Scary First Snow Experience. Is it me or AWD Problem ? - Page 2 - Subaru Forester Owners Forum
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post #16 of 78 (permalink) Old 12-14-2009, 06:14 AM
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+1 Nasarog. Good/bad/mismatched/over inflated tires or not, it sounds like you were driving too fast for the conditions.

Your first hint should have been when, as you put it, "When I just started driving at 15mph on a straight flat road, one of the front wheels was sliding to the side a bit and the rear wheels were pushing the car in that direction. It felt almost like a RWD car."

You should have slowed down, or at a minimum, maintained speed. Speeding up to 25 mph when the road conditions indicate you do not have adequate traction at 15 mph is foolish. Count yourself lucky you were able to stop before the tree.

Subaru's are great in snow and ice, compared to most other vehicles. But the rules of physics still apply. All the AWD/ABS/stability control in the world is worthless if you do not have traction between your tires and the road surface. You can keep that traction with proper tires, inflation, and driving technique.

It sounds like you haven't driven on snow and ice much before. One of the best rules for winter driving was drilled into my head when I drove transit buses in icy midwestern winters is to pretend that you have an egg between your foot and the accelerator and brake pedals and treat both with very gentle pressure. Rapid acceleration or braking on ice or snow is a recipe for lost traction.
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post #17 of 78 (permalink) Old 12-14-2009, 06:23 AM
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Quote:
Originally Posted by Kyle_B View Post
I'm in the same boat, but we got a foot of snow here. I've only driven FWD cars in the winter so I was not, and still am not use to what an AWD drive car does. I suggest waiting until there is enough snow on the ground so you can't see any dry spots, go to a parking lot (that has no cars) late at night, and mess around. Lose control, try and get it back, just see what the car does when you do certain things.
+1.

But your tire problem, Matrix, is a bigger concern.

The difference in wear will take a toll on your AWD system, and not only that, but as your fronts are brand new, you'll want to remember that they're still scrubbing through their mold-release compound, and will not have best-pressure.

In addition to the above "basic lesson" that Kyle suggested, I'd also recommend that you experiment with tire pressures (including the front-to-rear bias) a bit.

Our Subarus may be AWD - but a highly-regarded AutoX and track-racer once said, wisely, "remember that AWD is 'half RWD,'" that tail can and will come around to bite you, particularly under such conditions, particularly if you've over-driven the car's mechanical limitations.

Physics is physics, and our vehicles are bound by those undeniable laws.

Yes, getting better tires can help you either enlarge your safety envelope or help you push performance to that edge, but you'll still need to know and intimately understand the basics. Otherwise, you're simply setting yourself up for a failure scenario at even higher speeds, with even more potential damage and harm.

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post #18 of 78 (permalink) Old 12-14-2009, 06:30 AM
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Originally Posted by adc View Post
You are destroying your gear train by having 2 different size tires. When a front tire and a rear tire can not match speed one of them must slide to make up the difference. This means serious wear on the gear train.
All four tires are all the same BFG Traction T/A. I assume that "at 80%" means the rears have that much remaining tread compared to the new fronts.

That does not seem like a big difference. The Owners Manual is silent on the allowable difference, but if over 1/4" should be where the AWD problems start, and if 20% wear represented that 1/4", the new tread thickness would be 1.25", whereas it is actually more like half that. (I have the same tires on another car.) So his 20% wear is a negligible 1/8".

Also I don't think that different sized rear tires would slide on the pavement or a steel gear be damaged. The "soft" components in the drive train should accommodate the difference -- the viscous couplings, clutch packs, etc. They would be the parts that would eventually be damaged by a difference in tire sizes and rotational speeds.

Quote:
Originally Posted by Matrix11229 View Post
... When the road wasn't straight anymore and I was at 25mph, suddenly the front end slid to the left, and the rear end pushed it further to other side of the road... I tried to correct but no luck; it felt like the right side wheels spinning faster then the left. So I slammed on the brakes to avoid the tree and finally stopped at the curb...
I have driven my AT Forester in snow briefly with Geolandar tires and then Michelin Primacy, neither of which are winter tires, and the traction of both under AWD is amazing. But one should get familiar with what AWD does for the handling in snow, and let the AWD do its job for which it needs some throttle. I think taking foot off the gas, corrective steering without throttle, and braking all negate the benefits of AWD. And AWD can't always overcome too much speed.
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post #19 of 78 (permalink) Old 12-14-2009, 06:44 AM
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Quote:
Originally Posted by Nasarog View Post
Poor tire condition = driver error.
Tire quality/inflation = driver error.
Too high of a speed in poor conditions = driver error.
No practice driving in poor conditions = driver error.
Not using your head = driver error.

