Last post on Mar 04, 2013 at 6:34 PM
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Subaru Forester, Tires, Wheels
#270 of 298 Re: new year-any good tires [xwesx]
Jan 24, 2011 (3:34 pm)
I have a 2002 Forester S, automatic, with limited slip differential. I have religiously rotated my tires to keep their circumferences matched.
I agree that in theory at least all 4 tires should be changed at the same time. The manual says: "For safe vehicle operation SUBARU recommends replacing all four tires at the same time". There is a warning after that that mixing tires can produce damage to the drivetrain.
However, even the flat tire change procedure described does not specify that the spare (I have full size spare on my 2002) needs to go to he back, nor are there any mileage or speed limitations stated in the manual. There is no requirement (as i have seen in many posts) to put the vehicle in FWD mode. Basically the manual describes the flat tire replacement as it would be for a FWD or RWD car. If a different spare tire circumference could cause problems, why isn't it stated in the manual?
I am wondering if the manual transmission Subarus (50-50 traction F/R) have the more stringent requirements, while the autos do not.
#271 of 298 Re: new year-any good tires [daniell]
Jan 24, 2011 (3:30 pm)
The automatics are definitely sensitive to differences. The speed limitation of using a "donut" tire is from the tire itself, not due to the stress it places on the drivetrain. If you have one tire that is of a different circumference, the stress will be on that axle's differential. If you have both tires on one axle different in circumference from the other axle, the stress then goes to the center differential.
I can't imagine anyone driving on a donut for an extended period of time, but were that to happen, it could damage the differential because of the added stress, especially if the vehicle is driven on paved roads. On snow, ice, and gravel, the stress is more often relieved through tire slippage.
#272 of 298 Re: new year-any good tires [xwesx]
Jan 24, 2011 (3:42 pm)
Edited my previous post a bit to include specifically some info from the manual...
I agree with everything you say, theoretically. All I am saying is that the manual does not seem to put any serious restrictions on the tire circumference. If so, it would require installing the spare in the back, putting the car in FWD mode etc. as described in other posts. The only cautionary words about tire replacement are: "Have the wheel nut torque checked at the nearest automotive sevice facility. Store the flat tire in the spare tire compartment". If circumference was such a big concern, they would have said something like "repair your flat tire at the closest facility if possible, and install it back". No such words, they allow you to use the spare permanently.
#273 of 298 Re: new year-any good tires [daniell]
Jan 25, 2011 (12:37 am)
It seems as though they have become more detailed in the owner's manual in recent years. In the 2010 manual, beginning on page 9-2, it states:
CAUTION: Never use any temporary spare tire other than the original. Using other sizes may result in severe mechanical damage to the drive train of your vehicle.
The temporary spare tire is smaller and lighter than a conventional tire and is designed for emergency use only. Remove the temporary spare tire and reinstall the conventional tire as soon as possible because the spare tire is designed only for temporary use. Check the inflation pressure of the temporary spare tire periodically to keep the tire ready for use. The correct pressure is 60 psi (420 kPa, 4.2 kg/cm2).
When using the temporary spare tire, note the following.
. Do not exceed 50 mph (80 km/h).
. Do not put a tire chain on the temporary spare tire. Because of the smaller tire size, a tire chain will not fit properly.
. Do not use two or more temporary spare tires at the same time.
. Do not drive over obstacles. This tire has a smaller diameter, so road clearance is reduced.
. When the wear indicator appears on the tread, replace the tire.
. The temporary spare tire must be used only on a rear wheel. If a front wheel tire gets punctured, replace the wheel with a rear wheel and install the temporary spare tire in place of the removed rear wheel.
Precautions for AWD models with automatic transmission:
Your vehicle is equipped with the AWD (All-Wheel Drive) system. In addition, if your vehicle is an AT model, before driving your vehicle with the temporary spare tire, deactivate the AWD capability of the vehicle as follows.
1. Turn the ignition switch to the “LOCK” position.
2. Pull any one spare fuse out of the spare fuse holder in the engine compartment. Spare fuses are attached on the back side of the fuse holder cover. You may pick up any one fuse in the spare fuse holder.
3. Put a spare fuse inside the FWD connector located in the cabin and confirm that the All-Wheel Drive warning light “ ” illuminates. The All-Wheel-Drive capability of the vehicle has now been deactivated.
NOTE: After reinstalling the conventional tire, remove the spare fuse from the FWD connector in order to reactivate All-Wheel Drive. Make sure to restore the removed spare fuse in the spare fuse holder located in the engine compartment.
#274 of 298 Re: new year-any good tires [xwesx]
Jan 25, 2011 (4:04 pm)
Thank you for the detailed answer.
It seems that most if not all of those additional precautions are caused by the doughnut spare tire being vastly different from the remaining 3 tires. My car came with the full size spare from the factory, and again, I see none of these additional steps in the manual. Based on that, it seems that the AWD can accomodate fairly different circumferences. Another pointer in that direction - the "light load" tire pressure recommended in the back is 28 psi, "heavy load" is 36 psi, and always 29 psi in the front (numbers from memory, but I believe I am right - wife has the car now). Big difference between 29 psi and 36 psi. Bottom line, acording to my 2002 manual at least, restrictions when replacing with same size tire as my spare is seem to be rather related to driving dynamics and traction characteristics than circumference.
