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Old 15th November 2015, 21:52   #391
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Default Re: Driving all four wheels: how is it done?

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Originally Posted by SS-Traveller View Post
Both vehicles have manual transmissions, so I am not sure why either cannot be towed in neutral, with engine switched off too.

What does the owner's manual say?
This is from the Duster's manual , it expressly recommends not to tow the AWD if any of it's fours wheel is in contact with the ground. It specifically mentions that if the vehicle is stuck , it can only be towed for a short distance to pull it out , and mentions about possible mechanical damage. Any idea about the reason ?


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Old 15th November 2015, 22:46   #392
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Default Re: Driving all four wheels: how is it done?

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Any idea about the reason ?
Thanks for the image, DriveTrain. I'm afraid I really cannot figure out what kind of mechanical damage can occur, if the vehicle being towed is a manual transmission with gearshift in neutral, and the drive selector is in compulsory 2wd mode.

Shall look forward to someone explaining this.
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Old 17th November 2015, 10:28   #393
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Default Re: Driving all four wheels: how is it done?

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Originally Posted by SS-Traveller View Post
if the vehicle being towed is a manual transmission with gearshift in neutral, and the drive selector is in compulsory 2wd mode.
In the Yeti, it in permanent 4WD mode.

I have voluntarily tried a couple of times to launch it, even on debris [with tarmac below] and I have always got a clean start, no wheel-spin.

That is why I am asking that if the engine is running & its [whatever] mode/s are active, how can mechanical damage occur?

Sutripta Sir..?
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Old 8th January 2016, 09:24   #394
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Default Re: Transmission Windup

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Originally Posted by ex670c View Post
Transmission windup is caused by lack of differential action between the front and Rear Wheels, this is further aggravated by the use of LSD/Lockers/Diff-Locks.

Wrt to Mahindra JEEPs and Maruti Gypsy.
I just had a chat with some one in Pune who has restored a old LandRover Discovery. This car has full time 4WD with no central differential, but there is a ratchet araingment in the front propeller shaft. So front wheels can be rotated forward freely when on a ramp, but rear wheels are rigidly fixed to the transmission.
On turns when the front axle will travel more than the rear, the ratchet will be allowing the slip, but when a rear wheel spins in place, and rear axle cannot apply torque, the front axle will automatically be taking up.
Only two issues I forsee in such an arangement are
1. If a car has worn rear tyres, the front axle will continuously take up traction and lead to windup.
2. Engine braking will be applied only to rear axle.
In this the first can be taken care of by having a slightly taller reduction ratio in the front differential.

Rahul
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Old 13th January 2016, 17:56   #395
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Default Re: Transmission Windup

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Originally Posted by Rahul Rao View Post
I just had a chat with some one in Pune who has restored a old LandRover Discovery. This car has full time 4WD with no central differential, but there is a ratchet araingment in the front propeller shaft. So front wheels can be rotated forward freely when on a ramp, but rear wheels are rigidly fixed to the transmission.
On turns when the front axle will travel more than the rear, the ratchet will be allowing the slip, but when a rear wheel spins in place, and rear axle cannot apply torque, the front axle will automatically be taking up.
Only two issues I forsee in such an arangement are
1. If a car has worn rear tyres, the front axle will continuously take up traction and lead to windup.
2. Engine braking will be applied only to rear axle.
In this the first can be taken care of by having a slightly taller reduction ratio in the front differential.

Rahul
Another issue can be added to the above - when reversing in a prolonged U-turn. Not frequently confronted perhaps, but I've faced it in steep narrow hill tracks without the option of 3-point turns. The whole point of the central diff/viscous coupling I feel is to be able to circumvent this torque wind-up issue. This 'ratchet' arrangement (paul & ratchet presumably, allowing uni-direction rotation) in the old Discovery appears to be a partial solution at best.
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Old 13th January 2016, 23:21   #396
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Default Re: Transmission Windup

Quote:
Originally Posted by Rahul Rao View Post
I just had a chat with some one in Pune who has restored a old LandRover Discovery. This car has full time 4WD with no central differential, but there is a ratchet araingment in the front propeller shaft. So front wheels can be rotated forward freely when on a ramp, but rear wheels are rigidly fixed to the transmission.
On turns when the front axle will travel more than the rear, the ratchet will be allowing the slip, but when a rear wheel spins in place, and rear axle cannot apply torque, the front axle will automatically be taking up.
Only two issues I forsee in such an arangement are
1. If a car has worn rear tyres, the front axle will continuously take up traction and lead to windup.
2. Engine braking will be applied only to rear axle.
In this the first can be taken care of by having a slightly taller reduction ratio in the front differential.

