The ST165/ST185/ST205 4WD systems are really FWD? Weigh in!

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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby Sluice Box Hero » Mon Aug 12, 2013 9:35 pm

phattyduck wrote:
Sluice Box Hero wrote:ok, so i did this today, not clutch drop at 6k, but i did get the car to 5k. and nothing, the front end spun and the rear just sat there.

if i put the rear on stands and start to pull the car forward the car pulls itself off the jack stands. this proves to me that it is a fwd car with a tag along rear axle.

now, where can i have this viscous couple rebuilt??? mine obviously is done.
Doesn't prove anything. If your viscous coupler is not working, the car wouldn't move in either case - the 'off the ground' wheels would spin front or rear.

Fix that selector switch...

-Charlie


well, not exactly. the front wheels always have power from the transmission. 100% of the time, just like a FWD car, there is a pinion gear that rotates a bull gear, which is connected to a carrier with spider gears inside, its a 100% normal FWD transmission. what makes it 4wd/awd is coupler for the center diff. there is another set of spider gears attached to the FWD diff that is the center diff. the inside of the viscous coupler attaches the the side gear that is attached to the carrier (the part where the bull gear is attached on the FWD diff)on the FWD diff, this is going to be the left side gear in the center diffs spider gear set. the outside of the viscous coupler (the part where the ring gear for the rear output attaches) it attached to the right side side gear in this set of spider gears. the inside plates of the viscous coupler get power all the time from the FWD diff carrier. the outer plates only get power when there is a difference in speed between the plates, IE, the front wheels are turning a different speed than the rear wheels. now where this gets interesting is the outer plates, or the carrier for the viscous coupler is ONLY powered from the inner plates. when driving straight down a road the inner plates and the outer plates are moving the same speed, so there is effectively no "coupling" happening, if you started up an icy hill and the front tires started slipping then the change it speed would tighten up fluid and start to transfer power to the rear wheels until they were traveling the same speed again, at which point, the coupler would "unlock".

if you were to set the front of the car on jack stands and the rear on the ground the inner plates would spin with the front diff since they are 100% conntected to the front diff carrier all the time. and since the outer plates are 100% connected to the rear wheels (well, the ring gear of the ring and pinion that make up the rear output of the "transfer case") they will want to stay still, just like the rear wheels, this is of course until the viscous fluid begins to heat up and transfer power to the outer plates in essence creating force on the rear wheels. if the fluid is trashed, or leaked out, then the rear wheels would just sit there and do nothing while the fronts spun normally. now, if one were to lift the rear of the car in the air and set the front tires on the ground the car would pull itself off the stands immediately just like a normal FWD car, because, it has a normal FWD diff that gets power 100% of the time from the transmission.

basically what im getting at is that you could NEVER, and i mean NEVER EVER NEVER EVER get the rear wheels to spin without the front wheels spinning faster. unfortunately this is a FWD car with a RWD assist.

the way many other AWD cars get a rear bias is with either clutches and gears, or the front wheels are power through a viscous coupler rather than the rear wheels.

if toyota wanted to get this correct they would have built the unit backwards. the transmission pinion gear would have driven a bull gear that shared the carrier with the outer blades of a viscous coupler and with the ring and pinion for the rear output to the rear wheels. and the carrier for the front carrier would have been driven off the inner blades of that coupler.
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby athousandleaves » Tue Aug 13, 2013 1:09 am

Possibly suffered a bad tow at some point... Flatbed or nothing!

Edit

^^^ you're totally mistaken... The car is 50/50 split all the time. The center diff gets power first and this is why its 4WD not AWD or FWD with RWD assist.
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby Sluice Box Hero » Tue Aug 13, 2013 2:17 am

athousandleaves wrote:Possibly suffered a bad tow at some point... Flatbed or nothing!

Edit

^^^ you're totally mistaken... The car is 50/50 split all the time. The center diff gets power first and this is why its 4WD not AWD or FWD with RWD assist.


you are very wrong.

the center diff gets power at the same time as the front diff, the difference being that the front diff is not viscous coupled to the front drive shafts, it is mechanically coupled, so they have 100% power all the time. the inside plates in the viscous coupler has power all the time, but the rear wheels MUST move at a different speed that the front, and at the very most you can get 50% torque to the rear tires. you cannot get the rear tires to spin unless the fronts are spinning faster. this is also true in decel, if you were to come into a corner in a engine brake induced slide, the rear would create drag as well. never the less, if you were to line lock the front tires and lift the rear tires off the ground and put the car in gear it would stall, the rear tires would never spin, you can never have the rear tires spin without the fronts spinning. it is a FWD with rear assist.

i understand it hard to comprehend, but, its true, and its how 90% of all AWD cars are.

one way you could possibly get around this, and i would only do this in a low traction race, is to run a slightly larger rear tire, or even a slightly numerically higher rear diff ratio. this would trick the VC into thinking the front was spinning faster than the rear all the time and make it lock, then at the point where it was locked the rear tires would be traveling slightly farther per rotation, or give you a rear bias feel.
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby athousandleaves » Tue Aug 13, 2013 2:35 am

I'm not wrong but if that's how you understand the 4WD system of the alltrac then so be it. I'm not here to debate it with you but I wouldn't mind hearing your opinion again once you can afford to swap in a newer transmission or viscous coupler.

