Spare Tire impact on differential question

[HondaRD]

Hi all, I was hoping some of you experts out there can shed some light on a concern I've had. Here's my scenerio:

I have Subaru equipped with a manual transmission with full time AWD. The front and rear differentials are open while the center differential is viscous coupling (standard Subaru setup). I've noticed the temporary spare tire has about a 1" smaller diameter than the full size tires. I was wondering if using the spare tire will cause premature wear to the center diff? My thinking for why Subaru would equip theirs vehicles with a smaller sized spare is that having an open diff would prevent excessive wear to the center diff. Does this logic make sense? My fear is that I have to use the spare when I am 100 miles away from civilization and end up causing expensive damage to the drivetrain, all because of a flat tire.

OEM tire size: 225/45-17 (24.97" diam)
OEM spare tire size: T125/70-17 (23.89" diam)

Should I not lose sleep over this or should I consider purchasing a larger spare tire for an added margin of safety?

Thanks!!

 

[Zaedock]

The spare is only 1/2" shorter and it came with the vehicle from the factory. With the open diffs, I wouldn't worry about it.

 

[bepperb]

I know the manual AWD system is different than the automatic. With open diffs and a viscous diff in the center, the only worry I would have is overheating the center diff. But, I would assume Subaru has tested that. I would probably drain and refill it after your scenario.

This would be a great question for nasioc.com, and probably already covered there.

 

[WishIhadatruck]

I know on some of the automatic trans Subaru's the manual says to remove the fuse for the AWD if you put the spare on. The autos have an electronically controlled center diff I believe. The manuals have the viscous coupling in the center like you said. What does your manual say?

 

[mechtech2]

Go slower than normal and get the real tire fixed and back on as soon as possible. This is for anybody.

WishIhadatruck brings up a very good point.

 

[FL_Rob]

It's a high vis coupling, not a locked coupling.Short term is no problem.Whats the center bias anyway? 30%?

 

[oggb]

To answer your question - yes, there's a fairly probable chance that if you drove on that for more than a few miles you could cause damage. I also own a Subaru and have often times wondered the same thing. It seems ridiculous they don't include a spare that is at least within THEIR OWN service limits for tires. If I remember correctly Subaru says your tires need to be within 1/4" in diameter.

If you're running that donut tire, the center diff IS going to be heating up. How far you'd have to drive to do damage is questionable. Most Subaru technicians would tell you that running a tire that is a full inch smaller in diameter than the rest is a huge no-no.

Heck, my service department near me preaches about how I need to keep my tires rotated so that I don't chew that diff up.

 

[HondaRD]

Thanks for the input fellas. Basically the manual gives general precautions such as keeping speeds below 50mph and to only use the spare tire on the rear. I've seen these same precautions on other non-AWD vehicles so I'm not sure if they are specific to the concern for drivetrain wear but I can see keeping speeds as low as possible to be a positive for the diff life.

My understanding of how an open diff works is that the drive shaft will spin at a speed that is equal to the average of the left and right wheels. So if the front left wheel spins 4% faster than the front right wheel, the front drive shaft will be turning 2% faster than the rear drive shaft which could result in partial activiation of viscous coupling (depending on activiation threshold speed I assume). Please feel free to let me know if I'm off base with this idea

 

[FL_Rob]

The center diff baises front to rear only,thats why they say to run the donut on the rear only,which means the front must have a vis coupling(side to side) also and the rear must be an open diff.Am I right Subie guys?

 

[Rhymingmechanic]

"Only use the spare tire on the rear."

So when a front tire goes flat, Subaru wants you to jack up the rear, replace a good tire with the spare, then jack up the front and replace the flat with the good rear tire? That's the kind of thing I can see a BITOG obsessive doing in his driveway, but not most people on a freeway shoulder or deserted highway.

 

[gathermewool]

Originally Posted By: FL_Rob
The center diff baises front to rear only,thats why they say to run the donut on the rear only,which means the front must have a vis coupling(side to side) also and the rear must be an open diff.Am I right Subie guys?


The OP's Subie has three differentials: an open-type front differential, a viscous-coupling center differential, and an open rear differential.

