jdm svx trans, will it work?

[svxistentialist]

In my layman's opinion it is not safe to suggest whether a car will understeer or oversteer based on the percentage drive to the front wheels in an AWD configuration.

It is usual for designers of FWD vehicles to set up the cars to understeer when driven beyond tyre adhesion primarily to curb over-enthusiastic driving in a safe fashion. It scrubs off speed and limits what a driver can do without the risk of spinning the car.

What I'm saying is the inherent tendency of most cars to understeer is a function of the chassis and suspension, not primarily the split of the power to the wheels.

In the case of the SVX the VTD gearbox by giving a higher percentage of power to the rear does tend to offset the natural tendency of the chassis to understeer.

Joe

[NiftySVX]

Originally Posted by svxistentialist

In my layman's opinion it is not safe to suggest whether a car will understeer or oversteer based on the percentage drive to the front wheels in an AWD configuration.

It is usual for designers of FWD vehicles to set up the cars to understeer when driven beyond tyre adhesion primarily to curb over-enthusiastic driving in a safe fashion. It scrubs off speed and limits what a driver can do without the risk of spinning the car.

What I'm saying is the inherent tendency of most cars to understeer is a function of the chassis and suspension, not primarily the split of the power to the wheels.

In the case of the SVX the VTD gearbox by giving a higher percentage of power to the rear does tend to offset the natural tendency of the chassis to understeer.

Joe

This is true because any car is going to be balanced so as to not loose the tail end, all things equal, if for no other reason than most cars have a heavy forward weight distribuition(well ok there are exceptions, think 1980s 911) . However, I think what he was trying to communicate was that the us SVX, as are all subarus that use the ACT-4 setup, handle very predictably at the limit and will not swing the tail around like in a RWD car, due to the fact that under engine power, the rear wheels will never slip ahead of the front wheels In a car with a center differential, it is theoretically possible to direct all of the power to one axle or another, assuming there is no device to prevent such an occurance (obviously there is in the VTD). This can easily cause rear wheel speed that is greater than 100% of total front wheel speed, meaning that the speed of the propeller shaft is higher than that of the pinion shaft. Now let us all recall the big rule: In a ACT-4 car, you will never have a propeller shaft that is greater than the speed of the front pinion gear. This is not mechanically possible. Therefore, to get the rear wheels to slip, the front wheels must also be slipping. In my 10 years driving subarus with this setup, I can tell you that this almost never happens unless you are trying. Quite honestly, I don't even think the SVX has the horsepower necessary to break the rear end loose on dry pavement, and even if it did, the transfer clutch in a stock setup is no where near strong enough to make this a possibility. However, with a valve body mod and some better friction discs in the transfer clutch, this thing can actually lock the front output to the rear, which makes it so the car is less stable. I can easily swing the rear of my car around nowadays if I get on a low friction surface because the open front diff will allow the inside front wheel to turn much ahead of the outer one, which makes the front end stay planted, but also causes the pinion speed to be equal to the propeller shaft speed in a conditon when the rear wheels have broken traction. Because of the LSD action at the rear I get both of them spinning and out comes the tail end in a classic oversteer fashion.

The SVX was always intended to debut the VTD system to the world, but at the time the US would not allow a car to be full time 4wd with no means to provide 2wd for emissions testing on a dyno, which made the VTD not even an option for a vehicle on US roads. Once this requirement was abolished, VTD came over to the US.

ANYWAY, back to the subject of this thread, I am really not sure that you can simply put in a solenloid and have it work correctly. It may work, but I doubt it will work just as good as it was intended, because the torque split duty ratio can not be the same between the two, or I cannot imagine them to be so. However, It can be assumed with a fair amount of certainty that both control units will behave in an identical manner when there is a difference in wheel speed between front pinion and rear propeller shaft. As a matter of fact, I am not even sure that there exists a complex logic for the vairable apply in the VTD unit as this would not seem to be necessary. And if there is, I would be surprised if it was as agressive. This is because in a
VTD unit you have mechanical torque split that is fairly close to neutral. As a matter of fact, the slight rear bias would seem to be ideal for a tarmac setup, because of the weight shift to the rear. If I were to guess as to the programming in a vtd setup I would say that it probably has a map that tells the clutch to be at rest much more often than ours, but to apply under conditons of large throttle opening and low wheel speed, such as a full throttle launch, but to let the gears roam freely about their business under any conditon that is what you would see in a normal, casual drive.

