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

athousandleaves

New member
Luke, as RedCelicaTRD said; when launching your front right tire is experiencing a lack of traction and due to your front differential being open you are sending most of the 50% of the front axles torque to that wheel. The rear wheels are still receiving 50% of engine torque and you will continue to move forward because of this. With regard to esracing's launch he was able to break traction on all four wheels with half engine torque to the front and half to the rear. Even though all four wheels were slipping they were all gripping an equal amount and all received identical power.



I took my car to my friends shop tonight and we tried a few tests to see how the 4WD system reacted. Ideally we would have reproduced the test used in the ramp video but we didn't have such a nice ramp available to play with and opted instead to try and duplicate this test: http://www.youtube.com/watch?v=9B0ANOI3GcM using these positioning jacks and another test up on the lift.

Before commencing these tests my car sat outside in ~6°C weather for over 3 hours so everything was completely cold. Also to my knowledge my car is equipped with all stock differentials, open front, viscous center, open rear.

Firstly when the car was put on the lift and then put in first gear with all wheels receiving 0 traction; all 4 wheels moved in the same direction simultaneously. No detectable delay in the rears catching up.
We tried the lift test with the e-brake on (by accident) and when dropping the car into gear only the front wheels moved but the engine was beginning to stall as the power being sent to the rears wasn't being used and therefore the bias on the viscous coupler caused engine braking.

We then propped the front wheels up on the jacks to simulate slipping across the entire front axle. Unfortunately the jacks acted more like an e-brake for the front wheels as they weren’t really designed for being driven on however when the wheels did get rolling the jacks would meander and the car would start to push them out, bringing the test to a halt.

Initially when put on the jacks and driven in reverse the car began driving backwards with only the rear wheels moving!
The car acted in a similarly to when the e-brake was engaged up on the lift, it wanted to stall.
Eventually with some more driving in reverse the car did manage to briefly spin the wheels on the jacks as well as the wheels in the rear however they spun at different velocities, the rear's were constantly turning whilst the fronts only got a quarter turn in before stopping again.

We then tried driving forward with the front wheels on the jacks and managed to get all four wheels moving. The rears were getting the power out much more easily with the fronts once again struggling to put the power down because of the jacks refusing to roll fluidly.

I took some videos while completing all these tests so if anyone REALLY wants to see them I can put them on youtube when I have time.

As a result of these tests I stand by my conclusion that the viscous coupler is a limited slip type differential that has the ability to mechanically sense the front and rear axle speed/grip and apportion up to 50% of engine torque to the wheels on those axles. Furthermore, the alltrac's 4WD system is not FWD until slip occurs it is a full time 50/50 4WD system with the ability for a limited difference in front and rear axle speeds.

Again, the reason our system is not FWD based 4WD is because of where the transmission is putting the power first.
  1. The transmission directly drives the center differential.
  • The left side gear of the center differential drives the front differential
  • The right side gear of the center differential drives the rear wheels via the transfercase
  • The amount of power sent to either the front or the rear is decided by the center diff via the driving conditions mechanically sensed.

The Viscous coupling connects the left side gear and right side gear of the center differential. When there is a bias of rotation on either side of the unit it will 'lock up' and put the car in full 4WD mode until equilibrium is achieved again.
 

4rsnduction

New member
athousandleaves":25ovtpkh said:
As a result of these tests I stand by my conclusion that the viscous coupler is a limited slip type differential that has the ability to mechanically sense the front and rear axle speed/grip and apportion up to 50% of engine torque to the wheels on those axles. Furthermore, the alltrac's 4WD system is not FWD until slip occurs it is a full time 50/50 4WD system with the ability for a limited difference in front and rear axle speeds.

Again, the reason our system is not FWD based 4WD is because of where the transmission is putting the power first.
  1. The transmission directly drives the center differential.
  • The left side gear of the center differential drives the front differential
  • The right side gear of the center differential drives the rear wheels via the transfercase
  • The amount of power sent to either the front or the rear is decided by the center diff via the driving conditions mechanically sensed.

The Viscous coupling connects the left side gear and right side gear of the center differential. When there is a bias of rotation on either side of the unit it will 'lock up' and put the car in full 4WD mode until equilibrium is achieved again.

