I got a Subaru last year for the safety and the all wheel drive. My roommate who also drives a Subie said if I blow a tire I will need to get 4 new tires if I have decent mileage on them because I could blow a differential and have 4 differentials in an AWD car.
I know pretty much nothing about cars. What I know about differentials I learned from a Google overview. Why would I need all new tires? Can’t my tires still rotate at different speeds if one has more wear than the other?
inb4 “Why didn’t you ask your roommate?”
Yeah, I thought of that as I typed this post. I really don’t know, but now I have a break at work so here I am.
Comments
Yes, the tires can and will rotate at different speeds but that causes more stress on drivertrain components because they aren’t designed to operate like that for extended periods of time.
Some tire places will shave down the new tire to match the old ones.
Most differential gear sets are not designed to operate at different output speeds constantly.
Think of it like taking smaller steps with one of your legs constantly. Sure, you can do it, but after a while things are going to get uncomfortable.
This becomes even more serious when you start looking at limited slip differentials, which are designed to assist with traction. These will wear out prematurely, at best.
There are tire shops that will ‘shave’ a new tire to match your other three tires. That may or may not be an ideal option, depending on your tire wear.
Edit: forgot I was in ELi5 and added some text.
Shouldn’t it be 3 differentials, not 4?
You have 3 differentials: front, center, and rear. They allow your wheels to spin at different speeds during turns. The differentials use sets of clutches (clutch packs) to transfer power to different axles. Those clutches generate some heat when they operate. They are designed for a specific duty cycle; and they are usually fine with a small difference in tire diameter. But once you get a diameter difference greater than about 3/32″, that’ll cause the tire to constantly rotate at a significant different speed. And that’ll cause the clutches to overheat and wear out much quicker than normal.
Generally with Subarus, you want all four tires to be within 3/32″ diameter, or 1/4″ circumference.
Can confirm.
I drove my Outback for too long without rotating the tires, so they wore unevenly. This caused my front and rear tires to rotate at different speeds all the time, rather than when slipping, as intended.
Here’s the ELI5 answer: differentials are only designed to have different wheels at different speeds for short periods of time. If your wheels spin at different speeds for long periods of time they will cause chronic stress on various parts of the differential which, over time, will lead to failure. Tires being of unequal diameter will cause this.
Think of it like holding a brick at arms length, with your arm straight. Yes, you can do it. But you can’t hold it there for hours at a time. Your arm will fail. That’s the most simplistic way I can think of.
The not ELI5, Subaru specific answer (this isn’t really ELI5able): The centre differential has what’s called a viscous coupler, it’s part of the centre diff and it’s what drives power to one set of wheels if the other set begins to spin. It’s a component common to all Subaru AWD systems, as far as I know. It works with a fluid that, when heated, binds two clutch plates in the differential. When one side of the coupler is spinning faster than the other (which happens when one set of wheels is spinning and the other isn’t, or if there’s a significant speed difference), its friction changes to be more viscous, binding up those clutch plates. When the fluid cools down, it returns to a less viscous state and the wheels are then free to spin at different speeds as the clutch is now disengaged.
In my case, I left the tire rotations for too long, so one side of that viscous coupler was always spinning faster, so the fluid was always heated, which wore it out. Now when the transmission is warm, the differential binds up, and the front and rear wheels always turn at the same speed. In tight turns, the issue is bad enough that my tires skip. It’s putting horrible stress on my driveline. It’s probably a $1000 repair for the part alone.
This will also happen in a front or rear wheel drive car with a limited slip differential. It’s just far more likely that the front and rear tires have a different diameter than the left and right.
You don’t need new tires per se, what you need is equal tires. This can also be accomplished by shaving your tires to the same diameter. Whether or not that’s financially worthwhile is very situationally dependent.
Take this with a grain of salt, I’m just a backyard mechanic. But it’s a problem I’ve researched a fair bit.
(edited for spelling and grammar)
No you don’t need to change all four tires. With just normal driving your tires are going to wear unevenly, and the difference in speed of rotation is going to be so miniscule as to not even count.
Almost any place that sells used tires would be able to match the tread depth of the damaged tire, no need to buy all new.
This is only true if there are extreme wear differences between the new and old tires.
The stories of blown differentials are a little over-sold as risks, if diffs were that fragile then a normal drive would ruin them completely and god help you if you need to put the donut on. In general you should replace all 4 tires on a car if one is flat simply because you get even and more traction – which is ultimately more important because it is a safety issue. Even if you don’t have AWD, if three tires are worn and you put one new one on, you aren’t doing yourself any favors.
What you want to avoid is to continuously run with uneven tires, but if you are out and you blow a tire and the shop only has 1 of your type and you need to come back later or drive to your home city or whatever you will be fine.