AWD/4WD Miss-conceptions...

"hangus77" (hangus77) 10/31/2014 at 12:02 • Filed to: None

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So, I work with this fellow who just laid a very interesting statement on me. I will try to relay what he said, word for word:

"I don't like AWD systems because when you get a flat tire and you have to replace a tire, you have to at least replace TWO tires. It will cause the AWD system to fail and that's bad. Mmmmkay?"

I think that is horseshit because unless you have a locked differential, an AWD system is MADE to allow wheels to spin at different speeds, i.e. when you go around a turn and the inside wheel spins slower, etc. My opinion is that 1/4 - 1/2" of tread depth difference on one tire versus its counterpart on the other side of the vehicle is not going to do shit to wear out your AWD system faster. I know that there have been some very descriptive AWD/4WD explanations on here, so I figured it was appropriate to share.

Please share thoughts/opinions/facts/jokes/pokes/smokes.

Thanks!

Will with a W8 races an E30 > hangus77 10/31/2014 at 12:06

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It Depends.

That's probably not the answer you were looking for, but of the multitude of AWD systems, some do require matched tires, some do not. And sometimes it is minute design details of seemingly similar systems that makes the difference. In the end it is very much vehicle specific, but in many cases your friend is correct. Check the manual.

ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) > hangus77 10/31/2014 at 12:08

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He's correct.

The issue is having different circumference damaging the drive-line. The spare on an AWD system needs to be the same overall distance around as the normal wheel/tire combo.

So when you replace one tire, if the other tire on the same 'axle' is worn enough the change in overall circumference can result in damage to the gearing since one wheel will always travel faster than the other.

AWD systems are designed for short-bursts of varying speeds across the axle, not prolonged differences.

crowmolly > hangus77 10/31/2014 at 12:08

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1/2" of tread depth is a lot IMO. That's most of the tread on a BFG Mud Terrain tire, and into the cords on car tires.

Unless the good tire on that axle is almost new you'd be replacing 2 tires anyway.

hangus77 > Will with a W8 races an E30 10/31/2014 at 12:08

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I would assume that it greatly depends on the type of system that it is and how the center differential works. I just have a hard time fathoming that having one wheel spin just a liiiiiittle faster than the other during normal driving makes that much difference. They spin differently all the time, if you think about it.

HammerheadFistpunch > hangus77 10/31/2014 at 12:09

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Generally speaking, he;s right. if its a fully open system with no electronic aid an only mechanical system (no clutches) then you will be fine, but 99% of the time there is either a clutch pack, a viscous coupler or another form of limited slip in the system, and having 1 wheel spin at a different rate can mess shit up. Good news/bad news is that you can replace one tire, but you will have to shave it match the others, reducing the value of the new tire you purchased. Still, cheaper than 2.

deekster_caddy > hangus77 10/31/2014 at 12:09

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This is quite standard fear-mongering supplied by many chain tire stores. Wheels are turning at different speeds all the time. It's true that different tire sizes can give the systems fits, but different tread depths are usually not enough to make that difference, unless you needed all 4 tires anyway.

Pro-tip - you can usually find "rotations per mile" on most tire manufaturer's websites in their tech areas. It's interesting to compare that number across different brands and models with the same size - sometimes they are significantly different, although usually not.

RamblinRover Luxury-Yacht > hangus77 10/31/2014 at 12:10

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It depends. Mostly BS, but... If you have as much of a size difference as that 1/2" you mention, that's an extra 4% or so faster that one side is turning. That will be constantly crabbing your spider gears for extra wear more than normal driving (average relative motion around zero, some wear in both directions). In addition, that extra 4% will possibly be enough in a sharp turn that with a managed AWD system, it may think you are slipping when you are not, and abuse the tires accordingly. It's more of a bad idea in some respects with a classic 4wd system used when there's some snow on the ground - straight driving will include a lot of slip and might make you break free when you wouldn't have otherwise.

The big issue would be if the replacement tire actually had a different profile, which if one buys used tires sized "whatever" is crazy likely.

hangus77 > RamblinRover Luxury-Yacht 10/31/2014 at 12:14

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I mean, if the difference between the new tire and the flat/damaged tire was 1/2" I would just replace both of them anyway, but he was saying that regardless of tread wear, you have to buy two tires. I just don't think that is the case.

RamblinRover Luxury-Yacht > HammerheadFistpunch 10/31/2014 at 12:14

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It'd also depend on what the engagement of the system is. His example is around 4% wheel to wheel difference, and about 2% axle to axle difference, which could well be low enough that a mechanical clutch pack, viscous coupler, etc. wouldn't engage at all.

RamblinRover Luxury-Yacht > hangus77 10/31/2014 at 12:16

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It'll depend on the system, but if you're replacing with the same exact tire and wear is less than severe, there won't be a meaningful difference with any but maybe 5% of AWD systems.

HammerheadFistpunch > RamblinRover Luxury-Yacht 10/31/2014 at 12:16

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its true, but generally speaking its not a good idea to mix and match different diameters of tires, even small difference. That having been said, I have more tread on the 5th tire I started to rotate in too late than I do on the other 3 (plus spare) that are on my cruiser right now. Its an open rear diff though and a very loose VC unit so Im not too worried.

hangus77 > crowmolly 10/31/2014 at 12:16

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Yeah, I get it. I just was putting out an arbitrary number. 1/2" is a LOT. haha. If the difference was that much, I would certainly replace both or all tires.

hangus77 > RamblinRover Luxury-Yacht 10/31/2014 at 12:17

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That's basically what I was thinking. Thanks!

