Tire Replacement...all four Mandatory?

[phatline]

[...]The real point is, the tires have to match in circumference (within the tolerance). Even further, one should say "effective" or "rolling" circumference. Strictly speaking, they do not have to be replaced as a set. Circumference varies by brand and model, tread wear and inflation. [...]

 

The 99 forester 5MT I just bought has two different brands of tires (michelins on back, bridgestones in front). The tread depth is very close between them (maybe 2/32" more in the back).

 

What is freaking me out is that there's no way the circumference is within 1/4" and I don't think it would be even if the tires were all 4 new and of-a-kind because the front end is so much heavier. Also general variations in tire inflation and cargo load would easily overcome this miniscule tolerance. I just measured with a string (tires on the car) and got a circumference of 79" in front and 82" in back---12x the tolerance! PSI is 28.5 front/26.5 rear (sub spec is 29/26).

 

What should I do? Replace all four tires? I estimate they have 30k miles left on them... And what about the issue that even with new tires, the circumference is not going to be perfect? I'm not trying to beat a dead horse, here. If this has been covered adequately in another thread please be kind .

 

As far as symptoms go, the only time I notice anything is a brief feeling of "hesitation" when starting from a dead stop. It happens between 0-3 mph (my estimate) and I do get a sensation that power is moving front-back-front-back. It's all over in less than a second.

[Broyer]

This thread has all of a sudden got me concerned... I just replaced one tire on my 00 OBW - 5 speed. The others have about 30K on them, but they look like they have alot of life left on them. I replaced the one tire because a tire specialist told me it was bad. I was having them rotated and balanced and he noticed this particular tire was not running true. I have put about 7K on the car since the tire replacement, mostly highway. The only thing I have notice lately is that sometimes, it will not slip easily into first gear when I am moving slowly. I just replaced the clutch, and was wondering if this was related to the new clutch, or maybe me not quite used to the new clutch. What should I do? I can't afford new tires all around, but I definitely can't afford a new transmission. I will try to measure the circumfrence of all tires this afternoon. I agree with one of the posts that stated you could easily get beyond the 1/4" circ. tolerance from normal wear... Any advice?

 

Bob:confused:

[1 Lucky Texan]

The 99 forester 5MT I just bought has two different brands of tires (michelins on back, bridgestones in front). The tread depth is very close between them (maybe 2/32" more in the back).

 

What is freaking me out is that there's no way the circumference is within 1/4" and I don't think it would be even if the tires were all 4 new and of-a-kind because the front end is so much heavier. Also general variations in tire inflation and cargo load would easily overcome this miniscule tolerance. I just measured with a string (tires on the car) and got a circumference of 79" in front and 82" in back---12x the tolerance! PSI is 28.5 front/26.5 rear (sub spec is 29/26).

 

What should I do? Replace all four tires? I estimate they have 30k miles left on them... And what about the issue that even with new tires, the circumference is not going to be perfect? I'm not trying to beat a dead horse, here. If this has been covered adequately in another thread please be kind .

 

As far as symptoms go, the only time I notice anything is a brief feeling of "hesitation" when starting from a dead stop. It happens between 0-3 mph (my estimate) and I do get a sensation that power is moving front-back-front-back. It's all over in less than a second.

 

If u read some of the archived threads on this issue, you will find there a re a lot of variables and often, folks do get by without damage. It's a little like the issue with oil. COULD someone get to 250K miles only using the cheapest oil and changing every 10,000 miles? Yeah, maybe somewhere someone could. But would you recommend it as a normal practice?

 

Just because your car isn't displaying any symptoms now, might not mean the lifespan of the center diff hasn't been cut in half or that it will break tomorrow. So much depends on the type of driving too. If its been on soft roads gravel etc. the forces are relieved before the drivetrain builds up any stress. Lots of surface street driving might do the same. Long distance highway travel would be much worse.

 

Also, the actual distance from the center of the axle to the ground is the issue, that's why(in addition to other factors) the fronts usually list a higher pressure than the rears, exactly as you noted, it's heavier and more pressure will keep the 'proper' inflation in the tire for the load that's on it.

 

the 1/4" is some engineer's safety margin compromise number. probably not just made up out of thin air - but probably has some margin of variance that will also yield satisfactory performance/life. You just want to minimize the stress on the drivetrain when operating strictly on hard/dry surfaces for long periods of time. That requires no speed differences among the 4 axles.

