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Replacing *some* tires on an AWD car


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Posted (edited)

Hi all,

It's been a while since I posted here. Hopefully the forums are still active.

Long story short, can I just replace 1 or 2 tires on an AWD car instead of replacing all 4 tires at once?

I understand why a tire shop recommends to replace all 4 (safety, wear & tear on vehicle, and profit motive). I'm cheap, and I'm curious, so I want to try to salvage what I have.

I am hoping to find a used tire to closely match the other 3 on a set (same size with a similar tread level remaining) with the goal of minimizing the difference in circumference. So how much leeway would I have here? I found one decent reference, but other than that, just a lot of fear on the internet. This article indicates 2/32 - 4/32nds should generally be okay: https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=259

Would it matter if I put the different tire on the front vs rear? What about replacing 2 tires so that at least each axle has a matched pair? Would that be better than replacing a single tire, or with open diffs would it not matter? I'm thinking if I had to replace two, then it might actually be better to put them on opposite corners so as to even out the speed front-to-back. I believe I'm overthinking this now.

For background, I have an IS250, which I believe has open diffs at the front and rear. Centre diff appears to be clutch pack type. The reason I ask is that I have 2 sets of wheels and tires (winter/summer) and each set now has 1 tire with an irreparable leak. :angry:I really don't want to replace 8 tires this year, especially considering I might sell this car soon. 

I realize there will be no perfect solution here other than to buy a matched set of 4. I am also aware I can buy a 1 new tire and pay to have it shaved down. This post is not about those options. Those will be my backup plans. Curious what others have done in this situation and if there have been any real life lessons learned.

Edited by Beau
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Maybe I just drank the Koolaid for a bit too long, but here is my 2 cents.

As a Parts guy in my previous life, I was always the bad guy when it came to this with customers. I was taught that while you can do it, and maybe never have problems, it is strongly suggested to do all 4 tires at once on any AWD/4x4. The biggest reason I ever heard was that it can cause issues with the drivetrain somehow by having different tires, because a "new" tire is not going to be exactly the same diameter as whatever is already on the car, due to previous wear. I honestly don't know if they are that sensitive, and I have been there too, where I only needed one tire. Some places will straight up refuse to do it. 

My personal recommendation... that's a nice car, and I would go with a whole new, matching set, especially if you are taking it out on the track. New tires are always a great feeling on any ride. Unless you are really hurting, buy the entire set.

 

Dason  

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Thanks for weighing in on this, Dason. I appreciate the recommendation. I may end up going that route, just curious if there are other viable alternatives. Like you, I also don't know if my AWD system is that sensitive. I suspect there's gotta be some allowable margin. Just how much is the question.

FYI this is not a track car by any stretch. It's just a boring "daily" driver, although these days it's more like "once a week" driver since we don't go anywhere! :P

 

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Is the car full-time AWD? or does it only send power to the front when slippage occurs? If it's mainly RWD (part-time AWD), you can probably get away with swapping just the front or rear two. But if it's sending power to all four all the time, yeah, I wouldn't have just one tire spinning at a different rate.

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I believe it is full time AWD. But I don't have a lot of info on how it really works. Lexus doesn't really "promote" their AWD system technology like some other MFRs. I agree with you that part-time (detect slippage) systems are probably quite different when it comes to mismatched tire sizes.

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Posted (edited)

Realistically, it just depends on the rolling diameter of the replacement tire compared to the rest of the tires. 

A lot of tire and car dealers have focused on the point that different circumferences of the tire cause driveline wind-up, and therefore must cause driveline damage. Therefore, all tires must be replaced at the same time...which happens to be a great scource of income to those who follow that theory! However, that statement is taken out of context by looking at the extreme side of the story.

On every car (2WD, AWD, 4WD) the tires are not exactly equally tall, anyways. Equal tire pressure on all four tires will give you a more compressed tire on the axle with more load on it.  That can be temporary, like under hard braking or cornering, or it is all the time due to static weight distribution. Furthermore, people don't check their tire pressure on a regular basis, and their tires won't be equally inflated, meaning one or more tires will have a smaller rolling diameter than the other one. 

Cars are also not always travelling in a straight line. The only reason why vehicles have differentials is to allow for different wheels speeds as required when travelling through a curve. Therefore a difference in wheel speed is a constant occurence for the drivetrain of the vehicle.

It is for that reason that manufacturers are allowing to replace only one tire as long as its difference in wear is within reason to the other three. The previously mentioned 2 to 4/32nds are quite common. Of course the idea is to replace with exactly the same tire, since tire height differs somewhat from model to model. It is also logical that they expect a tire matching the rest of the set, since different model tires also change the handling of the vehicle.

 

So what is happening with one tire being a litlle bit taller (newer) that the other three?

- on a 4WD: when mounted on the non-driven axle, just a minor amount of walking of the side and spider gears. If your differential has oil in it, then you won't hurt anythng at all, since most axle disconnect systems do exactly that, anyways.

- on a AWD: part-time AWD see above. Full-time AWD: the speed difference is neglegible to the axle differential, and is reduced by half before it goes to the center differential. Small differences in speed between the axle shafts or the two drive shafts do not trigger any limited slip or locking devices, because small differences are happening constantly while the vehicle is being driven. No damage will occur.

- on a 4WD in 4WD, or a part-time AWD in 4WD: those transfer case settings are only to be used in low-traction scenarios. In those conditions it is extremely common that one wheel will spin faster the the rest. Because of the conditions the vehicle is travelling in, driveline wind-up and consequent damages are not an issue at all. Spinning one tire too fast because of heavy throttle is causing the most damage in those situations. Different tire diameters play no role in low traction environments.