WiscoProud > hangus77 10/31/2014 at 12:17

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As I understand it, as long as the tires are the same nominal height, the system will be fine. I suppose an extreme case would be if a completely bald tire was replaced with a brand new one, the difference could approach 1" in height, but since most people rotate their tires the actual difference should be negligible. I certainly don't think its going to wear out the components though.

crowmolly > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 10/31/2014 at 12:21

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That's not just AWD though. If the difference is great enough between two tires linked by, say, an Eaton posi you will cook it as well. R&P probably wouldn't suffer but the diff will.

RamblinRover Luxury-Yacht > HammerheadFistpunch 10/31/2014 at 12:22

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Correct. It's a good idea to replace tires in pairs, generally, but his co-worker's contention that regardless of the condition of the other tires you'd have to replace two is... not really correct. I made a point to mention that if he's got, say, over half the life left in his tire and replaces with the same brand and tread of tire, any manufacturer diameter/etc. differences are likely going to be smoothed over to the point of not being different than a set that hasn't been rotated. That's more critical - having a tire which is actually the same size and shape, regardless of some variance in tread wear. If somebody holes an expensive MPSS or something not that far into service life, replacing a pair of MPSS is probably not going to be necessary as long as the replacement is also an MPSS.

HammerheadFistpunch > RamblinRover Luxury-Yacht 10/31/2014 at 12:26

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excellent point.

BJ > Will with a W8 races an E30 10/31/2014 at 12:28

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This is essentially true for any drive system when the tire to be replaced is on a driving wheel. It has nothing to do with AWD, although it's perhaps more important in that case because AWD can be complicated and expensive to work on?

I worked in a tire shop when I was 16 and when someone came in with a wrecked tire we almost always sold a pair. My boss was extra cautious and wanted to make sure that the customers' cars were always safe and functional. It didn't hurt the bottom line, either, of course.

samsmith > hangus77 10/31/2014 at 12:33

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Yeah I agree with most statements on here RE tread depth. Example: The SAAB haldex 4 XWD system is very delicate and anything more than 1/2" of tread change could (and usually is) very dangerous to the TC and the axle. And if you have the eLSD with the XWD system you're pretty much fucked, but that's for many many reasons!

Will with a W8 races an E30 > hangus77 10/31/2014 at 12:40

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It's the long, straight highway drives that make the distance. Also important to note, many modern AWD systems do not have center differentials. Center diff is just one solution.

Will with a W8 races an E30 > BJ 10/31/2014 at 12:42

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Agree completely. Tires should be dismounted and mushroom patched when possible, otherwise replaced in pairs.

KusabiSensei - Captain of the Toronto Maple Leafs > deekster_caddy 11/15/2014 at 08:17

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Yes, the wheels will turn at different speeds during turns, but the idea is that there isn't any force on the spider gears needed when going straight ahead (i.e. both output shafts are turning at the same rate).

For the following, I'm considering the rear axle, mainly because the angle of the wheel to the driveshaft is fixed.

At the very least, on an open diff, you have the opportunity to have enough difference to be engaging the spider gears in the carrier to deal with the difference in output rotational speed. With a limited slip diff, you will wear the limited slip mechanism (worm gears, clutches, viscous couplings, etc) as it may trigger an engagement due to difference in speed.

Again, it depends on how much of a difference in speed there is. Think of it this way: What's the maximum permissive difference in speed in full traction conditions? That would be speed differential on the smallest radius turn the vehicle can handle. Now you may want to have it begin locking up at that smallest radius while at speed, but not enough to affect parking maneuvers (i.e. percentage speed differentials are smaller in actual numbers at low speeds), but not below that percentage difference in speed at higher absolute speeds.

#ItsAllVeryInterestingMrMay

deekster_caddy > KusabiSensei - Captain of the Toronto Maple Leafs 11/15/2014 at 09:11

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Yes, you are right that the differential will be turning. But the differential is ALWAYS in flux a little, and that's what it's designed to do, even in a limited slip. (That's why it is called limited slip, it's supposed to be able to slip a little at any time) The difference in mildly worn tread vs full tread is such a small percentage in the overall picture that it's a non-issue. Yes, you should not put different tire sizes at each corner, but replacing a single tire when the others are half-worn isn't going to blow up the car or void your warranty...

KusabiSensei - Captain of the Toronto Maple Leafs > deekster_caddy 11/15/2014 at 11:20

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Yes, however, you will be using the spider gears moreso than normal until it is fixed. You might have a differential rebuild slightly sooner than you otherwise would, but that's all theoretical.

#JamesMayOnOneSideKittensOnTheOther

deekster_caddy > KusabiSensei - Captain of the Toronto Maple Leafs 11/15/2014 at 16:33

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Sooner than otherwise? Wow, they must make differentials like crap these days. Change the oil and they should never wear out within the reasonable life expectency of any car. Even with an oddly worn tire or two. You aren't talking about _different size tires_. Just uneven wear on the same size. A non-issue, really.