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Heres a pic of the HVAC and controls in my
-juice
Its hard for me to grasp why the on/off acceleration device is used at all.
The device is mounted ahead of the rear diff before the left/right split. My assumption is that it always over-drives the entire back end. Your Lexus example is over-driving by roughly 4.5%. The press material for the RL claims 5%. (What's a half percent between friends...) So, in a straight line, I'm thinking the Acura and Lexus are doing essentially the same thing.
Not sure how the Acura avoids binding, though.
When the clutch packs in the SH-AWD rear diff are locked on one side and not the other, the outside wheel is being over-driven instead of both. That creates the inward yaw moment, which keeps the nose in line.
As applied in the RDX, the rear wheels are pushing the vehicle around the corner (like a RWD vehicle). However, it doesn't waste any power on the inside wheel.
Think of it as an over-achieving rear LSD.
Googled for:
sh-awd
And the video was the first hit.
The other question is what does the RDX give up? The twin clutches on the rear axle?
I like that JDM Odyssey that they show at the very beginning. A diplomat that goes to my church owns one.
I also wonder if there is a concern about wear on the twin clutches on the rear axle. Sounds like they're going to absord a lot of changes in speed pretty much every time you go around a curve.
Very neat, though, and more informative than any video I've seen from Subaru or Audi. I've managed to get my hands on some videos from Japan for Subaru, but they tend to focus on the concept of AWD and a low center of gravity, rather than the specifics of any particular AWD system. They do put out PDFs like this one, at least:
http://210.101.116.115/fisita/pdf/G347.pdf
Audi tends to do demos out on ski slopes/ramps, but they hand pick the competition in order to show their strengths.
-juice
http://hondanews.com/CatID3007?mid=2005081756995&mime=asc
Positioned at the front of the RL rear drive unit, the Acceleration device typically passes torque rearward to the rear axle at very close to a one-to-one ratio. In cornering, however, the Acceleration device's output shaft spins faster than its input shaft.
The Acceleration assembly uses a compact planetary gearset to achieve its speed increase. Hydraulic actuators operate clutch packs that control the planetary gearset. When the input shaft is locked with the planetary gear carrier, there is no ratio change (this is the straight-line mode). During cornering, the carrier is coupled with the case, and the output shaft speed increases up to five percent. A speed sensor at the hypoid gear, downstream of the Acceleration device provides a feedback loop to the SH-AWD Electronic Control Unit to ensure that the system is working properly.
So, the accel device is controlled via hydraulic actuators. The ECU makes the decision to enable those actuators, though I'm still not clear which input stream(s) will have an impact. Could be yaw. Could be wheel-speed sensors. Could be something else. Could be all of them.
The RDX gives up only the acceleration device. The RL's version can manage three things.
1. Torque control front to rear.
2. Torque control left to right (rear axles).
3. Ability to over-drive the outside rear wheel.
The RDX can manage 1 & 2 from that list. It will bias torque from 30/70 to a 70/30 split from front to back. And it can use the electromagnetic clutch packs on either side of the rear diff to send up to 100% of the rear axle torque to one wheel. Or, if moving in a straight line, the rear axle torque can be split evenly (50/50) between the two rear wheels.
Wear on the clutches is actually addressed in the very last paragraphs from the link in the previous post. The system includes sensors which will effectly tell the ECU to apply more pressure to the clutch packs if they start to wear down.
But I don't think that would be a significant problem. The two 'halves' of each clutch pack are going to moving at the same speeds. Even if one of the wheels is not getting power from the engine, it will be 'dragged' along at pretty much the same speed as the rest of the wheels. Honda and other manufacturers have been using wet clutch packs like these for a long time without issue. They probably see lower forces than transmission clutch plates.
To rotate the car going around a turn. Think controlled oversteer under throttle.
And I suspect that if the acceleration device really has been eliminated from the RDX then that's how it's done.
Correct me if I'm wrong but I believe that if you wish to have the rear "take up" 70% of the engine torque in straight line driving then both rear wheels MUST be driven (at least slightly) faster than the front.
