[Nfsjunkie91]

Hey all,

I'm about to take my new-to-me '03 OBW on a trip about 3 hours away, and I had a concern. I noticed that the tire brands front to rear don't match, and that the rear tires are in much better shape tread-wise than the front. I just went and test-drove it with the FWD fuse in and the issue that I had (drive-train shake under sudden throttle) went away.

Last night, before pulling the FWD fuse, I tested the car in a parking lot, turning it in extremely tight circles. The car had no problem doing this, and did not require any throttle. Part of me thinks that the issue I'm having with torque bind may actually be drive-train slack, and my mind is tricking me into thinking that I fixed the problem with FWD fuse, or have not noticed it yet.

The main issue I thought I had (I thought it was torque bind) was actually a CV joint going in the front (no big deal).

So, here's my question: Is it better to drive the car with the FWD fuse in to avoid any drive-train issues, or put it back in AWD and inflate the front tires to say, 35 PSI instead of 30 and see if that makes a difference?

 

[Saint J VT]

I believe the 2WD fuse is meant for a temporary measure when you must use the spare tire. In such an instance, you would typically restrict your top speed to that printed on the spare tire (40 -45 MPH seems correct).

I think, if you keep your speed down, the fuse should not be an issue. The faster you go, more than likely, the faster things will begin to be a problem, IMHO.

Whatever your manual states is the TOP SPEED when using that fuse should be strictly adhered to!

If THAT is the case, you may want to consider buying four new tires, just to be on the safe side and so you don't have to be crawling along with your four way flashers the entire trip.

 

[Saint J VT]

I suppose you could make a vertical mark on the 6 o'clock position of each tire in a long, flat parking lot, then drive straight for a distance, get out and see if the vertical marks are all at the same bottom position. If they aren't, adjust the air accordingly -- marks that went faster than others would have to have some air added to the tires, as those tires are a bit smaller in diameter resulting in the tire to spin each revolution at a more rapid pace than the other tires.

OTOH, if marks are going SLOWER than other tires, the diameter of the tire is greater than that of the other tires, in which case you would let some air out.

I think this is correct. The goal is to have every tire end up with the bottom vertical marks all staying in sync with each other, i.e., none too fast and none too slow!

Then you will be certain that all the different tires are spinning at the closest rate of speed, in which case you could most likely drive the car normally.

Remember: All things being equal - smaller diameter tires will spin at a faster rate than larger diameter tires!

It may take driving at least 1/4 mile or further to get the most accurate readings of the marks! The road surface should be flat, even, straight.

 

[Nfsjunkie91]

I suppose you could make a vertical mark on the 6 o'clock position of each tire in a long, flat parking lot, then drive straight for a distance, get out and see if the vertical marks are all at the same bottom position. If they aren't, adjust the air accordingly -- marks that went faster than others would have to have some air added to the tires, as those tires are a bit smaller in diameter resulting in the tire to spin each revolution at a more rapid pace than the other tires.

OTOH, if marks are going SLOWER than other tires, the diameter of the tire is greater than that of the other tires, in which case you would let some air out.

I think this is correct. The goal is to have every tire end up with the bottom vertical marks all staying in sync with each other, i.e., none too fast and none too slow!

Then you will be certain that all the different tires are spinning at the closest rate of speed, in which case you could most likely drive the car normally.

Remember: All things being equal - smaller diameter tires will spin at a faster rate than larger diameter tires!

It may take driving at least 1/4 mile or further to get the most accurate readings of the marks! The road surface should be flat, even, straight.

This is great advice, I will try this out. I remember reading that Subaru said all 4 wheels should be within 1/4" of each other in circumference, so even just rolling the car forward one rotation of the tire and measuring the distance should be a good starting point. I'll give this a shot later. Before I get a chance to do that, I'm going to inflate the front tires 5 psi higher than the door sticker, since they're more worn.

 

[plain OM]

The mark on the tires idea is very good; although I would note that the way tires roll at slow speeds isn't necessarily the same at high speed.

The symptoms described in the first post are the same as torque bind, and I wouldn't doubt the possibility that it's due to the different tires. Torque bind is often checked by doing tight circles, as you describe. That works when the AWD clutch is tightly engaged when it shouldn't be, such as circling with the engine idling.

