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Old 27th August 2009, 14:27   #46
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@Guna - thanks, enjoyed that immensely. Now who will build that set up here for testing? Pretty simple, but very effective!
In another post yesterday, I had raised the subject of cars that were ideal for India, but failed. I listed the Fusion and the Peugeot 309, and today I remembered the third on seeing this video - the Forester. The car that GM brought to India and messed up. It was a lovely, well built car, and with timeless lines. Even today, it looks contemporary. Perfect for highway oriented use, to take the rough with the smooth. As good as the modern SUVs are on bad highways, and equal to most equivalent sedans on the good ones. Lots of GC, but no stability issues. Not an offroader, but never claimed to be one. A car to buy for the long haul.
Btw, the CRV in the video really cuts a sorry figure. I can understand why the Subaru did well, it is a genuine AWD because all 4 wheels get equal power all the time, with the balance then moving around with traction loss. So the rear wheels could keep the car moving, because they did not have to start with no power, as the CRV had to.
Can't figure out why the Toyota full time 4wd didn't make it, that was a surprise.
Thanks once again!

Last edited by Sawyer : 27th August 2009 at 14:35.
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Old 27th August 2009, 16:12   #47
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Can't figure out why the Toyota full time 4wd didn't make it, that was a surprise.
Thanks once again!
Sawyer:

Toyota essentially has three 4WD platforms with some variations of it.

1. Full Time with lockable Center Diff. This is the version that is in the LC80/LC100/LC200/Prado/Fortuner.

2. Part Time without Center Diff. This is the unit in the lower end LC80/LC70/4Runner/Hilux/Seqoia etc.

3. Light Duty AWD. This is the system used in the Celica/Supra AWD, RAV4, Highlander/Harrier, Sienna Minivan and some of their PUV (Pansy Utility Vehicles). The system offers center lock only with the manual transmission and not with the automatic. This system was really meant for improving the handling and not for their off-road vehicles. Remember that the AWD Porchce/Supra can hug curves faster than the rear wheel drive one. But as PUVs became more popular and Manufactures started to go after the Subaru Market, they put these light duty AWD with fancy names to milk the customer. Even this light duty unit used to have torque split 50% front and back but in the automatic without the center differential lock it becomes pointless as the power will all be routed to the diff that is without traction. If this test was done using the Manual RAV4 which shares the same light duty AWD system you could lock the center diff and it would have climbed with 50% traction in the rear wheel. However, it would have failed the single wheel traction test with the front wheels as the front has open diff.

This light duty system which used to be full time AWD has now been updated to be an on-demand unit like the Honda CRV. To be honest I am not much into PUVs so I haven't bothered to reasearch this AWD system. One thing I can tell you is that it is pretty effective in the AWD Supra/Celica which is what it was initially developed for.

Last edited by 4x4addict : 27th August 2009 at 16:13.
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Old 27th August 2009, 16:18   #48
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I guess the Toyota Full time 4wd failed in the video because the center diff was left open, so no drive was able to reach the rear wheels. They did not run the test in the center locked mode at all. Its a Subaru ad, of course that video, but still useful to get a sense of the outcomes.
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Old 27th August 2009, 16:23   #49
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Quote:
Originally Posted by Sawyer View Post
I guess the Toyota Full time 4wd failed in the video because the center diff was left open, so no drive was able to reach the rear wheels. They did not run the test in the center locked mode at all. Its a Subaru ad, of course that video, but still useful to get a sense of the outcomes.
Center locked mode is available only with Manual Transmission, not sure why. This Highlander was probably an automatic which most of the cars sold in the US are anyways.
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Old 28th August 2009, 13:45   #50
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Your confusion is understandable. Most people who own 4WD vehicles, don't really understand how it all works. Even the Mahindra & Maruti service stations where I service my CJ340 and GV, the mechanics don't understand the exact usage of 4WD system. It took me more than a year of 4WD ownership, regular offroad experience and frequent discussion with fellow offroaders and 4x4 section to get a decent understanding of just my vehicles.

Since Fortuner 4WD system appears very similar to my Grand Vitara, that is torque sensing LSD in the central diff, let me dare to assume I understand it.

H mode - This is used for your regular driving. Here the front and rear diffs are open. The torque is sent almost equally to both front and rear drive shafts. However, the center differential has torque sensing type limited slip system. It acts as open diff until some lack of traction is detected. When that happens, more torque is sent to the drive shaft with higher traction. The amount of torque is decided by the torque bias ratio. Downside, if the traction is zero in the front or rear, the other side gets nothing either*. Then it is time to lock the center diff.

