Method for equalizing tire diameter

[Jorge-789995]

I think the whole thing is blown out of proportion. This site is a magnet for those that have an axe to grind about their recently failed XC. If the bevel gear was failing at even a low rate, we'd have heard about it by now. If you need evidence of the propensity to grumble, then look at all the folks that register for this site with an 'IHateVolvo' screen name and their first post is threatening a class action lawsuit against Volvo, Ford and anyone associated.

3/32? I agree that there are not that many cars out there that are meticulously maintained. I'd guess that if you loaded up an XC, you may get 3/32" in tire diameter difference front to back. Until I see these things failing in larger numbers, I think it's overblown.

[mbsl98]

If you haven't heard of AWD system failures at statistically very significant rates, you are not following very closely. They are being reported regularly on all of the boards focussed on Volvo, with much more problem with the pre-haldex. It's not 100%, or even 50%, but it is clearly a significantly increased risk of failure that is heavily affected by owner maintenance. Separately, the load in the car, or the pressur in the tire, cannot actually change the rolling circumference of the tire. Only actual wear of the rubber does that. Remember that the tread is backed by several layers of steel belts. The steel is not capable of stretching or shrinking due to pressure or weight, and the pressure merely holds the tire in more or less of a circle shape. The mismatch issue is when one tire covers more ground than another for each revolution, ie: difference in rolling circumference.

[philosophicaldreamer]

If you haven't heard of AWD system failures at statistically very significant rates, you are not following very closely. They are being reported regularly on all of the boards focussed on Volvo, with much more problem with the pre-haldex.

You overestimate the importance of Volvo boards like this one. You have so few Volvo owners who participate on these boards that it is not even worth mentioning it. From a statistical point of view boards like these have little value as the basis of determning anything because people who participate in them are a self-selected group of owners. You have basically two types of people who login into boards like this one: those who love these cars and like to discuss them and those who have had a lot of problems with them and like to vent about them. Most people who drive them without experiencing any problems don't even give a second thought to posting their impressions on boards like this one.

Ta-ta, j.

[Sasquatch]

...You have basically two types of people who login into boards like this one: those who love these cars and like to discuss them and those who have had a lot of problems with them and like to vent about them. Most people who drive them without experiencing any problems don't even give a second thought to posting their impressions on boards like this one....

You forgot the most important third group. The group which is researching the vehicle to help in their purchasing decision.

[Jorge-789995]

Separately, the load in the car, or the pressur in the tire, cannot actually change the rolling circumference of the tire. Only actual wear of the rubber does that. Remember that the tread is backed by several layers of steel belts. The steel is not capable of stretching or shrinking due to pressure or weight, and the pressure merely holds the tire in more or less of a circle shape.

If that is the case, then the radius from the wheel center to the road would not change as the tire pressure went to zero. The tire deforms where it contacts the road. Less pressure = more deformation = shorter radius = shorter circumference. Tire pressure does matter. Recall, the tire is never completely flat, just flat on the bottom.

[philosophicaldreamer]

You forgot the most important third group. The group which is researching the vehicle to help in their purchasing decision.

I see your point. However, I would argue that those who use such boards for researching their vehicles before they buy them do not constitute statistically significant number in regard to our discussion, since they have no experience with Volvos and thus they cannot post any meaningful information about their experiences with cars that they have not purchased yet.

Ta-ta, j.

[volviste]

Hi, sorry about my english but I have to tell my recent experience
with my XC2001 and the tires.
My car have 130000kms and the dealer have change the front tires
at 126000. when I bought the XC, he said he have changed the
disc of the casetransfert. After my first kms with the car, around 120 to
140 km/h, a lot of vibrations arrived, more and more to a strange noise
(something like a "sklong!") and after every thing seems ok!
The dealer of my region said : "classic, we have to put new rear tires"
They do it, and now my car is fantastic! I payed nothing for the tires.
And he explain that is necessery to rotate the tires every 15000kms.
Michel from France.

[philosophicaldreamer]

Hi, sorry about my english but I have to tell my recent experience with my XC2001 and the tires.
My car have 130000kms and the dealer have change the front tires at 126000. when I bought the XC, he said he have changed the
disc of the casetransfert. After my first kms with the car, around 120 to 140 km/h, a lot of vibrations arrived, more and more to a strange noise (something like a "sklong!") and after every thing seems ok! The dealer of my region said : "classic, we have to put new rear tires" They do it, and now my car is fantastic! I payed nothing for the tires. And he explain that is necessery to rotate the tires every 15000kms.
Michel from France.

Michael:

No need for you to apologize for your English. All you have to do is to scan our posts on this board, and you will discover that many of us here lack basic grammar, punctuation, and spelling skills, but this does not stop us from writing. Aglo-Saxons desperately need the French Ministry of Purity of Language to address the educational shortcomings of the English speakers. Anyway, your experience with tire rotation sounds about right. I have been rotating tires every ten-thousand miles and it works out marvelously.

