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SUVs 4WD & AWD systems explained
1376 messages, Last post on Nov 23, 2009 at 9:11 AM
You are in the SUVs Forum. Your Hosts are steve_ & tidester
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Replying to: ateixeira (May 20, 2008 6:48 am) just saying...  Wwest does a trick with jack stands and 2x4s but I'm not sure the rest of us want to go there.
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Replying to: steve_ (May 20, 2008 8:04 am) Getting 4 wheels in the air is fairly easy, though. The only tricky part is getting the jack under the 2nd axle once the first one is lifted. If you've ever cross-rotated your tires, you've probably already figured that out. I have a few Jeep buddies and will ask if they know if there's one of those ramps around here.
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I actually did this test on my Miata, to see if the Diff was still working (I bought it used, so I wanted to see if it was, well, abused). Mine has a viscous coupling limited-slip differential on the rear axle. It's not front-to-rear, but rather side-to-side and on the rear axle only. All I had to do was jack up the rear axle (after blocking the front tires). Any how, the two sides are linked mechanically. So you can test them with the engine off. With an open diff, you turn one tire in either direction, and the other tire will turn in the opposite direction. This way the average speed for the axle remains zero, and the diff absorbs the difference in speed from side to side. With a viscous LSD, the tires should turn in the same direction. So I tried this. By hand, I spun the tire forward, and sure enough, the other side spun in the same direction. Woo-hoo! I was excited because that meant my viscous LSD was working! It's pretty neat, actually, you feel it engage. You need to increase the force after the weight of the 2nd tire is added to your work. I don't recall exactly, but it did not take even half a rotation of the tire, in fact it was very nearly immediate, maybe less than 1/10th of a turn of the tire. Heck, I'd do it again, just for the fun of it, and to see if the diff is still working, 8 years after I bought the car. I'll try to video tape it. The manual transmission Forester uses a viscous coupling acting as a center diff, so it works exactly the same way. If the rear axle is spinning faster than the front axle, they would lock together in the same way. It may be harder to test because with the vehicle off, both axles have open diffs (VDC is inactive). You might need to manually push both tires on one axle in the same direction to test if the center viscous coupling engages the front axle and makes it spin in the same direction, sort of like the Miata, only rear-to-front. You would still need all 4 tires off the ground, though, but in theory you could test the center viscous coupling with the engine off. To test an automatic Forester you would definitely need the engine on. I don't think the hand-spin test would work in this case. |
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Replying to: ateixeira (May 20, 2008 8:48 am) I rotate front to back - just jacking up one side is nerve racking enough.  We could all descend on a Land Rover dealer lot that has one of those little demo areas. They usually have a steep dirt ramp to play with.
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Replying to: steve_ (May 20, 2008 12:29 pm) 
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Replying to: kurtamaxxxguy (May 20, 2008 7:28 am) I think we are getting confused here between L/R and F/R movement of power. If you see a spinning wheel and then it shifts the power from that wheel, it could be shifting it L/R which is slower to react than F/R. The F/R transfer is a lot smoother and more fluid than the L/R movement of the power. A true LSD will be more fluid in transitioning power from L/R, however it still won't transfer 100% of power left and right and in addition as the LSD (either mechanical clutches or the viscous fluid in the Viscous LSDs) wear down over time the percent of power that is actually transferred L/R diminishes and the reaction time increases. Take for instance a Subaru with 100k miles on a Viscous rear LSD, the reaction time and % of Limiting of the slip will be approaching that of an open diffy. Same goes for a mechanical LSD that employs clutches in the diffy. The ABLS (Anti-lock Brake Limited Slip) found in the VDC and other "traction control" systems do not loose any of their reaction time or capabilities over time, perhaps your brake pads will wear out slightly quicker but they are a $50 item to replace. Rebuilding a the Mechnical LSD is about $300ish maybe more and a Viscous LSD requires an entire Diffy swap out. -mike
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Replying to: paisan (May 20, 2008 5:22 pm) On a few they stop intentionally to open the doors, to demonstrate chassis rigidity. I'm wondering if that pause is due to the driver. This hill climb is pretty amazing, note the angle of incline (37.5 degrees) and that there is no wheelspin whatsoever: http://www.youtube.com/watch?v=feFRI9juwV4 The hill climb occurs at 3:00, though there are two instances. If the Forester were nearly FWD, as some have alleged, the front wheels would spin and only then would the rears bite down and climb. At that angle, all the weight has shifted to the rear axle, so the front axle has very little traction and there's no way it could pull the car up. Here's a funny (and very unscientific) comparison of the Forester and Outlander. They have a tug-of-war and the Forester wins, FWIW: http://www.youtube.com/watch?v=HdXjw5hACVI&feature=related The tug-of-war happens at 6:48.
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I'd like to observe the behavior of both of these in the various modes with all 4 wheels up in the air. We could test and verify several things on the Outlander: * FWD only mode - the rear axle should not spin at all if it truly disengages * 4WD lock - both axles should spin at the same speed * Auto mode - does the front axle spin faster? Does the rear begin right away, or react only after the front axle spins? * Stability/Traction control - we could repeat all 3 tests with this On and Off For the Forester, we'd have 3 scenarios to test: * FWD mode with VDC off - again, is the rear axle totally disengaged? * AWD, VDC on - any side to side variance? * AWD, VDC off - reactive, proactive, same axle speeds, or what? The interesting part would be to observe the differences specifically. |
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Replying to: ateixeira (May 21, 2008 5:59 am) Hey, wait a second, this is just as scientific as other videos that have been posted. Maybe even moreso. Furthermore, it's manufacturer neutral. I just think the judge, let this video be entered into evidence in the case Outlander vs Forester.
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Replying to: kdshapiro (May 21, 2008 7:32 am)  Let's get 'em both up on jack stands. I'm curious to see how both rear axles behave, especially Auto mode for the Outlander. And to see if Lock is any different.
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