Method for equalizing tire diameter

[glansberry]

After reading the posts about having equal tire pressure all around, and for having the same wear on each tire - I went out and checked my car - I see that the tread depth is about 1/16 deeper in the rear than the front. Pressure in rear was 39, front was 36.

I measured from the driveway up to the rim - I was suprised to see that the rear rim was 1/4" higher off the driveway than the front. I lowered the rear tire pressure to 28 PSI - this equalized the radius of the wheels. Does this seem like a sensible method of protecting my viscous coupling, or did they already take into account that the front would be riding lower due to the weight of the engine, by adjusting the gear ratio? In that case, making the loaded wheel radius the same would not be optimum.

Any thoughts?

[Ragman]

You have more weight on front tires than the rear because of the engine weight which would explain your measurements. The sticker on your fuel door will tell you whatsafe pressures to maintain for the loads you carry in the back.

Beyond that, I wouldn't go too crazy with tire circumference. With 15,000miles on my tires, i hit a cinderblock and blew one tire. You really think I'm going to buy four tires? Nope. Only one and I drove it another 25000 before replacing the set. No troubles with the tranny.

I'd be more concerned with tire circumference as it pertains to the antiskid braking system than the transmission.

[dlr97]

1/16" tread depth difference amounts to less than 0.5% difference in diameter. Not a problem.

[Jack]

FWIW, changing a tire's pressure won't change it's circumference which is the actual distance the tire must travel per revolution. Your viscous coupling doesn't care how tall the car is, it just gets unhappy when the four wheels don't rotate at the same rate.

[mbsl98]

1/16 inch equals 2/32nds, or about 25% of the usable tread depth. That is at the upper end of the tire tread differences that Volvo says is OK, and is a result of not rotating your tires. If you leave the better tires on the rear, you will soon be beyond the allowable diffrence spec's and you will also face real issues when you need to replace the worn fronts - you won't be able to buy two "new" tires that will be close enough to the partly worn rears. You will be much better off to rotate regularly so that you keep all of the tires within 1/32nd or so until they are all worn to replacement level. Separately, Volvo stresses that if there is measureable difference, the bigger tires always go on the front to avoid AWD damage (see owners manual).

[glansberry]

You are right, and I am embaressed - since this is true, why is everybody crazy about tire pressures vs AWD damage? Shouldn't the focus be on tire wear and proper rotation. I will start measuring tread depth and rotate whenever the fronts have more wear. And I'll go back to recommended equal tire pressures.

[JRL]

Another post I'll make crystal clear.
If you have more than 3/32" tread difference and you have the old viscous coupling AWD , (no Haldex pre 2003), you're making a good case for your Angle gear or your driveshaft to go south. (fail)
You MUST keep the tires that close, rotate as often as you can (every 4K miles is good) and for God's sake do NOT use the spare donut for more than 20 miles as it is WAY off in diameter.
Oh and you CANNOT buy ONE tire as a replacement or TWO, you MUST buy all four (4)
JRL

[philosophicaldreamer]

Is it me missing something in regard to angle gear? Or the whole thing is blown out of proportion? You don't hear so much about the catastrophic failures of those gears, and I cannot believe that all those early XC are meticulously maintained by their owners. I am certain that there is very high percentage of XC owners if not most of them who are not rotating their tires religiously, and they don’t seem to have all the purported problems. On top of it Subaru had used the same technology, and they have not encountered any problems. Are we really talking about something that the owners of early models should worry about? What is the real story here? Is this just another urban legend?

Ta-ta, j.

[JRL]

I sell these, trust me after 100K miles and if the tires were not kept close to equal, there have been/are many problems.
But, hey if you don't care, don't bother to do what's required to maintain these XC's
Subaru technology is quite different and who cares, the rest of the car falls apart after 100-125 miles abyway.
THEY WILL START TO GIVE PROBLEMS if not maintained this is NOT an urban legend.
I have had to replace driveshafts in two of these in the last 4 months, both had about 100K miles and BOTH had grossly mismatched tires when I bought them.
Why don't you ask a large dealer who services a lot of these how many they have repaired
You just keep on philosophically keep on dreaming and don't care for your XC, but don't come crying when you have to spend upwards of $1500 (or more).

[philosophicaldreamer]

You just keep on philosophically keep on dreaming and don't care for your XC, but don't come crying when you have to spend upwards of $1500 (or more).

What you are pointing out is the problem: I have talked to two dealers in Nashville, and they all denied any unusual problems. To be honest I am more inclined to believe you than them, since, in my humble opinion, there is a reason why dealers are one of the least trusted professions in this country. This is why I stick with my trusted independent Volvo mechanic, who has managed to save me hundreds of $ by doing what actually is needed and not what dealers want me to do.

Ta-ta, j.