There seems to be some confusion about exactly how the Volkswagen
Syncro/Audi Quattro I system works. This document aims to set that record
straight, and may or may not succeed, but it tries very hard, so give it
some credit : ) This document applies to the following cars, and probably
a few others that I've forgotten:
Audi 4000 Quattro
Audi 5000 Quattro
Volkswagen Quantum Syncro (all years) Audi Turbo Quattro Coupe, aka ur-q, TQC This document does NOT apply to the Vanagon Syncro, which has a completely and utterly different viscous-coupling system.
These cars have three differentials: a center, rear, and a front. The center diff divides power between the front and rear diffs. The front diff divides power between the front wheels, and the rear diff, predictably, divides power between the rear wheels. These diffs are all conventional open diffs, which means that they split torque evenly. Under normal circumstances, all four wheels will spin at approximately the same rate, all powered. However, say if the right rear wheel of the car lifts off the ground, the center and rear diffs will have one output shaft that is really easy to spin, and the front diff will never see any motion at all (although it will get the same amount of torque as the rear diff, but it won't spin anything - this is kindof confusing, but if you think about it for a while, it makes sense). The upshot of this is that only that wheel that is off the ground will spin, since it takes less torque to spin that wheel than it does to move the other wheels and move the car. This is what's bad about conventional diffs, and it's why when a normal car gets stuck in a ditch, only one wheel spins and it's the "wrong" one.
This is where differential locks come in. Without them, the Quattro would be nothing really special. When a diff is locked, both output shafts are forced to turn at the same speed, with up to potentially 0/100% torque split, in theory (except that it always takes some torque to turn wheels, even when they are off the ground). So, if the center diff is locked (knob pulled out one notch, one green light lit), one front wheel and one rear wheel, at least, will be forced to spin. With the rear diff locked as well (knob pulled out all the way, both green lights on), both rear wheels will be forced to turn at the same rate as well, and the car becomes effectively 3-wheel drive under any circumstances. This isn't true AWD, one could argue, but it's certainly better than conventional "4wd". (Just to set the record straight, when you put a non traction-control or anything new and fancy equipped truck/SUV/Jeep/whatever in "4wd", it becomes what an Audi Quattro I is with the center diff locked). The first Audi to have "true" AWD is the new Quattro IV's, which have a Torsen center diff and electronic traction control which utilizes the brakes front and rear.
So why not leave the diffs locked all the time, you say? Well, your front wheels describe a larger arc than your rear wheels when turning, and the outside wheels turn faster than the inside ones. So when you have your diffs locked, the tires and road surface have to scrub off this difference, causing "hopping", and lots of wear and tear on tires, diffs, and much of the steering and suspension system. This problem, and the fact that your average consumer has no idea how Quattro I works or how to use it, is why Audi switched to Quattro II with the 80/90 and 100/200 Quattros, which has a torque-sensing Torsen center differential.
Questions, comments, suggestions, corrections, additions? Email me.
Page last modified on Jan 30, 2000Back to QSW page