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2019 Forester Touring CVT
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Discussion Starter #1
Wondering if the 2019 Forester can forgo the set of tires or new tire shave down when replacing a tire? I am led to believe other manufactures no longer have this issue due to fluid coupling AWD vehicles.:nerd:
 

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Where did you get that?

Just a quick one I checked

2019 Acura RDX manual:
Tire and Wheel Replacement
Replace your tires with radials of the same size, load range, speed rating, and maximum cold tires pressure rating (as shown on the tires' sidewall). Using tires of a different size or construction can cause certain vehicle systems to work incorrectly. It is best to replace all four tires at the same time. If that isn’t possible, replace the front or rear tires in pairs.
 

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Discussion Starter #3
Guy I work with had 3 Audis with AWD. On the 3rd Audi he claimed no longer had to shave tires or replace a set of tires due to the fluid coupling. I am no expert so this is the reason I asked in forum.
 

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There may be a 'fluid coupling' (aka viscous coupling) component in the CVT transmission, but the differentials have not changed on these vehicles so that the AWD is 100% safe from non-equal rolling circumference on a tire. Any potential issues of causing damage to the AWD system would arise after long term use of a non-equal set of tires, so it is something you (or your Audi friend) could get away with for a while. Long enough perhaps to preach about how new cars are immune to differing tire sizes... but I suspect Audi continues to use regular mechanical differentials and needs matched rolling tire sizes also.

Additionally, different rolling sizes of the tires make for a potential a safety issue in inclement weather. AWD and VSC dont need the added challenge of an unequal tire when trying to keep the vehicle straight in a dynamic maneuver or slick road.

Kinda far fetched, but I also would imagine an insurance company could void a claim in an accident where they find the vehicle was fitted with tires outside of the manufacturer recommendation. That liability is enough for me to not roll the dice on tires.

Fortunately Tire Rack (and others surely) will shave a tire for you if you call them and place the order. Have not tired this but got a quote; I got a screw in the sidewall of 1 OEM tire at 5,000 miles and despite adequate remaining tread to have one shaved and installed, I went with a new set of 4. In 2016 the OEM tire was garbage on the XT, and expensive ($200+) for a single shaved replacement, so a new set of Colorado capable tires was the best route. Tire Rack Tire Shaving

I suspect the reason the donut spare states to not run the spare for more than 50 miles, and no more than 55 mph, is to protect differentials from damage from that tire, I didnt measure when using it but theres no way that little donut is the same rolling circumference as the real tires. So its ok briefly to get home, not good as a long term plan IMO.

Pull quote from a Tire Rack article:

"Four-wheel drive and all-wheel drive vehicles are equipped with additional differentials and/or viscous couplings that are designed to allow momentary differences in wheel speeds when the vehicle turns a corner or temporarily spins a tire. However, if the differentials or viscous couplings are forced to operate 100% of the time because of mismatched tires, they will experience excessive heat and unwarranted wear until they fail."

So viscous coupling or no, tires need to be of matched sizes
 

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Guy I work with had 3 Audis with AWD. On the 3rd Audi he claimed no longer had to shave tires or replace a set of tires due to the fluid coupling. I am no expert so this is the reason I asked in forum.
Forgive my blunt nature- so one guy mentions his Audi not needing it- which you didn't bother to verify and then come on a forum and say it seems other manufactures don't require all 4 tires. That's a pretty good stretch.
 

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Discussion Starter #6
Personally I see no reason for getting worked up about a question that I was asking. Please have a much better day!
 

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Discussion Starter #7
There may be a 'fluid coupling' (aka viscous coupling) component in the CVT transmission, but the differentials have not changed on these vehicles so that the AWD is 100% safe from non-equal rolling circumference on a tire. Any potential issues of causing damage to the AWD system would arise after long term use of a non-equal set of tires, so it is something you (or your Audi friend) could get away with for a while. Long enough perhaps to preach about how new cars are immune to differing tire sizes... but I suspect Audi continues to use regular mechanical differentials and needs matched rolling tire sizes also.

Additionally, different rolling sizes of the tires make for a potential a safety issue in inclement weather. AWD and VSC dont need the added challenge of an unequal tire when trying to keep the vehicle straight in a dynamic maneuver or slick road.

Kinda far fetched, but I also would imagine an insurance company could void a claim in an accident where they find the vehicle was fitted with tires outside of the manufacturer recommendation. That liability is enough for me to not roll the dice on tires.

Fortunately Tire Rack (and others surely) will shave a tire for you if you call them and place the order. Have not tired this but got a quote; I got a screw in the sidewall of 1 OEM tire at 5,000 miles and despite adequate remaining tread to have one shaved and installed, I went with a new set of 4. In 2016 the OEM tire was garbage on the XT, and expensive ($200+) for a single shaved replacement, so a new set of Colorado capable tires was the best route. Tire Rack Tire Shaving

I suspect the reason the donut spare states to not run the spare for more than 50 miles, and no more than 55 mph, is to protect differentials from damage from that tire, I didnt measure when using it but theres no way that little donut is the same rolling circumference as the real tires. So its ok briefly to get home, not good as a long term plan IMO.

Pull quote from a Tire Rack article:

"Four-wheel drive and all-wheel drive vehicles are equipped with additional differentials and/or viscous couplings that are designed to allow momentary differences in wheel speeds when the vehicle turns a corner or temporarily spins a tire. However, if the differentials or viscous couplings are forced to operate 100% of the time because of mismatched tires, they will experience excessive heat and unwarranted wear until they fail."

So viscous coupling or no, tires need to be of matched sizes
Correction: I did not preach I was asking?
 

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2019 Forester Touring CVT
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Discussion Starter #9
During my education as a youth. Educators would say to the class. The only dumb question is the one that was never asked.
 

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Maybe- but in the age of the web there are plenty of lazy questions. You know the ones you can easily find the answer to with a quick search vs. posting on a forum wanting someone to give you the answer.
 

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2014 Touring with Eyesigh CVT
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During my education as a youth. Educators would say to the class. The only dumb question is the one that was never asked.
That is one of the many reasons that education is failing because we are loosing the self reliance, the ability to do research and expand our knowledge beyond asking.

You can use Google or look up the information in your owner's manual and read Subaru's recommendation.

Laughing at oneself and with others is good for the Soul
 

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Discussion Starter #12
I just ordered my first Subaru and was collecting decision data for the 2019 Touring. Have a forecasted delivery in late April.

The more I drill down into the forum threads here the more I discover necessary research sources.

Thank you Sir can I have another! 🐒
 

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I'm generally a fan of doing your own primary source research instead of just going to forums. One of the problems on forums is the same guy that told you Audis don't need 4 tires/shaving (or etc.) may be the first/only answer or the dominant voice on a forum. Another problem on forums is that answers pile up, there can tend to be more varying of the answers so there is less consensus and harder to decide if its good advice or not. There is some potential that a "primary" source like a manufacturer may take a more restrained position because they have more responsibility for the results of their answers.
 
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