Once I have time I'll upload those videos I took of my car a while back.

Either my car and everyone else's are broken or yours is. :shrug:
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby Sluice Box Hero » Tue Aug 13, 2013 2:40 am

i would love to see your car with just the rear wheels spinning.
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby athousandleaves » Tue Aug 13, 2013 2:52 am

I don't recall saying that was possible?

50/50 4WD therefore if your transmission was functioning properly then your car would have pushed off the jack stands.
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby Sluice Box Hero » Tue Aug 13, 2013 3:02 am

oh, most def my coupler is shot, any time the front is spinning and the rear is stationary the rear should ramp up to match the front quickly.
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby ST_6396 » Tue Aug 13, 2013 3:37 am

The 88+ 4wd/awd system is a pretty generic system for that era. For newer cars that have been coming out in the last 10 years or so, yes there is a huge difference between 4wd and awd but back when these cars were made there was no readily available system to control torque splitting and things like that but at the same time a "true" 4wd system would completely lock the speed of the front diff to the speed of the rear diff. The VC will allow for a slight difference in speed between the front diff and the rear diff. Although the viscous coupler will allow for a slight difference, the greater the difference the harder the VC will work to eliminate the difference.
http://www.youtube.com/watch?v=ONnhNgUzMjo (video showing how a viscous coupler works)
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby CMS-GT4 » Tue Aug 13, 2013 5:48 pm

The videos in the first post by Toyota explains how the system works in extreme detail. Video 3 explains the VC operations of our vehicles and how power is transferred.
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby Theman88b » Mon Aug 19, 2013 2:29 am

Having a hard time understanding how it is FWD most of the time. The center differential gets power FIRST correct? The LEFT side gear on the center diff is connected to the front differential and the RIGHT side gear on the center diff is connected to the VC which works the rear diff. If all wheels have equal traction, power is split equally right? I say this because the right side gear on the center diff is connected to the rear diff, doesn't go through the front diff to get to the rear. Once a tire losses traction that is when the VC is needed to send more traction to the opposite axle(front or rear). I am tired and may be wrong here. I am trying to find more information on how the system works. I will try and get a picture of what I am talking about.
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby CMS-GT4 » Mon Aug 19, 2013 2:40 am

Its not. Its full time awd. The power is split 50/50 through the open diff. If there is excessive slippage from one end the viscous coupler locks to maintain the 50/50 split. Normally an open diff would transfer power to the faster moving side, but the VC prevents that keeping power to front and rear. The first two videos cover the manual locking system and show how without the locked center that the car can get stuck. Then in the last video it shows how the center diff allows the car to not get stuck.
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby Sluice Box Hero » Mon Aug 19, 2013 3:03 am

50% goes to the front tires, 50% goes to the center differential, the VC reacts when it detects difference in speed between the rear differential and the center differential. so up to 100% of the 50% that goes to the center difff CAN go to the rear diff, but, under normal, non slipping conditions, 0% is sent to the rear because its not needed. the center diff gets 50% of the power, but the rear diff gets nothing until its needed. the front diff on the other hand get 100% of its 50% of the power.

in a FWD bias AWD car: if you were to line-lock the front tires and try and burn out the car would stall. the rear tires would not start to spin or break traction, actually they would get zero power since the front tires have to be moving for the center diff to get power to the VC since the front and center diffs are directly connected to eachother. now, if you were to line-lock the rear tires and try and burn out the front tires would break traction and roast UNTIL the VC heated up enough to sufficiently lock and stall the engine or, im my case, since my VC is smoked, the front tires would just roast and roast and roast......

in an alltrac the front tires need to be moving at a faster speed than the rear tires for the rear tires to put down any power.
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby athousandleaves » Mon Aug 19, 2013 3:10 am

^^ :crazy:
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby ___Scott___ » Mon Aug 19, 2013 3:44 am

Just as a data point, when I would ice-autocross, a quick pull on the e-brake could help rotate the rear end, but if held too long it would lock all four and stall the engine. :doh: That turned out to be a bad technique anyway and was only useful if I had already screwed up and needed a plan-B.
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Re: The ST165/ST185/ST205 4WD systems are really FWD? Weigh

Postby CMS-GT4 » Mon Aug 19, 2013 4:11 am

The BGB and the video in the first post contradict the statements Sluice box makes about the AWD system in the viscous equipped gtfours.
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