Torque on Demand

Drive that new Acura RDX hard, you'll feel the difference. Having the outside wheel dig in on a hard turn, rather than slipping the inside one is a huge advantage. The RDX is no lightweight, but the awd they are using makes it seem smaller

My two cents:

Cent #1: Acura's system isn't all that new - I had its one-axle progenitor on my 1997 Prelude SH. It does not work like an LSD, by the way. An LSD keeps the inside drive wheel from spinning too much in a turn compared to the outside wheel, since the spinning wheel decreases the amount of torque going to the outside wheel in a conventional differential. The SH system actively transfers torque to the outside drive wheel to get it turning faster than the inside wheel so that neither of them spins, and you get extra turning help when you need it (that's where the yaw sensor comes in). To illustrate the differences numerically, consider these three cases:

a) a regular differential in a hard turn under power with poor traction under the wheels: the inside drive wheel will spin, so up to 100% of the torque will go to the spinning wheel, and 0% torque will be delivered to the ground.

b) a limited slip differential in the same conditions: the inside wheel will start spinning, but the LSD will only allow it to spin a certain amount compared to the outside wheel. With the 40% LSD on my other car, the outside wheel will get something like 50% of the torque, the inside wheel will get 40% of that (20%), and the rest (30%) is lost to spinning the inside wheel, resulting in only 70% of the torque getting transferred to the ground (these numbers are for illustration purposes only! I have no idea how to calculate the actual values, which probably involve differential equations, the friction coefficient of the road surface, etc.)

c) the front-axle SH system in the same conditions: the system will sense the slippage and yaw rates and apportion torque to the outside wheel so that neither wheel spins, and the car goes where the tires are aiming. Let's say that amounts to 80% to the outside wheel and 20% to the inside wheel, resulting in transfer of 100% of the available torque to the ground.

It's a great system - another example of Honda engineers cheating physics (the much-copied V-TEC valve train being another good example). I beat on my SH system for 145,000 miles and it never gave me any problems. In the SH-AWD system, I think they are using the same principle to apportion torque not just left-right but also front-back.

Cent #2: The Isuzu TOD system works pretty well, except that it doesn't hold the engagement of the front axle long enough when driving on snow. When starting from a dead stop on a snow-covered road, it senses slip in the rear, it engages the front, then sees that there is no speed differential between the axles, and disengages the front 0.1 seconds later. Of course, the rear axle starts spinning again, it engages the front for a tenth of a second, sense no speed difference, etc... It would be great if after fully engaging the front axle, it held it there for a couple of seconds before gradually decreasing torque to the front axle. It operates much better on dirt, though, I'll give you that.

Great commentary! But how do we countenance the 90% front drive bias of the SH-AWD system? They claim it's for fuel efficiency sake, but I think that only compensates for the energy expended spinning a rear wheel drive drive shaft, which I believe is a neglegible energy expenditure compared to having the handling advantage provided by the full-time rear wheel drive biased VX? Also, the Acura vehicles don't offer the option of going "combat" and utilizing raw 4x4 locked differential low range torque, which they claim is made superfluous by their SH-AWD system. Again, I think that's probably overpromise as a true lowrange mode behaves so much differently than any automatic device does, it's just brute rubber eating force and I noticed that Acura in their literature make not statement about consuming precious tire rubber on pavement with their system. If it really had that brute force available I think they would have cautioned against its effects. The VX literature does.

Thanks!

As for ToD vs. SH-AWD, I guess I would not compare the two systems so directly. ToD is an appropriate system for enhancing traction in off-road vehicles like the VX and Trooper, while the SH-AWD system is appropriate for enhancing cornering and handling in a sports sedan, while at the same time giving it a measure of all-wheel-drive traction enhancement, much like Audi's Quattro system.

I don't think any AWD system is a suitable replacement for true 4WD on a off-roading truck. They are nice enhancements to traditional non-truck drivetrains, and may help you get home in the snow, but I don't think Acura or Audi or even Subaru intend or market their AWD systems to be substitutes for true 4WD.

First, I wanted to say that this has been a most informative dialog. I've learned a lot from this groups positive exchange of knowlege on this topic. This board is a great way to share perspectives.

Second, I wanted to point out that the Acura while offering sexy transfer of power technoadvancements (if there is such a word) still relies on unit body costruction instead of a body on frame constuction like the VX. I'd have to guess that the use of unit body is why that MDX/RDX is biased towards front drive as the hull lacks the torsional rigidity found in body on frame constuction for effectively sending power to the rear except under difficult surface situations when it's required. We all know front drivers tend to understeer due to weight distribution balance issues. Nevertheless, the VX has near neutral weight distribution and the torsional strength to drive the rear wheels most of the time. When it comes to performance driving, like road racing, I don't think there are any teams using solely front drive configurations due to understeer considerations. Just some additional perspective.

I think hyundai or kia is using tod on one of their suv's

It sure sounds like good 'ol TOD if you read their website. They also call it an all new system??? However, theirs is front wheel drive biased like the Acura, that's how they probably get away with calling it "all new".

