Car Tire Question Smart Guys/Girls

[firerescuefin]

Took a tire into Discount Tire today for warranty replacement. They were willing to replace the tire, but stated because the new tire would have more than 4/32 difference to other tires on the car (A4 quattro) they recommended replacing 2 or all 4 tires.....or possibly do damage to the drivetrain. I get the general idea regarding not wanting a tire with a greater circumference on one wheel, but I am having a hard time buying this one.

[AngryScientist]

Took a tire into Discount Tire today for warrant replacement. They were willing to replace the tire, but stated because the new tire would have more than 4/32 difference to other tires on the car (A4 quattro) they recommended replacing 2 or all 4 tires.....or possibly due damage to the drivetrain. I get the general idea regarding not wanting a tire with a greater circumference on one wheel, but I am having a hard time buying this one.


they speak the troof. your audi uses fairly sensitive electronics to sense wheel slip with the traction control system, different diameter wheels can throw off the balance with the abs and traction control. additionally, you'll put additional stress on the cars differentials with a different diameter tire, as the wheels will rotate at a slightly different speed, every. single. mile.

that said, each awd manufacturer has a recommended allowable diameter difference. if i remember correctly audi's is more generous than subaru. your owners manual will definitely tell you exactly what that difference is.

if you search this on tirerack there is an article detailing the same information. additionally, tirerack will "shave" a new tire for you if you're out of spec to get you to the right diameter, so you only have to buy one tire.

[AngryScientist]

from tirerack:

"I often hear from customers who have damaged one tire on their AWD vehicle that they would like to buy one tire to replace the damaged tire.

Simply installing one new tire runs the risk of drivability problems or expensive drive line damage. I recommend they consider a set of four depending on availability and the tread depth of the three remaining tires. But replacing the other three partially worn tires along with the damaged tire does increase the cost. Another option to consider is shaving the new tire to the current tread depth of the tires that remain on the vehicle.

By matching the tread depth of the replacement tire to the tread depth of the partially worn tires that will remain on the vehicle via the removal of tread rubber from the new tire using a specialized machine that operates as a tire lathe may seem counterintuitive, the value of the mileage sacrificed is considerably less than the cost of rebuilding worn drive line components.

Tire shaving will range from $25 to $35 per tire and is significantly less than the cost of unnecessarily replacing the remaining two or three good tires that still have lots of mileage remaining. But if the tread depth on the remaining tires is 5/32nds or less, shaving may not be the best choice.

Some manufacturers have set parameters for matching the tires used on their four-wheel drive and all-wheel drive vehicles.

* Audi: As stated in their owner's manual, "rolling radius of all 4 tires must remain the same" or within 4/32-inch of each other in remaining depth
* Porsche Cayenne: Within 30% of the other tire on the same axle
* Subaru: Within 1/4-inch of tire circumference or about 2/32-inch of each other

Refer to your owner's manual for additional recommendations from other Original Equipment vehicle manufacturers"

[firerescuefin]

AngryS,

Thanks for the quick response....The guys at Discount have never done me wrong....and are willing to give me credit on the other 3 towards new ones....looks like that's direction I am going...thanks again.

Geoff

[AngryScientist]

AngryS,

Thanks for the quick response....The guys at Discount have never done me wrong....and are willing to give me credit on the other 3 towards new ones....looks like that's direction I am going...thanks again.

Geoff


thats what i'd do geoff, i tend to keep my cars forever, and like to minimize anything that accelerates drivetrain wear. add to that, you'll probably never know the problem until an emergency (panic) situation where you are relying on the cars abs and traction control, and thats not the time for surprises. cheers.

[wintermute]

I've got nothing but good stuff to say about Discount. I had a set of Kumhos on my (now deceased) Volvo S60R that I wasn't happy with. I'd driven them a few thousand miles and discount gave me a killer trade in on a new set of tires. Plus they will match prices and do the free rotations (or in my case, putting on the snow tires). Great place!

[dookie]

they speak the troof. your audi uses fairly sensitive electronics to sense wheel slip with the traction control system, different diameter wheels can throw off the balance with the abs and traction control.

by my math, 0.125" (4/32") tread depth difference is ~0.79" circumference on a 225/50-17 tire (current A4 spec).

sure, modern traction control systems can detect this difference. that said, the designers are a bunch of fools if this 1% difference actually affects the performance of the system...much less causes drivetrain damage.

so, i'm calling BS. checked with your local audi dealer to see what they have to say?

