Circumference variation in new tires

[Randol]

I just had to replace a tire on a Mazda CX30 AWD, my wife took out a curb and chunked a tire. The tires had about 8500 miles on them.
I replaced with the same brand and model.
Of course, I researched all the data about differences in circumferences affecting the AWD system.
Most of these talk about differences in tread depth, which would definitely contribute.
I saw anything from 2/32 to 3/32.
That got me thinking about how precise the circumferences are on a given set of 4 tires, or the variation in them, new condition, right off the rack.
Do manufacturing lot codes make a difference, Tooling variation and so on.
Both unloaded and loaded roll out.
How about air pressure? The vehicle calls for 38 Psi front, 36 Psi rear.
Is this an attempt to equalize the roll-out due to differences in weight bias?
Any experts out there?

 

[AZjeff]

Calling @CapriRacer

 

[CapriRacer]

Calling @CapriRacer

Thanks for the vote of confidence.

Even when I was working, I didn't have access to this kind of information

- BUT -

We can make some reasonable guesses based on the available information.

I have heard that if the tires are with 4/32nds of an inch tread depth, even the most sensitive AWD systems don't experience problems. I take that to mean the tires within 1/4" are OK. I know that the diameter of a tire is more or less 3% less than the actual freestanding (unloaded) diameter. My guess is that also applies to differences in diameter due to tread depth. 3% is just too small to worry about, but I thought I would mention it.

1/4" sounds too large for my taste, so kind of think that tires nowadays - that is modern steel belted tires can be made within an eighth or so.

Is there variation in rolling diameter? Yes, that's the basis of those ABS systems that use the system to determine if one of the tires is low. How much? Clearly this would be dependent on the difference in inflation pressure. I did this once and cannot remember, but it was significant - more than 1/2". I will try to find the info and if I do, I'll report on it later.

Shouldn't that affect an AWD system? I would think so, but I suspect that this only becomes a problem if the warning light is ignored.

Differences between batches of tires? First, the "batches" analogy kind of falls apart because of the way tires are made. There are so many different components that variation in production is more or less the sum of the total and relatively stable over time. It's only when a significant event takes place - like a new machine - that you'll see differences from different production dates.

How about air pressure? The vehicle calls for 38 Psi front, 36 Psi rear.
Is this an attempt to equalize the roll-out due to differences in weight bias?

The tire pressures specified by car manufacturers is really complicated, but generally it's tied to the max weigh t- they call that GAWR (Gross Axle Weight Rating). I think it's just happy circumstances that rolling diameter kind of follows. It's not intentional at all!

 

[grasscutter]

I wonder what tire cupping does to an AWD system. Probably wreaks havoc on it.

 

[andrew_j]

Just older Subaru is most sensitive in center diff with to variations . They used more mechanical full time systems . Majority of them now are FWD and occasional kick of traction as needed to rear wheels.

Other makers have more tolerance.

 

[Rand]

Just older Subaru is most sensitive in center diff with to variations . They used more mechanical full time systems . Majority of them now are FWD and occasional kick of traction as needed to rear wheels.

Other makers have more tolerance.

what is your basis for saying subaru is a part time AWD?
AFAIK they still use a MPT clutch in the cvt, just no more viscous coupling in most models since the MT have gone away.

You may be right for Hondas and Toyotas.

 

[edyvw]

Just older Subaru is most sensitive in center diff with to variations . They used more mechanical full time systems . Majority of them now are FWD and occasional kick of traction as needed to rear wheels.

Other makers have more tolerance.

Subaru is still full time AWD, so are longitudinal Mercedes, BMW, most Audi’s.
Toyota and most VW’s are FWD based.
Not sure about latest Pilot. It has very capable AWD that really behaves like full time AWD.

 

[andrew_j]

what is your basis for saying subaru is a part time AWD?
AFAIK they still use a MPT clutch in the cvt, just no more viscous coupling in most models since the MT have gone away.

You may be right for Hondas and Toyotas.

Sorry to clarify I meant majority of modern systems out there are part time with very quick engagement . Subaru I believe retains full time AWD however no idea anymore if sensitive to tire circumference issues.

 

[spasm3]

Thanks for the vote of confidence.

