Ase, I understand what you’re describing, but Subie uses a viscous coupling. It would seem that with adequate heat dissipation and proper fluid the viscous coupling should be able to convert the differences to heat energy and dissipate it harmlessly.
Without actually tearing down a failed unit and seeing exactly what went wrong inside, the answer will remain elusive. This would be a perfect question for our old friend transman.
If I constantly drive my AWD in big circles in a parking lot, how long till my differentials disintegrate?
I’ll bet I’d go through several sets of tires first, or maybe even wear out the rest of the car.
Driving in circles won’t do any harm to your differentials or transfer case, because the front tires and the rear tires are doing the same things, they’re driving in the same circles. But the point is a modern AWD transfer case is not supposed to allow a great degree of slippage between the front and rear propeller shafts. If they did, you’d be in the snow with your front wheels stopped and your rear wheels spinning. Defeats the point of AWD. If you put 2 different size tires on an axle with a posi-trac diff in it you would soon have problems.
Yes, Subie uses a viscous coupling, and I agree about the heat dissipation and fluid. The fluid on failed units is almost always black and burnt.
FWIW, I’ve seen far more transfer case troubles caused by mismatched tires on GM and Ford products than Subaru.
“Driving in circles won’t do any harm to your differentials or transfer case, because the front tires and the rear tires are doing the same things”
No, do this in a light snow and look at the tire marks, the front and rear do not follow the same path. But close.
@BillRussell has it exactly. It’s a rare moment when a car has any two wheels rotating at precisely the same speed, except maybe on a straight highway that requires no corrections in direction for anything. So, to me, the possible failure of a component of an AWD because of wheel rotation variation can be avoided with adequate lubrication and dissipation of heat. If any mass produced auto fails to do that, it is inferior and should be avoided.
I think asemaster is correct in that it is a matter of heat dissipation. My guess is that the differentials and fluid couplings are designed for a certain amount of thermal dissipation, and anything beyond that results in overheating and frying the lubricant. Towing the car would result in a large amount of heat dissipated. And I think driving around in a continuous circle would eventually cause damage. My guess is that the designers calculated the thermal load for a typical driver, added a margin, and designed to meet that load.
But I think Subaru (and others) are lax in these designs. Same with turbos. They need to be able to dissipate much more heat, or use higher temperature lubricants. You really can’t expect Joe driver to know all of these limitations. Just as you can’t expect all drivers to drive conservatively.
In the same vein, they also need to find a lubricant that doesn’t increase the viscosity so much at cold temperatures, so the gas mileage doesn’t drop so dramatically in the winter.
The front and rear tires never follow the exact path, as shown in the snow picture, but that’s irrelevant. Despite not following the exact same path on a turn, the front and rear inboard tires are turning at the same speed, and the front and rear outboard tires are turning at the same speed, albeit a little faster. The front and rear differentials slip enough to make up the difference, the result is that the front and rear propeller shafts are both turning at virtually the same speed.
Now this is an exaggeration, but lets say that someone puts 225/60R15 tires on the front of an AWD, and 205/60R15 on the rear. The front wheels will always be rotating slower than the rear, then result being that the front propeller shaft will be turning slower than the rear. The AWD transfer case will have to make up this difference by slipping beyond what was designed, causing excessive heat, etc… Does that make sense?
This isn’t a design flaw or a manufacturer defect, it’s just a fact of engineering. Any AWD system can easily deal with variations in wheel rotation variation, as long as the same size tires are on all 4 corners. It’s when you replace 2 tires with new and leave 2 tires that are half worn out that you begin to have problems.
“The front and rear tires never follow the exact path, as shown in the snow picture, but that’s irrelevant. Despite not following the exact same path on a turn, the front and rear inboard tires are turning at the same speed, and the front and rear outboard tires are turning at the same speed, albeit a little faster.”
Exactly
Following a path that is slightly different is an entirely different animal from turning at different speeds.
“The front and rear differentials slip enough to make up the difference, the result is that the front and rear propeller shafts are both turning at virtually the same speed.”
No they don’t. The average of the wheel speed of the two rear wheels in a turn is slightly slower that the average of the two front wheels, so the front propeller shaft has to turn a little faster than the rear propeller shaft.
I once had an 86 Toyota Tercel 4wd wagon. It’s transmission was similar to the Subaru design except that when the 4wd was engaged, it locked both ends of the transmission together. There was no viscous coupling or spider (differential) gears in the transfer section. If you put it in 4wd on a good traction surface, it would lock up the transmission in short order. You had to jack up the rear of the car to release the tension.
keith is correct. If they follow different paths, ie, the rear wheels have a smaller radius of the turn then the front wheels, then the rotation rate is different. Simple Physics.
It’s much more obvious in reverse, but @keith and @BillRussell are correct.
No they don’t. The average of the wheel speed of the two rear wheels in a turn is slightly slower that the average of the two front wheels, so the front propeller shaft has to turn a little faster than the rear propeller shaft.
Right. Which is why I said “virtually the same” and not “exactly the same.”
We all know what happens when you drive a chain or gear driven 4WD on bare dry pavement. But such antiquated machinery isn’t the discussion here. A modern AWD system can and will easily tolerate the differences in turning radius when the car is driven with 4 matched tires. Throw different diameter tires on the front and rear axles and all bets are off.