Based on all of this, IMO it does not seem completely unreasonable that replacing just 2 tires as opposed to all 4 is possible.
#275 of 298 Re: new year-any good tires [daniell]
Jan 25, 2011 (10:27 pm)
Well, it is possible, but it will place additional stress on the center differential. If left in that state long enough, the differential is likely to fail. The driver might not even notice it happen, but that one time AWD is really needed, the front tires are going to be spinning and the rear will be doing nothing to help.
If you have a full-size spare, there is much less of a difference in circumference and therefore much less stress on the system. If you keep the spare rotated in with the other tires, you could even drive on it regularly with no problems. But, if you only use it as the spare, you should swap it back off the car as soon as is reasonable.
You're right - most of the precautions in the 2010 manual are specific to the donut spare. There would be no location or speed restrictions on a full-size. I did not realize that the 2002 model included a full-size from the factory. The 2009+ models can accommodate a full-size in the well, but come with a donut.
#276 of 298 Re: new year-any good tires [xwesx]
Jan 26, 2011 (1:21 pm)
One can put 2 new tires on the same side of the car, or opposite diagonally. That would move the mismatch from the center differential to the front and rear differentials. Lots of people with FWD and RWD cars change just one tire and differential failures as a problem are generally rare.
Here is a very relevant article I found. It's not about Subaru or AWD, but shows real, measured impact of tire pressure. The author found that while the impact of tire pressure on circumference is very small, "The “roll-out” of the rear tires was reduced by 1” by simply reducing the tire pressures from 25 psi to 14 psi." Basically the same tire with 25 psi covered an extra inch on the ground with each rotation, compared to when inflated to 14 psi. One inch per rotation is huge, and that's just the impact of tire pressure. Below is the link. The guy even shows pictures of his measurements.
Again, the range for the rear tire pressure recommended by Subaru is 28 psi to 36 psi based on load, quite big itself. Based on all of this, I will have no problem replacing just one tire (and perhaps fine tune the roll-out using the tire pressure) if needed.
#277 of 298 Re: new year-any good tires [daniell]
Jan 26, 2011 (4:57 pm)
There are a lot of assumptions there!
First, yes, you could put a new tire on each axle and take pressure off the center differential and transfer it to the other two differentials. I did replace a single tire as I didn't realize it would cause a problem; that was about 20,000 miles into a set of tires that were on the car for 75,000 miles. When on dry roads, I didn't even notice the difference. On slick roads, the axle with that tire would slip every 3-4 seconds and create a noticeable "wobble" in the movement of the car. At about 185,000 miles (on the odometer - I put a new set of tires on at around 196,000), I started to get some differential noise from the rear diffy that became more consistent over time, but it still operated fine on my open-differential '96 Outback through 220,000 miles when I ceased to own it.
Second, you are making the assumption with tire pressure that the contact patch and sidewall height change in a linear fashion with tire pressure, which is not the case. The difference between 14 and 25 psi is worlds different from 28 to 36 psi. If you operated the car at the former difference for any length of time, it would create problems. At the latter, it would not. However, were one to drive the car at, say, an even 28 psi all around but carry a heavy load in the back at all times, it may cause additional stress on the drive line.
In the end, it really all depends on your priorities. If you want a well-functioning and maintained car, it doesn't pay to scrimp or neglect. If you don't care and can pass it on to some unsuspecting victim before any problems become easily apparent, scrimp away.
#278 of 298 Re: new year-any good tires [xwesx]
Jan 26, 2011 (5:53 pm)
Not to worry, I completely realized the difference between 14-25 psi and 28-36 psi, and the likely non-linearity I am a mathematician, and I deal with stuff like that a lot in my job. Just did not want to go there. But I'd be willing to bet that the 28-36 increase in psi with a full load will cause more than the 1/4" Subaru demands.
What bugged and still bugs me is that there is little information available about this issue. My suspicion is that the Subaru AWD can deal with more than the 1/4" circumference difference required, easily. Your own story proves that the Subaru AWD system can take a lot. I can only dream to reach that kind of mileage with my car (too many problems, in spite of being extremely well maintained). And simply we don't drive enough (reaching 90k miles in 9.5 years of ownership).
I suspect a lot of requirements in the manual are due to the litigious world we live in. For example, the manual specifies front to back rotation on each side. I got non-directional tires from Sears, with lifetime rotation and balance, but they refuse to do a FWD-pattern or X-rotation, because of the specs in the manual. So I go to Sears, have my tires balanced, then go home and do a proper FWD rotation. That doesn't make sense. I assume the only reason that is in the manual is because some OEM tires they use are directional. Just for the record, I am doing the FWD rotation pattern rather than X (cross) rotation because my auto Forester has 90% forward bias. So far that has worked very well.
#279 of 298 Re: new year-any good tires [daniell]
Jan 26, 2011 (7:02 pm)
Having worked on Subarus for 10+ years, raced them for 10+ years and owned them for more than 12 years. I can tell you that you DO NOT want to drive a Subaru with a tire that is mis-matched for any extended period of time. You can drive it for say 200 or 300 miles but to drive it 30,000 or 40,000 miles WILL definitely cause your center diffy to fail eventually over time. If your subaru is equipped with a front or rear Limited Slip Differential, you should also put the mismatched tire on the axle w/o the LSD.
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