+1 and Agree
 

klue

New member
Looks good, I took a look at the BGB, and the vc looks like its driven first, you guys are on to something!
One thing we should correct is how we are referring to the torque split. If the car is 4wd its not delivering 25% torque to each wheel. or 50 front 50 rear. Its actually 100% torque delivery to each wheel. comments?
 

phattyduck

New member
athousandleaves":hr6l4ewr said:
phattyduck":hr6l4ewr said:
An Alltrac would pass the first set of tests (ramp, front or rear without power), but not the second set (only one wheel with power) because of the lack of locking F/R diffs and/or traction control.
For arguments sake would an Alltrac with front and rear torsens not be capable of climbing with one wheel grip?
I feel that it would be able to as 50% of the power will be lost on the slipping front or rear axle while the other axle with one wheel having grip will receive up to 75% of the 50% of power into that one wheel depending on the bias for the torsen (approximately 37.5% of total engine power).
I guess it would depend on the amount of power you need to actually move the vehicle up the ramp so power to weight is more of a concern once grip is available.
A Torsen can only multiply available grip from one side to the other. If one side is on 'ice' or is in the air, the other wheel will not get any torque (0x2.5 = 0). Well, it will get almost no torque, maybe a few ft-lbs. If the car had a a clutch type LSD front and rear or a really tight viscous LSD, it could transfer power around better, but then you get maintenance and wear issues. Add in a little bit of smart traction control (not available on the Alltrac, of course) and you could do just about anything (that's actually how the H1 Hummer is setup)

Don't confuse torque with wheel speed.

A viscous coupling unit will transfer torque based on the difference is shaft speed of the two inputs/outputs (like 50 ft.-lbs. at 25 RPM shaft speed difference), up to the torque limit of the unit.

A torsen LSD will transfer torque up to a pre-definded multiple (like 2.5x) of torque/grip to the opposite input/output.

A clutch-type LSD will transfer torque up to the limit of its torque handling capacity (a specific number, like 50 ft-lbs.) no mater what the grip of one side or the other is when there is difference in shaft speeds.

Each has its own uses and best use cases.

-Charlie
 

phattyduck

New member
klue":1x7uwe4r said:
Looks good, I took a look at the BGB, and the vc looks like its driven first, you guys are on to something!
One thing we should correct is how we are referring to the torque split. If the car is 4wd its not delivering 25% torque to each wheel. or 50 front 50 rear. Its actually 100% torque delivery to each wheel. comments?
Torque delivery is 50/50 when the front and rear axles are going the same speed (i.e. straight driving, not wheel spin).

When there is a difference in wheel speed, then the viscous coupler transfers torque to the side that has more traction (i.e. is spinning slower).

With an open front and rear diff, you can only get a max of 50% power to any given wheel (actually less, as there are losses in the viscous unit that leave some power going to the slipping axle). The open diffs are stuck at 50/50.

With an open front and a Torsen rear, you could get up to say 75% power on either rear wheel (assuming a 3:1 torque multiplier in the Torsen and a 100% transfer on the center viscous unit). The fronts can still only get 50% max each.

-Charlie

PS. That spinning front wheel on those launches is what allows the viscous coupler to transfer the power to the rear and get such a great launch...
 

klue

New member
That would make sense in theory, but using the car on the dynometer I get the same torque output at each of the four wheels?!?!
 

Redrkt01

New member
Andrew, I REALLY want to see the video. Please post!

Luke, would the dynometer be able to accurately apportion a percentage of total torque to any individual wheel?
 

klue

New member
Yea thats what we use a dynapak (hub type 4 separate pods), torque is the same on each wheel. Which is why I was saying that toque is 100% each wheel

thoughts?
 

Redrkt01

New member
klue":1hzseh4p said:
thoughts?

I'm thinking that the maximum torque delivered from the output shaft will be the same torque value at each wheel. On the other hand one could assume that torque could be divided per wheel. But I don't think that's right. I imagine that if I uninstalled the prop shaft I would still get the same torque value on either of the front wheels. It's getting late and I'm tired :shrug: , but 100% torque at each wheel sounds right to me. Luke, I think your results are correct.

Edit: Just wanted to add one thing about how we refer to the torque split. I don't think that referring to the torque split as 50/50 runs counter to saying that torque is equal at all wheels. Considering what phattyduck outlined regarding how much power can be sent through the open diffs I liken the 50/50 split to a ratio. Seen as 50:50 it can then be reduced to 1:1, meaning just as much goes to the rear as the front and vice versa. Of course, I am getting sleepy and that might have been a big mental belly flop. :twisted:
 

athousandleaves

New member
Luke can you give some numbers for total engine torque vs the torque measured at each wheel?

For example:
  • were you seeing 50ft-lbs @ each wheel when engine tq is 200-220ft-lbs
    or
    were you seeing 200-220ft-lbs @ each wheel while engine tq is 200-220ft-lbs
    or
    were you seeing 100ft-lbs @ each axle while the engine tq is 200-220ft-lbs


I don't have much dyno experience so I wouldnt be able to comment on how the system is integrating the sensors but I would find it strange if you were actually seeing the full crank power on each wheel.


Might have to give me some more time on the videos, I'm really busy with work right now.
 

klue

New member
Redrkt01":1zus198y said:
klue":1zus198y said:
thoughts?



Edit: Just wanted to add one thing about how we refer to the torque split. I don't think that referring to the torque split as 50/50 runs counter to saying that torque is equal at all wheels. Considering what phattyduck outlined regarding how much power can be sent through the open diffs I liken the 50/50 split to a ratio. Seen as 50:50 it can then be reduced to 1:1, meaning just as much goes to the rear as the front and vice versa. Of course, I am getting sleepy and that might have been a big mental belly flop. :twisted:




Yes exactly! I'm like playing devils advocate, its bringing out the goods in this topic. saying we are splitting torque does not actually work like that. Power in the drive train is never reduced to any wheel unless its slipping. To further that, a wheel can only ever have as much torque as the final drive delivers it wont it does not stack on top if anyone is thinking along those lines.


Andrew, the toque we see on the dyno is measured at each of the four wheels. The measurement is equal to the the total torque/horsepower output. So if its 200ftlbs at the front left wheel, its the same out put at every other wheel, and the same as the total output. This output is measured at the hub on the dynapack, so we should see a good percentage more at the crank considering each wheel is accounting for the losses of all the other ones in the drive train.
 

phattyduck

New member
klue":nugkvd2f said:
Yea thats what we use a dynapak (hub type 4 separate pods), torque is the same on each wheel. Which is why I was saying that toque is 100% each wheel

thoughts?
You are getting the same torque at each wheel, yes. Now imagine 3 of the 4 hubs now have no resistance... what would the torque output be at that 4th hub?

If 3 open diffs, you get no torque - the last hub gets the same torque that each of the other 3 can put down - thus the "50/50" split. Each wheel has to apply the same torque.
If 3 full-locking diffs, you get full torque at that last hub with 'traction' - thus 100% transfer of power to that last hub.

Those are the two edge cases - everything else is in between those two.

So, if the resistance to movement at each hub is the same, they will all put down the same torque/horsepower no matter the diff setup. Once there is differing traction between wheels/hubs, things get complicated.

-Charlie
 

klue

New member
phattyduck":2obl0xdo said:
So, if the resistance to movement at each hub is the same, they will all put down the same torque/horsepower no matter the diff setup. Once there is differing traction between wheels/hubs, things get complicated.

-Charlie


yea that pretty much sums up the whole thread lol

Yes redrkt01 we are in the same boat, i think everyone else is too

We could play around with this on the dyno, I know we can apply load to the hubs individually and see what happens with the output. This would be the best way to test the system.

Who wants to pay for the dyno time, ill donate my car lol
 

Sluice Box Hero

New member
Rpz123isme":2xpopo47 said:
Put front end on stands and leave the back on the ground. Get in and fire it up and put it in gear and let go of the clutch at 6k with your buddy in front of the car. And if he is right then no harm done. If he is high as a kite then he will learn a lesson......lol :D


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.
 

athousandleaves

New member
Perhaps you should double check the position of the drive selector switch on the transmission before jumping the gun and ripping into your trans?

Would really suck if someone left it in FWD for a few years...
 

athousandleaves

New member
I'm not referring to the early JDM manual drive select switch from inside the car, I'm talking about the one that is ON the transmission... Only supposed to be used on the dyno...please don't make me dig up the BGB page :doh:


Edit
Can we move this to an emergency help thread to keep from cluttering up this one?
 
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