cookie Posted May 17, 2005 Share Posted May 17, 2005 fairly seriously. This is probably why I have better automotive luck than some of my friends. There have been a few folks on the board in the past that posted that they had thier VCs fail. By keeping my tires as close to the same size as possible I hope to do my best not to join them. Here in CA there are flea markets and junkyards that have a good selection of used tire when you need to match up a used one. the San Jose flea market and the one in Oakland have a pretty good size tire selection. Sometimes you can even find a similar used tire at the local gas station. One of my last Jeeps had this problem. I bought it used and dead with about three brands of tires on it. Since it had quadratrac I had to match up used ones by measureing them. This worked for about the first three years until I finally rewarded it with new tires for the last few years I had it. There is really no sense in argueing about it. One should just try to keep the tires as close to spec as possible. Quote Link to comment Share on other sites More sharing options...
Johngenx Posted May 18, 2005 Share Posted May 18, 2005 As The Dude pointed out, Subaru's spec is circumferance. You failed to multiply by 2pi (2 x 3.14 = 6.28). 1/32" tread depth variation and you are still ok (~0.2" on circumference). But 2/32" tread depth variation and one is over Subaru's spec. Reliably measuring the tread to this accuracy is difficult IMHO. I was faced with this exact situation a few years ago when I had a sidewall puncture on a fairly new tire. The wear was "around" 1 or 2 32nds of an inch. I ran the fuse until the weekend. I then jacked up the car and wrapped a tape measure around all 4 tires. The new tire was right at the 1/4" allowance. I pulled the fuse and did not experience any trouble. Commuter Um, why by 2Pi? I'm using the diameter, not the radius. Pi*D = C. (Or Pi*2R = C) Our .28" difference in D equates to (3.14 * 0.28) = 0.879" Now, that does exceed the 1/4", but I have to agree about the impact of that .879". I wonder how realistic it is? Quote Link to comment Share on other sites More sharing options...
~01~Outback Posted May 18, 2005 Share Posted May 18, 2005 Quite simply if the diameter of one tyre is, lets say 1% smaller than the other tyres - over 100km that tyre has travelled 1km more; over 10000 its travelled 100km more!! That certainly cant be good Quote Link to comment Share on other sites More sharing options...
pinister Posted May 18, 2005 Share Posted May 18, 2005 hi guys 1/4" of the wheel diameter is almost 1%. every time your forester makes a turn of 19m radius (the centerline of the car) the front wheels turn 1% faster then the rear ones. when turning the steering wheel to the maximun the front wheels will turn about 3% faster. how many times do you reach that 1%? it seems that for these turns we need larger wheels at the front... isn't there any developed damage? i know that it's against the theory, but, at least from that aspect, if we replace just 2 wheels isn't it better to put the new and bigger ones at the front??? Quote Link to comment Share on other sites More sharing options...
brus brother Posted May 18, 2005 Share Posted May 18, 2005 And now you have to catch up with that other tire! Quite simply if the diameter of one tyre is, lets say 1% smaller than the other tyres - over 100km that tyre has travelled 1km more; over 10000 its travelled 100km more!! That certainly cant be good Quote Link to comment Share on other sites More sharing options...
Johngenx Posted May 18, 2005 Share Posted May 18, 2005 I would imagine that the driveline can tolerate a 1% difference in tire circumference. If not, I think that the roadsides would be littered with broken Subarus. Think of all the owners that drive along with one tire low on pressure. What about tires deforming from heat, and the possibility of it being at different rates? Warranty costs would be insane. Quote Link to comment Share on other sites More sharing options...
