posted by
someone claiming to be Hugh
on
Wed Mar 2 12:04 CST 2005 [ RELATED]
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Hello,
I have a '99 XC with 60,000 on the odometer. I regularly rotate the tires to insure even wear. I follow the air pressure recommendations that Volvo has printed on the inside of the gas tank flap. Volvo recommends, to achieve the best fuel economy, to set the air pressure at 36 lbs in the front tires and 41 lbs in the rear. I've done that since I obtained this car (used) 5 years ago when it had 12,000 miles on the odometer.
I have read posts, where people have said you must keep the air pressure the same in all 4 tires, so that the circumference is identical. I do want to obtain the best mileage I can, but don't want to risk damaging the AWD. Yet, it appears, based upon my experience, I haven't destroyed the AWD. Of course, I may be lucky, and now my luck will change due to the years of having the front and rear tires set at different pressures. I would appreciate hearing other people's thoughts on whether I can safely follow Volvo's recommendations regarding economy air pressure settings, or should I stick with keeping all 4 tires at the same pressure. Thanks to all who respond.
Hugh
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posted by
someone claiming to be Bob
on
Thu Mar 3 11:06 CST 2005 [ RELATED]
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Personally I think the issue is exaggerated...
Not to say the system doesn't fail, but I'd be willing to bet in the vast number of failures, it's because the owner either drove for too long on the compact spare, replaced a bad tire with one of a different make than the others or didn't want to spend the money for two or four new tires when a tire went bad, so they put a brand new one on with three much more worn tires on the other wheels.
I did the latter (same brand and type of tire) on my Jeep Grand Cherokee AWD with 30K miles (there was no mention in the manual not to) and burned up the viscous coupling within a few hundred miles. Fortunately I was still under warranty. My business partner had the same thing happen on his Grand Cherokee two months out of warranty and it cost him $1600.
This is obviously a common problem in many AWD systems and seems to be a dirty little secret. At least Volvo warns you about it. I think the manufacturers are afraid to tell people they should replace all four tires if they get a flat and figure any failure of the AWD system as a result of mismatched tires will most likely occur out of warranty. It will only happen if the other tires are worn down enough to put too much strain on the system as it tries to balance power to the new tire, which is larger as it has more tread. If the other tires are that worn, most likely the car has too many miles on it to still be under warranty.
Subaru has there own failure mode under these circumstances--it depends on which part is the weak link.
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posted by
someone claiming to be Hugh
on
Thu Mar 3 11:41 CST 2005 [ RELATED]
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Bob,
Thank you for your thoughtful and thorough response. I think you are right about the "dirty little secret" senario. If buyers were told up front, tha by buying a vehicle with AWD, they may at a mininium be forced to buy two new tires, should they get a flat, or possibly four, they would pause or think twice about buying a car with AWD. I recall asking a veteran Volvo Service Manager, about what happens if you need to replace a tire on an XC. This was about a year after I bought the '99 XC and Volvo was selling the next generation of the XC that was based on the S80 platform. That model came with as original equipment, the Pirelli Scropian tires. They looked huge to me. The service manager said nonchalantly, that all four tires would have to be replaced, and the cost for the tires alone, would be nearly a $1,000. I was staggered by this comment. I had actually purchased the '99 XC for my father-in-law, who drives my small children to school each day. So I was "locked in" already.
I liked and still do the '99 XC, (my father-in-law's last car had 4WD - a Toyota Carolla wagon and he wanted another car with that capability) but I personally, would think twice about buying a car with AWD. You must constantly monitor the tire wear by rotation, and when necessary, be required to replace two if not all four tires (that otherwise still have tread) to protect the AWD system. I have two '96 850 N/A wagons with 5 speed manuals. I enjoy a significant advantage in gas mileage over the '99 XC. I realize the '99 XC's mileage is handicapped by the automatic transmission and the AWD system (together with its additional weight), versus the manual transmission.
Bob, I think you are right, that with the exception of Volvo, most car manufactures don't discuss the inherent weakness of a AWD system, because when it fails due to improper tire size, the vehicle will be out warranty.