All this means is that power for an open differential will go through the path of least resistance. A vehicle with an open differential will experience zero traction if one of the wheels attached to that differential has zero traction - the other wheel with traction will remain still. Most FWD vehicles have an open front differential, and we all know what happens when you punch it in the snow - one wheel spins like crazy while the other just sits there, doing nothing!

The viscous-coupling center differential is filled with a viscous (thick - resists flow) fluid. When the speed of the drive shaft to the rear wheels increases by a certain amount in relation to the front wheels, the viscous coupling, which is not a mechanical connection - not locked - will cause more power to be directed to the front wheels. If one wheel in both the front and rear lose traction, the car will not move.

Originally Posted By: Rhymingmechanic
"Only use the spare tire on the rear."

So when a front tire goes flat, Subaru wants you to jack up the rear, replace a good tire with the spare, then jack up the front and replace the flat with the good rear tire? That's the kind of thing I can see a BITOG obsessive doing in his driveway, but not most people on a freeway shoulder or deserted highway.


This is good practice for any car with a flat in the front for two reasons:

1. The front wheels are the wheels used to steer the car. It's always safer to have the smaller, less-trust-worth spare in the rear to minimize the risk of it blowing out.

2. The front wheels turn, have more of the weight of the car over them, and see more abuse than the rear tires.

To answer the OP:

I haven't researched this in a bit, so bear with me on the explanation. How the rear (open) differential works is the drive shaft enters the differential and spins a pinion gear. The pinion gear spins a ring gear, which is attached, via gearing, from one side of the shaft to the other. The open differential will remain still when the left and right wheels are spinning at the same speed - only the input pinion gear, the ring gear and the coupling will spin. When a difference in speed occurs, such as when you turn a corner - outside wheel spinning faster than the inside wheel - the differential itself will spin. The average of the two speeds is the same, but I'm not sure how this would affect drive-shaft speed, and thus, center differential operation.

If you have the spare installed I'm not really sure what happens. It is my educated guess that the differential would spin due to the difference in left/right wheel speed, just occurs during turns. This leads me to believe that the drive shaft would also change speed, and thus, activate the viscous coupling in the center differential transfer power to the front wheels. I'm not sure exactly how much or how bad it would be, but I'd say, IMO, that it wouldn't cause any permanent damage if you kept the speed low.

 

[FL_Rob]

I'd just get a full size spare,theres no telling how much heat/wear would develop in the center diff,I'm only familiar with heavy duty 4wd and exactly how it puts power down,via open or limited slip, or lockers etc.,where its a giant no no to use offsized tires.

 

[HondaRD]

Thanks for the comments everyone. I really appreciate the education from it. It's strange that Subaru warns that all four tires should be within 1/4" circumference and yet they equip their vehicles with an off-sized spare tire. Subaru does offer a slightly larger diameter spare tire for a different model which I'll probably see about getting, maybe a slightly used one. Knowing my luck I'll have to throw on the spare at the worst possible time on the side of the road.

 

[HondaRD]

Ha, I just realized I should have measured the difference in tire size between spare and full size after taking into consideration compression of the sidewall once installed on the vehicle. The donut spare has a smaller sidewall height and is pumped up to 60psi so the sidewall will not compress as much as the full size tire with a taller sidewall pumped up to 32psi. I think the in overall diameter between spare and full size is meant to cover this difference thus making the rolling radius between the two much closer than I thought. Jeesh, sorry for not thinking about this before making my original post.

 

[gathermewool]

I know we've all seen people driving around on donuts for long periods of time, but that's not what they're designed for. They're designed for low speeds - some only rated to 35 mph - and for as long as it takes to get to a tire store. Full size tires or spares take up space, weigh more and cost more.

I wouldn't think the viscous coupling would be damaged at all from short term spare use. I don't think that 100 miles on a spare is equivalent to extended driving on excessively unevenly worn tires.

 

[mjoekingz28]

So, if your front tire blows how are you suppoed to get the spare on the rear with only one jack, or do they supply two jacks?

 

[mjoekingz28]

nevermind, it came to me while reading the spare tire/donut thread