Not keeping up your car in poor weather and not preparing for harsh days is driver error. I am not critisizing him, but his initial post places all the blame on the car,not the driver.
That's right, I was being overly nice or something..
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post #20 of 78 (permalink) Old 12-14-2009, 07:03 AM
 
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I run two different brands (one same brand pair on front, another brand pair on back) on both my Subarus, they are however the same size, no problems in the last three years on both. I think 1/4" rule is bollocks, that tolerance alone is broken by the "suggested" psi (32/26!).

The forester does kick out tail wise when it engages, just keep it in 2nd (if you are auto?), this keeps split at maximum so you don't have sudden power from the rear.

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post #21 of 78 (permalink) Old 12-14-2009, 07:43 AM
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FWIW, I have BF Goodrich Traction T/A's on both my Civic and Forester. Both are currently at about 8/32 tread.

Honestly, the civic handles better in the snow until it bottoms out or gets stuck in ruts. The forester slides all over.

Have considered getting snow tires for the subaru, but it doesn't make sense when I'm driving on plowed roads most of the time.
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post #22 of 78 (permalink) Old 12-14-2009, 08:45 AM Thread Starter
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Thanks for all your comments guys !

Quote:
Originally Posted by vintage42 View Post
I think taking foot off the gas, corrective steering without throttle, and braking all negate the benefits of AWD. And AWD can't always overcome too much speed.
That was exactly my mistake - "taking foot off the gas, corrective steering without throttle".
I will practice and shift into "2" in deep snow and see what happens. I didn't realize that subaru awd is not evenly distributed all the time.

Quote:
Originally Posted by vintage42 View Post
Also I don't think that different sized rear tires would slide on the pavement or a steel gear be damaged. The "soft" components in the drive train should accommodate the difference -- the viscous couplings, clutch packs, etc. They would be the parts that would eventually be damaged by a difference in tire sizes and rotational speeds.
Quote:
Originally Posted by TSi+WRX View Post
But your tire problem, Matrix, is a bigger concern.
The difference in wear will take a toll on your AWD system, and not only that, but as your fronts are brand new, you'll want to remember that they're still scrubbing through their mold-release compound, and will not have best-pressure.
In addition to the above "basic lesson" that Kyle suggested, I'd also recommend that you experiment with tire pressures (including the front-to-rear bias) a bit..
Can I compensate the thread size difference by playing with front tire pressure ? Currently I'm running 34psi all around. If I will deflate the front tires by 4 psi, will it compensate the difference ?

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Last edited by Matrix11229; 12-14-2009 at 08:54 AM.
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post #23 of 78 (permalink) Old 12-14-2009, 09:10 AM
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Quote:
Originally Posted by Matrix11229 View Post
Can I compensate the thread size difference by playing with front tire pressure ? Currently I'm running 34psi all around. If I will deflate the front tires by 4 psi, will it compensate the difference ?
I honestly don't know..... I would not know how to do the math,

Also, there exists a more than potential problem in that if adjusting the tire pressure too low, you may incur not only additional wear on those tires, but also again shift handling traits to be more tail-happy.


---


Quote:
Originally Posted by Atomic1 View Post
FWIW, I have BF Goodrich Traction T/A's on both my Civic and Forester. Both are currently at about 8/32 tread.

Honestly, the civic handles better in the snow until it bottoms out or gets stuck in ruts. The forester slides all over.
^ That's honestly very strange - what tire-pressure are you running on the Forester? Have you tried optimizing pressures for your needs, or are you still going off the door-card/manual? Also, are the two comparable fitments?


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post #24 of 78 (permalink) Old 12-14-2009, 09:15 AM
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You could always see if you could have the newer tires "shaved" to get the tread depth to match up better.

Just to throw another point in, you always want to have the tires with better tread on the rear. I'd at least start with that.
See here: Tire Tech Information - Where to Install New Pairs of Tires? and #9 here: Top Ten Tire Safety Tips - MSN Autos

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post #25 of 78 (permalink) Old 12-14-2009, 09:19 AM
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Ah, thread serendipity -- I just spent a long weekend up in the Mt Shasta area driving across back-country roads and trails covered with snow varying from an inch or so to maybe 6 inches in my new(ish) 2010 X. I came back with the overall impression that this is an amazingly stable and predictable car on snow and ice, even when you do dumb things like brake sharply on turns or randomly turn the steering wheel this way or that (I tried all these things on isolated stretches). My previous experiences in these sorts of conditions have been with a Nissan Pathfinder with heavy-duty tires, and for the most part, the Forester did much better than I expected, even with the stock Geolandars. It's no coincidence that so many of the cars in the local supermarket parking lot were Subarus....