HL mode - This basically disables or locks the center LSD, this is the normal 4WD high mode in Jeeps. Both front and rear get same torque irrespective of slip.

LL mode - Same as HL mode but you will get almost twice the torque in every gear at half the speed. Generally used for descent control and slow climbing.

N mode - All wheels will be disengaged from the transfer case. Useful for towing only, never engage this lightly. If the vehicle is towed by traffic cops in normal fashion (lifting one side), your vehicle will be severely damaged.

*The clutch based or viscous type LSD behaves differently under similar conditions, but that is off-topic here. When to use the modes can only be understood by frequent offroad driving. Yes, it is possible to damage the system in HL & LL modes, exact explanation is again off-topic here.

If this is all confusing, don't worry. I took a year of hob-nobbing with experts and offroad driving before the basic understanding sunk in.

Hi Samurai

I could not follow the whole thread because it is too technical for me. Would be great if you can clarify the following

1. If I drive my GV in 4H Lock on tarmac, will this damage the drive train
(or create wind up ? )

2. If Ans to above Question is yes, then what is way to undo the wind
up ?

3. When I engage GV in 4L Mode does this also lock the center
differential?. I ask this because some time one need higher torque on
perfect tarmac. Due to Torque requiremet I may like to engage Low
range gear, but if I do that my drive train will be damaged (as I am on
tarmac and engaging 4L Lock means locking the center differential also)

Regards
GRV
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Old 28th August 2009, 15:15   #51
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Quote:
Originally Posted by GRV View Post
1. If I drive my GV in 4H Lock on tarmac, will this damage the drive train (or create wind up ? )
Yes. When you lock the center differential, the front and rear drive shaft will turn at the same speed. However, the front wheels and rear wheels don't always turn at the same speed unless under perfect condition which rarely exists. When you turn, drive over bump or pothole, front axle will turn differently than rear axle. This is the cause for wind up. On slippery roads however, the wheels will slip and release the strain. On hard tarmac there is no such release.

Quote:
Originally Posted by GRV View Post
2. If Ans to above Question is yes, then what is way to undo the wind
up ?
Unlocking the center diff should do it, the normal 4H. But, don't rely on this like it is a feature. Don't engage lock unless you are in slippery ground. Beach/wet grass/slush/loose gravel are terrains where you can use locking without worry.

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3. When I engage GV in 4L Mode does this also lock the center differential?.
Of course, and it will cause the axle windup with twice the force.
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Old 28th August 2009, 15:49   #52
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If you realize in good time, that the transmission is stressed, then unlocking the center diff will help get it back to normal. But if you haven't, and persist in keeping it stressed, the wind up could end up doing permanent damage like fused or broken gear wheels, or even broken drive shafts.
The lack of the open diff in 4l mode does come in the way of twisting steep climbs on narrow tarmac, as I found out once, where I had to stay in 4H, and slip the clutch in 1st to climb out. No other way around this in that situation. On the other hand, if you are climbing/descending steep tarmac in a straight line, you can still engage 4lc. Wind up will not happen, because you are travelling in a straight line. It is when you turn that the axles start to turn at different speeds.
PS: You should try this out once, on straight tarmac. When climbing slowly with the transmission in 4lc, if you turn the steering wheel in either direction, you will feel/hear the resistance. It gets easier to feel this in a car that does not have power steering because you have to then fight the steering wheel a lot harder to hold the car in the turn. If you are strong enough to, and the tyres have grip enough to, something that sits in between your torque on the steering wheel, and the grip of the tyres on the ground, will have to yield on the turn. Or until it actually yields, start getting wound up, along with the muscles on your forearm! Power steering takes this feel away from you, so it gets to be even more of an issue in such cars.

Last edited by Sawyer : 28th August 2009 at 16:00.
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Old 28th August 2009, 16:19   #53
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Quote:
Originally Posted by Samurai View Post
Yes. When you lock the center differential, the front and rear drive shaft will turn at the same speed. However, the front wheels and rear wheels don't always turn at the same speed unless under perfect condition which rarely exists. When you turn, drive over bump or pothole, front axle will turn differently than rear axle.
Another situation when the front and rear wheels spin in different speeds is while taking a turn (especially a sharp turn). Front wheels make a sharp arc (arc with smaller radius) and cover more distance (and have to spin more to cover the extra distance) then the rear wheels which make much shallower arc.
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Old 28th August 2009, 16:57   #54
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Thanks to all (Samurai, Sawyer, Guna) for your inputs. To Sum up what I understood