Rahul
Hi Rahul,

Very interested to learn about the ratchet arrangement. Would you be able to provide a simple diagram or pic to clarify this? Normally without a centre diff, would'nt you risk blowing your transfer case the moment you engage 4wd , if your front and rear diff ratios differ.
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Old 14th January 2016, 09:21   #397
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Default Re: Transmission Windup

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Originally Posted by shashanka View Post
Another issue can be added to the above - when reversing in a prolonged U-turn. Not frequently confronted perhaps, but I've faced it in steep narrow hill tracks without the option of 3-point turns.
This technology did reduce wind up issue not eleminate it. This is probally why it was available only in basic full time 4 WD vehicles, and was retired in favour of other devices.

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Originally Posted by Bump-Stop View Post
Very interested to learn about the ratchet arrangement. Would you be able to provide a simple diagram or pic to clarify this? Normally without a centre diff, would'nt you risk blowing your transfer case the moment you engage 4wd , if your front and rear diff ratios differ.
The ratchet gives drive only in one direction, it is like the arangement in the rear sprocket of a bicycle where you can stop cranking your pedals on a slope, and the bicycle can freewheel. https://en.wikipedia.org/wiki/Freewheel
Now with a slight difference in front axle ratio, only your rear axle is driving you, and the ratchet in front is slipping, as the difrential shackle is moving faster than the propeller shaft but the moment you have rear wheel slip your front axle which is slower will take up traction.

Rahul

Last edited by Rahul Rao : 14th January 2016 at 09:25.
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Old 29th February 2016, 14:37   #398
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Default Re: Driving all four wheels: how is it done?

Driving all four wheels: how is it done?-lettertoed.jpg

So, someone writes to the editor of an auto magazine, pointing out the fact that many car reviewers are ignorant of the fact that there are different types of 4wd systems, and all 4wd/AWD vehicles are not the same - pretty much what this thread set out to explain!

In addition to the 3 major systems of 4wd that have been described at the beginning of this thread (Driving all four wheels: how is it done?), there is also the unique system seen in the Jeep KL Cherokee Trailhawk, which uses twin 2.92:1 low-range planetary gearboxes, one at each differential. The system is explained through the following diagram:

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The Cherokee system for four-wheel grip includes AWD, locked 4WD-high, locked 4WD-low, and a rear disconnect with sliding fork engagement (it is built by AAM, according to Automotive News). Bob Sheaves wrote that the patent applications showed it to be similar to the New Venture 242 transfer case, with a 90-degree output to the rear axle. The front-axle differential is separate from the transmission (but possibly inside the transaxle housing), with the bolt on the power takeoff (PTO) box along a split line (near #30 on the following diagram), separating it from the main housing. Not shown on the diagram, because it is a “flattened view,” are the two front halfshafts, which are splined to the front-differential side gears. Note: Chrysler refers to the PTO and associated gearing as a PTU, or Power Transfer Unit.

To the left of #30 on the diagram, everything would be common, enclosed components with the 2WD transaxle and case. To the right of #30, everything is enclosed in the PTO housing.

The diagram shows why a special gear reduction is needed, and why there is no need for a neutral position in the PTO housing, which is only used to disconnect the drive line (to use the PTO). The planetary gearset provides the main reduction. Locking the ring gear to the sun gear gives a 1:1 ratio to both differentials. Locking the rear gear or planetary gears so they’re stationary gives a reduction ratio.
Mr. Sheaves pointed out, “With the range select ahead of the AWD differential, the differential can be smaller, lighter, and less expensive than if placed downstream from the splitter. The faster you spin the interaxle propshaft, due to multiple reduction stages, ... the more vibration you incur due to mismatch of joint angularity caused by movement of the axles (since they do not move up and down and are rubber mounted to the body) relative to each other, forcing the need of a multiple jointed propshaft with additional carrier bearings to shorten the span and stay below each individual shaft's critical speed. ... You don't design a component without considering the effect on the system.
“One thing missing from both designs is a torque proportioning differential in both axles and between the axles in the transfer gearing. It can be done, but not in the shown differential housings or PTO. This would be more important to provide a biased system, as you want in high performance cars.”

