Tire Tread

jeep
tires
cherokee

#1

I drive a 2004 Jeep Cherokee with 112,000 miles. Yesterday, I had a bad vibration and an oil burning smell. When I took it to the dealer who just saw the car last week, he told me it was the differential had burned up. He told me that the cause was the different tire tread between the front and back axals. Is he pulling my leg? The tires are the same size but from different manufacturers so they have different tread type. Does that really make a difference beyond driving control?


#2

The TREAD is unimportant…It’s the diameter of the tires that’s important, Measure the circumference of a front and rear tire. They should be within 1/2 inch of each other.

Now a few questions…Is this an AWD vehicle, full-time 4wd?? Or can you shift into 2wd?? What differential burned up??


#3

Caddyman’s advice is sound. Many years ago I put 2 new tires on my wife’s 86 Dodge Colt. Same size but different manufacturers. I measured the tires and the size difference was considerably more than tread wear would account for between the two manufacturers. Four new tires solved the problem.

Is your Jeep Full Time or Part Time 4wd? Full Time or AWD is very sensitive to tire size.

Ed B.


#4

I can shift it to 4 wd part time and 4wd full on the fly. It is the front differential. How exactly does the size of the tire matter as long as the same tire is on the axal? Not being a machinic or playing one on TV, it is hard for me to understand why.


#5

That Jeep has the Quadra-Trac full time 4WD wheel drive system. It should be cleared up that the Quadra-Trac system is orders of magnitude more robust than the AWD of the Dodge Colt.

A continuous extreme difference in the rotation rate of the front axle and rear axle would result in burning up the clutches in the transfer case, not the front differential.

To quote a previous response to a similar question:

[i]Are you allowed absolutely zero toe in angle? (Toe in makes the front tires rotate slower than the rear tires… much more so than the change in diameter of 25k miles of wear).

The full time four wheel drive system in your Jeep allows a reasonable amount of slippage between the rotation of the front axle input and the rear axle input.

This slippage is facilitated by clutches (not unlike automatic transmission clutch packs) that are bathed (and cooled) in automatic transmission fluid.

This slippage is necessary to prevent overstressing the driveline while driving on pavement, due to the things like toe in, differing tire pressures, slightly different tire diameters, etc. All the things that make the front axle turn at a very slightly different rate than the rear axle.

Quite frankly, in order to burn these clutches out, you’d have to put go-cart wheels on one axle and drive down the highway in full time 4WD.[/i]

The problem is that your differential (which was very likely a more complex limited slip differential) finally let go after over 100k miles.

The fluid in the limited slip differential is NOT regular gear oil. It has to allow the clutches to grab without slipping very much. Which means it doesn’t lubricate as well. Combine this with all those miles of clutch plate friction material floating around in there grinding on the gears and you should be surprised it lasted this long, but it did.

The transfer case allows a certain amount of slippage to persist before pumping up the clutches to lock in the front axle.

In short, no way. The dealer wants to sell you 4 new tires in addition to rebuilding the axle.

$$$$$+$$$$=$$$$$$$$$

To put this in perspective, I did a little calculation using typical alignment data and found that with identical tires, moving down the road at 70 mph, the front axle spins 16RPM slower than the rear axle.

That’s the equivalent of running tires on the front axle with 1/2" larger circumference than the rear (provided toe in is eliminated).

Something breaks at over 100k miles and suddenly the owner has done something wrong and it’s his fault and he NEEDS to buy $700 of tires…


#6

Thank you. Now this makes sense to me. I am not sure how I am going to bring this up to the dealer, but you are right. How can after 100,000 miles can this be my fault? Nothing has really changed over those miles. Thanks for explaining it to me.


#7

Placing fault doesn’t really change anything. You still have to pay for the repair. In many AWD systems, having well worn tires on front and hardly worn tires on back is enough to cause a problem. If for instance, you never rotated the tires, this condition could exist.


#8

If you have been using 4wd full time then it will burn up your differential dependent on the difference in wear between the front and rear tires. The item that counts is not size of tire but the actually rolling circumference.

Basically you have a center differential that allows some slippage between front and rear axles for turning. When you drive straight it barely slips however with turns more and when you do a sharp turn you near the limit of slippage. Adding a different size tire puts you near the limit slippage more often and when making sharp turns likely beyond the limit damaging the part.

Every car maker is different on how much difference in circumference is allowed. Subaru officially states 1/4" my mechanic say 1/2" is okay, he said 1" physically breaks the internals of differential over a period.


#9

Have you read the owner’s manual? Likely it warns you about mixing tyre sizes due to this problem. If they warned you and you failed to read the book or follow the instructions, … well I don’t think there is any question of who failed. Sorry.