Tire shaving / buffing - how to avoid replacing ALL your tires when one goes on an AWD?

I think you'll find circumference is independent of inflation. The tread does not change length.

Think of a flat tyre like a tank track.
 
@ateday IIRC, you had to seriously "massage" your spare wheel well to fit that profile in there?

There is no question of doing that with mine. My towbar has straps about 30x5mm that are attached to the outside of each side of the wheel well. There is simply no way that these could be reshaped without an oxy spanner, and after removal from the car.
 
Ratbag said:
@ateday IIRC, you had to seriously "massage" your spare wheel well to fit that profile in there?

There is no question of doing that with mine. My towbar has straps about 30x5mm that are attached to the outside of each side of the wheel well. There is simply no way that these could be reshaped without an oxy spanner, and after removal from the car.
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I ended up with a combination of a partially deflated tyre and cut away a lot of the foam spare wheel well sides. The tyre is more difficult to remove than fit in but it works.
 
Ratbag said:
Bridgestone Turanza Serenity Plus
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Hey @Ratbag - are they directional? If so, how do you handle using a spare on the 'offside'? And rotations? (Or do you not bother with such things?)

They reason I ask is that I've just looked closer and the treads aren't symmetrical.
 
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Not as far as I know. Most tyres since about 1980 have had asymmetric tread patterns. They have an inside and an outside.

My spare is the original one that came with the car in ~2006.

I just use it as necessary, without a second thought.
 
duncanm said:
I think you'll find circumference is independent of inflation. The tread does not change length.

Think of a flat tyre like a tank track.
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[head aching]. Really? Thinking out loud:

The height of the hub off the ground will decrease with a deflated tyre. That means the radius of the circle of where the rubber-meets-the-road decreases - that rubber-meets-the-road is a circle with a smaller circumference than an inflated tyre. So the wheel has to turn faster to keep up with the speed of the other 3 wheels. And so there's drive chain stress.

A deflated tyre is like an over-worn tread - just makes the rubber-meets-the-road circle smaller, and the wheel turn faster at a given speed.

The amount of rubber on the ground will increase, like a tank track. All nasty things to the tread.

I think.
 
Ratbag said:
Not as far as I know. Most tyres since about 1980 have had asymmetric tread patterns. They have an inside and an outside.

My spare is the original one that came with the car in ~2006.

I just use it as necessary, without a second thought.
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that's kind of funny. If Subaru says we're supposed to keep all tyres the same type (tread pattern, size etc), to my mind we should be buying symmetrical treads only. But if most tyres these days are asymmetrical treads (but non-directional), then Subaru drivers have been in violation of this since the 80s: the same tyre will have treads pointing the opposite way depending on their being mounted on the left or the right of the car.

Presumably nothing has happened to these Subi owners.

Really lines up with your skepticism about this, @Ratbag. The tyreshop guy was baffled when I asked him, too.

Subaru is supposed to have said keep all tyres within a quarter inch rolling circumference of each other. 1/4" is about 6mm on the circumference. That translates to keep all tyres within ~1mm difference on the radius. 1mm. That's got to be impossible.

That drive train has got to be a lot more robust than Subaru is letting on.
 
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As I said before, it is the job of a differential to allows wheels to rotate at greatly differing speeds! Rotating at slightly differing speeds is a piece of cake ...

Consider my 1968 Landcruiser. Front open diff. Rear open diff. Open or locked centre transfer case gears. If you drive it on a bitumen road in 4wd - essentially centre diff locked (or other non slip surface), you get transmission wind up, and you will (usually) break one of the front driveshaft universal joints, dropping the drive shaft onto the road. I have had this happen to me.

If it had a differential in the centre, the diff gears would simply allow the front and rear driveshafts to rotate at slightly different speeds.
 
Random Twitch said:
[head aching]. Really? Thinking out loud:

The height of the hub off the ground will decrease with a deflated tyre. That means the radius of the circle of where the rubber-meets-the-road decreases - that rubber-meets-the-road is a circle with a smaller circumference than an inflated tyre. So the wheel has to turn faster to keep up with the speed of the other 3 wheels. And so there's drive chain stress.

A deflated tyre is like an over-worn tread - just makes the rubber-meets-the-road circle smaller, and the wheel turn faster at a given speed.

The amount of rubber on the ground will increase, like a tank track. All nasty things to the tread.

I think.
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Nope.

The circumference of the tyre is inelastic (the tread bends, but it doesn't change length). Therefore the same number of rotations are required for a given distance travelled regardless of inflation.

When the tyre is flat - the circular approximation fails.

Consider the following thought experiment.

Take a couple of 2pi length tyre treads.
Wrap one around a sold circular wheel of radius 1, wrap the other over two spaced axles like a tank track.

The second has an axle to ground distance < 1, but they both have the same rollout of 2pi.

(caveat - the tank axles rotate faster.. this is different from a flat tyre on a single axle).
 
@duncanm

[sits back, baffled] Look, I refuse - refuse, I say - to accept this! [pounds table theatrically]

I agree that the tank tread thought experiment shows that a given length of tyre (2pi) means the tank axles roll faster. But that's not the point.

