TOD discussion

[bobmumgaard]

I've spent a few hours studying the patent and look at the maintenance manual and I think I have a pretty good idea about how this thing is assembled and how it operates. Though I would love to get one on the bench to take apart and examine.

About torque:
mbeach is correct that you can only transmit as much torque as something (say a wheel) can hold. This means that if the wheel was on ice, you could send torque to the wheel until traction faltered and then the wheel would just spin, So if you are sending torque to the rears and they are spinning, trying to send more torque causes the wheel to spin faster, nothing more. Now if the wheel is locked, say the VX is strapped down on a dyno and you send alittle torque to the front, nothing will probably happen. If you send more and more torque eventually the wheel will either start slipping against the ground, the straps will snap or you will break something in the drivetrain, say an axle.

Construction of the transfer case:
It seems to me that the clutch works very much like a Cusco or Kaaz limited slip differential. This means that they do allow a difference in rotational speed front to rear. And this makes sense because otherwise it would be the equivalent to a true mechanical lock. If the front and rear HAD to spin at the same speed it would be like driving in 4Low all the time, lots of scrub.

The transfer case appears to take the output from the engine and transmits it to the rear wheels through a shaft, much like there is just a driveshaft and no transfer case. This means the rear is powered all the time, no matter what and always spins at the same rate as the transmission output. (unless we activate the planetary gearset for 4low) On this shaft is a sprocket, this sprocket is connected by a chain to another sprocket on the front driveshaft. Inside the first sprocket (the one on the mainshaft) is the clutch. The clutch appears to consist of 13 friction disks. 6 of these are attached to the mainshaft, (and thus spin at the speed of the rear wheels) and 7 are attached to the sprocket (and thus spin at the speed of the front wheels) These disks alternate, one for the rear next to one for the front next to one for the rear and so on..

There is then an inductive coil that creates a magnetic field when we send current through it. This coil squeezes all the friction disks together. So when we power this coil up the squeezing action increases the friction between the front and rear shafts, this means we take some of the torque we were sending to the rear and send it to the front. Now the front might not move, if the wheels were in a tough spot then the clutch would just slip. Squeeze harder and you get more friction=more torque. Keep squeezing and eventually the front wheels will either move, something will break or you will be squeezing your hardest and the friction disks in the clutch will still just slip. However, in normal situations, if you squeeze the front will start to spin at same rate as the rear, and if it doesn’t, the TOD ecu just squeezes harder until they spin at the same speed (or the ecu is happy with the speed difference)

The system is the same in a clutch type limited slip differential like Cusco or Kaaz, we just modulate the squeezing electronically; they do it with elaborate shims. And the same principle as the multidisk clutches you see on high torque cars.

In fact, this is almost the same system we see on any car’s clutch. In that case, your foot controls the squeezing, and the clutch slips, it is still transmitting some torque even though it’s slipping. Hopefully the clutch eventually stops slipping and is then transmitting 100% of the engine’s torque to the wheels.

The ball ramp system appears to be there to make sure that the clutch doesn’t self activate. (Imagine if it did at 75 mph, dangerous liability there) and it doesn’t appear to be able to transmit very much torque, atleast not enough to break a driveshaft. Though I am still looking the whole ramp thing over, but this is my initial assessment, though I could be wrong. In addition to its small size, I don’t think that the ball ramp system is involved with any automatic torque transfer because if it was the clutch would be a VERY expensive redundancy.

So… the moment we are waiting for….can we get true 100% 2wd? In my opinion not 100%, but maybe 95%

Because the friction disks are always in some contact, and there are so many of them they will always create some friction, even when there is no squeezing from the solenoid. How much friction (and thus torque) probably depends on how fast everything is spinning.
This means that if you take off the front driveshaft, the front output of the transfer case will still turn. Also, if you break a CV joint and have the TOD disconnected everything will still spin due to this residual friction. An interesting experiment would be to disconnect the front driveshaft and connected the transfer case output to a brake (like an engine brake used my manufacturers to dyno engines) and measure this torque.

A more practical experiment would be to put the TOD control we are talking about in a VX (I think we are on the right track with electronic systems discussed above). And take the VX to a brake based dyno. In fact, dynopack is the most common brake based system. This dyno attaches a large brake to each hub of a 4wd car and applies a variable resistance to the wheel. This resistance measures the torque each wheel is putting out. (most dynos are inertia based (use rollers) and measure power and then infer torque, the opposite strategy as a load (brake) based system)

If someone really wants to find out the exact characteristics of the TOD system, find a dynopack system. They are used for rally cars most often, so the best bet is to find a place that works on STi’s and Evos. Unfortunately for me I’ve never located a dynopack system within a few states of Nebraska.