Even when I was working, I didn't have access to this kind of information

- BUT -

We can make some reasonable guesses based on the available information.

I have heard that if the tires are with 4/32nds of an inch tread depth, even the most sensitive AWD systems don't experience problems. I take that to mean the tires within 1/4" are OK. I know that the diameter of a tire is more or less 3% less than the actual freestanding (unloaded) diameter. My guess is that also applies to differences in diameter due to tread depth. 3% is just too small to worry about, but I thought I would mention it.

1/4" sounds too large for my taste, so kind of think that tires nowadays - that is modern steel belted tires can be made within an eighth or so.

Is there variation in rolling diameter? Yes, that's the basis of those ABS systems that use the system to determine if one of the tires is low. How much? Clearly this would be dependent on the difference in inflation pressure. I did this once and cannot remember, but it was significant - more than 1/2". I will try to find the info and if I do, I'll report on it later.

Shouldn't that affect an AWD system? I would think so, but I suspect that this only becomes a problem if the warning light is ignored.

Differences between batches of tires? First, the "batches" analogy kind of falls apart because of the way tires are made. There are so many different components that variation in production is more or less the sum of the total and relatively stable over time. It's only when a significant event takes place - like a new machine - that you'll see differences from different production dates.



The tire pressures specified by car manufacturers is really complicated, but generally it's tied to the max weigh t- they call that GAWR (Gross Axle Weight Rating). I think it's just happy circumstances that rolling diameter kind of follows. It's not intentional at all!

I am correct in remembering that you once said that tires grow larger in diameter over their life? Not sure that would happen evenly with all four tires, so some variability in circumference is probably built into the AWD system.

 

[SeaJay]

I replaced a single new tire on my Subaru AWD with the others have more than 8,500 on two different occasions. The owner's manual seemed to suggest certain ruination of the vehicle if I did that. Never had a problem for many tens of thousands of miles after those two replacements. I trusted my 3rd generation tire shop who told me I would be fine, and I was.

YMMV

 

[bonjo]

there are tolerances built in to allow for tyre wear; the open diffs can cope with this.
You should aim to avoid tyre mismatch on the same axle but IMO an O/D difference of less than 10mm would & should not affect the AWD and ABS system

 

[jadatis]

Mayby it would be an idea if carmakers build in a warning if differential satelite wheels are turning on a straight road. Looks easy to me to build in sensors to check that.

Normally those satelite wheels should not rotate on a straight road. That is what is important for wear of the diferentials if rolling-circumference of tires are different.

Goes for 2 and 4 wheeldrive , 4 wheeldrive have 3 diferentials ( if I use the right word for it).
Mayby it can also be done with use of the rotationspeed sensors used for ABS and more.

Then if different rolling circumference between left and right on axle, and between front and rear wheels, it can be warned about, maiby only to the service system for garage.

Then a pressure-difference can be done to compensate so same rolling circumference reached.
I rather call it rolling distance,gives lesser confusion with circumference.

Made a rolling-circumference/distance calculator spreadsheet in wich also deflection is included in the calculations.
Shall not be 100% acurate, because aproaching formula used, I found on agricultural site.
But better indication then only comparing circumference of tire.

For those who interest it .
RC= (overall diameter minus deflection) x pi ( 3.14159)
With my calculator you see that same circumference can still give different rolling-circumference/distance.

Calculators on internet simply give circumference as RC ( overall diameter x pi).

The sizes of tires given on sidewall are rounded,
So a 255/50R18 can be yust as whell be in real 257/49R 18 ( rimmdiameter is always exact).

here an example

 

[repairman54]

Jack up a rear wheel and turn it back and forth. See all that play, that is the tire tolerance in the system. Its not a rock solid front to rear or side to side connection.
Lost a RR tire to a screw in the shoulder, not reparable. Only 8-9k miles on a new 4 tire set on my '16 AWD Equinox at 50k miles. Put a new same brand tire on it in that position. Wore that set out, new 4 tire set again. At 115k miles now and zero issues.
Biggest issue would be a size mismatch not tread depth. Donut spares and not a precise match either.
Servicing diff. and T case fluids is more important via the severe schedule than tire wear worries.