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What exactly, is not true?

If you are talking about the increase in fuel burn when using 4 wheel drive, you are partially correct.
There are additional losses using 4 wheel drive that don't exist in 2wd. Driveline losses, the additional rotating mases, the friction of the joints (be it u-joints of C/V's) the additional gears and the resulting surface friction, etc. Audi found that the decrease in rolling resistance (by driving all four wheels) overcame the increased mechanical losses ONLY at speeds over 100 MPH.

I believe that was 100 KM/H, and the difference below was not consistent. It has more to do with turning corners than anything. In a straight line, 4wd was slightly more fuel efficient.

Actually I was working for Audi as a Technician at the time and it was 100 MPH.

I haven't seen the research to be able to form an informed opinion on it...But to have any potential of not being proven very wrong by empiricle evidence (www.fueleconomy.gov) it must have been at 100+mph, or maybe on ultra low grip surfaces where the reduction in tire slip is more significant than the drivetrain losses, or who knows what else since we're relying on nothing more than memories of 20+ year old data. Either way 100km/h is a holding-up-highway-traffic 62mph, which certainly falls under the conditions where 2WD is consistently proven to provide superior fuel economy in the real world.

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-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

I'm modeling this AWD system off of Audi's Quattro AWD system which uses a Torsen center differential to distribute power to both ends, mine will most likely be a Peloquin unit. The advantage to the Torsen setup is that it automatically biases torque based on which side has more traction. If the front wheels are getting more traction the diff will send more torque to the front up to a certain multiplier. For example some are 2:1 so 67% of the torque goes forward and 33% back or 4:1 where it's 80/20. I don't know what the ratio for a Peloquin unit is, and it's something I'll have to research. However the rule of anything x 0 = 0 still applies so if there's wheels with no traction (like on ice) I won't go anywhere without a special trick (light braking) to give "traction" to that wheel.

If you don't think you can get stupid with AWD just check out this video:


I'll most likely put some form of a clutch type LSD in the rear end and leave the front open (I do want to be able to do what that Audi is doing from time to time).

That Audi uses a Subie style transmission... completely different from what you are intending to use...


If you haven't already, do a search here on all wheel drive locosts...

The only vehicle I can think of, available in the US that hasn't been discussed and wouldn't require massive re-engineering of a frame plan or risk of destroying expensive diff/axles is the e30 325ix... I'm not sure if the front diff is located in the oil pan, or in front of the motor... but super rare...

I have found some info on the North American Subaru Impreza Owners Club (NASIOC) forums about how the Subaru AWD system works and it is a Viscous Coupling, similar to the Haldex system in an Audi TT or A3, not the Torsen setup in the larger longitudinal engine Audis such as the A4. It may be Subaru-like in that the diff is incorporated into the Trans but the AWD system used is very different, using a Torsen center diff (follow the Link above).

As long as you remember that the quattro (Always with a little q) was and is the longer, longitudinal layout.
The engine, mounted to the front of a transaxle, with half shafts coming out of the transaxle driving the front wheels, and a output shaft coming out of the rear of the transaxle driving a rear differential.
The output shaft that comes out of the rear differential is drivinig at final gear speed (transmission) not final drive speed.
That means that the output going to the rear differential, is the same speed as what the front differential is seeing.

With the layout that you are using, turning a 020 trans 90 degs and the outputs from what would normally go to the front wheels you are now driving to the front and rear differentials.
One issue that I am seeing is one that you and I have already mentioned: finding a 1:1 differential that is capable of handling the load.
I think that you are right (at least I haven't found any data to presume otherwise) that the differential mounted in the rear of the syncro Passat, is a 1:1 differential.

Another problem is rotation direction.
If you mount the engine towards the front, the shafts will be turning clockwise (as seen by the rear differential). Which, if I remember correctly, is the wrong direction for the back end of the car, but will work just fine for the front end (using the Passat diffs).

Something else that was mentioned before was the load direction at the diffs, you were correct, the teeth in the diff are designed to be driven in only one direction. There is a drive side and a coast side. If you try to drive the coast side, you will probably blow up the diff case as the gears will try to push each other apart.

Sorry...I'm late to this tea party. Commenting two pages back, I wouldn't make gearing/tire choices in order to make your speedo correct. That's probably one of the easiest problems to fix, with an calibratable, aftermarket speedo, or a lowcost index card and sharpie with new numbers to paste behind the stock needle.