 

Carl

[1 Lucky Texan]

The 99 forester 5MT I just bought has two different brands of tires (michelins on back, bridgestones in front). The tread depth is very close between them (maybe 2/32" more in the back).

 

What is freaking me out is that there's no way the circumference is within 1/4" and I don't think it would be even if the tires were all 4 new and of-a-kind because the front end is so much heavier. Also general variations in tire inflation and cargo load would easily overcome this miniscule tolerance. I just measured with a string (tires on the car) and got a circumference of 79" in front and 82" in back---12x the tolerance! PSI is 28.5 front/26.5 rear (sub spec is 29/26).

 

What should I do? Replace all four tires? I estimate they have 30k miles left on them... And what about the issue that even with new tires, the circumference is not going to be perfect? I'm not trying to beat a dead horse, here. If this has been covered adequately in another thread please be kind .

 

As far as symptoms go, the only time I notice anything is a brief feeling of "hesitation" when starting from a dead stop. It happens between 0-3 mph (my estimate) and I do get a sensation that power is moving front-back-front-back. It's all over in less than a second.

 

If u read some of the archived threads on this issue, you will find there a re a lot of variables and often, folks do get by without damage. It's a little like the issue with oil. COULD someone get to 250K miles only using the cheapest oil and changing every 10,000 miles? Yeah, maybe somewhere someone could. But would you recommend it as a normal practice?

 

Just because your car isn't displaying any symptoms now, might not mean the lifespan of the center diff hasn't been cut in half or that it will break tomorrow. So much depends on the type of driving too. If its been on soft roads gravel etc. the forces are relieved before the drivetrain builds up any stress. Lots of surface street driving might do the same. Long distance highway travel would be much worse.

 

Also, the actual distance from the center of the axle to the ground is the issue, that's why(in addition to other factors) the fronts usually list a higher pressure than the rears, exactly as you noted, it's heavier and more pressure will keep the 'proper' inflation in the tire for the load that's on it.

 

the 1/4" is some engineer's safety margin compromise number. probably not just made up out of thin air - but probably has some margin of variance that will also yield satisfactory performance/life. You just want to minimize the stress on the drivetrain when operating strictly on hard/dry surfaces for long periods of time. That requires no speed differences among the 4 axles.

 

Carl

[phatline]

Also, the actual distance from the center of the axle to the ground is the issue, that's why(in addition to other factors) the fronts usually list a higher pressure than the rears, exactly as you noted, it's heavier and more pressure will keep the 'proper' inflation in the tire for the load that's on it.

 

Hi Carl,

 

Thanks for your reply. I understand what you're saying (BTW I have some mechanical engineering education). It seems to me that keeping proper inflation (so as to keep the axle-to-ground heights equal front and back) is actually more important than having matched tires. Improper inflation could easily throw new tires out of spec, and likewise judicious inflation could "correct" for improperly matched tires.

 

Is there an approved method for measuring the axle-to-ground? It's not quite as easy as it sounds. I can easily measure from the center of the rim to the ground, but it isn't perpendicular to the ground surface. Doing so yielded 32 mm rear and 31 mm front--I'm going to play around with inflation (a couple PSI up or down should be safe to do) and see how that affects the heights.

[1 Lucky Texan]

Hi Carl,

 

Thanks for your reply. I understand what you're saying (BTW I have some mechanical engineering education). It seems to me that keeping proper inflation (so as to keep the axle-to-ground heights equal front and back) is actually more important than having matched tires. Improper inflation could easily throw new tires out of spec, and likewise judicious inflation could "correct" for improperly matched tires.

 

Is there an approved method for measuring the axle-to-ground? It's not quite as easy as it sounds. I can easily measure from the center of the rim to the ground, but it isn't perpendicular to the ground surface. Doing so yielded 32 mm rear and 31 mm front--I'm going to play around with inflation (a couple PSI up or down should be safe to do) and see how that affects the heights.