He was a Mech. Engineer and a lot of it went way above my head, to be honest.
Any how, very cool concept. Here's a link:
http://www.christini.com/index2.php
-juice
I think you are incorrect. At least, if I understand what you wrote. What you're suggesting is that the rear wheels turn faster (over-driven) than the fronts. If that were possible, eventually the back end would pass the front. :surprise:
Applying torque to a wheel, or set of wheels, does not mean those wheels will turn faster. They will turn harder - with more force.
Picture two moving a cart down a road. One is pulling, while the other is pushing. It doesn't matter if one is applying more force than the other, because it is the combination of their efforts which moves the cart.
Now get both pushers behind the cart. One on the left, the other on the right. If the guy on the right pushes harder, the cart's nose will begin to turn to the left. In this situation, the right wheel is allowed to move faster than the left based on how much force is applied.
This is what would happen if torque were applied to an outside wheel. The RDX (even without the accel device) can do this.
The RL takes that same concept and adds the ability to over-drive the wheel. With this in place, the right wheel is getting more power, but it is also geared higher via mechanical advantage. This exaggerates the speed of the wheel, turning the cart's nose even more dramatically.
Front to rear or side to side torque split is whole another issue. With 70% of the engine power going to the rear wheel, all you're getting is a greater push from the rear axle compared to the pull from the front (and it reverts to 70/30 under normal cruising).
When my AWD 94 Ford Aerostar is in 30/70, normal, mode and with two tires off the ground on the same side and I manually rotate the front wheel then the rear wheel turns about 5% farther in rotation. If I then activate the 50/50 solenoid the wheels turn at an equal rate.
And keep in mind that if the rear wheels are overdriven and you're on a high traction surface the center differential (or whatever takes its place/function) takes care of, prevents, any driveline windup.
That's the sole advantage, or maybe definition, of AWD. If both front and rear are to be "driven" simultaneously on a high traction surface then there must be some sort of "rubber band" to prevent driveline windup.
My 2001 AWD RX300 uses a center open differential to prevent driveline windup but with a VC, Viscous Clutch/Coupling, across the two output shafts of the center diff'l to limit the "slip" between the two shafts if the front, primary driven wheels develop wheelspin/slip.
Since true (part time) 4X4 vehicles almost always use a planetary gearset in the transfer case I suspect the planetary gearset serves the purpose of an open diff'l unless it is locked.
See:
http://dwolsten.tripod.com/articles/honda4.htm
But my wife (who does not research much) wants something different/European. We need to buy within the next couple of months.
I hate to say that we are considering the following choices. (ramdom order)
1. Volvo 06 V50 Wagon (Ford Reliability and problems?)
2. Saab 06 9-3 SpotCombi Wagon (GM Reliability and problems?)
3. Acura 06 TL (I really like to get 07 but we can't wait)
4. Acura RDX (Coming out this Summer? July-August)
5. Lexus 06 IS 250 (too expensive and small)
Help me guys with value, reliability, sproty and luxury; which car should we buy?
Have your wife drive the RX400h..
Not any different than a RWD or FWD vehicle where one pair of wheels get 0% of the torque, even though they are all being driven at the same rate.
Torque distribution does not determine rate at which the wheel is turning. Imagine lifting the vehicle up, and trying to apply force to stop the wheels. The one with greater torque will require greater effort/force to stop.
It's possible because torque is a measure of force (twisting force). It is not a measure of speed (work done over a length of time).
You and I both push on a block of granite. You push twice as hard as I do. The granite block weighs 40,000 lbs. Even though you are applying twice the force, our speed is the same (going nowhere).
When a vehicle's power split is something like 30/70, the rear wheels are providing the majority of the force. Yet the overall speed of the vehicle is the same. So long as you're driving in a straight line, all wheels are linked into a single task and move at the same speed.
Now, when the vehicle enters a turn, the outside wheels are not doing the same task as the inside wheels. The outside wheels must move a slightly longer distance than the wheel on the inside. They swing in a wider arc. Now that extra force is allowed to operate. The outside wheel has the freedom to move a wee bit faster than the other wheels.
