this problem, not just Subies. 4WD cars (light trucks ... ?) with open diffs and a transfer case aren't as likely to spit the dummy as AWD cars, but these have rarity value, these days. They also have problems with axle wind-up between front and rear wheels that can cause propeller shafts/universals or the TC to break.

Basically, there is no such thing as a "perfect" system that's foolproof to boot.
 
Only one undersize tyre on either axle will make the centre diff wear but this takes time, you would be fine to make it to the tyre shop.

Btw, it helps us answer your questions if you post your Subie type in your profile. If you have a manual, you need to be careful of your centre diff. That means keeping correct tyre pressures & sizes. Also means no handbrake turns...sorry lol :lol:

If yours is an auto, you can put in the fuse to disable the centre MPT making it FWD not AWD till you get the tyre fixed. They're also not so sensitive to different sized tyres as the manual viscous centre diff
 
Ratbag said:
Gidday MAS

It can be 1,000 Kms (or more) of dirt roads from one tyre place to the next here in Oz. True, this isn't the norm any more, but it isn't hard to find commonly used roads where the distance is over 500 Kms ...
Click to expand...

Say what? That is awesome:biggrin:

I am lucky to live near the most remote areas of the lower 48. Even so, it is hard to be more than about 60-80 miles from a paved road. Once there, it could be 100 miles or so to a tire shop but that is about it. And it will take going to the most remote of places to get this far, about 300 km, from a tire shop.

The best places to go off-pavement, whether in a Subaru or a rock crawler, depending on taste, are actually either close to the 4wd capital of the US in southeast Utah or to a big enough town in southern Colorado. The third area of interest to me, northern Arizona/southern Utah is more remote than either of those. Still, aside from a little known road somewhere, I can only think of two zones, one on the northern side of Grand Canyon and one in the Maze district of Canyonlands, where one might end up about 300 km from the closest shop. But that's about it.
 
NachaLuva said:
If yours is an auto, you can put in the fuse to disable the centre MPT making it FWD not AWD till you get the tyre fixed. They're also not so sensitive to different sized tyres as the manual viscous centre diff
Click to expand...

True, but not anymore. There used to be a switch, then a fuse, now I am told that this is not true anymore. I guess I will ask again.
 
"went straight to Bob Jane and got two new Michelin Energy XM2s for the rear. We got some breakfast and coffee while we waited. Once the car was ready we were on the road again. It was a fairly long day of driving through Molong, Wellington (where we stopped for lunch), Dubbo, Narromine, Trangie and Nyngan."
 
Bit worried about your tyres. You only replaced 2 but your sig says you have a 5MT? Having tyres with different wear front to back means different diameters which will bugger your centre diff. How old are the older ones?
 
NachaLuva said:
Having tyres with different wear front to back means different diameters which will bugger your centre diff.
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That's a myth (fortunately). The tread depth has less than a second order effect on rolling circumference - it is the underlying belt which determines it. There are numerous references "out there" which explain this in detail.
 
Thanks for that info, Zippo.

The tyre calculators show that the tread depth from new to bald has very little effect anyway.

Towing with either the front or rear wheels off the ground is a whole different ball game ...
 
zippo said:
That's a myth (fortunately). The tread depth has less than a second order effect on rolling circumference - it is the underlying belt which determines it. There are numerous references "out there" which explain this in detail.
Click to expand...

I'm not willing to risk it lol :iconwink: Have heard of too many people needing new centre diffs with different tyres front to rear

Tweaksta, if the others still have most of their tread I'm sure it would be fine, its more if 2 are old & 2 new...
 
zippo said:
That's a myth (fortunately). The tread depth has less than a second order effect on rolling circumference - it is the underlying belt which determines it. There are numerous references "out there" which explain this in detail.
Click to expand...

How so? Thread depth does have an obvious impact on overall tire diameter. Unfortunately, my alignment issues this year have made me measure these things more times in one year than in the previous 20 combined!

