True or False?

[GrtArtiste]

My job is in automotive marketing research. I read customer surveys. Today I read something I've never seen in over 10 years. The claim was that the dealer defrauded the customer by insisting that the blowout of one tire on an AWD vehicle necessitated the replacement of all four tires. Any truth to this?

GrtArtiste

 

[d00df00d]

If and ONLY if the tires had really deep treads AND all of the remaining tires were thoroughly used, then I can see how the dealer could argue that all four should be replaced.

Otherwise, yes, sounds like fraud (or at least a gross exaggeration) -- intentional or otherwise.

 

[sciphi]

My mom had to get the center differential on her Subaru replaced when she ran 2 different sets of tires, with different tread depths, on the car. The car would shudder violently around even small turns because the differential had locked itself shut from the speed mismatch of the different-diameter tires. She saved $200 on tires right then to spend $1000 on that differential and another $400 on new tires 6 months later. It's a "feature" of the car that it needs 4 new tires when one blows out.

Uninformed customer ranting. Ignore.

 

[Stu_Rock]

True.

The other choice is to obtain an identical replacement tire and have it shaved down to the tread depth of the remaining three.

Only specialty tire shops are going to have equipment to do that, so I can see a dealer wanting to keep the work in-house and just replace all four tires. The ethical thing to do, however, would probably have been to mention the tire shaving option.

Here is Tire Rack's page on it:
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18

 

[tinmanSC]

There is some truth to this. Different size tires (even just different tread depths) can make the center diff work extra hard and wear, another issue is excess heat. All that energy from one tire rotating faster than the other needs to be dissipated. It ends up as heat (entropy, it's a killer). This generally does not apply to tires that are "new-ish", as the difference in size is minimal.

 

[Boomer]

Think you mean enthalpy, not entropy.

 

[Quattro Pete]

Originally Posted By: GrtArtiste
My job is in automotive marketing research. I read customer surveys. Today I read something I've never seen in over 10 years. The claim was that the dealer defrauded the customer by insisting that the blowout of one tire on an AWD vehicle necessitated the replacement of all four tires. Any truth to this?

GrtArtiste

Yes, it's possible under certain conditions. See second half of this article:
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18

 

[lasmacgod]

The owner's manual on the wife's 2005 Explorer with the automatic 4wd specifically stated that all 4 must be replaced at the same time when there is 4/32" or more tread depth difference, IIRC.

 

[PeteTheFarmer]

Some systems do require all four tires to match closely in diameter, so this was not necessarily fraud. I remember when my cousin bought her Jeep this was the first thing her husband researched. Some more info here.

http://www.4x4abc.com/4WD101/diffautoAWD_AWD.html

 

[d00df00d]

Well, I guess I stand corrected. Thanks.

 

[lyle]

Just buy one new tire and have it shaved to the tread depth of the other three. No need to buy four...save yourself some dough...

 

[PandaBear]

I don't think it is a fraud if they say it is their policy to only replace all 4 (for liability reason), and ask the customer to take the service elsewhere otherwise.

Can it be done safely? Sure, but how much can they charge by shaving the tire down and send the work elsewhere? What about the mismatch of difference in tire age related traction difference?

One reason I'll never own an AWD or 4WD vehicle.

 

[JOD]

Originally Posted By: Stu_Rock

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18


There's quite a bit of misinformation, or more accurately error by omission in that article.

Differentials in all cars have to be able to cope with a difference of a couple centimeters at an absolute minimum. Otherwise, you'd have to adjust your air pressure every time you load the car with cargo or have passengers in the back. Rollout can very significantly within the cargo and passenger limits of the vehicle, 10mm or more (generally, sidewall stiffness is in the order of 230N/mm or so, depending on the tire).

If a car were sensitive to 1/4" tire differences (as Subaru claims) you'd literally seem them strewn by the roadside in droves. It's part CYA on the part of the auto manufacturers (something to blame in case of failure), and part urban legend: "my Volvo bevel gear blew up because I had the wrong size tires!". No, it blew up because you were either driving like an idiot or the bevel gear is under-designed...or both.

Secondly, the article doesn't differentiate between completely manual and electronically-controlled AWD systems. On Haldex units (where wheel speed is measured and sent to the system's computer) and other electronically controlled AWD systems, the difference is even less important. The manufacturer of Haldex states the system will tolerate a difference of up to 8% of rollout diameter. In an 18" wheel w/normal sized tires, that a difference of a couple inches, not mm's.

I certainly agree that putting on a completely different size tire (or donut spare) can damage some (though not all) systems. Claiming that a 3mm difference in tire is going to cause problems though is completely ridiculous.

 

[The motor guy]

I have this problem on my dodge ram and was wondering. I my alignment was off and it sort of wore down one tire more than the other. I think it wore it down by about 6/32" more than the other tire. Is 6/32" going to cause a problem? I rotated my tires to the back after having an alignment done. Will my differential still apply power to both wheels even though one tire is spinning slightly faster than the other?

 

[CapriRacer]

I have struggled to understand the physics involved in this.

If one assumes that everything is rigid and positively connected and that there are only differentials in the axles and not in the transfer case, then even the smallest difference in tire diameter will cause the center tranfer case to bind. That would also mean that small differences in inflation pressure and small differences in tread depth should also result in binding - but they don't!

I have searched for a study that shows how rolling diameter is affected by tread depth as well as rolling diameter vs inflation pressure - and no luck so far. It seems intuitive that both of these would have an effect - and someone must have studied this to the point where they can quantify it enough to address potential issues in ABS, TPMS, and AWD systems. My search continues.

But typically 4X4 systems are only used where there isn't a good connection between the driving surface and the tires. That is, the vehicle is slipping to some extent and the purpose of using the 4 wheel drive is to utilize all the traction available. Keeping in mind that maximum traction occurs at 10% to 15% slip, then it makes perfect sense that differences in rolling diameter are acceptable in 4X4's.

But AWD systems are a different animal and the center differential (viscous coupling or whatever) has to absorb the differences - and that allowable differences would be highly dependent on the pieces involved. Some would be sensitive and some would not.

I would assume that failure rates would be dependent on how much difference there was - that is, a 6/32nd TDR (Tread Depth Remaining) would have much more than twice the failure rate of 3/32nds - and that even then, the failure rate would not be 100%.

I would also assume that the temperature being generated by the system is what would drive failures. So ambient temperatures, vehicle speed, number and severity of turns, etc. would all affect the rate of failure. - AND - it is conceivable that not all systems of otherwise identical units would be equally sensitive. In other words, this is not a "black and white" situation where either the unit fails or it doesn't - it's more a situation where a certain percentage of units fail. It's been my experience that consumers react pretty strongly to even hundredths of a percent failure rates if the repair cost is high.

 

[d00df00d]

Good post.

 

[tinmanSC]

Originally Posted By: Boomer
Think you mean enthalpy, not entropy.

The Second Law of Thermodynamics, Entropy, says that all energy must eventually become heat.

My understanding (I'm an economist, not a physicist) is that Entropy, not Enthalpy, would govern the increase in heat energy as a result of kinetic mismatch in a diff.