Torque on Demand

[Chopper]

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

[VehiGAZ]

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.

[blacksambo]

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.

[VehiGAZ]

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.

[blacksambo]

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.

[AREA 51]

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

[blacksambo]

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".

[Chopper]

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

[Jolly Roger VX'er]

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

I've come across this a few times myself when researching vehicles for a co-worker who was shopping. Case in point:

http://consumerguideauto.howstuffwor...ow-kia-cga.htm

.......the new sorento will again be available with rear- or four-wheel drive and come in two trim levels: lx and upscale ex. the lx 4wd system is part-time, has a low range, and isn't designed for use on dry pavement. available on ex is kia's torque-on-demand (tod) full-time 4wd system. the tod system detects wheel slip up and automatically transfers power between the rear and front wheels. tod may be used on dry pavement. for hardcore off-road junkies, a limited slip differential for the rear axle is available.

It sounds like their version of "TOD" excludes low-range; and, I just got to laugh about the statement "for hardcore off-road junkies, a limited slip differential for the rear axle is available"...

I noticed most of you hard-core off-roaders here waiting to replace your lockers with limited-slips..lol.