TheBigJ Posted May 18, 2005 Share Posted May 18, 2005 hi guys1/4" of the wheel diameter is almost 1%. every time your forester makes a turn of 19m radius (the centerline of the car) the front wheels turn 1% faster then the rear ones. when turning the steering wheel to the maximun the front wheels will turn about 3% faster. how many times do you reach that 1%? it seems that for these turns we need larger wheels at the front... isn't there any developed damage? i know that it's against the theory, but, at least from that aspect, if we replace just 2 wheels isn't it better to put the new and bigger ones at the front??? Ok... in the name of keeping the information here accurate, the 1/4" refers to circumference, NOT diameter or radius. While it's true that front tires wear out faster for various reasons, you are supposed to rotate them before the difference is so extreme. If you are just replacing two tires (for whatever reason, although not recommended) try to find some that are the same size and have similar wear as cookie said. If you are forced to get some with more tread, then putting them on the front to wear them out faster might not be a bad idea. There might be some reason not to do this I haven't considered. Johngenx: tread depth variations are changes in radius (NOT diameter). Therefore a 1/32 difference in tread depth = .03125 change in radius, or .0625" change in diameter. That's why we are multiplying by 2 pi, not just pi. You can multiply change in diameter by pi if you want, but in that case, your 9/32=0.28" is really a change of 18/32=0.56" in diameter. IMO the 1/4" is conservative and slightly exceeding it is no big deal. But there's no reason not to follow it - if you are considering running mismatched tires because you can't afford new ones, you should reconsider your vehicle of choice. Get something that's 2WD. New tires (not great ones, but at least all the same size) can be had for around $200 for a full set last I checked. Quote Link to comment Share on other sites More sharing options...
frag Posted May 18, 2005 Share Posted May 18, 2005 At first sight insisting that there should not be more than a 1/4 inch difference in circumference front to rear seems an excessive requirement. But according to my measurements, it's not. After 4-5 years, my old Toyo Spectrums have only 1/40th of an inch of difference in circumference. How do I know ? Simple test: put a chalk mark on front an rear tires side where they touch the ground. Drive forward in a straight line for 40 wheel revolutions. Compare the marks position. 1/4 inch of difference in diameter = marks 10 inches apart = very easy to measure and takes real rolling radius into account. Marks 1 inch apart (my case) = 1/40th of an inch of difference in front and back circumferences + peace of mind. Quote Link to comment Share on other sites More sharing options...
cookie Posted May 18, 2005 Share Posted May 18, 2005 are tested to a certain standard. Subaru has determined that these couplings can take a certain amount of heat before they get so hot the silicone fails. Since they are always constantly compensating for differences in revoloution they are going to build up heat that is acceptable. If you ask them to compensate more than the requirements you head to the failure threshold. All you have to do is cause them to overheat and they are dead. The standards are there to keep you from destroying these couplings, and if it did not cause problems nobody would care what size tires you put on the car. I personally do not have to hit myself in the head with a hammer to determine that it would hurt. Other people have done this and I am willing to believe them. Quote Link to comment Share on other sites More sharing options...
bjwirth Posted May 18, 2005 Share Posted May 18, 2005 are tested to a certain standard. Subaru has determined that these couplings can take a certain amount of heat before they get so hot the silicone fails... Other people have done this and I am willing to believe them. I'll state again for the record, that I'm NOT an engineer at subaru. But my limited knowledge of determining specs goes something like this. Usually there is some statistic used to measure failure rates. they most likely have data which correlate differences in sizes of tire circumference to center differential failures. But this data is never a "hard line." it's not like "well at 0.25", the CD is ok. at 0.26", the CD is toast." it's probably a bell curve. at 0.25" no failures out of 100 tests; at 0.5" 10 failures; at 0.75" 50 failures..... so why isn't it a straight cut off? probably because driving conditions vary. I suspect even if you had 100,000 cars that drove the EXACT same way, you would still see variation- why, because some cars were built on monday and some were build while the QC guy went to take a d*mp. So you take all these factors together. Now from what I know about engineers, they get hardly get the last say. there's always lawyers and bean counters who get the last say. can you imagine this conversation: Lawyer: so at 0.5" there's a 0.1% chance of failure? Engineer: yes Bean counter: so that will cost us $XXXX in warentee claims E: yes, but the cars all have new matching tires so there's really no chance of that L: so at what level would there be NO claims E: well according to our data 0.35 had no occurances BC: let's make it 0.25- if a claim comes in and the difference is greater, we can deny that claim L: OOOHHHH good thinking!! that's why you make the big bucks!! OK the above was TOTALLY fictional. I just read too much Dilbert. Quote Link to comment Share on other sites More sharing options...
cookie Posted May 18, 2005 Share Posted May 18, 2005 at the meeting. I am not sure at all that Dilbert is fictional. Since I am often the guy who has to take an old device and keep it going way after the manufacturer thinks it is junk I tend to err on the side of caution. I'm the guy who has to spend my money and get dirty when it fails so if possible I'll go with the spec that makes me sure it won't. It may anyway, but at least I tried. Quote Link to comment Share on other sites More sharing options...