For all the complaining people do about having to keep matched tires on their AWD car, does anyone want to go back to the days of having to get out of the car to lock or unlock the hubs, or having to lay in the snow to chain up the car to get to Grandma’s house on Christmas Eve?
Viscous coupling is based on a special fluid that gets thick as the speed of the slip increases.
This fluid fills the space between two sets of disc. One set of disc have holes machined in them; the other set have slots. When turning on dry pavement, the speed difference between the discs is so small that the special elixir acts like a liquid and flows between the holes and slots.
When one end of the differential starts to slip, the speed difference becomes so large that the fluid becomes a solid like substance. Power is transmitted between the holes and slots through this thickened liquid.
(You can make a liquid similar to that at your kitchen. Fill a cup with corn starch and add very little water to it. You can slowly move a fork up and down in it. Jerk the fork and it becomes a solid. Researchers have suggested making bullet proof clothing, not just a vest, based on this kind of liquid.)
When you tow with one set of axle locked to the tow truck while the other set drags on the ground, this solid like liquid can do damage to the discs. Remember when people with AWD cars mention about grinding noises after the cars had been towed with one set of wheels on the ground? Stuff are really grinding against each other. Discs that aren’t supposed to be touching are touching because of that liquid.
Chunk, good explanation. Except I don’t get the last part:
“Stuff are really grinding against each other. Discs that aren’t supposed to be touching are touching because of that liquid.”
Should’ve explain the last part better. The discs aren’t suppose to touch each other. But they would grind against each other if they are deformed by the thickened liquid. The thickened liquid would not damage the disc if the two axles are free to turn together when the thickened liquid forces them to. But this is not the case when one set of wheels are locked and the others are on the ground
Thanks for the technical explanation. Now we’re left with a real issue: is technology that will almost definitely self destruct in situations that happen to cars at least once in a while appropriate? Should consumer items like cars have devices in them that will fail in “normal” use?
Maybe you and I treat our cars like the technical miracles they are, but you know well that most drivers have as much respect for their cars as they do for their refrigerators.
My answer to the question I’m asking is “No”.
I agree with wentwest. And I put turbos in the same category, something that will fail in heavy use.
Maybe you and I treat our cars like the technical miracles they are, but you know well that most drivers have as much respect for their cars as they do for their refrigerators.
Many consumers are dopes. This does not mean it’s the manufacturer’s fault when their dopishness messes things up.
I recently specced out a home theater for my aging mother. She wanted the works. Surround sound, TV, DVD, satellite, media server, streaming movies from Netflix and Amazon Prime, etc etc. I told her what kind of complexities that adds to the user experience - you can’t just hit the on switch and turn a dial to change channels any more. With increased complexity comes increased user requirements.
Well, the inevitable happened and she was mad that she couldn’t control the whole system with one button. Even giving her a super fancy Logitech Harmony touch screen remote where all she has to do is push the button that is labelled for what she wants to do didn’t ease her frustration. She wanted all the modern marvels and she wanted it to be as simple as the old 15" (giant for the time) Montgomery Ward TV we had when I was growing up.
She even called in a high-end home theater installer, who showed up and told her “this is set up as simply as it can get until you start removing features.”
Same thing with the Subarus. People want automatic AWD and all the other modern conveniences, but then they get upset when those added features require maintenance that you don’t have to do if you don’t have them. If you want a simpler car to maintain, start removing features.
Personally it boggles my mind that people spend 5 to 6 figures on something that they must have in order to earn enough money to not be homeless, but refuse to learn even the basics about how to keep it in proper working order. A car is the second largest single purchase an average person will ever make, only exceeded by a house. You would think people would be smart enough to protect their investment, yet you always hear stories of people breaking their cars doing things that the owners manual clearly says not to do. It’s absolutely baffling.
Shadowfax: It’s not the maintenance that is the problem, it’s the risk of severe damage if my car gets towed. There is no way I can guarantee it won’t be towed at some point. I’ve parked the car in a good spot only to have the city post no parking signs afterwards and tow soon after that.
@BillRussell if I parked in a legal spot for a legal amount of time, and the city then came along and made it illegal without notice and then towed my car and damaged it, I would file suit.
I’ve had similar experiences, usually in college towns. They’ll come along and put up no parking signs in places where it’s normally perfectly legal to park. Then they’ll immediately ticket everyone parked there. They count on people just shutting up and taking it (college students don’t have much money or world experience), and make a lot of money that way. One university police department wasn’t too happy when I hauled them to court over it and not only won, but got them, shall we say, enthusiastically chastised by a very angry judge who had apparently told them in a previous case to stop doing this.
This is why responsible municipalities require advanced notice of a temporary parking restriction. If you’re gonna have a parade on Saturday, you’d better have signs up on Wednesday saying that there will be no parking on Saturday. That way anyone who’s parked there on Saturday can be legitimately towed because they aren’t supposed to park on the street for 72 hours straight anyway.
In short, if the city is doing pop-up no parking zones and then immediately towing cars parked there by people who had no way of knowing about the coming no parking restrictions, that’s a BS tow and you should fight it vigorously.