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post #26 of 78 (permalink) Old 12-14-2009, 09:24 AM
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Originally Posted by TSi+WRX View Post
I honestly don't know..... I would not know how to do the math,

Also, there exists a more than potential problem in that if adjusting the tire pressure too low, you may incur not only additional wear on those tires, but also again shift handling traits to be more tail-happy.


---




^ That's honestly very strange - what tire-pressure are you running on the Forester? Have you tried optimizing pressures for your needs, or are you still going off the door-card/manual? Also, are the two comparable fitments?

I would first set the tire pressures according to the settings indicated in the owners' manual or door placard and go from there.
After that, you might go back to basics and measure the height of the tires (front and back) and compare the older, worn tire height versus the newer tires. (Different brands will be different heights/widths even with the same marked size) After measuring, figure out if you really need to shave or purchase tires or even do anything. Real world measuring and comparason is worth much more than making guesses based on estimates. IMHO

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post #27 of 78 (permalink) Old 12-14-2009, 09:42 AM
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I've been driving my 03 Forrie AT in all sorts of snow/ice over the past 7 years of ownership, it's my first Forrie and my first true AWD vs. the previous CRV-Real Time system.

I have to say that I've never experienced any of the issues that you've described and my drives include windy, white out conditions at up to 40 MPH (at nite, almost no other cars out, plows yet to get out)........

Quite the opposite, I've been amazed at how good the Subie AWD system is. The only thing that I can suggest might be the problem, other than your own lack of familiarity with AWD, must be that you replaced 2 tires NOT all 4.

My tires, thru much of the long winter drives, were the original Geos, all worn equally (I always rotate them every 10K miles).....and my driveway, back then, was a 40 degree sloped hill.......300 feet long......never had a problem going up although it was a scary trick....

Start off with four identical tires.....and play in the empty parking lots as already advised. It's loads of fun and really helps you to get used to the car's capabilities.

Be careful, drive carefully.

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post #28 of 78 (permalink) Old 12-14-2009, 10:08 AM
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Originally Posted by ArcadeTom View Post
I would first set the tire pressures according to the settings indicated in the owners' manual or door placard and go from there.
After that, you might go back to basics and measure the height of the tires (front and back) and compare the older, worn tire height versus the newer tires. (Different brands will be different heights/widths even with the same marked size) After measuring, figure out if you really need to shave or purchase tires or even do anything. Real world measuring and comparason is worth much more than making guesses based on estimates. IMHO
He'd need to lift the car, or take the rims off the car, right?

That seems to make sense.

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post #29 of 78 (permalink) Old 12-14-2009, 10:54 AM
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About matching tire size by measurement

I believe it's not the measured diameter or measured circumference that is the issue -- it's the rolling circumference (RC). When the tire is under load, it collapses somewhat at the bottom. The effect, which seems to be the subject of much tire technology debate, is that the number of turns of the wheel/axle is not the same as what would be calculated using the measured diameter (or radius) of the tire/wheel, both unloaded and under load.

This is confirmed in tire manufacturer data, where they often provide a static radius or diameter figure, occasionally a static loaded radius, and then a dynamic or rolling figure (usually in number of turns or rotations per mile or km). The latter does not correspond to any of the other measurements.

The issue for the AWD is that all four tires/axles be turning at the same rate, within a reasonable tolerance. Subaru, and other AWD manufacturers, interprets this to be no more than 1/4 inch difference in RC. The RC is variable, depending on brand, model, size, tire pressure, load, wear and even speed. But you can get a idea of whether your tires are turning at a rate reasonably within the spec. Park on a relatively flat straight surface, at least 100 feet long. Mark all tires at the bottommost point with chalk. Move the car forward while someone watches as one of the wheels turn exactly 10 revolutions (mark at the bottom). See where the marks on the other tires end up. If they are all within 2.5 inches of their bottommost point, your're okay. (If possible, I suggest doing it a few times to ensure repeatability because any steering change, acceleration or braking can introduce differences.)
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post #30 of 78 (permalink) Old 12-14-2009, 11:05 AM
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Quote:
Originally Posted by Real Lyte View Post
I run two different brands (one same brand pair on front, another brand pair on back) on both my Subarus, they are however the same size, no problems in the last three years on both. I think 1/4" rule is bollocks, that tolerance alone is broken by the "suggested" psi (32/26!).
.....
Ummm, no. The difference in tire pressures compensates for the fact that the front of the car is heavier than the rear and equalizes the effective rolling radius of the tires.

Different brands in the same size can differ by 1/4" or more in DIAMETER, and the 1/4" is supposed to be applied to circumference, which is diameter times 3.14159... So you may be way out using different brands. Why would you consider using different brands in the first place?

George

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