In summary, I think that a TCU meant to operate a VTD setup, assuming that the solenoid is the proper one for that type would make the front wheel bias even more pronounced than it is with the US tcu.

[NiftySVX]

I apparently felt so strongly about my post that I found it necessary to put it twice. Whoops.

[Trevor]

Originally Posted by svxistentialist

In my layman's opinion it is not safe to suggest whether a car will understeer or oversteer based on the percentage drive to the front wheels in an AWD configuration.

It is usual for designers of FWD vehicles to set up the cars to understeer when driven beyond tyre adhesion primarily to curb over-enthusiastic driving in a safe fashion. It scrubs off speed and limits what a driver can do without the risk of spinning the car.

What I'm saying is the inherent tendency of most cars to understeer is a function of the chassis and suspension, not primarily the split of the power to the wheels.

In the case of the SVX the VTD gearbox by giving a higher percentage of power to the rear does tend to offset the natural tendency of the chassis to understeer.

Joe

Joe,

What I have detailed in my post is based on safe grounds. Yes all road cars are designed with a tendency to understeer. I refer specifically to excessive understeer, in line with US members complaints. This tendency is the result of excessive front drive, as occurs with the US models, but not VTD cars.

The tale of front drive pulling a car round a corner, is old hat by 70 years. This was an advertising claim by Citroen when they brought a truly beautiful car to the market.

[b3lha]

I just posted this in the gearshift maps thread. It is relevant to some of the discussions above.

http://www.subaru-svx.net/forum/show...&postcount=182

[svxistentialist]

Yeah Phil

I'll venture an opinion.

To me it looks like the parameters are dictated by chassis engineers. By people who understand a little bit about handling.

Once the weight transfers to the back, so does the power.
I like it.

Joe

[Trevor]

Originally Posted by b3lha

I just posted this in the gearshift maps thread. It is relevant to some of the discussions above.

http://www.subaru-svx.net/forum/show...&postcount=182

Thank you Phil, we are lucky to have your expertise.

Statements which make sense and are correct, are being added to the thread.

First let us again point out, that this particularly relates to the USDM SVX. In this regard you have correctly concluded that ---

“I think the TCU is looking at how fast the input shaft is turning compared to the amount of throttle applied. The sweet spot of the map is in the middle, when you are travelling at a steady speed, high duty cycle = power up front.”

This explains why those not applying some power through a corner, i.e. using poor technique, experience excessive understeer. More so with large cross section tyres, not inflated to a maximum level.

“Based on what I've found, I think that the theory about torque split being based on a percentage difference in speed between front and rear wheels, is wrong.”

I totally agree and have never suggested that this is the case. Again we have had incorrect theory broadcast by persons claiming authority.

“Alternatively, when you are accelerating against the car's inertia, the input shaft will spin slower relative to the amount of throttle applied. Again, the map lookup gives a lower duty cycle and power shifts to the back.”

Every reason to corner properly and under power.

“There is a bit more to it that I don't yet understand, but I think I've got the basics right. Any mechanical gurus want to offer an opinion?”

Everything you have advised makes sense and adds up exactly, both on the basis of mechanical, as well as handling dynamics.

Thanks again, Trevor.

[NiftySVX]

[QUOTE}

“Based on what I've found, I think that the theory about torque split being based on a percentage difference in speed between front and rear wheels, is wrong.”

I totally agree and have never suggested that this is the case. Again we have had incorrect theory broadcast by persons claiming authority.

