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Guest Message by DevFuse

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Flat Tires and AWD

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5 replies to this topic

#1 cannonball


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Posted 30 September 2004 - 01:07 PM

I was reading the AWD and spare thread and it got me thinking. It kinda concerns me that the tire diameters have to be within such a tight spec. It's been stated that you have to put the FWD fuse in an AT Model, but what can you do with a MT. Say you have a slow leak in the front that is not noticeable until it's way past the 1/4" rule. Since the MT can deliver more torque to the other axle during slippage and the front is open diff, what could happen if you drive 10 or so miles before realizing there's a flat? It seems to me that the MT would be more forgiving than the AT. A flat or partial flat tire would have a higher rotation speed because the tire is a smaller diameter. Correct me if I'm wrong but wouldn't that mimmick a tire slowly slipping. Can anyone enlighten me on this one?

#2 Setright


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Posted 30 September 2004 - 02:25 PM

I dont think tyre pressure will affect the rolling circumference that much. The tread is belted (steel/nylon) underneath and this wont change length. So even though the tyre sags, one rotation is still all the way round :-)

#3 Commuter


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Posted 30 September 2004 - 04:18 PM

This has been discussed before. I was even in the midst of a 'debate' on tire circumference a couple years back.

What matters is effective circumference, or rolling diameter. And yes, it does change with tire pressure. One guy proved it to himself by rolling his mountain bike a certain distance (5 revs of the tire as indicated by the tire valve position). When he let most of the air out of one tire and then repeated the test (with him on for weight), he found a noticable difference.

I know that radial tires do not 'sag' the way the old bias ply tires did (who remembers those :) ). On the old bias ply tires, even just a few psi drop would show up as a noticable 'bulge' at the pavement. With todays tires, I find that I usually won't notice it (by eye) unless there has been a 10 psi or more drop. And I'm quite sure that that would be enough to create the 1/4" circumference difference.

So... Why don't we have a slew of failures on the Subaru center differentials? A number of reasons. Most of this is just my own thoughts from my engineering background, knowing how companies spec things and from hanging around boards like this for a few years.

First of all, the center diff (whether clutch pack or VC) has to tolerate "something" or it would never work. Every time you turn a corner, it has to allow slippage. Second, manufacturers are going to rate things conservatively. 1/4" variance translates into about 0.3% difference. With liability laws and lawyers being as they are, I'm sure the real number that can be tolerated is higher. Third, all of this is "for real". I can quote a couple of examples I've read over the years.

It's generally accepted that the VC is more forgiving than the clutch pack. That certainly seems true from my message board travels. However, I recall reading of one person who had 2 VC's fail and a 3rd one was showing heat damage. I'm not sure of the time frame, but I think this was over several months, perhaps a year. They discovered that he had a pair of tires with a 1.5" difference in circumference from the other pair! :eek: Even at that (6 times Subaru's spec), it took a while for the damage to occur. On the other hand, I've read a couple of times of people blowing a tire with an automatic and just throwing something on, and then frying the clutch pack. One guy was coming back from a trip. He blew a tire. He got a used tire from some junk yard as a quick fix and continued on his way home. 2 or 3 hours of driving as I recall. The following week, he discovers that his clutch pack is shot. (I don't know how different the tire was, or how many miles, etc. Eg, the clutch pack might have been on it's last legs for example.)

I just changed my clutch pack on my 97 OB, but I have 410,000 km. I can't complain. I did have a tire get down to about 15 or 20 psi a couple of times as I had picked up a nail and had a slow leak and didn't realize it. I probably drove a few thousand kms as the air slowly leaked. (I use to put on 5,000 km a month.) I had 2 nails in the same tire within a 3 week period. Did it do any damage? Not that I could tell. If it hadn't happened, would I have gotten to 420,000 km, or 450,000 km? I have no way to know.

One needs to be aware of the "potential for damage" to the Subaru center differential. It's rather expensive to find out the hard way. With the autos (mine anyway), I'm surprised that there isn't some placard right there with the spare tire. I honestly didn't know about the FWD fuse for about 2 years until I got onto some of the message boards. And I HAD looked over the manual. I found it buried at the 'end' of the "how to change a tire" section. D'oh! I never read that far. I've changed many a tire in my day and nothing in the diagrams or my quick skim of the material suggested that this car was any different. I'm just glad that I didn't "find out the hard way".


#4 Setright


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Posted 01 October 2004 - 12:55 AM

Mountain bike tyres are cross-ply.

#5 Ranger83


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Posted 01 October 2004 - 07:39 AM

[quote name='cannonball']I was reading the AWD and spare thread and it got me thinking. It kinda concerns me that the tire diameters have to be within such a tight spec....QUOTE]You can prove to yourself that the system can handle such variations pretty easily. Go to a wet or dusty parking lot and make a full-lock circle. You'll see that the outside front tire travels quite a bit farther than the inside rear.

The math is not at all difficult, and most tire manufacturers list the rolling diameter and tire height on their website. The Michelin HydroEdge 205/70x15 on my car have an overall diameter of 26.1"" and turn 797 revolutions per mile at 45 mph. This yields an effective rolling diameter of (5280*12)/797/(3.14*2) = 25.32 if I used the right numbers? So the "rolling radius" is about 12.67" instead of 13.05".

You can even calculate the difference by measuring the circumference of the circles, but I think you'll agree that, by inspection, when you turn the wheels travel different distances at different rates. So the system is robust enough at a minimum to accomodate this.

Your ABS brakes take this into consideration as well - that's why you can turn while under heavy braking. Some new cars including Subaru VDC with traction control or directional control even use these differences to sense and correct for sliding in a corner. The relative rate of rotation of each of the four wheels relative to the sterring angle can tell the compter where to apply power or scrub it off (some systems work by braking a wheel, while some newer cars with fly-by-wire throttles can reduce or add power). You'd notice the heavy steering, list, and noise long before a low tire damaged your drivetrain.

I bought an extra wheel for a full-size spare.

#6 cannonball


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Posted 01 October 2004 - 08:08 AM

Hmmm... Interesting thoughts. Thanx for the replies.

I agree that there has to be what I call fudge factors involved in the engineering.


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