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 How is power transmitted from rear to front in AWD

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wernerk
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Number of posts : 160
Age : 55
Home City : Flinders Park, South Australia
Model and year : 06/94 Toyota Hiace Super Custom 3.0L TD AWD
Registration date : 2012-01-19

PostSubject: How is power transmitted from rear to front in AWD   Fri Mar 02, 2012 11:16 pm

Hi all.

I'm trying to find out how the power is transmitted from rear axle to front or vice versa if one of the wheels starts to spin, say in slippery mud. It was a question my mechanic asked me.

Might try and clarify my question.

If you're driving on a muddy surface say and one of the wheels hits a slippery patch, how does it affect the remaining wheels? Are they all rigidly connected via the driveshaft and hence spin at the same speed? Or is there some mechanism e.g. a centre differential, between the front & rear axles which enables one to pick up the "grip" when the other axle loses it, so to speak?

Do the SC's use a centre diff? Viscous coupling? Something else?

Hmm, looking at an article on Wikipedia, it would appear that there would have to be some sort of centre diff type mechanism, otherwise you would get axle windup with a constant 4WD.

I'm not a mechanic at all, just trying to get my head around how these things work.

I tried Google to find out what's in the SC's, but drew a blank.

Hopefully someone can enlighten me!

Thanks.
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WGWarburton
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Home City : Central Scotland
Model and year : 94 2.8 Noppo HiTop Super-Long
Registration date : 2012-02-07

PostSubject: Re: How is power transmitted from rear to front in AWD   Sat Mar 03, 2012 11:50 pm

Hi,
As I understand it:

With P/T 4WD on earlier SCs there's no centre diff. If the 4WD is engaged the front & rear are directly linked and you'll get drive to at least one front and one rear wheel.

With F/T AWD there's a centre diff- either open or a viscous coupling. I'm not sure which (I have the P/T system above). If it's open then it's possible for only one wheel to be spinning, if it's a viscous coupling then some power will still go to the wheels with grip, even if one is spinning.
You can't use the P/T system in 4WD mode on dry tarmac as you'll get transmission windup, bad handling etc. The F/T AWD is useable (and useful) on road but less effective when things get gnarly.

Hope this helps. Corrections welcome!

[edit]: Aha here you go:
http://hiace-super-custom.free-boards.net/t1102p15-not-a-real-4wd

This thread gives you more to go on- look at the last few posts. The AWD system has a viscous coupling in the centre, so you'll get a transfer of some power to the front axle if the rear loses grip. The older P/T system has a transfer case with a low ratio setup but can't be used on dry roads and needs you to get out and manually lock the hubs to enable the capability.


Cheers,
W.
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wernerk
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Number of posts : 160
Age : 55
Home City : Flinders Park, South Australia
Model and year : 06/94 Toyota Hiace Super Custom 3.0L TD AWD
Registration date : 2012-01-19

PostSubject: Re: How is power transmitted from rear to front in AWD   Sun Mar 04, 2012 3:56 am

Hi W.

thanks for your response, quite informative.

I'm going to run the info by my mechanic and see what his thoughts are.

regards,

Werner
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