Thanks to SH-AWD, that wheel also has the torque to take advantage of that running room. Meanwhile the inside wheel, which is just along for the ride during a turn, is not getting any power. All the power is going to the wheel which is in the best position to make use of it.
Yep, it does affect which wheels are accelerating.
Even though you are applying twice the force, our speed is the same (going nowhere).
-juice
* How many kids? Other passengers? (passenger space)
* How much cargo? (cargo space)
* Any pets? (crate size?)
* What is your budget? ($)
* Do you get a lot of snow? (AWD)
First I would decide on sedan vs. wagon/SUV. I prefer the latter, you get a lot more versatility, cargo space can be up to 4 times greater and plus you have a flat roof for a roof rack and even more capacity.
The 4 things you list as priorities are asking for a lot, sporty and luxurious usually means expensive, not value priced. On top of that you want reliable, so that limits the options, too.
On the value end of the scale, try a Forester XT, a Mazda CX7, or even a Legacy GT Limited wagon. Maybe a Saturn Vue Redline (Honda 3.5l V6) though the interior probably would not be nice enough for you.
On the luxury end, RDX, MDX, and RX350, though the latter two aren't sporty. The X3 fits but would not meet my definition of reliable. FX35 is a good one, but watch the price with options. Maybe a Murano or Tribeca if you want to go out on a limb with styling?
Try this, go to a car show. Test fit everything you mentioned plus the others mentioned here. That should help you narrow it down to maybe 2-3 top picks. Then go test drive those.
Put a *lot* of weight on the test drive and test fit. That's really what matters, not what a bunch of internet jockies like us think. Good luck.
-juice
"Yep, it does affect which wheels are accelerating." - Juice
Only if the capability for individual acceleration is present.
When a vehicle is cornering, the outside wheels have that freedom and the effect of the extra force on a single wheel can be realized. The outside wheel is free to move faster than the others.
When driving in a straight line, all the wheels are working as a group on one task. The efforts of one wheel cannot be distinguished from the total amount of work begin accomplished by the group.
In order to shift torque from left to right, ATTS used a single method. It applied mechanical advantage via gears and such. The advantage is given to either the left or the right wheel.
The RL's version of SH-AWD accomplishes this in two ways.
1. There is the acceleration device placed ahead of the diff. This uses mechanical advantage in a manner similar to ATTS. However, it's a much smaller unit and manages only a 5% variance in speeds, not 15%. This device drives the entire differential. In this case, the over-driving is happening fore/aft, not left/right. The torque coming out of that device is not split. That doesn't happen until step 2.
2. There are also electromagnetic clutch packs on either side of the diff. These connect the east/west half-shafts to the torque supply. If one of those clutch packs lets go completely, the wheel on that side gets no torque. Nada. Zip. Zero. At the same time, the clutch packs on the other side of the diff stiffen up and drive 100% of the torque to the wheel on that side.
So, here's what happens if we were to follow the path of the torque from engine to wheel.
Torque is delivered from the engine, through the transmission, to a center differential up front. This splits some power to the front and some to the rear. The front end is just a basic open diff. No mysteries there. We're concerned with the rearward torque.
The torque then travels down a prop shaft to the acceleration device.
We'll pretend the car is cornering at this moment.
The acceleration device "speeds up" the output. The prop shaft (input) might be turning at 3,000 rpms, while the output shaft is turning at 3,150 rpms. That torque is sent to the rear diff where it gets split left and right.
The torque does not go left because the clutch packs have let go. The wheel on that side of the diff is freewheeling. It is not powered. It is no longer connected to the engine.
The torque takes a right turn because the clutch packs on that side have clamped shut. Torque travels through the clutch packs, along the half-shaft to the wheel. That wheel is now being driven with 70% of the engine's torque trying to move it 5% faster than the wheels up front.
The result is that one side of the vehicle (the right side) is being pushed both harder AND faster than the other. This helps push the car around to the left.
And I suspect that the acceleration device and the two L/R clutches used in combination, working in concert, make up for the total and complete lack of a center and rear differential.