[/QUOTE]



The torque split is calculated by a map, I don't see anyone who has suggested otherwise. However, it is clear that a difference in wheel speed between the speed sensors is how it detects a slipping condition. It says that plain as day in the repair manual. A normal driving pattern is calculated by the principle of predictive modeling, as I stated before. A difference in wheel speed between speed sensor 1 and speed sensor 2 is how wheel slip is detected. It is a very simple system, requiring little to understand the basic operation, providing you have a grasp on the aforementioned modeling.

[Trevor]

Originally Posted by NiftySVX

[QUOTE}The torque split is calculated by a map, I don't see anyone who has suggested otherwise. However, it is clear that a difference in wheel speed between the speed sensors is how it detects a slipping condition. It says that plain as day in the repair manual. A normal driving http://www.subaru-svx.net/forum/imag...mad.gifpattern is calculated by the principle of predictive modeling, as I stated before. A difference in wheel speed between speed sensor 1 and speed sensor 2 is how wheel slip is detected. It is a very simple system, requiring little to understand the basic operation, providing you have a grasp on the aforementioned modeling.

I do not think that the explanation in the manuals necessarily has relevance. Often the explanations are simplified rather than the complete process being explained, which is what Phil is outlining as follows:-

“I think the TCU is looking at how fast the input shaft is turning compared to the amount of throttle applied. The sweet spot of the map is in the middle, when you are travelling at a steady speed, high duty cycle = power up front.

If there is wheel spin then there will be less resistance to the engine's torque and therefore the input shaft will be spinning faster relative to the amount of throttle applied. The map lookup gives a lower duty cycle and power shunts to the back.

Alternatively, when you are accelerating against the car's inertia, the input shaft will spin slower relative to the amount of throttle applied. Again, the map lookup gives a lower duty cycle and power shifts to the back.”

As I see it the point Phil makes, is the inclusion of throttle position within the equation, rather that the simple use of front/rear wheel speed difference. He will no doubt explain/comment further.

[NiftySVX]

Much good information has been given and I am not disputing any. Input shaft speed could be used to calculate load in a round about way because there is going to be a higher difference between input shaft speed and crankshaft speed, and thus, wheel speed when the engine is running hard against the resistance of the drivetrain, as is obvious to anyone who has ever driven an automatic transmission vehicle. However, engine load is traditionally calculated as a function of the mass air flow meter. But, our cars do not have an input shaft speed sensor, only VSS1 and VSS2 which are on the rear output and the differential case. Input shaft speed=turbine speed. Turbine speed=engine speed (minus the slipping of the converter obviously) and thus does not vary with road speed, as the speed of the input shaft is before there has been any gear involved. The main purpose for this sensor that I have seen in practice, as a technician, is to monitor the lockup clutch, the torque converter, and even to detect slip in the clutches between the input shaft and the output shaft. Interestingly, if our 4eat were equipped with such a sensor I think we would see our cars illuminate the MIL often due to an excessive difference in speed between the input shaft and the output due to slipping internals.

The service manual is actually quite accurate as to how the ACT 4 system works, aside from being fairly vauge about the map specifics. It is pretty logical, and has served well since introduced in the XT in 1987.5. I often explain it to people who ask in this manner:

Think of it as if the tcu were a person. If you asked the TCU when it would think transfer apply should be high it would tell you the same thing you would think. It would say, well, I bet there is a high likelyhood of wheel spin when the wheel speed is 0 and the tps is 100%, with the trans in 1st. You would agree, because that would be a car that is at wide open throttle at a stand still. Conversely, you would ask the TCU what it thinks it should be doing when you've got a speed of 40mph, range D speed 4, throttle 10%. The computer would tell you it's not going to need a high duty because the car is cruising and is not likely to experience wheel slip. Each time though, the TCU is gonna ask, hey, how fast are my rear wheels going? (VSS1), and what about my front wheels? (VSS2) because if they are turning at too much of a different speed I am gonna go ahead and lock that clutch down tighter than a (insert your favorite euphemism) and forget the rest of that stuff for now, because none of my predictions matter if the wheels are slipping, because that is what we are trying to prevent in the first place.