1. There is no wind up even one engages 4H or 4L lock on a perfect
tarmac (off course always avoidable), if one is driving in a straight line

2. In case Wind Up situation occurs, unlocking the differential (i.e. coming
back to 4H mode) will bring the transmission back to normal condition

Thanks once again

GRV
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Old 28th August 2009, 17:11   #55
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Two corrections - in the first para, in a straight line, as achieved with zero steering wheel input. An occassional bump is not the issue, continuing rotation of the two axles at different speeds is, and that starts as soon as you depart the straight line, by giving input to the steering.
And unlocking will prevent more wind up from happening for sure. Will it bring it back to normal depends on what permanent damage any preceding wind up may have caused.
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Old 30th August 2009, 17:27   #56
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Quote:
Originally Posted by Sawyer View Post
Two corrections - in the first para, in a straight line, as achieved with zero steering wheel input. An occassional bump is not the issue, continuing rotation of the two axles at different speeds is, and that starts as soon as you depart the straight line, by giving input to the steering.
And unlocking will prevent more wind up from happening for sure. Will it bring it back to normal depends on what permanent damage any preceding wind up may have caused.
Thanks Sawyer

Can wind up trigger leakage in Front differential seal ? I have the front differential seal leaking in my Vitara and service person could not explain it.

Making a wild co relation, if a Wind up Transmission purs extreme pressure on differnetial seal , which if not strong can start leaking ?

Thanks
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Old 30th August 2009, 21:11   #57
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I don't know if something else that is a weaker link should show up before you burst the seal, but it could be the reason. I assume you are referring to the oil seal - if so, and if it is because of wind up, I'd say you got away lightly! On the other hand the seal could be leaking simply because it was not good enough, and needs a replacement.
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Old 30th August 2009, 22:11   #58
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Quote:
Originally Posted by GRV View Post
Can wind up trigger leakage in Front differential seal ? I have the front differential seal leaking in my Vitara and service person could not explain it.

Making a wild co relation, if a Wind up Transmission purs extreme pressure on differnetial seal , which if not strong can start leaking ?
Now that is highly unlikely in the first instance. Windup starts with the gears, both of the gearbox and the differential. First damage will occur to synchronisers. Tho oil seal does not take any load as such, all it does is sit between a fixed and a rotating part and prevents the oil from dripping out.

When you engaged 4HL/4LL, the front diff started churning the oil inside more than usual. The pressure of the oil on the oil seal caused it to fail again, because the seal coupler was faulty.

Maybe - just maybe - the front diff was overfilled with oil in the first instance, which led to this.

BTW, the OTR/4x4 gurus are absolutely right, never ever engage 4HL or 4LL on hard tarmac, even for a straight line run. Maybe a 100 metre run is just ok with locks engaged in an emergency (though I can't foresee any), but not as a routine.
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Old 31st August 2009, 03:46   #59
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BTW, the OTR/4x4 gurus are absolutely right, never ever engage 4HL or 4LL on hard tarmac, even for a straight line run. Maybe a 100 metre run is just ok with locks engaged in an emergency (though I can't foresee any), but not as a routine.
Now that is not quite correct. A 100 metre run in each mode, once in a month or so, is essential to keep that part of the car in good shape, via use of it. Just the way the AC should be kept in good shape by not letting that system lie idle for more than a month or two at a stretch. And one does not have to go into 4wd country to do that, a stretch of open tarmac where you can do this by the side of the road, without any steering input is perfect for this. The whole wind up thing starts only when you give the front axle steering input. If you drive the car with the center diff locked, on hard tarmac, without touching the steering wheel, wind up cannot happen. Because the natural tendency, the way the car is designed, is for both axles to turn at the same speed, which is what they do in the absence of steering input on hard tarmac. It is when you give steering input that the axles start turning at different speeds to enable the turn to happen, and that is what causes the wind up if not relieved via tyre slip or an open differential.

Last edited by Sawyer : 31st August 2009 at 03:50.
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Old 31st August 2009, 08:01   #60
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The whole wind up thing starts only when you give the front axle steering input. If you drive the car with the center diff locked, on hard tarmac, without touching the steering wheel, wind up cannot happen.
Well, it can happen if the hard tarmac is not perfectly smooth. If the road contains potholes and uneven undulations, one wheel can turn less or more than the other. Say if left wheel goes through a depression and the other wheel doesn't, it creates a similar effect as turning but to a much lesser extent.
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