Drove 70 miles on a spare doughnut all below 50mph

"71MGBGT Likes Subarus of Unusual Colors" (adamprotter)
06/12/2015 at 08:06 • Filed to: None

Unpacked my entire dorm room and loaded up the car only to find my tire was flat. Had to unpack everything and change the tire. Also had to learn how to change a tire. I knew what to do, it’s just that I’d never actually done it before.

DISCUSSION (29)

WRXerFish - WRX-Wing pilot > 71MGBGT Likes Subarus of Unusual Colors
06/12/2015 at 08:09

That sucks, congrats on the new skill. Always important to be able to change a tire in an emergency!

71MGBGT Likes Subarus of Unusual Colors > WRXerFish - WRX-Wing pilot
06/12/2015 at 08:15

The only part that gave me any trouble was finding the right jacking point. There were a couple of little indents where it could have been and it was just a matter of finding the correct one. Also, trying to fit the majority of my belongings into the car with the full tire was interesting.

64Mali > 71MGBGT Likes Subarus of Unusual Colors
06/12/2015 at 08:29

Is the tire toast or can you find a place to plug it? And congrats on tire changing, at least it looks like it wasn’t raining.

Short-throw Granny Shifter is 2 #blessed 2b stressed > 71MGBGT Likes Subarus of Unusual Colors
06/12/2015 at 08:36

That’s some bullshit that the WRX doesn’t come with a full size spare. There is a guy at my work that has been driving his on a donut for the last 3 weeks now, so it must be one tough donut.

Boxer_4 > 71MGBGT Likes Subarus of Unusual Colors
06/12/2015 at 08:39

Luckily for you, with the continuous AWD system of the manual transmission cars, it doesn’t matter which tire is flat.

Not so with the Active AWD system of the automatics. If that was me, I would be changing two tires, as the temporary spare can’t be in the front. I would also need to pop in a fuse to disable the AWD.

71MGBGT Likes Subarus of Unusual Colors > Boxer_4
06/12/2015 at 08:43

WTF, that’s ridiculous!

71MGBGT Likes Subarus of Unusual Colors > 64Mali
06/12/2015 at 08:47

It might be that the old plug failed, but I’m honestly not sure. I kissed a curb a couple days ago but the tire was fine. I went on a drive this morning and it was all right too. Moving it from my parking garage to the dorm did it, so I’m not sure if the curb thing and the loss of air are correlated.

64Mali > 71MGBGT Likes Subarus of Unusual Colors
06/12/2015 at 08:50

I carry one of these in my cars, (well did I used them all up and have to reorder). It says for ATV but I’ve had patches hold for years including a sidewall plug. http://safetyseal.com/catalog/index.…

71MGBGT Likes Subarus of Unusual Colors > Short-throw Granny Shifter is 2 #blessed 2b stressed
06/12/2015 at 08:50

After driving all the way home, it still looks to be in good condition. It just needs to get me to the shop today and I’ll retire it. Maybe send it to a farm upstate or something.

71MGBGT Likes Subarus of Unusual Colors > 64Mali
06/12/2015 at 08:51

That’s a good idea. I already had fix-a-flat and a portable air compressor in there, but that’s a good addition.

64Mali > 71MGBGT Likes Subarus of Unusual Colors
06/12/2015 at 08:53

Fix-A-Flat is the mechanics worst nightmare. It slimes the whole inside of the tire with glue and makes changing it a mess. Also has a tendency of wrecking valve stems.

71MGBGT Likes Subarus of Unusual Colors > 64Mali
06/12/2015 at 08:55

I think of it as a last resort. Not going to use it unless I really have to.

64Mali > 71MGBGT Likes Subarus of Unusual Colors
06/12/2015 at 09:00

Yeah I used to carry it when I was doing long drives and the though of being stranded in rural VA was not high on my list. The last can I had I used on a golf cart at a job site.

71MGBGT Likes Subarus of Unusual Colors > 64Mali
06/12/2015 at 09:04

I’m just glad I checked the pressure on the doughnut. It was down 20 psi

Captain of the Enterprise > 71MGBGT Likes Subarus of Unusual Colors
06/12/2015 at 09:10

It’s at times like these that I’m glad my car came with a full size spare, have used it at least 4 times and sometimes for a couple weeks (sister borrowed car and cracked rim twice in three months) and appreciated having a fully operational wheel.

TylerJ > Boxer_4
06/12/2015 at 09:12

Not knowing anything about Subarus, I assume it becomes FWD with the fuse?

OPPOsaurus WRX > Boxer_4
06/12/2015 at 09:13

really? what do you drive?