Commuter Posted May 18, 2005 Share Posted May 18, 2005 Um, why by 2Pi? I'm using the diameter, not the radius. Pi*D = C. (Or Pi*2R = C) Our .28" difference in D equates to (3.14 * 0.28) = 0.879" Now, that does exceed the 1/4", but I have to agree about the impact of that .879". I wonder how realistic it is? From your original post: Suppose we have driven our four tires down to the wear bars, meaning we have 2/32" of tread remaining. (Since the majority of posters are in the US, I'll use non-metric units not to tax you...) We have a flat. The tire is shot. We buy a new one, and it has 11/32 of tread. The height difference is about 9/32 of an inch. You were speaking of tread depth, right? That is my interpretation of your post. Tread depth is a difference in radius, not diameter. Your formulas are correct. For those that don't mind mixed units... 1mm change in tread depth = 1/4" change in circumference. Just happens to work out nicely. As others have stated, and as I have stated in prior topics, I believe that Subaru is being conservative with the 1/4" circumference spec. What can you get away with? I don't know. And I don't plan to test it either, especially not with my automatic. A few years ago, a guy on old i-club board reported that his third VC (manual tranny) was failing. The 2 prior ones had failed due to heat damage. He 'discovered' that the 2 new tires he'd bought (same brand/model/size apparently) were about 1.5" different in circumference than his 2 old tires. I don't recall how long it took for the damage to occur, but it did. So... It's only a sample size of one, but there is one "number" that exceeded the 'safe' territory for a VC equipped manual Subaru. Commuter Quote Link to comment Share on other sites More sharing options...
Scottbaru Posted May 18, 2005 Share Posted May 18, 2005 I'm also not an engineer at Subaru. I am an automotive engineer, and I design and build industrial machinery (and I love it, and I've been doing it for a long time). Most of what I've read here is useless, all that matters is what speed the mechanism in question has to turn, and how hot that makes it. I've heard enough about chalk marks and pi. You've got the numbers, you're not doing the math. Do the math for your wheel size, figure out the difference in rpms the axle diff and center diff have to accomodate for different wheel sizes. It only matters for the max speed you expect to maintain. Viscous fluids and clutch plates aren't your everyday mechanisms, so it's hard for most of us to guess from experience what speeds will generate how much heat. I did the math on a previous post, I think 1/4" difference in circumference is very conservative. I agree the lawyers and bean counters probably had something to do with the specs. I don't think any of us will be able to come closer to setting safe specs with what we have to work with. Quote Link to comment Share on other sites More sharing options...
pinister Posted May 19, 2005 Share Posted May 19, 2005 hi guys i admit that i exaggerated a little with the need to bigger wheels at the front, but there is a little truth in it. when the tires die naturally, means wear until they die, there is no problem. from time to time we replace the front wheels with the rear so all af them wear and die at the same time. but it happens (too much, unfortunately) that one tire is being damaged and got to be replaced. in this case it's pity to change the whole 4. what i would do is (since i have 2 spares) to buy 2 new tires when the used 4 reached half of their wear and put them in front (i know, it's recommended to put the new tire at the rear), and to use the used tires as spares. this way i can keep 2 size tires with small differace between them, and if one tire goes off, i can replace just one or two (depends on the mate's shape) and store the smaller two. by the way, is there any way to measure the central viscose LSD when not hearing unusual noises? Quote Link to comment Share on other sites More sharing options...