-dave

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...nowadays people are so intellectually lazy and lethargic that they can't build ANYTHING with their hands. They'll spend hours watching whiny people marooned on an island, but won't spend a second adding anything to the world. -weconway
Visit my [Locost 7 build log]

Thanks for the input. I definitely don't want to have an index card with speeds taped to my dash. I'll probably be going with a full aftermarket gauge cluster since the stock Golf one is pretty sparse (speedo, odo, temp (without actual degrees), fuel, a clock and warning lights). I'll have to look into how the various aftermarket speedos work and are calibrated. One thing I would kinda like to keep is the mechanical speedo, they just seem more reliable and more accurate to me. Another reason for me to use full aftermarket gauges is that I can make the layout to my liking and keep track of the engine better. I'll probably have the following gauges in the car:
Speedometer
Tachometer
Oil pressure
Oil Temperature
Coolant Temperature
Exhaust Gas Temperature
Boost pressure
Fuel level
Voltmeter

Probably in the following config:
.:O.O:.

Judging by the direction of rotation you're talking about, it sounds like you're talking about the engine on passenger side of the car. This would actually put the transmission towards the front and engine towards the rear, unless VW makes engines that spin 'backwards'. As I previously mentioned both diffs would have to be flipped, not just the rear. The reason for this is that while the direction of rotation provides natural loading on the 'front' diff, you also don't want the front diff propel in it's natural direction since that is now towards the rear of the car. So the diff would still have to be flipped upside down to propel the car in the direction you want it to go, just like the rear diff that is additionally being loaded in reverse. As to whether or not this would necessarily create lubrication problems on for a given differential is pretty much anybody's guess.


While I won't pretend to have enough technical background to be able to provide an informed opinion on the potential service life concerns of running a diff upside down and/or backwards: I will note that differentials are designed under the knowledge that they can/will see significant amounts of reverse loading over the course of their lifetime. Thus there is a threshold of reverse loading that every differential should be able to reliably tolerate....Again, provided proper lubrication is able to be ensured.

_________________
-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

This is true, however the amount of loading that the diff can handle when driven on the coast side is NOT the same as full torque from the engine. The coast side is designed to power the vehicle in reverse (and not at full power for extended periods of time) or to spin the engine during lift throttle, in gear coast.
Running the coast side of a gear set is not a good thing or a cheap experiment.

Alright, so back on the first page I posted this (pretty crude) diagram of the basic drivetrain layout. For those who are discussing the direction of rotation from the diffs I'm reposting it here (front of car is on top as though standing behind car):



As far as I can tell the Sycro diff is mounted in the car with the diff on the passenger side and the angle gear on the driver's side of the propshaft. The input to the diff from the trans is supplied from a right angle drive bolted to the passenger out-drive of a normally FWD transmission with the gear to the passenger side of the prop shaft. This means that the propshaft will be turning clockwise into the rear diff unit. When the front diff is rotated upside down around the axle line, the right angle stays in the same relative position and thus loads the gears the same as long as the input spins clockwise into the unit. The rear, unfortunately, has a counterclockwise input in my application, which requires the unit to be flipped on the input axis and loads the gears on the opposite faces.

Here's a Picture from BrokeVW.com of all the gears from a disassembled 020 with syncro right angle in place. This view is from the front of the car (passenger side is on left).

as you can see, when the car is moving forwards, the propshaft to the rear of the car will be spinning clockwise into (looking at the input shaft) the diff. This is how I figured out where the gears in the diff units were and then touched up the drive train to show this:

Here's a redone outline of my proposed drive train with the gears of the diff units in place:
Front

Back

Find the episode of monster garage where they make the corvette mud racer.
You can turn any trans motor combo (longitudal) into a 4 wheel drive.

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The biggest thing in motocross.

While it may not be able to handle the same torque in reverse that it can forwards, many differentials are still designed to handle significantly more torque than the stock engine they were originally mated to produce. Also it's important to realize that "coasting" provides a more significant amount of loading than most people suspect, and is something that the differential spends a significant portion of its life doing. As an experiment to demonstrate how much reversed loading engine braking makes relative to WOT drive loading, take whatever manual transmission car you have access to and time how quickly it goes from 75% of redline to redline. Subsequently time how quickly it goes from redline to 75% of redline, without using the brakes, which at first gear speeds should be sufficiently close to being almost entirely due to "coasting"...Or engine braking. At the race track drivers can even get the rear wheels to 'lock up' a bit due to the added engine braking, be it intentional or not.


Exactly. Also I'd like to note that the disassembled 020 trans photo is certainly helpful to show how the VW syncro system bolts to the FWD transmission, but could be a little confusing/misleading for anybody who trying to determine it's orientation in the car as shown, due to the way the rest of the transmission shafts/gears laying bass-ackwards relative to the differential and PTO assembly...As I know it had me scratching my head for a minute before realizing that.

_________________
-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie