Prolonged uneven tires== differential damage??

[Legback]

I read somewhere that even tire pressure was critical in keeping the 4wd diffs from suffering. I had crappy tires on for four years that I was always pumping and they were wrong more than they were right. When I did my oil separator cover, I noticed a silverish ooze coming out of the bottom of the circular cover around the drives on the sides of the front diff. Connecting the dots....prolonged bad tire pressure and low diff fluid leads to that whining noise.... what next? Start tranny shopping?

[Fairtax4me]

Change the fluid first.

But yes the long and the short of it is, different size or improperly inflated tires will wear the differential.

One tire spinning faster than the other means the spider and side gears in the diff are constantly moving. This wears the shaft and bearings that the spider gears ride on, eventually it throws off the alignment of the gears which causes the teeth to wear quickly and unevenly. They start filling the fluid with "silver" (metal particles), which floats around and causes more wear. Causes wear on the ring and pinion gears, as well as the diff carrier bearings. The silver causes more wear. The wear creates more "silver". It's a snowball effect.

 

Change the fluid, and ride it 'til it dies.

[subietank]

ya had a 92 legacy with three diferent tires...... ya not good. It's fine in the winter when there some slippage happening. mine made all sorts of noise...

[rpholz]

 

an extreme case. but still a side affect of odd ball tires

[rverdoold]

Of course it is true to have 4 matching tires but, the front diff is open so if the front left tire has a different inflation than the front right will it be a big issue. Since that differences is handled by the front diff? Same for the rear wheels. My rear diff is open so center diff is only harmed if both my rear tires are bigger then both front ones.

(this is what i am thinking but is this true????)

[johnceggleston]

Of course it is true to have 4 matching tires but, the front diff is open so if the front left tire has a different inflation than the front right will it be a big issue. Since that differences is handled by the front diff? Same for the rear wheels. My rear diff is open so center diff is only harmed if both my rear tires are bigger then both front ones.

(this is what i am thinking but is this true????)

 

i have had the same thought, but when i had a nail in my right rear tire and it got way low on pressure before i noticed it i could feel a difference. minor torque bind. and if it is binding then it is wearing the drive train. so it matters.

 

however, if you do have 2 pairs of matching tires you are better off putting the different sizes on different sides of the car. this will help. but there is no cure for one odd size tire except the FWD fuse. and that is not recommended.

[Legback]

Thanks for the input! I think I'll put the front diff on an every 4 or 5 month fluid change to delay it's demise. IS there a recommended brand or is NAPA 85-90 good enough?

[Fairtax4me]

the front diff is open so if the front left tire has a different inflation than the front right will it be a big issue. Since that differences is handled by the front diff?

Yes, it can be an issue. The difference in speed between the two tires is translated to the over all speed of the axle (including the diff). As one turns faster than the other, the differential carrier has to turn at a different speed to accommodate that. Assuming that both rear tires are the same size and inflated correctly. The rear differential will spin at a lower or higher speed than the front, depending on teh circumstances.

 

You can turn it into an equation to see the difference mathematically.

Lets say your tires turn 800 Revolutions Per Mile properly inflated. In a straight line the diffs will also turn 800 rpm. One front tire loses a few psi now its turning at 790 rpm. By averaging wheel speed, this lowers the overall axle speed of the front axle to 795 rpm. 800 + 790 = 1590. 1590 / 2 wheels = 795.

But your rear axle is still 800 rpm.

Not a huge difference, but when you add up 500, 1,000, 5,000 miles like that (or more in some cases) all of a sudden you have several thousand extra rotations of one differential. Now at city speeds, this is probably fine. But when cruising down the highway at 70 mph, you start making a lot of excess heat due to friction between the gears. The heat is what ultimately kills the differential.

[rverdoold]

Yes, it can be an issue. The difference in speed between the two tires is translated to the over all speed of the axle (including the diff). As one turns faster than the other, the differential carrier has to turn at a different speed to accommodate that. Assuming that both rear tires are the same size and inflated correctly. The rear differential will spin at a lower or higher speed than the front, depending on teh circumstances.

 

You can turn it into an equation to see the difference mathematically.

Lets say your tires turn 800 Revolutions Per Mile properly inflated. In a straight line the diffs will also turn 800 rpm. One front tire loses a few psi now its turning at 790 rpm. By averaging wheel speed, this lowers the overall axle speed of the front axle to 795 rpm. 800 + 790 = 1590. 1590 / 2 wheels = 795.

But your rear axle is still 800 rpm.

Not a huge difference, but when you add up 500, 1,000, 5,000 miles like that (or more in some cases) all of a sudden you have several thousand extra rotations of one differential. Now at city speeds, this is probably fine. But when cruising down the highway at 70 mph, you start making a lot of excess heat due to friction between the gears. The heat is what ultimately kills the differential.

 

Haven't looked at it that way, you got a point.

I have one slow leaking tire. Advantage is that i know it and check the weekly (does not leat that quick) and keep tires on pressure.

It is already doing this for 5 years and they are nearly gone so checking what to buy next after winter when the summer season starts again.

[idosubaru]

IS there a recommended brand or is NAPA 85-90 good enough?

any gear oil of the appropriate weight is fine, so you're golden. Edited October 9, 2010 by grossgary

[johnceggleston]

any gear oil of the appropriate weight is fine, so you're golden.

 

any advantage to adding some special purple / red/ synthetic/ special blend?? i don't really know what i'm asking but i have heard stories of some "better" oil making old 5 speeds quieter and smoother so i was just wondering.

[Legback]

Cool. Thanks again. If a tire looses air pressure, in effect you're making the tire SMALLER which would mean the low pressure tire has to turn FASTER to keep up. OR is the flab of the flat tire causing resistance thus slowing it down? AND is the problem with the central drive shaft fighting with the diffs or the L and R sides of the diffs fighting each other?

[rpholz]

Thanks for the input! I think I'll put the front diff on an every 4 or 5 month fluid change to delay it's demise. IS there a recommended brand or is NAPA 85-90 good enough?

 

or why don't you just buy some tires?

 

 

tire deals always going on somewhere

[Legback]

or why don't you just buy some tires?

 

 

tire deals always going on somewhere

 

Already got new tires!!! Just to prevent further damage, I thought I'd look to make more improvements.

[Fairtax4me]

Cool. Thanks again. If a tire looses air pressure, in effect you're making the tire SMALLER which would mean the low pressure tire has to turn FASTER to keep up. OR is the flab of the flat tire causing resistance thus slowing it down? AND is the problem with the central drive shaft fighting with the diffs or the L and R sides of the diffs fighting each other?

 

It was just meant as a hypothetical example. I can see it going either way.

I think in theory the tire essentially gets smaller diameter-wise since it begins to ride on the sidewalls as it loses air pressure. But what's on paper and what really happens can and often do differ substantially.

 

As for diffs fighting each other. In an Awd car any differential motion is eventually transfered to the center diff/transfer unit, which means all 3 will experience the side effects of a problem on one axle. The center diff has to balance power output between front and rear. When one axle is "loaded" the limited slip action of the diff transfers that motion to the "unloaded" axle. But if the other axle can't physically spin any faster the diff has to take up that motion, and transfer it back into the transmission instead.

Basically when one wheel spins faster or slower, the differential assembly as a whole reacts accordingly. Then each differential becomes a "wheel", one spinning faster or slower than the other.