 

Well, I do not have the training you do and it took me a long time to realize the actual issue (there's long thread about this from a month or 2 back, very interesting) but the problem with altering pressures to get the proper - um - 'operating radius' - is, you may affect tire safety negatively, too little and sidewall flex will increase the heat and outer edges will wear faster, too much pressure and center tread will wear faster, etc. I think 1-2 psi different from 'ideal' is probably OK for most of us. best is too just get new tires and rotate them, or have a newer tire 'shaved' to mix in with 3 older ones ,etc.

 

Again, long high speed trips on the highway are probably the main risk(to a center differential). After all, even making a turn is gonna vector some mass to one side, changing the axle to ground distance some. I think you can probably make some pressure adjustments to 'skate by' for a while, but getting new tires brings a lot of pluses - you know the history/age of them, you can choose the type tire you want for your intended driving style/area, increased safety, etc. I wouldn't delay too long.

 

if you're interested in seeing me display my ignorance;

 

http://www.ultimatesubaru.org/forum/showthread.php?t=51194&highlight=radius+tank+tread

 

lol!

 

 

Carl

[frag]

Hi Carl,

 

Thanks for your reply. I understand what you're saying (BTW I have some mechanical engineering education). It seems to me that keeping proper inflation (so as to keep the axle-to-ground heights equal front and back) is actually more important than having matched tires. Improper inflation could easily throw new tires out of spec, and likewise judicious inflation could "correct" for improperly matched tires.

 

Is there an approved method for measuring the axle-to-ground? It's not quite as easy as it sounds. I can easily measure from the center of the rim to the ground, but it isn't perpendicular to the ground surface. Doing so yielded 32 mm rear and 31 mm front--I'm going to play around with inflation (a couple PSI up or down should be safe to do) and see how that affects the heights.

 

I know a relatively easy way of measuring real world rolling radius (including all the variables one would have to measure independently otherwise).

Find a vacant parking lot or a traffic free length of street and scribe a chalk mark on all four tires sides (two on the same side would do in my opinion but why not be anal about it) where the tire contacts the ground. Drive in a straight line for 40 revolutions of one of the tires (you will maybe need a partner here). If there is a 1/4 inch difference in cricumference between front and rear tires, the marks will be 10 inches apart from each other at the end of the 40 revs. Much more precise than string measurment of the tire's circumference and it takes into account ALL of the variables including inflation.

When I tried this with my old Toyo Spectrums (one year away from their dismise), the radial diff between the front and rear tires was less than half an ich after 40 revs. Only way in my opinion to know the hard truth about where your tires stand on this question of circumference difference.

It's much easier and faster to do than to explain.

[phatline]

[...] the problem with altering pressures to get the proper - um - 'operating radius' - is, you may affect tire safety negatively, too little and sidewall flex will increase the heat and outer edges will wear faster, too much pressure and center tread will wear faster, etc. I think 1-2 psi different from 'ideal' is probably OK for most of us. best is too just get new tires and rotate them, or have a newer tire 'shaved' to mix in with 3 older ones ,etc.

 

http://www.ultimatesubaru.org/forum/showthread.php?t=51194&highlight=radius+tank+tread

 

Carl, that was an entertaining thread but I feel more confused coming away from it! I now agree with the original idea that shape doesn't matter--one revolution of tire tread equals one shaft revolution, and tire tread "length" (circumference) does not change much with inflation (steel belts limit the extent to which the tire can deform). It will certainly change some (as evidenced by uneven treadwear), but the shaft-to-ground distance should not in itself have any effect on rotational rate. It can't, unless the rubber is slipping.

 

I guess Frag's suggestion is the only way to actually verify this. Measure the rotations/distance and calculate circumference that way. Has anyone done this test with varying tire pressures? My guess is there will be a change, but it's entirely due to actual tire stretch (which is minimal) and nothing to do with the axle-to-ground height.

[frag]

Has anyone done this test with varying tire pressures? My guess is there will be a change, but it's entirely due to actual tire stretch (which is minimal) and nothing to do with the axle-to-ground height.

 

This has been thoroughly discussed a few years back and axle-to-ground height (rolling radius) does make a big difference. I tested this with a bicycle then and just did it again in the hall to refresh my memory. Just four revolutions of the wheels.

1) Same pressure front and rear: marks are both exactly at 6 o'clock at the end of the experiment.

2) One third the pressure in the front tire : when the front mark touches the ground on the fourth revolution, the rear one is 5- 6 inches late. Of course you have to put weight on the biccyle to have the lower pressure tire flatten somewhat.

