Torque on Demand

[Chopper]

I've never had issues with the TOD. It's pretty slick in that you are all but RWD till you need more, then it's there, pretty quickly, and fairly smoothly. I like it at least as much as the Audi/VW system...maybe more...but it's light years behind Acuras' new system. I am talking only about road manners here, because I know absolutely nothing about offroading. My comments were directed at the transmission itself. I generally don't like automatics, and do not care for this one at all. Lexus and MB make some amazing gear boxes, but I'm with Dino, I'd rather do it myself. A manual conversion is tempting. Has been for six years now.

[blacksambo]

I've never had issues with the TOD. It's pretty slick in that you are all but RWD till you need more, then it's there, pretty quickly, and fairly smoothly. I like it at least as much as the Audi/VW system...maybe more...but it's light years behind Acuras' new system. I am talking only about road manners here, because I know absolutely nothing about offroading. My comments were directed at the transmission itself. I generally don't like automatics, and do not care for this one at all. Lexus and MB make some amazing gear boxes, but I'm with Dino, I'd rather do it myself. A manual conversion is tempting. Has been for six years now.

Thanks for the transmision clarification. I'm not so certain about that Acura SH-AWD system. First, it sticks you 90% into front wheel drive until something tricky pops up? Second, that yaw control stuff sounds a little overblown. Meaning they state versus conventional 4WD its better because it automatically splits power between left and right tires. Doesn't limited slip differential do this? So, the only time the Acura's system is of benefit is versus locked-differential mode 4WD's. Is that really all that advanced versus VX's TOD which is biased 90% rear wheel drive with full-time LSD? I'm not so sure TOD has been really eclipsed yet, especially since it's been fairly bullet proof dependibilitywise for 11 years running. Not trying to pat Isuzu on the back here, just trying to guage the true lay of the land today.

[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.