The tyre treads can be inelastic, but they deform. Consider a half-flat tyre - it sags and bulges so that the axle sits lower to the ground. That lower height is the radius of the circle that is actually doing the turning. Its smaller than the radius of a full tyre. The tyre bulges around the flat in an effort to accomodate all the spare rubber. The circumference has been effectively reduced.

So the axle of a flat tyre will have to turn faster to keep up with the fully pumped other wheels. Equals drivetrain stress.

We might be in furious agreement.
 
this is just a coda to my experience here, in case someone reads this later on wondering what happened.

I bought 5 road tyres, Bridgestone Turanza Serenity Plus for ~$800 (4 for 3 deal). I plan to do a 5 wheel rotation every 10000km. They run very smoothly and quietly, and rolling resistance is noticeably reduced.

Am I happy with that? No, not really - I have lost my offroad tyres, and that really sucks. But they were ~$1200-1400 with a couple of weeks' delay, and I couldn't afford either.

Food for thought, if you are contemplating a choice like this.
 
@Random Twitch
As I said before, I've never been anywhere where my high speed road tyres were the limiting factor.

With my old Landcruiser, I ran road tyres on the front, and full bar lugs on the back.
But, I also towed a 2 tonne tandem, tandem axle horse float behind it, on all sorts of "roads".
 
Ah the good old days, towing a horse float when I was 17 years old on my L's cause my uncles horse placed and he decided it would be better for me to drive home.

@Random Twitch I went through these same thoughts. I messed up one of my Bridgestone Duellers while off-roading. I bought a single new tire and then realized it was ~6mm larger. Too use the new tire or not to use it. My OCD was going nuts. For now I am just keeping as a spare. Unless I go on a long trip, in which case I'll get 5 new tires ( and thats just an excuse to buy a set of Maxis that I want to try).
 
Random Twitch said:
Hi all

I've been reading about avoiding the need to replace all 4 tires when only one needs replacing on an AWD like a Subi.

If this is new to you (like it was to me a little while ago), the basics are these: AWD systems like the Subi need all four tires to be pretty equal in diameter, i.e. they should have the same degree of wear. This avoids mechanical damage to the drive train. So, when you need to replace one tire, you should be replacing all of them so all four tires are the same size.

This is expensive, and therefore sux.

One alternative solution I read about on this forum is 'tire shaving' (I can't find the post now). Its a.k.a. 'tire buffing'. You buy a single replacement tire. A specialist can then shave down (or buff) the tire to take a few millimeters off - your brand new tire would then have the same diameter as your worn ones.

It sounds simple. But does anyone have actual experience doing this with their Subi?

How far does the shaving go - how does the technician know when enough has been taken off the new tire to match the wear of the existing ones? How many millimeters are we talking about, anyway?

It seems this service is more common in North America than here in Australia (something about the snow)? It *seems* only specialist services do it here, shops that service motorsports. I've found two in Melbourne that advertise it, and none in Adelaide.

I read on a motorsport site that shaving improves traction on a dry track, but it rules out the tyre for use in the wet. Clearly this is not what you'd want on a road vehicle! Is it just because they shave off so much material for motorsports?

In which case, is it even possible for Subis here in Australia?
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Some truck tyres can have new tread fitted to the carcass. I don’t know, if someone in Australia does that. If there is maybe they could do it.
To establishing the circumference of a tire, at a specified pressure can be done by measuring the rolling distance of the tyre That is the important thing.
 
Ratbag said:
As I said before, it is the job of a differential to allows wheels to rotate at greatly differing speeds! Rotating at slightly differing speeds is a piece of cake ...

Consider my 1968 Landcruiser. Front open diff. Rear open diff. Open or locked centre transfer case gears. If you drive it on a bitumen road in 4wd - essentially centre diff locked (or other non slip surface), you get transmission wind up, and you will (usually) break one of the front driveshaft universal joints, dropping the drive shaft onto the road. I have had this happen to me.

If it had a differential in the centre, the diff gears would simply allow the front and rear driveshafts to rotate at slightly different speeds.
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Subarus are different to the Landcruiser. AWD there is always some drive to all wheels so there is wind up. For that reason they should never be flat towed.
 
Being an off road tire they have a chunky tread pattern That creates the extra noise. The only way to get rid of the noise is to refit road tires.
 
ateday said:
Retreads seemed to go out some years ago. Bandag did but don`t know if they still exist. Don`t like the idea myself though..
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Retreads are still available for trucks, but are never to be used on the front axle.
 
ateday said:
Retreads seemed to go out some years ago. Bandag did but don`t know if they still exist. Don`t like the idea myself though..
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Being an off road tire they have a chunky tread pattern That creates the extra noise. The only way to get rid of the noise is to refit road tires.
 
ateday said:
I am having a good run out of my (6) 215X65X16 Bridgestone 697 Duellers . About 50k kms so far and estimate another 30-40k left. All wearing evenly but a tad noisy on bitumen, not excessively so though.
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Being an off road tire they have a chunky tread pattern That creates the extra noise. The only way to get rid of the noise is to refit road tires.
 
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