I would be ready to bet money that the same experiment would yield the same kind of results with car tires.

[phatline]

This has been thoroughly discussed a few years back and axle-to-ground height (rolling radius) does make a big difference. I tested this with a bicycle then and just did it again in the hall to refresh my memory. Just four revolutions of the wheels.

 

Hi Frag,

 

Thanks for humoring me .

 

I agree that pressure will affect circumference. What I propose is that inflation pressure affects two quantities: (1) the actual circumference of the tire (stretching/deforming the tread shape) and (2) the rigidity of the tire's shape (affecting axle height). Portion (1) should result in a change of the axle's RPM, but not (2). What this means to me is that having two different tire sizes inflated to achieve equal axle heights does not guarantee equal RPM at the axles.

 

What do you think?

 

I will test this theory tonight by adjusting my tire pressures until the axels are of equal height, then checking the rotations/circumferences using the "parking lot" method suggested on these forums, and post back here. My tires are of different brand/treadwear.

[nipper]

Hi Frag,

 

Thanks for humoring me .

 

I agree that pressure will affect circumference. What I propose is that inflation pressure affects two quantities: (1) the actual circumference of the tire (stretching/deforming the tread shape) and (2) the rigidity of the tire's shape (affecting axle height). Portion (1) should result in a change of the axle's RPM, but not (2). What this means to me is that having two different tire sizes inflated to achieve equal axle heights does not guarantee equal RPM at the axles.

 

What do you think?

 

I will test this theory tonight by adjusting my tire pressures until the axels are of equal height, then checking the rotations/circumferences using the "parking lot" method suggested on these forums, and post back here. My tires are of different brand/treadwear.

 

Your right, but i think that we are all forgetting WHY not so much the how.

SUbaru AWD is unique in the fact that the clutch pack is always engaged (unless the fuse is installed). It splits power from 10/90 to 50/50. The clutch pack is designed to accept some slippage, max being 20% speed differnetial between the front and rear axles for realtivley short periods of time. Most the time its no more the n a few percent (roads arent straight).

a differntial takes the speed of the driveshaft and splits it mechancially by 2. We will assume a simple ratio. Straight both wheels are turning 1:1. Make a turn it may be 1.5 on the left, .5 on the right. The speed of the driveshaft has not changed. . Now have mismatched tires on one axle, you are stressing the differntial (especially on a manual).

The front and rear axles do not turn at the same speed. This is because the front wheels turn, where as the rear wheels do not. The front axle actually turns a bit faster speed. (im assuming a normally RWD car only because ive gone to far to rewrite this).

By compensating for circumfrence with tire inflation, you are adding saftey issues. An overinflated/underinflated tire can hydroplane easier. It can cause odd braking and emergency handling. the difference in inflation is magnified by load.You are adding more drag on the differnentials.

Human nayure being human nature, somone is going to forget one tire is supposed to be 5psi less then the other tire to balance the rotational speed. A shop may check your tires while the car is in for service and you nor know it.

There are too many things that can go wrong and the price can be too high.

The tire chalk mark is an acceptable method of checking circumfrence. ALso once its measured make a hard turn and see how the tires end up in all differnt positions.

 

:cool: i hope i explained things well, as i do this better in person then on line. SInce your tires are of differnt sizes, the chalk marks should get out of synch, which is very bad for the clutch pack.

 

nipper

[firstwagon]

By my understanding of open differentials, (front and rear, not centre or LSD) the wheels on the same axle can spin at different speeds all day long without a problem.

 

Therefore wouldn't it make sense that as long as one of the rear (for example) wheels was the same size as the front then there would be no problem. The rear open diff would simply allow the other to spin faster (or slower) then the other three.

 

Just a thought.

[1 Lucky Texan]

Try to imagine a 'car' with one normal wheel replaced with a 'tank tread' arrangement. A center, driven axle opposite the tire on the other side and some sort of idler/bogey wheels positioned one forward and one to the rear of the driven axle.

 

Let's say the tank tread is 4 times the circumference of the other tires in it's length. If the car is pushed straight ahead, the 3 tires move the same amount - let's say 1 revolution. The 'tank tread' has only moved 1/4 of it's lenght, but, if it's axle is the same distance from the ground as the other 3 axles - it's axle has made one revolution, just as the other axles have.

 

An under inflated but larger tire would behave the same way (sure, increased sidwall flex would cause more heat and , as pointed out, there could be other safety/wear issues) and lag behind in rotation but it's axle would turn the same amount as the other 3 tires if it's the same distance from the ground.

 

If you throw turning into the issue, I think it might present an opportunity for drivetrain stress to be relieved - as would a gravel or other low traction surface.

 

It took me some time to get my head around it as you all can witness in my embarassing participation in the previously mentioned thread.

 

It's an interesting academic argument, but in practice - get tires that are the same.

 

Carl

[nipper]

By my understanding of open differentials, (front and rear, not centre or LSD) the wheels on the same axle can spin at different speeds all day long without a problem.

 

Therefore wouldn't it make sense that as long as one of the rear (for example) wheels was the same size as the front then there would be no problem. The rear open diff would simply allow the other to spin faster (or slower) then the other three.

 

Just a thought.

 

This is for a raio controlled car but its the best explination i can find

http://www.rctek.com/general/differentials_basics.html

 

Your simple logic is right, but you forget the front wheels turn. therefore the front axle covers ground at a differnt speed then the rear axle.

 

try this too

http://auto.howstuffworks.com/four-wheel-drive.htm

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18

 

AHA

http://www.trucktrend.com/features/tech/163_0206_four_wheel_drive/

Why Differentials?

If we didn't have to turn and weren't concerned with noise and tire wear, trucks wouldn't need differentials. But every time a truck changes direction, the front wheels scribe a larger arc than the rear's, and the outside tires do the same relative to the inside tires. This can be accomplished without differentials, but makes steering difficult and locks everything together.

 

 

A differential allows wheels to operate with some independence, so that the inside tire doesn't travel the same distance as the outside tire or the rear tires the same as the front. In turn, the tire doesn't bark while it turns in place and the truck doesn't feel like it's trying to jack itself up (often referred to as "binding" or "driveline windup").

 

and better

http://www.4x4abc.com/4WD101/def_turnfull.html

(explains differnt wheel rpms)

 

 

i should have looked for these first this way i wouldnt have to explain it

[firstwagon]

Your simple logic is right, but you forget the front wheels turn. therefore the front axle covers ground at a different speed then the rear axle.

 

 

I didn't forget. The front axle travels at a diferent speed then the rear in turns even if all four tires are the exact same size. It's not relevent to my point that you could have one rear tire smaller then the other and the rear diff would make up the difference instead of loading up the centre differential. It's the same as when you have a mini spare on a 2 wheel drive car. The diff makes up the difference in tire rotation speed.

 

It is possible though that it would average out the speed between the 2 tires in terms of rear driveshaft speed creating a difference front to rear.

 

 

Hmmm, I'll have to think about this some more.

[Olnick]

It's an interesting academic argument, but in practice - get tires that are the same. Carl

 

That was a fascinating and educational discussion. I looked back over it thanks to your reference, Carl And I was impressed all over again with frag's straightforward logic and OB99W's patient and thorough explanations.

 

This is one great board!

[nipper]

I didn't forget. The front axle travels at a diferent speed then the rear in turns even if all four tires are the exact same size. It's not relevent to my point that you could have one rear tire smaller then the other and the rear diff would make up the difference instead of loading up the centre differential. It's the same as when you have a mini spare on a 2 wheel drive car. The diff makes up the difference in tire rotation speed.

 

It is possible though that it would average out the speed between the 2 tires in terms of rear driveshaft speed creating a difference front to rear.

 

 

Hmmm, I'll have to think about this some more.

 

 

thats only if you go straight forever. Once you turn the front wheels you have four tires running at four differnt speeds, feeding two driveshafts. But hey, its not my 1000.00 dollar repair if you decide to do this to the car , and not my car you going to bang up so have fun If your talking about this for s**t and giggles thats fine, but for driving in the real world, i strongly advise against it.

 

nipper

[firstwagon]

thats only if you go straight forever. Once you turn the front wheels you have four tires running at four differnt speeds, feeding two driveshafts. But hey, its not my 1000.00 dollar repair if you decide to do this to the car , and not my car you going to bang up so have fun If your talking about this for s**t and giggles thats fine, but for driving in the real world, i strongly advise against it.