(for the record, i am a mechanical engineer who has been involved in traction control systems design for grassroots motorsports applications.)

[Shoeman]

Driveability issues yes especially on the front. Damage to the drive train I say highly unlikely.

[rice rocket]

by my math, 0.125" (4/32") tread depth difference is ~0.79" circumference on a 225/50-17 tire (current A4 spec).

sure, modern traction control systems can detect this difference. that said, the designers are a bunch of fools if this 1% difference actually affects the performance of the system...much less causes drivetrain damage.

so, i'm calling BS. checked with your local audi dealer to see what they have to say?

(for the record, i am a mechanical engineer who has been involved in traction control systems design for grassroots motorsports applications.)


I beg to differ; my S2000 could detect much less: my alignment was off (okay, I bent the control arm, so it was -3 deg cambered in on one side, and 0 deg on the other) and it kicked on the ABS light.

[AngryScientist]

checked with your local audi dealer to see what they have to say?




do you really think any audi dealer is going to disagree with what's in the owner's manual to save someone $$ at a tire dealer? that information arrived in the owner's manual as a result of conservative engineering. how conservative? that's a good question, but would you risk it with your own car to save a few bucks?

as i mentioned, it might be perfectly fine right up until you want the abs to save you from hitting a telephone pole, or it might be the difference between 150k and 250k out of your drivetrain, again, why risk the unknown.

[dookie]

again...detect, sure. matter, should not. legalese? not my arena.

[vqdriver]

A few years into buying our MDX, which is AWD, my wife destroyed the sidewall on one tire so I replaced it, only that one. Later that year she did the same thing to another tire so I replaced that one too. the tread difference was large so the old pair were put in the rear with the two new tres up front.
but whatever that difference in diameter was it was completely overshadowed by tire squash due to the weight of the car. Front squashes more than the rear. This was back in 07 and my diff hasn't blown up yet and tcm hasn't had a hiccup. Which leads me to believe that there is some good amount of tolerance built in.

Of course each car is different so YMMV and FWIW and all that, but that's our experience.

[RPS]

as i mentioned, it might be perfectly fine right up until you want the abs to save you from hitting a telephone pole, ....snipped.....

What's your opinion on a compact spare's affect on ABS systems in general? If 4/32 of an inch is a potential problem, what about diameter differences measured in inches? :confused:

I don't know anything about A4s and drivetrain wear, but ABS having problems stopping a car seems like reaching to me. If that severe, I doubt compact spares would be allowed at all.

[rice rocket]

again...detect, sure. matter, should not. legalese? not my arena.
It matters for sure when it kills ABS.

Also, if you have clutchpack differentials anywhere on the car, you'd wear 'em out pretty quickly since they'll be constantly slipping. Might not matter on open diffs or Torsens as much.

[phlash23]

I own a Subaru WRX and it matters for my AWD car. Good friends with people who work at the dealer and have heard several examples (although a bit more extreme than your case) of transmissions and differentials being ruined by mismatched tires.
IMHO the cost of a coupe tires is cheap insurance against new drivetrain parts.

[dogdriver]

I can't speak to potential drive train damage, but I'll bet that different tire circumferences will drive the car's brain nuts. Anti-skid braking and the electronic stability control (or whatever Audi calls it) both function by comparing wheel rotation rate (through anti-skid transducers-- basically a rotation counter at each wheel), steering wheel position, and accelerator and brake pedal inputs. When the car senses a difference in rotation speeds with the brake depressed without a steering input, it assumes that the slower spinning wheel is skidding and activates the anti-skid (usually pulsing the brake) for that wheel. Opposite for a faster spinning wheel when accelerating, or a skid in a turn. A constant difference in wheel rotation due to different size wheels probably activates a fault mode eventually and shuts off the affected systems. No idea what the tolerance is, but I'll bet that you'll get an ABS and ESC light if the difference is too great.

At a minimum, you're denying yourself two very effective systems (I've played with my wife's Mercedes E320 in the Walmart parking lot on a slippery night and both are awesome-- yes, I'm a geek, but it was pretty cool) that increase your safety.

My opinion, I could be wrong, Chris

[rice rocket]

You give Audi too much credit.

Most systems only compare wheel speed sensors. Only Mitsubishi, Nissan, and a handful of supercar makers use yaw and steering angle sensors.