Source

Last edited by SS-Traveller : 29th February 2016 at 14:56.
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Old 29th February 2016, 14:59   #399
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Default

Quote:
Originally Posted by SS-Traveller View Post
The system is explained through the following diagram:

]

Thanks, this is a very clever system. Mind you sometimes I think all this high tech engineering to do a bit of off roading sort of defeats the purpose. It is a bit of rough and tumble kind of hobby.

My 1998 Jeep Cherokee had a very simple 4WD system, would lock up the axles too, so not be engaged on a normal tarmac road. But on snow and on the Kansas prairie it would just power through anything. It was phenomenal. So the question is really what all this technology does for the driver. My take is that it takes less skill from the driver as more and more technology kicks in. Maybe that is to cynical, but then again 99% of the 4WD cars never ever go off road in the first place. So the technology is primarily for what the Brits aptly call Pub talk.

I'm a sucker for technology any day, so I would love to have a go. Still, some old school of road driving skills comes in handy too!

Jeroen
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Old 11th April 2016, 14:03   #400
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Default Driving Modes of a(true :D) SUV- Simplified

I know that many BHPians are well-versed with the driving modes of a true SUV . However, I am pretty sure that some of us aren't too well versed with them. If you want to know about these modes in a simple yet enjoyable way, without getting too much into the technical details, you may want to watch this video-

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Old 11th April 2016, 14:44   #401
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Default Re: Driving Modes of a(true :D) SUV- Simplified

Similar thread exists:

http://www.team-bhp.com/forum/4x4-te...-how-done.html (Driving all four wheels: how is it done?)

Cheers,
Vikram
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Old 14th April 2016, 13:55   #402
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Default Re: Driving all four wheels: how is it done?

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Originally Posted by SS-Traveller View Post
Thanks for the image, DriveTrain. I'm afraid I really cannot figure out what kind of mechanical damage can occur, if the vehicle being towed is a manual transmission with gearshift in neutral, and the drive selector is in compulsory 2wd mode.

Shall look forward to someone explaining this.
Yeti uses a Haldex system that automatically engages when it detects slip. This could be a reason.

From research on Haldex, i found this:

Quote:
It is designed so that the AWD system operates as front wheel drive until the vehicle computer detects a loss of traction by one or both front drive wheels. It then sends an electrical signal that locks the rear clutch pack and begins powering the rear wheels. Once the computer determines that the wheels have regained traction, it turns off the rear clutch pack and the real wheel become free and only the front wheels begin pulling the vehicle.
Source: http://towtalk.net/forum/topic/towing-awd.htm

Another good read on towing:

http://www.towtimes.com/index.php/to...rive-vehicles/
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Old 29th May 2016, 10:14   #403
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Default Re: Driving all four wheels: how is it done?

In 4L, can we directly engage 2nd gear and or go till 3rd gear [from 1st and 2nd] w/o much/any A pedal input?

I had to encounter slush and 4H could have cleared it, but because 4L wasn't engaged for a while, I engaged 4L and gingerly tackled it in 2nd gear.
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Old 29th May 2016, 11:25   #404
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Default Re: Driving all four wheels: how is it done?

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Originally Posted by Sheel View Post
In 4L, can we directly engage 2nd gear and or go till 3rd gear [from 1st and 2nd] w/o much/any A pedal input?

I had to encounter slush and 4H could have cleared it, but because 4L wasn't engaged for a while, I engaged 4L and gingerly tackled it in 2nd gear.
Yes, in most cases you should engage 2nd gear in 4 Low unless you absolutely need the highest torque available in 1st gear.
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Old 29th May 2016, 11:28   #405
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Default Re: Driving all four wheels: how is it done?

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Originally Posted by DriveTrain View Post
This is from the Duster's manual , it expressly recommends not to tow the AWD if any of it's fours wheel is in contact with the ground. It specifically mentions that if the vehicle is stuck , it can only be towed for a short distance to pull it out , and mentions about possible mechanical damage. Any idea about the reason ?


Attachment 1440008
If the vehicle has a manual transfer case you can put the transfer case in neutral and then put the gear box in neutral thus totally disconnecting the engine from the drive train.

Unlike a true 4WD vehicle with manual transfer case, an AWD vehicle such as Duster has a viscous coupling that links all the four wheels. As there is no manual intervention to engage/disengage the AWD, there is no way to disconnect the drive train from transmission completely and in such a case if the vehicle is towed with any/two wheels on the ground it will damage the transmission componentry.

Therefore AWD vehicles should only be towed on a flatbed.
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