- on an AWD in AWD mode in low traction conditions: axle or centre differential may add the wheel speed difference to the occuring amount of tire slip, and may activate center or axle locks slightly earlier than it would otherwise. Since the system is made to do this, and the vehicle is on a surface where driveline wind-up is not an issue, no damages occur. The same holds true for electronic differential brake lock...the chances that the EBL will come on are next to nil, since some speed differential is normal.

 

In conclusion: Replacing all four tires is always the best option, since you "refresh" your ride, and you will continue to have an equal replacement interval on all four tires. If only two are an otion, put the new ones on the same axle. Replacing only one? Make sure its rolling diameter matches as closely to the rest as possible, and try to get the same tire for consistancy in sizing and handling.

Edited by donrolandofurioso
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Thanks for the long, detailed explanation @donrolandofurioso. I think along the same lines as you do. Get as close as possible, but it shouldn't be a big deal if one tire is a bit taller/shorter than the rest. To add a bit more - tires wear at different rates due to misalignments, uneven driving patterns, etc. I'm guessing 1-2 32nds difference in wear is normal. On a 25" tire, that works out to only 0.25% difference.

I thought more about this problem during the weekend. I think there's an additional reason that wise people recommend against mixing tires when have to replace just one. Once you start, when do you stop? You now have an uneven replacement cycle. Additionally, a car with mixed tires will have poor resale value. This is generally thought to be true, but I think even more so for an AWD vehicle.

Still, I'm interested in the technical details. If anyone has more to add on this subject, please do! How much difference in circumference is going to cause excess heat/wear on a centre differential? Are there other considerations?

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On the smart car forums, if the circumference is not to spec, the traction control will be trying to put the brakes on all the time for the affected wheel. May not be an issue without traction control.

 

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12 hours ago, Paceman said:

On the smart car forums, if the circumference is not to spec, the traction control will be trying to put the brakes on all the time for the affected wheel. May not be an issue without traction control.

 

Not sure which forums you are talking about, but fact is that I personally have driven numerous different vehicles with a semi-inflated tire, and unless the rolling diameter is dramatically different (10 - 20%) AND you are hard on the throttle, TC or ESC will not come on. The car needs to see a clear "wheelspin" for TC to activate, which requires a prolonged speed difference while the ECM sees a power demand.

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7 hours ago, Paceman said:

Smart cars are very sensitive to tire diameter.

 

They're also 2wd so not exactly relevant to the thread.

 

@BeauHow "irreparable" is the leak? Years ago I had a daily driver (no autox use) tire with a leak in the sidewall (irreparable) and I had it patched anyway, no warranty of course, and it held until I replaced the tire ~6 months later. If that isn't an option, I would replace a pair, front or rear, if you can find something close enough in diameter. I'm thinking about a table with two pairs of legs that are off by 0.5". I'm putting them on the same end.

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Now would the F sport awd have the same setup? On the is350 f sport they are staggered in varying sizes. 
That may be indicative of the tolerance of the system axle to axle. 

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16 hours ago, Curtis said:

They're also 2wd so not exactly relevant to the thread.

 

@BeauHow "irreparable" is the leak? Years ago I had a daily driver (no autox use) tire with a leak in the sidewall (irreparable) and I had it patched anyway, no warranty of course, and it held until I replaced the tire ~6 months later. If that isn't an option, I would replace a pair, front or rear, if you can find something close enough in diameter. I'm thinking about a table with two pairs of legs that are off by 0.5". I'm putting them on the same end.

Relevant as they sense rotation at all corners and rely on tire diameter with a staggered set up.

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Thanks for the advice, guys. I've temporarily solved my problem buy picking up a used set of tires.

@Curtis - I originally wanted to just patch my 1 tire, but due to a number of factors not discussed here, I decided not to bother with it.

RE: smart cars and traction control. Is that a "feature" of the car trying to work as an electronic LSD? I would think there's at least a little tolerance allowed or you'd never be able to turn. But how much is too much? That is kinda related to this thread. Whether it's your diff that gets all heated up and worn out or a computer that intervenes, or ABS failure, lots of stuff can go wrong if you have a bad mismatch. Some of them will be car-specific. In a perfect world, all tires are the same, but we all know that's not always true. I'm thinking there's a lot more tolerance for uneven tire sizes than the MFRs want you to believe. Is it 1/32nd (< 1 mm)? Doubtful... 2/32nds? 4/32nds? 7/32nds? 

Seems like nobody spends much time detailing these things because the general consensus is to just replace your tire(s). And that's probably the best advice overall. But from a technical point of view, I'm still quite interested in this.

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Is there a pressure sensor of hall effect sensor connected to the steering system in conjuction with the speed sensor that outputs to the ECU that would "suspend" traction control (within certain parameters)? Logic would dictate that in tight parking lot manuvering or slow open lot manuvering, the traction control would be suspended.

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Typically, TC is also monitoring engine rpm and load. If the engine rpm is congruent with the load placed on the engine, and none of the tires change their speed differential, then TC has no reason to come on. 

This feature also allows the speed sensor circuits to be utilized as a tire pressure monitor system: If one tire starts to turn faster than the rest, despite a lack of power demand, the tire must be changing its rolling diameter. That can only happen through deflation, therefore a warning light will get triggered (and no TC comes on). To my knowledge, this type of pressure monitoring does not meet current US standards and therefore is not as common in North America (although some Nissan had it), but can be found very often in other parts of the world.

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