How often do you drive in a totally straight line for any distance? Not long. So the inter/intra axle differential systems have to slip most of the time you're driving anyway - anyone know off hand how much difference there is in tire travel between the four positions when say, making a turn a stop light, or going down a tight highway entrance ramp? How much "tire diameter" difference does that translate to?

There's a question that maybe someone with a tape measure and too much time on their hands can answer...

 

I would beg to differ with that. Now, if you drive while drunk all the time and weave, or if you are a pro slalom driver, you would not drive in a straight line, but in any kind of a trip involving distance, I generally go pretty straight for many, many miles at a time.

These systems are meant to allow for some slippage during cornering, but the center diff, if run with different size tires for any length of time, will definitely heat up and cause problems.

To calculate the effective tire circumference distance around a corner, you'd just have to take the radius of your turn and calculate the difference between the circle made by the inner tire and the outer tire--that difference would be roughly the track width of the car. So if you are turning say, a 100 foot radius circle, the inner tire would be going around the hundred foot circle, and the outer tire about 105 feet. So the difference there would be about 5%. On a 30 inch tire, that is about 1.5" effective difference. (The Escape's tires are ~29"--235/70 x 16's)

George

 

I'm still working on the questions "If a tree falls in the forest and there's no one to hear it, does it still make a sound" and "Does the light in the refrigerator really go out when you shut the door". I'll get to your question as soon as I figure those out!

 

i have an idea to fix all these problems. just buy a used tire that has about the same amount of wear. problem solved......................................

 

This is a *perfect* solution IF you can find a used tire with the same amount of wear while your car is sitting there with three tires on it. If you need the car daily, this could be a problem.

But if you are in a big city, you might well be able to find a junkyard Escape with a useable tire.

George

 

chances are you could do it. lots of tire shops that are little have fairly old tires and they sale them. i bought a set of 4 for my old F100 before i could afford brand new tires and they were pretty worn.

 

"Pretty straight" isn't good enough. Next time you're driving down a "straight" road, hold the steering wheel perfectly still and see how long you stay on the road...


Hence my point - it was stated earlier that 1/4 difference in circumference was large enough to cause problems, yet your figures (which ignore steering axle to non-steering axle differences too) indicate that going around a 100 foot radius curve is much more significant. Now granted, you (hopefully!) don't spend your driving life going around in 100 foot circles, but... neither do you spend most of your time driving perfectly straight.

So I'd say that while differing tire diameter is something to be aware of, the effects are probably exaggerated.Those of us who have been running 4WD for decades have long known that things generally work better with nicely matched sets of tires. But there's a limit to how closely matched they have to be.

 

Thanks for your thoughts...

I would say that during a 100 mile freeway stretch, even if I do steering corrections almost constantly, I am "averaging" a straight run down the road. Do you think that any one tire on a vehicle, going down the road, is going to turn more than 1% faster/slower than the others?

I absolutely know that fronts and rears are different, but my 5% example was meant as a quickie estimate.

I do think that the need to match tires is based entirely on the type of 4WD/AWD setup. If you have open differentials on both axles as well as the center diff, you should have no problems with slightly mismatched tires other than asymmetrical handling/braking. With a locked transfer case in 4-HI, we both know that you end up with some issues with drivetrain windup.

With a front or rear axle limited slip unit, you can turn momentarily and have no problems, but I'm guessing that if you drive in small circles with a front or rear posi, you will wear out the clutches prematurely.

And finally, with regard to AWD, I know that Subaru uses a limited slip center diff on it's manual trans vehicles that will lock up if the fluid shears due to different front/rear axle speeds. If you mismatch a tire or two, the fluid in the center diff is gonna heat up and try to lock the differential up to some degree. So you will eventually fry the unit. Turning a corner is probably too short a time period to significantly heat the fluid in the unit.

I'm guessing the Escape (like auto trans Subarus and most other small SUV's) has an electronic center diff that will apply some degree of lockup if the front and rear axles are not turning close to the same speed. So with a mismatched tire (or one with low air pressure, or a donut spare), it may apply some small degree of lockup even when you're going straight down the road. And this would eventually wear out the clutches prematurely. If you get a flat in an auto trans Subaru, you are supposed to pull the 4WD fuse (which puts the car into FWD only) and put the spare on the back.

Keeping four matched tires on one of these vehicles is probably much cheaper than replacing center diffs or other driveline parts. I'm thinking that with various stability and traction control systems, it's even more important to keep the tires matched.

Regards,
George

 

At this point there's plenty of information, manufacturer recommendations and opinions expressed. Rather than turning this into another marathon argument of points each owner should make their own choice .............. agreed?

Now on this tree in the woods matter....................

 

NP, that's what I'm here for...

Realize that it really doesn't matter what you *average*, all the matters is the instantaneous difference in rotational velocity. During each of the small turns to one side then the other that produce an average of "straight", your differentials are making up the difference. Changing lanes or small corrections on the interstate are probably going to be insignificant from a practical POV. But driving a country road? By your own calculations (which don't seem unreasonable) curves may cause more "mis-matching" than a 1/4 inch off in tire diameter.

Don't get me wrong, a very well matched set of tires will always be better than a mismatched set. But IMO and IME (in my experience) the importance of having exactly matched tires is probably overrated...

Also, another interesting point about wear and rotation. With all wheel drive vehicles, the tires on the front (the steering axle) tend to wear faster. That means to equalize wear, you should move the more worn tires to the rear when you rotate - exactly the opposite of what you should for maximum safety. So pick your poison...

 

The only way I think we could really reach a conclusion as to the damage (or lack of damage) that a mismatched tire would really cause is to know the exact operation of the center diff and its control mechanism (and maybe add the traction/stability control as well). I would hope that you could get a good distance to safety using a spare tire without damaging anything at minimum. Would any of those scan gauge things possibly be able to read parameters like "% center diff lockup" as you drive? That'd be cool IMO.

As for rotating, front wheel drive cars are even more extreme in terms of front tire wear bias. As a practical matter, if you rotate tires every 5-7.5k miles, there is usually not enough wear that the rears would be much balder. And during the next interval, the rears will end up identical in wear to the fronts. (Then the next cycle wears one pair deeper, etc.) One pattern that I like is to do the first rotation at, say, 3750 miles, then the next at 7500, then every 7500 after that. This way the fronts and rears are never more than 3750 miles apart in wear.

George

 

Agreed. The sensors and thus the data would already be there for the ABS. I'm not sure what you'd do with the information though... but my only real point is that there already a mechanism in your vehicle to handle mismatches in wheel/axle speed, regardless of the source. So try and keep the tires matched, but don't get lose too much sleep over it.

You're much more conscientious than me, kudos to you for keeping such a consistent schedule. I'm lucky if I can get the oil changed on time, I'm always way behind where I should be on tire rotation...