Ta-ta, j.

[mbsl98]

If that is the case, then the radius from the wheel center to the road would not change as the tire pressure went to zero. The tire deforms where it contacts the road. Less pressure = more deformation = shorter radius = shorter circumference. Tire pressure does matter. Recall, the tire is never completely flat, just flat on the bottom.

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Don't want to beat a dead horse here, but pressure still does not change the circumference. When the tire deforms more, the use of the normal formula to calculate circumference is no longer valid - it is no longer a circle but an elongated shape that still has a steel belt of specific length around the outside (that length defines teh actual rolling circumference). In other words, changing the radius from center to ground is offset by changes in the radius in other directions, as the tire shape changes away from a circle.

[Jorge-789995]

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Don't want to beat a dead horse here....
In other words, changing the radius from center to ground is offset by changes in the radius in other directions, as the tire shape changes away from a circle.

I'll kill the horse one more time, and then that will be my last comment on the subject. Well... maybe.

We agree on at least one point, however I must not be communicating my point properly (This would not be the first time I've had that problem).

I agree with you that perhaps the actual tire circumference does not change much if any due to tire pressure. If you had the tire sitting on the garage floor and measured it's circumference at 0 psi and again at 35 psi it would probably not change much. But the tire circumference as it sits on the garage floor and calculated using the 'free radius', as defined below, has little to do with the distance the wheel will travel during one rotation (also defined below as the rolling circumference). Further, the "radius change in other directions" as you say, has no influence on the rolling circumference either.

The loaded radius is what is needed to determine rolling circumference and it is this rolling circumference that is the critical measurement. To bracket the extreme cases, think of a flat tire versus a tire with 50 psi. The flat tire's rolling circumference is based on the rim's radius, not the free radius. To parrot conventional wisdom, this rolling circumference must be exactly the same for each of the 4 wheels or else your bevel gear and the entire AWD system will be fried. I happen to disagree with this conventional wisdom to a degree.

I agree with Jack above when he says that the VC doesn't care how tall your tires are, but I obviously disagree with his assertion that tire pressure has no bearing on rolling circumference.

Getting back to my overall point-- I do not think that the VC is as sensitive to difference in rolling circumference as everyone seems to think. I'm looking for the Volvo documentation that describes the need to keep all wheels exactly the same diameter. If anyone has info written by Volvo, please let me know.
My 2002 owner's manual has the following to say about the AWD system. The electronic part we know is not true as the VC is not electronic.

All Wheel Drive (AWD) - general information
Your Volvo can be equipped with permanent All Wheel Drive, which means that power is distributed automatically between the front and rear wheels. Under normal driving conditions, most of the engine's power is directed to the front wheels. However, if there is any tendency for the front wheels to spin, an electronically controlled coupling distributes power to the wheels that have the best traction.
Tire dimensions
Volvo recommends that you always drive on tires of the same brand, size, construction (radial), tread pattern, load-, speed-, traction-, temperature-, and treadwear rating. Never drive on mixed tires, except for brief periods when the temporary spare tire is in use. Always use properly inflated tires of correct dimensions. Tire size and inflation pressures are shown on the tire pressure label located inside the fuel filler door.

It goes on to say tire rotation is recommended every 7500 miles and that the temp spare has a max distance of 50 miles. There is nothing in that paragraph that is not standard practice for any car.

During the course of normal driving there are times when the 4 wheels will rotate at different rates. Every time you turn the steering wheel for example. Different tire pressures and loads cause the same issue. Most people do not maintain their cars to a high standard so if the VC was ready to shred at a moments notice as some posts would have me believe, then where is the evidence? I don't recall reading post after post about VC bevel gears shredding and I've read this forum almost daily for the last 4 years. (Yeah, I should get a life)

I am open to the evidence that I am wrong. It'll give me the opportunity to adjust my thinking. There must be some basis of truth in the VC sensitivity claim, but I think it's entirely overblown. This issue is approaching urban legend status.



Deflection = Free radius minus loaded radius.

Free Radius = The radius of the tire/wheel assembly that is not deflected under load.

Loaded Radius = Distance from wheel axis of rotation to supporting surface at a given load and stated inflation pressure.

Loaded Section Height = The loaded radius minus half of the nominal rim diameter. Distance from rim seat to outer tread surface of a loaded tire.

Revolutions Per Mile = Measured number of revolutions for a tire traveling one mile. This can vary with load and inflation.

Rolling Circumference = The linear distance traveled by a tire in one revolution. This can vary with load and inflation.

Section Height = Distance from rim seat to outer tread surface of unloaded tire.