OPPOsaurus WRX > 71MGBGT Likes Subarus of Unusual Colors
06/12/2015 at 09:15

20 minutes later:

Kinja'd!!!

Back in the day, i ws picking up a girlfriend at college and when we went back to my Explorer I had a flat. Changed it no problem, drove off without the jack.

Also, when I was swapping out the winters, my scissor jack froze up. Had to get a new one (got it covered by warranty even tho they tried to pin user error on me)

ColoradoTaco > Short-throw Granny Shifter is 2 #blessed 2b stressed
06/12/2015 at 09:44

Funny enough my old B5 A4 had a matching full size spare while my B5 S4 had a donut. Both were 2001s. The S4 retailed for almost 15k more too.

I imagine they did it on both the S4 and WRX for weight savings.

RazoE > Boxer_4
06/12/2015 at 09:46

Came to say this. AWD + Unequal tires = DANGER TO MANIFOLD

Boxer_4 > 71MGBGT Likes Subarus of Unusual Colors
06/12/2015 at 09:56

It is a bit ridiculous, but it’s based on how the two AWD systems work.

The Active AWD system in the automatic transmissions use a computer controlled multi-plate transfer clutch. The transmission controller monitors the speed difference using a speed sensors; one for the front axle set, and one for the rear axle set. The transmission controller has a set threshold for what it considers slipping (greater than a 20% speed difference between front and rear), and adjusts power output to the rear accordingly. It also adjusts power distribution based on other factors such as throttle position, gear, ABS, etc.

The continuous AWD system uses a viscous coupling instead of a multi-plate transfer clutch. The viscous coupling consists of various metal plates that connect the front and rear axles, and a silicone fluid between the plates. A speed difference between the front and rear axles causes the silicone to shear and heat up, increasing its viscosity. This enables the power to be transferred front and rear without the need of computer control.

I can dig up more information on both if you are interested.

Back to the temporary spare placement, I believe I didn’t clearly separate two issues that are at play. Generally, a temporary spare tire should be used only on the rear wheels. This is mainly for steering and control purposes, since the temporary spare is usually a little bit smaller in diameter than the standard tire. I don’t typically think of this reason alone, since the Subarus we have now are the first to have temporary spares since our 1990 Geo Prizm.

The Subaru Active AWD adds an extra complication, since the transmission control unit will see the different rotational speeds of the full sized wheel and temporary spare as wheel slippage and try to correct for it. That’s the reason for the FWD fuse, which prevents the car from shifting power rearward. Putting the temporary spare in the back is mainly for car control reasons.

The continuous AWD system appears to be less sensitive to slight differences in tire diameter, which is good, since you cannot disable the viscous coupling. It is still probably a good idea to use the temporary spare on the rear, but it isn’t critical for your AWD system.

Boxer_4 > TylerJ
06/12/2015 at 10:01

Yes. A warning light will come on, too. Usually “FWD” or “AWD” depending on the model year.

71MGBGT Likes Subarus of Unusual Colors > Boxer_4
06/12/2015 at 10:02

Well then, thanks for the awesome response! I hope I wasn’t supposed to do that.

Boxer_4 > OPPOsaurus WRX
06/12/2015 at 10:03

09 Forester with the 4EAT.

See my explanation to 71MGBGT below. I was a bit incorrect earlier.

Boxer_4 > 71MGBGT Likes Subarus of Unusual Colors
06/12/2015 at 10:07

You should be fine. I don’t believe the continuous AWD system is as sensitive to differences in tire diameter as the Active AWD system is.

Boxer_4 > RazoE
06/12/2015 at 10:11

General rule for Subarus: nothing more than a 1/4” difference in tire circumference, or between a 1/32” and 2/32” difference in tread depth.

boxrocket > Boxer_4
06/12/2015 at 11:33

This.

At my old shop we had to replace all 4 tires on a customer’s Subaru (legacy or forester) because she shredded one and the difference in size between the new tire and the old (which had about 40-50k on them) would have ruined the AWD system, confirmed by the local Subaru dealership.

NoneOfYourBiz > 71MGBGT Likes Subarus of Unusual Colors
06/12/2015 at 11:45

YES! On my car, the notches are not clearly marked. Wrong spot and you fold the seam over or, possibly, do frame damage.

One thing next time this happens: put the donut on the rear. Move the rear to the front.

Boxer_4 > boxrocket
06/12/2015 at 11:54

Yes, the specification is no more than a 1/4” difference in tire circumference. I believe that translates into between 1/32” and 2/32” difference in tread depth. More than that and you’re putting more stress on the drivetrain than it was designed for.