[dookie]

fyi...

service manager @ audi in cary, nc says no worries with the stated tread depth differential.

[AngryScientist]

What's your opinion on a compact spare's affect on ABS systems in general? If 4/32 of an inch is a potential problem, what about diameter differences measured in inches? :confused:
.

Compact spare tires will be the exact same diameter as the car's stock tires, just less wide, that's what makes them compact, not the diameter. just for the record.

dookie - i hear what you're saying, i'm just putting out the "by the book" answer, which you really cant go wrong with.

[Bob Loblaw]

We are looking at a new differential, and already did the transmission, in our '99 Subaru Outback. The fellow who did the tranny said both problems were probably at least contributed to by inconsistent tire pressure. All the tires need to be within Subaru spec always, or it over-stresses the AWD system.

It makes sense to me.

BL

[benb]

I'm sure the reason there is more tolerance in motorsport for this kind of thing is lifetime of the tires...

On a race car you might mismatch the tires to save a little money, but the tires are destroyed in one day, possibly a couple hundred miles. So you put extra wear and tear on the car for a couple hundred miles or even less. And you end up replacing components on the car much more quickly anyway...

Say the streetcar is wearing tires with a 60,000 mile tread life. You get a flat after 15,000 miles and put a mismatched tire on.. now you're going to put acccelerated wear on the drive train for 45,000 miles... much bigger potential for wear.

[Ken Robb]

I think anyone with racing experience would say that 1,000 miles racing causes more wear on components than 50,000 miles of normal highway driving.

[blantonator]

I would only replace two tires, just for keeping the alignment correct. I know early Volvo V70 CC were very sensitive to tire diameter, but I've never heard about this for audis. Best check at audiworld.com or the vwvortex.com.

[firerescuefin]

All,

Thanks for your comments/expertise. I purchased 4 new Contis this morning. As stated previously, Discount Tire gave me a pro rated discount for my other tires. I am very happy with the way they handled it.


Geoff

[christian]

I think anyone with racing experience would say that 1,000 miles racing causes more wear on components than 50,000 miles of normal highway driving.If there were less than 6 miles left in a rally stage, we'd drive out on the rim, rather than change the tire, because the time you lose driving on a rim is less than the 1:30 it takes the driver and co-driver to change the tire.

This was, obviously, not good for the center diff (which, with the transmission, cost about the same as an Audi A4), but then, they were rebuilt after every race.

On the street, I would not run mis-matched tires on an AWD car with clutch diffs. On a FWD car, I just move the bent rims to the rear! :D

[blantonator]

If there were less than 6 miles left in a rally stage, we'd drive out on the rim, rather than change the tire, because the time you lose driving on a rim is less than the 1:30 it takes the driver and co-driver to change the tire.

This was, obviously, not good for the center diff (which, with the transmission, cost about the same as an Audi A4), but then, they were rebuilt after every race.

On the street, I would not run mis-matched tires on an AWD car with clutch diffs. On a FWD car, I just move the bent rims to the rear! :D

unlike haldex awd systems, the Audi is a Torsen differential and doesn't use clutches. I wouldn't worry about driving with slightly mismatched tires, other than alignment issues. Haldex systems are more sensitive. I dunno about the fluid clutch that subbies use.

[Ken Robb]

If there were less than 6 miles left in a rally stage, we'd drive out on the rim, rather than change the tire, because the time you lose driving on a rim is less than the 1:30 it takes the driver and co-driver to change the tire.

This was, obviously, not good for the center diff (which, with the transmission, cost about the same as an Audi A4), but then, they were rebuilt after every race.

On the street, I would not run mis-matched tires on an AWD car with clutch diffs. On a FWD car, I just move the bent rims to the rear! :D

You sound like my pal/neighbor/navigator/driver Satch Carlson

[RPS]

Compact spare tires will be the exact same diameter as the car's stock tires, just less wide, that's what makes them compact, not the diameter. just for the record.
Well, since it’s on the record. :no:

With all due respect, where do you look up information “FOR THE RECORD”? Or is stating as such suppose to make it sound more authoritative?

For the “correct” record, compact spares are almost always smaller in diameter than the regular size tires on most vehicles. On my Odyssey the sizes are 26.6 inches in diameter and 24.5 inches in diameter – or a difference of 2.1 inches. That’s like in the order of 8 percent. In cycling terms it’s almost exactly the difference between 700C and 650C tires in 23C widths. Needless to say it’s a lot of difference.

