Full time 4wd - what is it?

RobinS · Post by **RobinS** » Wed Oct 18, 2006 2:58 pm

In conjunction with my Snow Chains query I have been investigating - is this the sort of 4wd we have?

Automatic 4WD. Often falsely referred to as all wheel drive. Essentially a sophisticated 2WD system. Should be called part time AWD. Only one axle gets power most of the time. If that main driven axle (could be front or rear) loses traction a fully automatic system (hydraulic, mechanical or electronic) routes torque to the other axle as well. This means you have to completely lose traction in 2WD on your driven axle first and then the other axle will take over to keep the car moving. So, for a short moment you have AWD. Once the main driven axle regains traction and both axles rotate at the same speed again, the system reverts back to 2WD.

If so, and in the Bongo the main driven axle is the rearit would explain why the handbook says put chains on the back.

Robin

paulmac · Post by **paulmac** » Wed Oct 18, 2006 9:17 pm

old chinese proverb = wothefukruonabout???

Colin Lambert · Post by **Colin Lambert** » Thu Oct 19, 2006 12:55 am

Things like CR-V's and HR-V's have the system described above. i:e: car predominently 2WD until system detects wheel spin then puts the other 2 wheels into drive. You can't do anything about it and rarely know when they are in 4WD.
Discos, Rangeys all have permanent four wheel drive. take it or leave it.
Shogun sports (& presumably Shoguns & Delicas) are 2WD to the rear with manually selected 4WD.
My Bongo is a 2WD so I haven't a clue!

parabat2 · Post by **parabat2** » Thu Oct 19, 2006 10:04 am

Short answer is yes.

RobinS · Post by **RobinS** » Thu Oct 19, 2006 10:11 am

Thamks Tony - that points me in the right direction on what to expect using snow chains.

Robin

shutty747 · Post by **shutty747** » Thu Oct 19, 2006 11:29 am

I asked a similar question a while back, and the eventual consensus was the same - there's a diff that drives the front wheels when the back slips.
The stickers are lying!

(one question I didn't ask on my previous thread was: if it's 2WD most, if not all of the time on tarmac roads, why is the MPG less? Is it just the weight? Or are there some turning losses all the time, somewhere in the depths of the drivetrain?)

parabat2 · Post by **parabat2** » Thu Oct 19, 2006 11:38 am

why is the MPG less?

Because on the 4WD, the viscous coupler, front diff, front CV joints are all turning all the time, even if no power is being transfered through the viscous coupler. More resistance gives lower MPG.

moonshine · Post by **moonshine** » Thu Oct 19, 2006 11:03 pm

As I understand it, the MPG difference is barely noticeable anyway. So little even, that unless you do 20,000 miles a year plus, I wouldn't worry about it.

bigdaddycain · Post by **bigdaddycain** » Fri Oct 20, 2006 2:42 am

i dont get it,on a snow filled car park last year, i floored it ,with my head out of the window to check (the bongo is my first 4x4 so i had to check) and both front AND rear wheels span in inison till they found grip! plus, i test drove seven or eight two wheel drives before settling on my 4 wheel drive version,and i could definitely tell i was being "pushed" around corners in the 2wd's as opposed to a push/pull feeling in the fourby...

parabat2 · Post by **parabat2** » Fri Oct 20, 2006 11:38 am

You're really asking for the long answer

The front and rear drive systems are connected via a positively weird device called a viscous coupler. Inside are vaned plates bathed in a "non-newtonian fluid". If the vaned plates are turning at the same rate, the fluid is thin. No power is transferred between the plates. If one plate turns faster than the other, it mixes up the fluid, causing it to thicken, thus transfering power between the plates. (Try mixing cornflour with a little water to see a the effect.)

Although the change is quite sharp, it still allows the transfer of power to be progressive - if there is not much difference between the turning rate of the plates not much power is transferred.

Hence the effect you see on snow etc.

Hope this helps.

tigger · Post by **tigger** » Fri Oct 20, 2006 11:50 am

non-newtonian fluid

????

Does that make it a "quantum fluid" then???

Colin Lambert · Post by **Colin Lambert** » Fri Oct 20, 2006 11:55 am

No that's VW engine oil!

parabat2 · Post by **parabat2** » Fri Oct 20, 2006 11:59 am

I'm uncertain - I'll ask the cat (if I can find it)!

To quote Wikipedia: A non-Newtonian fluid is a fluid in which the viscosity changes with the applied strain rate. (http://en.wikipedia.org/wiki/Non-Newtonian_fluid)

shutty747 · Post by **shutty747** » Fri Oct 20, 2006 12:45 pm

wow - you learn something everyday on bongofury! That IS weird, but what a clever application.

I guess you can't just buy viscous coupler fluid at your local garage?

Veg_Ian · Post by **Veg_Ian** » Fri Oct 20, 2006 12:53 pm

Sort of. It's silicon based. The Townace employs a viscous fan for cooling the rad and kind of works in reverse to the viscous coupler. The hotter it gets, the thinner the fluid and the faster the fan rotation. I know of one chap that has replaced the viscous fluid in his unit. I think he bought it from a Toyota dealer.