firstwagon Posted May 23, 2005 Share Posted May 23, 2005 I had a leaky tire last week and after reading all this I decided to put the FWD fuse in rather the risk damage. Out of curiousity I left the fuse in for a week after I fixed the tire to see what difference there was. On the highway, no difference. It felt exactly the same. Our town, much worse. I'm tired of the torque steer and the tire spinning in the rain (a common occurrance in Vancouver). Mileage... Since I bought the car in febuary it has been very consistant at 9.5 to 10.0 L/100km (25- 25 mpg US gallon). In 2wd this week I averaged 9.8L/ 100km driving the exact same route I do every week. No advantage there. I'm happy to be back in 4wd now. Quote Link to comment Share on other sites More sharing options...
bertherr Posted May 25, 2005 Share Posted May 25, 2005 Would having different sized tires causing the damage to the clutch packs also cause the auot trans to leak? Thanks, Bert Quote Link to comment Share on other sites More sharing options...
pinister Posted May 25, 2005 Share Posted May 25, 2005 hey bert as i understand the driving chain is: engine, clutch, transmission, differentials, axles and wheels. differences in wheels diameters will effect (if) only the axles and the central differential. small difference won't hurt, because the central differential will work as "open differential". maybe it will after a long time, well, i don't know. when the difference is bigger the central l.s.d. works, and may be damaged. i don't think that any of it effects the transmission because the trasmission doesn't "know" what happens in the central diff and after. all this concerns cars with central diff and l.s.d. (like forester). with 2wd: no problem at all. 4wd without central diff: just for off-road. even with totally equal wheels there is a "war" between the wheels every time the steering wheel is turned, and damage might occur. i know that some people use welded diffs which have the same effect on asphalt as 4wd. well, it's their cars... pini Quote Link to comment Share on other sites More sharing options...
the_bard Posted May 25, 2005 Share Posted May 25, 2005 Viscous fluids and clutch plates aren't your everyday mechanisms, so it's hard for most of us to guess from experience what speeds will generate how much heat. It's a shame some of us more electrically orientated types couldn't slap a temperature sensing device (I'm thinking about thermistors) together, to tell the temp on that VC or CP. I imagine it'd be difficult to tell the actual temp of the CP, but the VC ought to be easier, since we want the temp of the fluid. I imagine we couldn't measure the temp of the casing, since it'll either be cooler or at least introduce some time lag, but even that would be better than nothing. If we could find a way to drill into the case, tap the hole, and run a probe into it... N.B.: I haven't had any coffee yet. Nor have I actually seen the guts of an AWD system. But I thought it might be an idea. P.S.: Disregard the CP stuff above... heat doesn't kill the clutch pack (well, I suppose if there was enough heat), wear does, so stickin' a temp. monitor on it won't help at all. Thanks, friendly_jacek. Quote Link to comment Share on other sites More sharing options...
friendly_jacek Posted May 25, 2005 Share Posted May 25, 2005 hey bertas i understand the driving chain is: engine, clutch, transmission, differentials, axles and wheels. differences in wheels diameters will effect (if) only the axles and the central differential. small difference won't hurt, because the central differential will work as "open differential". maybe it will after a long time, well, i don't know. when the difference is bigger the central l.s.d. works, and may be damaged. i don't think that any of it effects the transmission because the trasmission doesn't "know" what happens in the central diff and after. all this concerns cars with central diff and l.s.d. (like forester). with 2wd: no problem at all. 4wd without central diff: just for off-road. even with totally equal wheels there is a "war" between the wheels every time the steering wheel is turned, and damage might occur. i know that some people use welded diffs which have the same effect on asphalt as 4wd. well, it's their cars... pini There is no "central LSD" in subaru. If I understand subaru AWD correctly, the slip in AT AWD generated friction in the clutch pack and it is a wear item. The slip in MT generates heat and VC may overheat and fail. Either way, unneccessary slip is potentially detrimental. Minimal slip is probably harmless in MT but may shorten life of cluches in AT. Quote Link to comment Share on other sites More sharing options...