Vehicles I’ve owned previously with compact spares were all the same – spare much smaller in diameter than the regular tires.

[rice rocket]

If you have a smaller diameter compact spare, it won't be on an AWD car. If you blow a tire on a driven wheel, you're supposed to move a "good one" from a non-drive wheel and put the spare in it's place.

[victoryfactory]

My '92 Jeep Cherokee would not switch in/out of 4WD if even
one tire was under inflated. I guess that's the price we pay for computer
controlled AWD vs the old school mechanical stuff. I do believe that even
the most crusty off road Jeep types will eventually give in and the best off
road performance will be from auto tranny/computer controlled AWD vehicles.
The idea of shifting gears manually and choosing a 4WD mode will become a quaint throwback.

This morning I bashed out of a 16" snowbank in front of my driveway
in my '09 Forester. That thing has been up to any test (well there was that
sand pile in Utah. Had to back down but I blame the street tires, not the AWD)

btw.,.. Great tread, er thread

VF

[RPS]

If you have a smaller diameter compact spare, it won't be on an AWD car. If you blow a tire on a driven wheel, you're supposed to move a "good one" from a non-drive wheel and put the spare in it's place.
The question was limited in scope to the affect on ABS. And since all four wheels have brakes, the question stands regardless of where it's moved.

[AngryScientist]

Well, since it’s on the record. :no:

With all due respect, where do you look up information “FOR THE RECORD”? Or is stating as such suppose to make it sound more authoritative?

For the “correct” record, compact spares are almost always smaller in diameter than the regular size tires on most vehicles. On my Odyssey the sizes are 26.6 inches in diameter and 24.5 inches in diameter – or a difference of 2.1 inches. That’s like in the order of 8 percent. In cycling terms it’s almost exactly the difference between 700C and 650C tires in 23C widths. Needless to say it’s a lot of difference.

Vehicles I’ve owned previously with compact spares were all the same – spare much smaller in diameter than the regular tires.


this comes as a big surprise to me, honestly. any car i've ever owned has had a spare that is the exact same diameter as the stock tire size. that has definitely been the case on my audi, vw, bmw, chevy S-10, my wifes Jeep and a few other cars. compact spares are skinny or course, to save space, but having a different diameter seems completely wrong to me. strange.

[victoryfactory]

It's funny how the marketing/ergonomics people at the car companies seem
to over rule the engineers on this issue. They have decided that consumers
should just call AAA and then buy 4 new tires every time they get a flat.

Even the "full size" spare that they sell for the Subaru to replace the donut
is on a different rim with a different tire and size! (haven't measured the circumference)

It seems to be about storage space and $$ with them. They even slap run flat tires
on many models and eliminate the spare all together to save space!

Then the sneak in some language about the maximum miles or speed when using
those "spares"

The only real answer is to buy a real one for $500. and strap it to the roof of
your car.

VF

[RPS]

For what it’s worth, I just read all 10 pages of the Honda Odyssey owners’ manual dedicated to changing a tire, and the only related information stated is that the Traction Control System must be turned off. Apparently all other electronics, including ABS, take care of themselves. Newer models than mine may be different but I doubt it.

Perhaps surprising to some, the diagrams specifically show the compact spare being installed in the front; the driven axle. No mention at all about avoiding certain vehicle placement. My guess is that as long as the vehicle has open differential(s) it doesn’t matter too much.

[AngryScientist]

They even slap run flat tires
on many models and eliminate the spare all together to save space!


VF

tell me about it. my new car came with runflats and no spare, worse, no place to even put a spare! i have bought a full sized spare and it lives in my trunk now, eating up valuable space. my biggest gripe with the new car.

[RPS]

this comes as a big surprise to me, honestly. any car i've ever owned has had a spare that is the exact same diameter as the stock tire size. that has definitely been the case on my audi, vw, bmw, chevy S-10, my wifes Jeep and a few other cars. compact spares are skinny or course, to save space, but having a different diameter seems completely wrong to me. strange.
Check the sizes -- it surprised me too. I did extensive research on this subject prior to installing a compact spare (actually a smaller standard tire) on the back door of my full-size travel van in order to avoid the tire hitting the driver-side taillight when the door is fully open (and tire is also lighter -- full size weighs 75 pounds), and not one example I compared was even close to same diameter. Because my large van is a one-ton I did decide that if I ever flat I’ll only install it in the front, even if it requires a double exchange.

