The downside to an AWD car

[Bogger]

Daughter picked up a couple screws in one of the tires of the wife's Subaru Legacy and by the time she realized it was flat had shredded the sidewall... No biggy, I figured we'd throw a couple new tires on the front and call it a day. These are the original all seasons Goodyear Eagle and probably have 30K of the cars 70K km on them and are at 50-60% (we have separate rims/tires for winter).

Apparently the 5/32 difference between the old ones and new ones is out of spec for the sequential AWD and we now need to replace all 4, I expected having to replace 2 but not all 4. It seems like a waste both financially and environmentally to scrap 3 perfectly good tires with better than 50% tread, but apparently that is how it has to be on an AWD Subaru.

Maybe the next one we will go back to FWD with Studded tires for winter.

 

[Flapjack]

I don't know much about it but that sucks, and you're right, not very environmentally smart.

It's all wheel drive so you'd think the transfer case was all controlled by the ECM. Does it have traction control ?

 

[Bogger]

I don't know much about it but that sucks, and you're right, not very environmentally smart.

It's all wheel drive so you'd think the transfer case was all controlled by the ECM. Does it have traction control ?

Yup full traction control as well as AWD.... I spoke with the tech and he thinks it's BS too but unfortunately it's how the sequential AWD system works. All tires need to be the same make/model/tread pattern as well, I had never put much thought into it before. Even with the dummy tire (spare) on it they told us not to drive it in and arranged for a flat deck to come get it. The spare is only to get it to the closest repair shop and must be driven under 80km/hr - not because of the tire but because of the transmission and differential system. Guy drove from Saskatoon to Edmonton on his spare and by the time the car got to the dealer in Edmonton the transmission was pooched...... Things that make you go hmmmmmm

 

[tex78]

3-4/32 is enough to mess up any awd or 4x4

Too much difference in axle speeds, and too tight of tolerances so it's not banging and vibration in it

Viscous couplers are by far the worse for touchy axle speed differences

 

[Flapjack]

Traction control was an option on my wifes car in 05, I vetoed that.

 

[Bogger]

3-4/32 is enough to mess up any awd or 4x4

Too much difference in axle speeds, and too tight of tolerances so it's not banging and vibration in it

Viscous couplers are by far the worse for touchy axle speed differences

Yah I get the mechanics behind it, doesn't help the fact that 4 new tires are twice as much as 2....lol

 

[sweld]

Just put more air in the old tires.

 

[MACHINE]

You can’t really do donuts lol

 

[Stg2Suby]

The Subaru AWD is nothing special mechanically, similar to Audi and uses differentials front, center and rear with different variations on limited slips depending on model. I doubt you'd harm it with a slightly taller tire in 1 location. But ideally all 4 should be identical and the tire shops will definitely endorse this recommendation $$

 

[Couch]

Yup full traction control as well as AWD.... I spoke with the tech and he thinks it's BS too but unfortunately it's how the sequential AWD system works. All tires need to be the same make/model/tread pattern as well, I had never put much thought into it before. Even with the dummy tire (spare) on it they told us not to drive it in and arranged for a flat deck to come get it. The spare is only to get it to the closest repair shop and must be driven under 80km/hr - not because of the tire but because of the transmission and differential system. Guy drove from Saskatoon to Edmonton on his spare and by the time the car got to the dealer in Edmonton the transmission was pooched...... Things that make you go hmmmmmm

Curios to know how the ecu could determine the tires are the same when driving in slush??? I'd call bs too!

 

[Frosty19]

I had a Subaru in the past, and 4 tires will be a lot cheaper than rebuilding the diffs and or trans lol
I don't think the traction control or ECU has anything to do with it, it's more the mechanical guts that mesh the "symmetrical" system.
You may be fine for a short while as the tires can slip on a slippery surface, but as soon as you are going over whatever speed on say a highway no longer symmetrical inside

 

[Caper11]

This has been a problem with AWD for awhile. The older Honda pilot was the only SUV on the market that was actually 4wd, until they went AWD.

No different that a skid steer or a rock truck, the diameter of the tire must match or risk destroying the driveline.

 

[turbo392]

Buy a new tire and get it shaved or have Tire Rack do it for you.

https://tires.tirerack.com/tires/Shaving

 

[Flapjack]

Curios to know how the ecu could determine the tires are the same when driving in slush??? I'd call bs too!

Or icy road, I guess I overestimated the tech involved, so the car has wheel sensors that apply ABS, I guess that's the traction control, big deal.

Frick I thought TC could send power equally to where it was needed, not brake. Maybe it does IDK, tire size wouldn't matter if it did.

 

[Caper11]

AWD and traction control are two separate systems, depending on the design of the system.
There are Full AWD systems and part time AWD systems thats incorporated the AWD into the traction control, with different levels of traction control.

Than there is skid control.

So look at a 4WD truck with bigger tires. Sometimes the spare tire is not upgraded to the bigger tire. If the person puts the spare tire on the driveline is going to hate life, especially if 4wd is used.

 

[niner]

Had the same issue when we had Ford Flex. Found 2 tires on Kijiji that had exact wear. I think that was more luck than anything. But there are thousands of tires on Kijiji. Sold some junk tires on Kijiji also. Crazy
what some guys will buy.

 

[Bogger]

Had the same issue when we had Ford Flex. Found 2 tires on Kijiji that had exact wear. I think that was more luck than anything. But there are thousands of tires on Kijiji. Sold some junk tires on Kijiji also. Crazy
what some guys will buy.

I thought about that but the hassle is not worth the savings.... $700 for 4 new Goodyear Eagle LS2's installed and so long as we don't blow any more tires they should be good for the next 7 years... I'm going to keep the old 3, likely sell 2 and keep one as a backup incase we blow another tire at 8/32....lol

 

[Flapjack]

AWD and traction control are two separate systems, depending on the design of the system.
There are Full AWD systems and part time AWD systems thats incorporated the AWD into the traction control, with different levels of traction control.

Than there is skid control.

So look at a 4WD truck with bigger tires. Sometimes the spare tire is not upgraded to the bigger tire. If the person puts the spare tire on the driveline is going to hate life, especially if 4wd is used.

Exactly
I have 35's w/stock spare, if I get 1 flat drive tire I have to change 2 tires, I get a little jumpy at the dump lol

 

[Flapjack]

A trucker told me about trying a skid controlled big truck, I don't think they got that figured yet by his ​experience.

 

[Couch]

Still call BS. More likely problem is that most of the awd systems use components that are sealed (cheap + shop time) and have no manner of checking for routine maintenance. The system has to allow for slip otherwise it would lock up and not be able to turn. The wear difference between tires is effectively the same as slip - one tire travelling a further distance than another for a given period of time ....
Taken to the extreme it could pose an issue if it exceeds the design limits and capabilities but not likely in this case.