That is a simple, accurate way to explain what the TCU is doing and I have explained it in this way for years to a number of different people, including Subaru sales staff, my mother, my accountant (he has a subaru) and several exes (that may have been why they are exes, but i digress...)

The important thing to remember here is that it is not complicated. I know that we all agree on this matter, yet we are having some kind of communication error within a thread where a poor soul is just trying to make his transmission drive the car.

[NiftySVX]

Oh, and I read that in 2006 or 2007 they started using the wheel speed sensors to detect wheel slip for transfer application because it was more accurate and allowed an even faster response. I will attempt to locate this material and submit it for scrutiny.

[Trevor]

Originally Posted by NiftySVX

Much good information has been given and I am not disputing any. -------------------- drive the car.

Phil is referring to the use of the throttle position as a means of load indication. Your lengthy post has introduced an aside issue, adding to the complication of the thread.

Your simple, method of describing the systems operation is rather loose. The TCU is not a person and can not anticipate, it works by sensing the here and now. A setting is made only according to existing data.

You close with :-

“The important thing to remember here is that it is not complicated. I know that we all agree on this matter, yet we are having some kind of communication error within a thread where a poor soul is just trying to make his transmission drive the car.”

Be sure that there is no communication error in respect of my posts or understanding of the written word. My english is exact. Unfortunately the object of your post is unclear.

[oab_au]

There won't be a map for "torque split". This is the result of the TCU adding pressure to the clutch to prevent wheel spin. The TCU would not have a clue what the split is, it only sends a pressure to the clutch based on the three inputs, Road speed, Throttle opening, and selected gear. Thats why the two systems are alike as far as the TCU is concerned.

The VTD TCU sends a signal ranging from 5% to 95%, the C solenoid moves from one end to the other, the torque changes from 36/64 to 50/50.

The US Transfer TCU sends a signal ranging from 95% to 5%, the C solenoid moves from one end to the other, the torque changes from 90/10 to 50/50.

The difference in the two C solenoids, is just to suit the inverted signal. The mechanics of the AWD system produces the torque split.

Harvey.

[Trevor]

Originally Posted by oab_au

There won't be a map for "torque split". This is the result of the TCU adding pressure to the clutch to prevent wheel spin. The TCU would not have a clue what the split is, it only sends a pressure to the clutch based on the three inputs, Road speed, Throttle opening, and selected gear. Thats why the two systems are alike as far as the TCU is concerned.

I gather the above relates to NiftySvx post 473. I agree that torque split will not be pre determined. But there will surely be four inputs, i.e. including front drive RPM, rear drive RPM, rather than simply road speed.

The VTD TCU sends a signal ranging from 5% to 95%, the C solenoid moves from one end to the other, the torque changes from 36/64 to 50/50.

The US Transfer TCU sends a signal ranging from 95% to 5%, the C solenoid moves from one end to the other, the torque changes from 90/10 to 50/50.

"Moves from one end to the other." Something wrong here, the “C” solenoid is certainly not a spool valve. It is enough to mix up normally closed and open.

The difference in the two C solenoids, is just to suit the inverted signal. The mechanics of the AWD system produces the torque split.
Harvey.

I note that you appear to now agree that the two systems incorporate a different "C" solenoid, i.e. N/O and N/C . Hooooray.
But, this is so because of the fact that in both systems the signal is similar. There is certainly no inversion of the signal, as this is not required. The required difference/inversion is taken care of by incorporating different solenoids.

[oab_au]

Originally Posted by TomsSVX

I thought the ECU was the speed governor for the JDM models?? Hence the code written into them for it and the JDM ECU having the hardware to make it work.

Tom

Tom the bit that makes me think this is, the only things that know how fast the car is going, is the TCU and the speedo. I can"t see how the ECU could tell.
I think buried in among all the geek talk, there is a little routine that waits for the 187Kph to come up and pulls the torque control. Only having half the power, would produce a good speed limiter.

Another one for Phil, in his spare time.

Harvey.