Load based dyno info:
http://www.mustangworld.com/ourpics/News/nowheel.htm
http://www.dynopack.co.nz/

Limited slip info:
http://auto.howstuffworks.com/differential8.htm

Normal car clutch info:
http://auto.howstuffworks.com/clutch.htm

About the mechanical lock
When we put the transfer case in 4Low it slides a mechanical lock between the front and rear axles. (actually the mainshaft and sprocket) In this situation the clutch is completely out of the loop. This means both axles rotate at the same speed no matter what. So if both front wheels where on the ground and the rears where not (imagine the VX hanging off a cliff, scary aint it?) then the car should be able to pull itself up (assuming enough traction and engine power) in this situation the fronts receive 100% of engine torque because of the mechanical lock (there can’t be any slipping) and because the rears cannot transmit any torque because they would be spinning against air at the same rate as the fronts pull against the ground.

Well alright, sorry this was so long, just trying to hash out the exact nature of the things we are working with here.

And again, I could be completely wrong on all this. What are your guy’s thoughts?

[bobmumgaard]

I agree with VehiGAZ and mbeach that 4hi would be bad in dry situations, and the TOD handles theses with ease. It is the times when TOD is overactive, trying too hard on snow and ice that we are looking to correct. Not to mention the need of the mechanically inclined to have COMPLETE control over the machines they use.

So the 555 you speak of is some sort of signal generator capable of creating a PWM signal? Can it modulate it too?

The 555 is a timer chip. You can use it along with capacitance, resistance and a transistor to create a PWM signal. This signal is varied by varying a control voltage into the 555 chip. So yes it is exatly what you speak of.

I agree with mbeach's assesment of the trooper mod. The coil is not meant for the prolonged current a constant 12V source provides. And a PWM signal would be more inline with an OEM solution.

I was working toward a knob with 5 poles (and thus 5 positions): Auto, 0% torque, 15% torque, 30% torque and 50% torque. This makes wiring the lights up easier (since youre working with poles instead of an infinte knob). And it can be wired to elimate the manual button.

I really need to clean up my circuit diagram so I can post it.

[mbeach]

Poop.
Posts are out of sequence.

[mbeach]

Here's what I can do:
-A MANUAL/AUTO switch. Tie it in to the same place as the Trooper Power Mod has it located.
-Replace the relay in the power mod with an appropriate timer circuit/PWM generator.
-Implement a knob, that by varying the values (vdc) applied to the timer circuit, will vary the PW of the output signal to the e-clutch.

Things that I need.
-Values, voltage and duty cycle, from the TOD to the e-clutch. This weekend I am splicing in taps to some (o.k., a bunch of) wires. I will be driving around with a Fluke 97 on my passenger seat for a few hours.
-I also want to gather all of this data for each condition (15/30/50/4lo/N) from every conceivable input source (TPS, ABS, Front and rear VSS, etc.). I also want to peek at the 4hi and 4lo switches that I see on the schematic.
-A plan to control the dash lights. Transistors is the way to go for sure. I need to think this through later though. Must collect data first.

I'm not going to promise anything, but I should have a handle on those duty cycle readings by monday.

[VehiGAZ]

I think you two are on the right track here! Thanks for the layman's explanation of how the transfer case works, and what happens when you shift into 4WD-low, Bob. VERY helpful to me!

So I think we should be fine if we come up with the circuit that allows us to generate the right signal to the ToD transfer clutch. It is used to having some current on it all the time - but it's best to mimmick the "natural" signal it gets - VDC is probably a bad idea in the long run.

I think my concerns about having the full-4WD-Hi activated constantly are not a big deal as long as we pay attention to what we are doing. Whatever stresses we put into the drivetrain will undoubtedly release themselves in that clutch pack.

Alrighty then - you EE types figure out that circuit. It would be nice if we could find extra ToD ECU wiring harness connectors - that way, it could be set up to plug right in in-line.

Thanks again!

[mbeach]

Alrighty then - you EE types figure out that circuit.

I'm an MEtech working in an EE's position. I think Bob's an ME as well. That's why we're overanalyzing everything

It would be nice if we could find extra ToD ECU wiring harness connectors - that way, it could be set up to plug right in in-line.

I want these plugs soo badly. As it stands, this will not be a fix for the faint of heart. With the plugs, we could build them and just send them out to be plugged in.

I am at 'work' right now. Getting ready to build the interface cables that I need to test the TOD's ins and outs.
I'll take pictures.