 

nipper

 

I have 4 new tires on my car, I always replace all 4 at the same time even in a 2wd. Nothing brings an old car back to life like a set of brand new tires.

 

 

I still don't think turning has any bearing on my point. The system is engineered to allow turning and thus slippage in the centre diff. Even if you have 4 brand new tires.

 

It's going in a straight line where wear from unequal sized tires will occurs and my point is as long as one is correct then the action of an open diff will allow the other to spin at a different speed without affecting the speed of the driveshaft (and thus the centre diff).

[1 Lucky Texan]

I have 4 new tires on my car, I always replace all 4 at the same time even in a 2wd. Nothing brings an old car back to life like a set of brand new tires.

 

 

I still don't think turning has any bearing on my point. The system is engineered to allow turning and thus slippage in the centre diff. Even if you have 4 brand new tires.

 

It's going in a straight line where wear from unequal sized tires will occurs and my point is as long as one is correct then the action of an open diff will allow the other to spin at a different speed without affecting the speed of the driveshaft (and thus the centre diff).

 

I don't think you could ever get AWD (4wd - torque transfer) to occur if there were no effect on the center diff.

 

I dunno how a rear LSD would affect the question though - hmmmmm

 

 

Carl

[nipper]

I don't think you could ever get AWD (4wd - torque transfer) to occur if there were no effect on the center diff.

 

I dunno how a rear LSD would affect the question though - hmmmmm

 

 

Carl

 

Rear LSD only affects the two wheels on that axle, and it doesnt affect the equation at all. Now if the rear axle was locked, then it would have a huge affect. A limited slip (mechanical or viscous) still allows for the wheels to turn on corners.

 

nipper

[nipper]

I have 4 new tires on my car, I always replace all 4 at the same time even in a 2wd. Nothing brings an old car back to life like a set of brand new tires.

 

 

I still don't think turning has any bearing on my point. The system is engineered to allow turning and thus slippage in the centre diff. Even if you have 4 brand new tires.

 

It's going in a straight line where wear from unequal sized tires will occurs and my point is as long as one is correct then the action of an open diff will allow the other to spin at a different speed without affecting the speed of the driveshaft (and thus the centre diff).

 

No we are not talking about turning, we were talking about tire sizes and rotaional speed. i was trying to explain how tires roatate at different speeds and the system will allow ofor SOME SHORT DURATION speed differnces of up to 20% (do the math thats not alot of differnce). That is why the system can only tolerate a 1/4 inch variation in circumfrecne of tires.

In a traight line exceed that you will fry your clutch pack or your center differential, its that simple.

Also playing with tire inflation to match circumfrance can be dangerous.

Another possability is (this is on paper only i am not condoning this at all)

is to have two new tires on the same side of the car. Assuming the older tires are the same wear, this would cause the front and rear drivshafts (yes there is a driaveshaft in the front, technically its the transmission output shaft but for sake of argument its a driveshaft). Now if you have a visous lsd you may burn out the lsd. A mechanical LSD should be able to tolerate this small diffenence in wheel speed.

Best thing to do is either by 4 new tires, or have the replacemnt tire shaved. you can get four used tires and measure the circumfrance off the car and match them up also.

 

nipper

[1 Lucky Texan]

somewhat off-topic, but a fun video;

 

http://www.archive.org/details/Aroundth1937

[frag]

 

It's going in a straight line where wear from unequal sized tires will occurs and my point is as long as one is correct then the action of an open diff will allow the other to spin at a different speed without affecting the speed of the driveshaft (and thus the centre diff).

 

If that were true firstwagon, then the limitations imposed by all our user's handbooks about the doughnut spare wheel use would be absurd. Why then only 50 miles per hour and for a limited distance?

[firstwagon]

If that were true firstwagon, then the limitations imposed by all our user's handbooks about the doughnut spare wheel use would be absurd. Why then only 50 miles per hour and for a limited distance?

 

My old Buick SkyHawk also had the same warning and it's 2wd. It's the construction of the doughnut tire.

[frag]

My old Buick SkyHawk also had the same warning and it's 2wd. It's the construction of the doughnut tire.

Makes sense.