I do not try to keep all of this info in my head all the time, but if I remember correctly, the vLSD of the MT Subies are especially sensitive to mismatch. The recommendation of other forum members (over at the OB forum) is to put the two sets diagonally. Say, front left and rear right would be the newer and the opposite the older tires. However, I think this is for the ATs.

I also remember, including from this forum, that 1 larger tire is not a big deal, whereas one smaller tire is a big deal.

Finally, since nobody bothers with rolling circumference, tire shops in the US usually consider 2/32 difference safe with some willing to push it to 3/32nds.
 
MiddleAgeSubie said:
How so? Thread depth does have an obvious impact on overall tire diameter.
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yebbut ... It's NOT about diameter, OR axle height above ground, or your cat's age. It's about how many revs per furlong the wheel is going to do, i.e. rolling circumference. Tyre shops (sic) can't be bothered measuring it because they have to be fitted to a wheel and loaded somewhat to get the underlying tread belt to perform as it will on a vehicle. Mind you, if they actually asked the manufacturer as I did, they'd surely get the RC info (unless they are YumCha brands).

I'll give a simple analogy. The belt (ever wonder why the early Pirellis were labelled "cinturato"? Check "belt" in an English/Itie translator) acts exactly like a caterpillar track. Yes, flat on the bottom. Now picture that caterpillar track with a 50mm block of (say) wood fixed to each element. Will the distance travelled per rev of the track change? No.

The tread itself deforms as the tyre rotates, compressing longitudinally at the point of first contact and recovering at the point of release - this is what gives that "less than second order" effect of tread depth on RC. It's the same for those blocks of wood, which slide - as they are outnumbered by the ones already in contact. These two distortions cancel out anyway.

MiddleAgeSubie said:
tire shops in the US usually consider 2/32 difference safe with some willing to push it to 3/32nds.
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I've seen it stated in some forum (i.e. it's probably bull****) that Subaru reckon up to 2% difference is OK.

When re-rubbering my Foz recently I emailed both Yokie and Bridgestone requesting RC info on the three tyres on my shortlist, all being in the stock size. They quite happily gave that info, but the numbers were all different, and IIRC 2% didn't quite cover the field.

Anyway, people will believe what they want to believe. There's another forum where the topic of "airing down" on unsealed roads is being thrashed to death yet again, proving that in spades.
 
Hmmm - correct me if I'm wrong here, but we can probably determine if we are stressing our diffs by going for a freeway drive for 20-30 mins then pulling over and checking if they are generating excessive heat....

Exacerbated wear of a component (or set of components) is caused by friction. Friction generates heat, therefore we should be able to determine if we are cooking our diffs by a quick temperature check to see if they are hotter than normal.

All we'd need to do this is to get some baseline figures for "normal" operating temps from people running perfectly matched tyres.
 
Those "normal operating temps" will vary a lot from vehicle to vehicle, depending on so many variables, that the only reference point of any value is the use of the same vehicle for A vs B testing.
 
I'd agree with zippo here, too hard to measure the stress on a centre diff by temp. It works on tyres, bearings, etc but not diffs...

Also remember that a Subaru centre diff is a viscous LSD, doesnt work on friction. Plated LSDs use friction between the plates but viscous LSDs use a special gel that thickens as it heats up creating a binding effect
 
Also remember that vLScD is the exception, not the rule: MT only...:lildevil:
 
Zippo,

I do not know what I want to believe to care either way. I need to be convinced, either way.

I doubt Subaru's 1/4 in rule.

But I also think that they would not put a sentence about equal wear in the manual for no reason at all.

Subaru gains nothing from that very irritating requirement. Therefore, there must be some reason for it.

You may as well be right. But while my dealers here hve no issues with a variety of mods, they have been clear that Subaru would deny any claims about differential and related, if there are mismatched tires...

Right or wrong SoA has been very pedantic about this, I do not know about Australia.
 
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