Hugh
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How much is too worn? We had a flat with our '98 XC when the tires were all at 8/32nds of remaining tread (80% left). Beomg cautious as a resutlopf this board, I put the snows back on while I figured out the replacement, and contacted Volvo directly to see if new tire with only 2/32 (1/16th of an inch)was OK. they replied that I should buy all four new tires. fortunately, with teh freedom of teh snows, I looked around and found an exact brand, model, and depth of tire used on ebay for $45 delivered.
Otherwise, our '98 has been incredibly good, with almost nothing wrong either under warranty or subsequently.
--
Mike Sullivan ('91 745 (184k), '95 855T (70k), '98 V70XC (83k). Past Volvo's: '85 744 (256k), '86 245 (165k), '86 245 (195k), '88 745 (208k), '93 965 (147k) .
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posted by
someone claiming to be Bob
on
Fri Mar 4 02:50 CST 2005 [ RELATED]
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I think as long as you keep the two front tires the same and keep the two rear tires the same you would be OK with most AWD systems, so you would normally only have to replace two tires if one goes bad.
Also, if the tires aren't "too" worn, it's probably not a problem--but how do you know if the tires are too worn? I had about 30K on my Jeep tires and certainly wasn't planning on replacing them for another 10-15K. My partner had about 38K on his tires. This is a statistical issue--when it will break is determined by how much stress it has been under and for how long. If the tires are really mismatched (like when using the mini-spare) it can fail within hundreds of miles. If the tires are only a little out, it will take much longer and you may end up replacing all the tires before it fail, so you never see a problem.
I've had a number of both 4WD and AWD vehicles and I still prefer AWD, even given the tire issues, both for better handling and better gas mileage. With most of these systems you are normally in 2WD (front or rear, depending on the manufacturer--Volvo is front). That's why they handle better and get better gas mileage than 4WD. As wheel slip is detected, power is automatically transferred to the other wheels. Most of these systems simply aren't designed to be driving all the wheels all the time and will eventually break under those conditions (this is also true of part-time 4WD systems but you generally wouldn't drive a part-time system engaged all of the time as they tend to be obviously noisier and rougher). You can also damage limited slip differentials on 2WD vehicles with mismatched tires.
By having one tire significantly different in circumference, that tire is actually turning at a different speed than the other tires to keep up, the AWD system intereprets this as wheel slip and begins apportioning power to the other wheels all of the time. The kicker is, unlike part-time 4WD, where you have conciously engaged the system, you have no idea this is happening on an AWD vehicle until it breaks. The new Haldex system will disengage rather than damage itself, but you won't have AWD and you won't necessarily know that!
Personally I have found the AWD system on our 2001 XC to be the best I've had (even compared to my Jeep and Audi Quattro), but with nearly 50K on the odo and no extended warranty, I'm looking to trade-in before some of the really expensive problems start happening (AWD system, throttle body).
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Go with the manufacturers recommendation.
Slightly different air pressure front to rear does not effect tire circumference to any great degree. What is very important is to have identical pressure front to front and rear to rear, along with equal wear.
Armie
--
'94 850 154000kms '86 760GLE 272000kms
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posted by
someone claiming to be Hugh
on
Thu Mar 3 11:41 CST 2005 [ RELATED]
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Arnie, Thank you for your reply. Hugh
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posted by
someone claiming to be tgrumaj
on
Thu Mar 3 08:31 CST 2005 [ RELATED]
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Like you I have a 99 and have consistently used the alternative pressure of 32 front and 33 rear. (Gives a much less bouncy ride!). I think that even if you used the same pressure the weight distribution in the car would create differences in the size of the tire. (Note how the front tire often look flater due to engine weight.) Just keep the tires rotated...keep the pressure as required and you should be fine. I have 84k and so far so good.
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posted by
someone claiming to be Hugh
on
Thu Mar 3 11:46 CST 2005 [ RELATED]
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Tgrumaj,
Thank you for your reply. I'm glad to hear your '99 XC has made it to 84,000 without incident. I hope mine does the same. I actually purchased the '99 XC used for my father-in-law. He drives it in a conservative manner, and does take care of it. I help him with maitenance, i.e. oil changes, tune-ups, anti-freeze flushing and washing and waxing. My hope is the car goes for atleast a 150,000 miles with only ordinary maintence being required.
Hugh
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