[AngryScientist]

the Traction Control System must be turned off. .

good to know. that does confirm that differing diameters will affect the vehicles traction control system, (which likely takes at least some input from the vehicles ABS sensors). why they would have just not allocated a little more space and given you a full diameter spare in the honda is beyond me.

one thing is for sure, there is a full sized, real spare tire/wheel in my car that i can drive on for thousands of miles in the event of a tire failure on any of my tires. i work far from home, and often take prolonged road trips, i wouldnt have it any other way.

anyway, looks like the OP has solved his problem, all is well.

[AngryScientist]

Check the sizes -- it surprised me too. I did extensive research on this subject prior to installing a compact spare (actually a smaller standard tire) on the back door of my full-size travel van in order to avoid the tire hitting the driver-side taillight when the door is fully open (and tire is also lighter -- full size weighs 75 pounds), and not one example I compared was even close to same diameter. Because my large van is a one-ton I did decide that if I ever flat I’ll only install it in the front, even if it requires a double exchange.

you're probably right, now that i looked into it for compact spares. all of my cars have full sized, real wheels/tires for spares. they are definitely the correct diameter. in my book, thats the safest and most convenient option.

for what its worth, my new bmw didnt come with a spare, and i did extensive research on my options, and in the end, buying a separate wheel/tire to keep in the trunk was my best bet, considering how far from home i usually am.

also, depending on where you live, and when you get a flat (weekends/holidays) getting your real tire fixed while you have a doughnut on the car can be wildly inconvenient.

[palincss]

by my math, 0.125" (4/32") tread depth difference is ~0.79" circumference on a 225/50-17 tire (current A4 spec).

sure, modern traction control systems can detect this difference. that said, the designers are a bunch of fools if this 1% difference actually affects the performance of the system...much less causes drivetrain damage.

so, i'm calling BS. checked with your local audi dealer to see what they have to say?


According to everything I've read and heard about over the past 10 years or so, it's true. Here's what the Car Talk guys had to say on the subject:


Car Talk: Shortcut on tires will end up being very costly

Dear Tom and Ray: I recently had four half-worn snow tires on my '04 Honda CR-V with all-wheel drive. Living in upstate New York, I was driving in snow on back roads much of the time (as a rural letter carrier for the U.S. Postal Service). I had the bright idea to buy two new snow tires this year for the front wheels and use the worn tires for the rear, and then next season reinstall the newer tires on the rear and buy new tires for the front. That would eliminate having to buy four new snow tires at one time. However, the local Honda service manager advised against my plan, saying that the newer tires would have a slightly larger radius than the older tires on the rear (even though the actual tire sizes are identical), and this could cause problems with the all-wheel-drive system. Is he correct? -- Bruce

Ray: Sadly for you, Bruce, we have to say yes. However, think of all the tire dealers who are clicking their heels while reading this!

Tom: The anecdotal evidence suggests that all-wheel-drive systems can be damaged by using different-size tires on different wheels. Even if the tires started out as exactly the same size, if one pair is worn down, that difference in circumference can cause problems.

Ray: Here's what happens. On all-wheel-drive cars -- cars that are permanently in four-wheel drive or can be left in four-wheel drive all the time -- there's a part called a center differential. This is a clutch assembly or viscous coupling that's so complicated, we couldn't possibly explain how it works without using our hands. And maybe the hands of at least two or three innocent bystanders.

Tom: When the car is going straight down the road, the center differential doesn't have to do anything. It just sits there. But when the car turns, the front and rear axles need to turn at different speeds to keep the wheels from binding up and the car from flipping over. The job of the center differential is to allow the axles to turn at different speeds.

Ray: The problem is that if the front and rear tires are different sizes, then the front and rear axles are always turning at different speeds. Even when the car is going straight ahead. That means the center differential is always working -- and that wears it out.

Tom: And the center differential costs big money.

Ray: Maybe someday they'll make center differentials more robust, so they can take the constant use that different size tires require. However, that's not how they're made at the moment.

Tom: So it's better to be safe than broke, Bruce. Take the service manager's advice and replace all four tires at once. We would hate to see the Postal Service have to change its motto to "Neither snow nor rain nor heat nor gloom of night, nor a $900 center-differential replacement ... stays these couriers from the swift completion of their appointed rounds."
Sources: http://seattlepi.nwsource.com/wheels/316943_talk25.html