Neutral weight distribution? You're trippin'. Look, I love this car as much as anyone, but... Why do you think the gas tank is hanging under the rear bumper? This car will push like a rhino on wet clay, till the TOD reacts. We have lots of "shell"(sand) roads in this county, so I get to play with it alot. As far as the Low range thing...the Acura is not intended for such activity. It'll handle the sand roads, but if you want more, it's the wrong car. I got an 03 STI I am currently tearing the rubber off of. Had it for three days now, and I'm about ready to sell it. It is much faster,sticks better, and rotates much more easily, than 0177... and three days in...I'm done with it (sorry Wormgod) The engine sounds wonderful, the squeaks and rattles, not so much. It just lacks somehow. This VX appeals on some other level, beyond performance. Face it, tecnology has marched on..215hp pushing 4000 pounds cannot be considered a performance vehicle anymore. The family sedan is averaging between 265-300hp. My wifes Altima has 265 and it's NO performance vehicle. The VX stands on its own. It's unique. Trying to pump it up only cheapens it in my opinion. It's advanced for 1993 when it was designed, but it a far piece from cutting edge at this point. Compare it in context, it's great...arguing it vs. current tecnology sounds like "They don't build them like this anymore" I hear that every time some guy drags in a '67 Cougar or some such old junk. In my head I always say, "There's a good reason" ;) Wayne

Perahaps you've answered your own question? The more sheer numbers and technology you buy may not be the vehicle answer your seeking at all. Instead, it seems like the VX delivers some sort of magical or quasi sophistcated driving experience not found in these other contrived numbers-only driven package scenarios. One might say that the VX delivers some sort of very satisfying unique blend of sensory factors not easily duplicated by other attempts, no matter what the date of manufacture. I know this sounds somewhat mythical like dragons or something but don't deny its potential existence. These things due happen and its what SCCA vintage racing is all about.

Perahaps you've answered your own question? The more sheer numbers and technology you buy may not be the vehicle answer you're seeking at all. Instead, it seems like the VX delivers some sort of magical or quasi sophistcated driving experience not found in these other contrived numbers-only driven package scenarios. One might say that the VX delivers some sort of very satisfying unique blend of sensory factors not easily duplicated by other attempts, no matter what the date of manufacture. I know this sounds somewhat mythical like dragons or something but don't deny its potential existence. These things due happen and its what SCCA vintage racing is all about.

On the subject of technology marching onward. That Subaru you speak offers a system called VTD,right? Which if you read about it sounds an awful lot like the VX's TOD. So is the difference performance-wise really more down to Horsepower than a more advanced driveline? If so, don't the VXer's with the Alpine supercharger compensate somewhat for this difficiency by adding this extra? Just curious.

Naa... the Subie is much bigger. Anything more than 5lbs. of boost will grenade the Isuzu engine (without MAJOR mods) I've seen Subaru engines making 400hp. on pump fuel...different deal. The flat 4 was made to be boosted, the six was not...too much compression, among other things. You are right in your nostalgia take...the old mans' series two E-type is still a special car to drive, but a Miata will out drive it everywhere but the straights, and is a better car in every measurable way ;)

Come on a Miata? I think the car is a cute little sports like car and would not mind having one as a 2nd car in the garage ut to name it and performance in the same breath is a stretch. I don't want to insult you or your chosen ride but I have a 1974 JH in my garage that I am restoring and the stock specs of this car in 1974 out perform a Miata in everything but breaking power since they did not have discs back then

'74 is a twelve cyl. if I remember...the early ones all had that beautiful, straight six. I learned to drive a manual in my dads, at the Hatfield racetrack (long gone) in PA, when I turned thirteen. I only used a Miata as an extreme example of how much things have changed. I am not a Miata lover (I am 6'3" and 200lbs.) but I have "spec" raced them several times at Robling and Sebring road courses... I am pretty sure it is a faster way around the track. They allow you to brake sooo much later, and carry sooo much more speed thru a curve...you can even get back on the power earlier comming out. The Big Cat just can't compensate with enough in the straights. I dunno...never saw anyone race the two. (Pinks..road course edition) I never dared drive his E-type hard.... but I was in it with him goin' balls out, all opposite lock, chasing a little Alpha at Lime Rock. I think I coulda taken him in a '06 wearing Pilot Cups ;) ;) P.S. we didn't catch the Alpha

The only current production vehicle that still ships with a Borg-Warner TOD is the Jaguar X-type (that I know of) - they don't reference it as TOD but it's supposedly the same system. There was a brilliant commercial - showed a close up of a guy spinning around - camera pans back and you realize he's in a sedan that's spinning on an icy road - creeping up behind and eventually passing him is a Jag X-Type and then they go into the whole 4-wheel drive with sensors bit. Here's a different commercial:

http://www.youtube.com/watch?v=pfL3egQ61bU

There was also this video:
http://www.youtube.com/watch?v=2QvSymMUL4A

-- John