But it will also appear when accelerating rapidly in a straight line if there's tires with different circumferences. This is because at heavy throttle, the AWD clutch in the automatic transmission will be more tightly engaged (it's control is, in part, responsive to accelerator position and engine torque). As the car accelerates straight, the wheels will want to turn at different speeds due to their difference circumferences, but the clutch will not allow this (no slippage), so the drive train will shake as the tension builds up and is suddenly released when a wheel slips just enough.

You're not going to be doing any good for the AWD system and could end up with it being badly damaged.

If you intend to keep the car for some time, and given that you're not experiencing torque bind symptoms on tight curves now, I think the consensus would be to put on 4 identical tires.

 

[Nfsjunkie91]

The mark on the tires idea is very good; although I would note that the way tires roll at slow speeds isn't necessarily the same at high speed.

The symptoms described in the first post are the same as torque bind, and I wouldn't doubt the possibility that it's due to the different tires. Torque bind is often checked by doing tight circles, as you describe. That works when the AWD clutch is tightly engaged when it shouldn't be, such as circling with the engine idling.

But it will also appear when accelerating rapidly in a straight line if there's tires with different circumferences. This is because at heavy throttle, the AWD clutch in the automatic transmission will be more tightly engaged (it's control is, in part, responsive to accelerator position and engine torque). As the car accelerates straight, the wheels will want to turn at different speeds due to their difference circumferences, but the clutch will not allow this (no slippage), so the drive train will shake as the tension builds up and is suddenly released when a wheel slips just enough.

You're not going to be doing any good for the AWD system and could end up with it being badly damaged.

If you intend to keep the car for some time, and given that you're not experiencing torque bind symptoms on tight curves now, I think the consensus would be to put on 4 identical tires.

Well, with the FWD fuse in, I did not notice any difference in the way the car drives in tight circles, whether it's in AWD or FWD, the car turned the same way, no odd shudders - so fortunately, I think the tires are "close enough" right now. Unfortunately, the cost of new tires at this precise moment is cost-prohibitive, we're going on a trip and I just spent a chunk of change getting the vehicle legally registered in my name in MD - $300+ all things considered.

Top of my priorities is getting new tires, I'm giving thought to ordering the exact same tires as the rear tires (the newer ones) and having them shaved to circumference. Ultimately, it'll end up costing me less than ordering 4 brand new tires.

I did discover that it's a terrible idea to drive long distances with the FWD fuse, as the 4EAT documentation from Subaru says:

"The FWD switch changes the driving mode from AWD to FWD, and should be used for diagnostic purposes only. Do not drive the vehicle in this mode. The FWD switch is located on the left front shock tower. The Legacy FWD switch is located on the right strut tower. It is activated by inserting the spare fuse into the underhood connector. The FWD light on the combination meter verifies that the vehicle is in FWD."

Furthermore, there's no drive-train shaking like you (and others) have described, even when I've taken the car on the highway, only seems like there's "slack" when I goose the throttle - and I think that may have to do with the worn CV axle.

 

[idosubaru]

I got you covered, read this a couple of times:

#1 - put your better tires up front - eventually they'll wear down to match the rears and you're golden (unless struts/alignment is hosed and the rears are wearing faster than the fronts). i would guess, based on experience, the issue will go away. you could even entertain the idea of, after swapping the tires front to rear - of over inflating the rear tires and underinflating the fronts to cover the gap so to speak.

#2 technically speaking you can actually install two tires of the same tires on one front where and the OPPOSITE side rear wheel and be fine...you can even run mismatched tires so long as you maintain those TWO the same. i'm not recommending it, i'm just saying it's possible due to the nature of of the way the differentials work - the 4WD is really only 2WD - one front tire and one rear (the opposite side from the front) is actually being driven. what i'm not positive about (since i don't do this) is wether it matters which side you do it on. the shops around here do it all the time because there are quite a few Subarus and the economy is such that buying sets of new tires isn't an option all the time.

#3 - you can use the FWD fuse just fine. you can glance over the arm chair technical banter from those that haven't done this much or at all. you won't experience any issues due to the FWD fuse.

maybe if you were rock crawling, doing burn outs, drag strips...maybe you'd see issues.

but for normal daily driving it's no big deal. there are actual issues associated with torque bind - it causes...duh..drum roll..."binding" and therefore puts binding related stresses on the drivetrain - including stressing mounts, tires, axles, ujoints, and interal workings of the trans. the most common failure mode for folks that ignore this too long is the shearing off of the internal hub/drum next to the clutch plates inside the rear extension housing. those actually fail due to torque bind.

one could make a case against all the nay-sayers or simply quote Subaru books from their computer desk and have never actually torn these transmissions apart to see the failures that running without the FWD is a very bad idea.

might as well eliminate the binding and stresses on the system - i would if i were you.

but your first step - is #1 above. that is going to help you a lot.

 

[Nfsjunkie91]

I got you covered, read this a couple of times:

#1 - put your better tires up front - eventually they'll wear down to match the rears and you're golden (unless struts/alignment is hosed and the rears are wearing faster than the fronts). i would guess, based on experience, the issue will go away. you could even entertain the idea of, after swapping the tires front to rear - of over inflating the rear tires and underinflating the fronts to cover the gap so to speak.

#2 technically speaking you can actually install two tires of the same tires on one front where and the OPPOSITE side rear wheel and be fine...you can even run mismatched tires so long as you maintain those TWO the same. i'm not recommending it, i'm just saying it's possible due to the nature of of the way the differentials work - the 4WD is really only 2WD - one front tire and one rear (the opposite side from the front) is actually being driven. what i'm not positive about (since i don't do this) is wether it matters which side you do it on. the shops around here do it all the time because there are quite a few Subarus and the economy is such that buying sets of new tires isn't an option all the time.

#3 - you can use the FWD fuse just fine. you can glance over the arm chair technical banter from those that haven't done this much or at all. you won't experience any issues due to the FWD fuse.

maybe if you were rock crawling, doing burn outs, drag strips...maybe you'd see issues.

but for normal daily driving it's no big deal. there are actual issues associated with torque bind - it causes...duh..drum roll..."binding" and therefore puts binding related stresses on the drivetrain - including stressing mounts, tires, axles, ujoints, and interal workings of the trans. the most common failure mode for folks that ignore this too long is the shearing off of the internal hub/drum next to the clutch plates inside the rear extension housing. those actually fail due to torque bind.

one could make a case against all the nay-sayers or simply quote Subaru books from their computer desk and have never actually torn these transmissions apart to see the failures that running without the FWD is a very bad idea.

might as well eliminate the binding and stresses on the system - i would if i were you.

but your first step - is #1 above. that is going to help you a lot.

Gary - Absolutely, this was what I was thinking, should help. I was also thinking about No. 2 but I'm worried I would wear out the differentials proper, too much stress... but then again, it's not an LSD... Either way.

My plan of action is this: Do number one. Check tires with chalk marks, if too far out of whack over-inflate more worn tires (the ones that would now be on the rear). If I notice/start having torque bind issues, in the FWD fuse goes.

 

[Saint J VT]

I'm anxious to hear what you discover!

 

[canubaru]

for your trip, either swap one of the fronts to the rear or drive with the fuse in.

for the future. measure the tires and see what the difference is. the closer they are the better. and putting the larger tires on the front will start to even them up.

but if the difference is big, i would consider buying 2 new tires of the same brand and see if that gets you closer to the same size.

depending on your tire size, there are about 750 - 850 tire rotations in a mile. a 1/4 inch difference per rotation equals about 200 inches or over 16 feet per mile. a half inch difference would be over 32 feet per mile.

hth.

 

[canubaru]

different size tires on the same axle is no different than turning the car. the open diff will ''allow'' this. it may put a little more wear on the diff, but it is not violent wear. larger tires on the front (or rear) will stress the tranfer clutch as well as the drive shaft, u joints and maybe axles but maybe not.

i think the u-joints are the weak point but the transfer clutch plates are the most likely to wear out. just my opinion.

 

[Nfsjunkie91]

different size tires on the same axle is no different than turning the car. the open diff will ''allow'' this. it may put a little more wear on the diff, but it is not violent wear. larger tires on the front (or rear) will stress the tranfer clutch as well as the drive shaft, u joints and maybe axles but maybe not.

i think the u-joints are the weak point but the transfer clutch plates are the most likely to wear out. just my opinion.

Good point. If I start to have issues I'll have the tools with me to handle whatever situations arises -- I'm not the kind to drive through when the drive-train starts yelling at me, and it's only been whispering to date...

 

[Euchre]

These suggested 'fixes' and workarounds are all temporary - in the long term, you need to get 4 tires the same size. Most likely that means buying new ones, or at least shaving or finding a way to quickly wear the larger ones down to match the smaller ones as closely as possible. If the Outback had been designed to run long term in FWD, it would have come with a proper disconnect for the rear drive, like some sort of transfer case.

 

[Nfsjunkie91]

These suggested 'fixes' and workarounds are all temporary - in the long term, you need to get 4 tires the same size. Most likely that means buying new ones, or at least shaving or finding a way to quickly wear the larger ones down to match the smaller ones as closely as possible. If the Outback had been designed to run long term in FWD, it would have come with a proper disconnect for the rear drive, like some sort of transfer case.

Of course. Like I said, that's my priority - funds permitting. Right now I'm sticking with AWD because it seems to be handling it fine, no major torque bind symptoms under most conditions. I've already priced a new set of tires for the car out online.

 

[plain OM]

Furthermore, there's no drive-train shaking like you (and others) have described, even when I've taken the car on the highway, only seems like there's "slack" when I goose the throttle - and I think that may have to do with the worn CV axle.

I must have misunderstood this:

I just went and test-drove it with the FWD fuse in and the issue that I had (drive-train shake under sudden throttle) went away.

(bold added)

My point was that the drive train shake/shudder/vibration under sudden throttle could be caused by different circumference tires, and it would disappear when the FWD fuse is installed. It is caused by torque build up in the drive train due to the mismatched tires, and will occur when going straight. It's not related to a malfunction of the AWD system, which is the basis of the usual discussions about torque bind, and which applies stress to the drive train mainly when turning. In this case we're talking of signs of drive train torque build up when going straight. If the car is driven in AWD mode with this condition the same stresses that grossgary referred to can build up and lead to damage in the drive train, including the shearing of the internal hub.

But if you're certain the drive-train shake is due to a worn CV joint, then here's hoping it doesn't fail and leave you stranded during the trip, especially if the joint is on a front axle.

 

[Nfsjunkie91]

Hey all,

Took the car to Jersey. No noticeable issues with it, other than the CV axle being funky if I make too sharp of a turn with power. I've got all the tools with me to replace it if need be (hopefully not), and I'm picking up a new axle when I get back to MD.

Otherwise, the car's been doing great! Quite happy with how it handles the shocks of New Jersey roadways.

Furthermore, my experiments with the FWD fuse were not for as long of a drive as I had meant to take, so results weren't totally conclusive. On the highway, the car didn't have any issues, so I'm quite happy.

Thanks again for all the advice so far, these forums have been a great help to me.

 

[Saint J VT]

I am glad the car is working for you without any "surprises."

Also, thank you for keeping us posted!

 

[NoRoadtrippin]

I have a '98 OBS. I drove with the FWD fuse for months, maybe even a year non-stop. Didn't create a single issue. Finally felt like spending all the money to get the torque bind fixed and I have been good to go. I comment just to add to the earlier comment that the FWD doesn't harm the car. Sure, maybe it wasn't meant to be done since there isn't a disconnect, but they also didn't make it very difficult to do so it can't be that huge of a deal.

 

[Saint J VT]

I have a '98 OBS. I drove with the FWD fuse for months, maybe even a year non-stop. Didn't create a single issue. Finally felt like spending all the money to get the torque bind fixed and I have been good to go. I comment just to add to the earlier comment that the FWD doesn't harm the car. Sure, maybe it wasn't meant to be done since there isn't a disconnect, but they also didn't make it very difficult to do so it can't be that huge of a deal.

Thank you, that is good information to know!

 

[jp1203]

From what I understand, the worst case scenario is that using the FWD fuse continuously could potentially wear out the Duty C solenoid faster. Having said that, though, Duty C solenoid failures don't seem to be something that is very common, so it's probably not something to worry about too much.