pinister Posted May 26, 2005 Share Posted May 26, 2005 hey i don't know what foresters are being sold in the usa, but all foresters that are being sold in israel (1994cc, atmospheric ot turbo, all with awd) have a central differential with viscose lsd. pini There is no "central LSD" in subaru. If I understand subaru AWD correctly, the slip in AT AWD generated friction in the clutch pack and it is a wear item. The slip in MT generates heat and VC may overheat and fail. Either way, unneccessary slip is potentially detrimental. Minimal slip is probably harmless in MT but may shorten life of cluches in AT. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted June 3, 2005 Share Posted June 3, 2005 i've heard lots of hype over this issue over my 10 + years of owning dozens of XT6's. for the first half of those years i was in college and didn't know much about cars. i bought used tires all the time. i know for a fact i had all sorts of different tires on my AWD XT6's. i drove 30,000+ miles per year, typically well over that. never had any transmission problems then. had to replace one auto trans at 150,000 miles recently...now that i always run matching sets? go figure. i've seen 2 soobs towed hundreds of miles on a tow dolly improperly and know of one other in texas, none experienced any trans problems?? i am an engineer, but have nothing to offer because what i've seen doens't match what i hear from soob so i really don't know what to think. i just keep the tires matching and don't worry about it. in my case i have extra transmissions and can pick them up for next to nothing if need be so i don't really care, but i keep the tires matching anyway. i'm a moderator at http://www.xt6.net and i always tell people to match their tires and be careful but what i also know is that i can't really say i know of anyone who's fried a trans in the 6 or 8 years or whatever i've been in the XT6 community because of this. but as many people as i know it's hard to believe someone hasn't grossly overshot the recommendations. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted June 3, 2005 Share Posted June 3, 2005 oh and can that bean counter conversation be framed and mailed to me, that was hysterical!!!!! Quote Link to comment Share on other sites More sharing options...
Slinky Posted July 9, 2005 Share Posted July 9, 2005 Hi folks... I'm here for the first time tracking down the source of the the howling in my sweeties rear end (no the car silly) as it's a 1991 Subaru Legacy AWD. I"m told she recently had a flat tire on the front of the car and put on the donut tire and drove highway speeds. The "howling" is coming from the rear of the car... where yes... one of her rear tires has a constant air leak (all tires match). So... One main questions is is this safe to drive with till it fails ? The single mom deal has much to do with this... will it lock up and cause a accident or just quietly die ? Will change'n the gear oil help anything ? I have connections with the salvage industry and can get a replacement 4.11 gear rear soon. That wont be able to happen for at least a week or more sadly. Ramble'n Randy Quote Link to comment Share on other sites More sharing options...
The Dude Posted July 9, 2005 Share Posted July 9, 2005 What the hell do you call a person living in Polecat Hollow. A Polecat Hollowian? Just curious. Anyway, the howling from the rear is almost certainly the rear differential AKA "the pumpkin". Not that it will cure anything, or stop the noise, but did you check the oil level in the rear diff? USUALLY, if there's oil in the diff it will run for a long time, driving the car's occupants insane with the ceaseless howling. FIX the leaking tire, it's only $10 to fix a flat, and you might save the car's center diff. You don't want two problems, do you? And single moms, just like single dads, have made choices in the past that have lead them to their present situations. Quote Link to comment Share on other sites More sharing options...
Slinky Posted July 9, 2005 Share Posted July 9, 2005 >>What the hell do you call a person living in Polecat Hollow. A Polecat Hollowian? Just curious.<< Ok here's the story... when I first started out in the salvage biz I'd find cars way back in the boonies in forgotten junk yards. I would sell off parts and give the genereal location, but not wanting to give away the rest of the car sitting in someone elses yard, I'd just say... me and the cars are near Polecat Hollar. Believe it or not there is a road near by called Polecat Hollow Road. >>USUALLY, if there's oil in the diff it will run for a long time, driving the car's occupants insane with the ceaseless howling. << Oil is full. I plan to change the oil. I also read on this site a "option" is to disconnect the mechanics of the rear drive train leaving the car in 2 wheel front drive. And once again the biggest concern is "locking up" and I read you comment about it will still run for some time (this is good) and it's our hope to have a replacement auto sooner than later. Thank you for the comments so very much, Ramble'n Randy Quote Link to comment Share on other sites More sharing options...
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