This is how Acura describes it in their literature (emphasis added).
"A matched pair of Direct Electromagnetic Clutch systems, one on each side, send power to each rear wheel. These clutch systems can be controlled as a pair to alter the front/rear torque split; depending on the situation, the rear wheels receive between 30 and 70 percent of the total engine output."
Honestly, I don't know exactly how locked clutch plates can boost the torque split beyond a 50/50 split. That part of the engineering still eludes me. Honda's own VTM-4 system does this and I know several companies including Ford, Subaru, and Mazda have used clutch-based systems to achieve variable power splits. Maybe it has something to do with the torque transfer unit mounted up front immediately behind the transmission. It's a single speed transfer case, but in the video linked above they called it a center differential.
There is NO way the rear tires can "assume" 70% of the engine torque by simply coupling them at a 1:1 ratio into the driveline. The best you can do, absent some type of "uncoupling" of the front drive, with a 1:1 ratio is 50/50.
This is what Autozine writes.
"When both clutches engage, more torque will be sent to the rear axle. In this way, SH-AWD can very the torque split between front and rear axle, as well as between the rear wheels."
Pretty much everything I've read describes it the same way. And that's the same way it's described for the MDX, which also can power the rear wheels with a 30/70 split. Other systems gear the back end at a different ratio so that "fully locked" means more than a 50/50 split. But there's nothing in the literature telling us that's how Acura does it.
The best I can figure is that somebody else owns the patents for the technology, so they don't get into details.
Those are primarily FWD and then gradually engage the rear axle when needed. They act pretty quickly, an S60 with AWD feels significantly different than a FWD model during hard cornering, even with no slip.
Dana is another well known supplier, but they mostly focus on bigger 4WD trucks.
I find it interesting that Infiniti and Subaru both use the term "VDC" so I wonder if underneath it all they use the same supplier. The G35 is rear-biased and full-time just like the Subaru system.
Japanese manufacturers have strange parterships, they all seem to own a little peice of each other, or at least a subsidiary.
-juice
Borg-Warner used to do the TOD systems for Isuzu (and now for the Kia Sorrento).
Acura had the SLX, I wonder if that was one relationship they established back then and then kept it up?
-juice
Bob
(background: paisan loves Isuzus and hates Fords)
-juice
I have convinced my wife to stay with a Japanese car.
Reliability/Safety, Luxury and looks are important to my wife and I. She is a realtor but not like others; have to have MB or BMW. She drives an Infiniti I30t but breaking down as we speak so I rufuse to go with Infiniti again eventhough G35 Sedan in tempting to get.
Therefore, all we have in our own world; Acura and Lexus.
Lexus is nice but pricy plus can't negociate much. Acura TSX seem a bit small for the real estate signs or clients to sit. TL seems to be a good size but rear seats don't fold but we may not have to fold. We have 7 month infant, so safty is always our concern as well as cargo/trunk space. I narrow it down to 06 TL or wait to see RDX if my current Infiniti last that long. I can get 06 TL w/Navi for under $35K including Tax and lisence. Lexus IS 250 is about the same but w/o Navi. (so no comparison for value)
Any other cars should I consider or TL or RDX are good choices?
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You might try the G35. It's been a while since I've sat in the back of one, but I recall it being fairly spacious.
Back to our regularly scheduled program...
That said, TOD is a "beefier" design, while VTM-4 is more compact and weighs considerably less. They are similar, but not identical.
Comparing SH-AWD and VTM-4, we'll find that both have three sets of clutch packs. One in front of the rear diff and the second and third on either side.
In VTM-4, the front clutch pack controls fore and aft torque distribution. The side ones activate to connect the rear half-shafts so they behave like off-road lockers. There is no acceleration device and torque split is "reactive" to either throttle input, conditions, or driver request (button).
In SH-AWD, the clutch packs are actuated differently and the whole system is governed by an ECU with more input sensors than VTM-4. And, of course, we have the acceleration device and the ability to run a permanent or "proactive" power split. It does not have the ability to be locked like VTM-4.
Best amongst...?? :confuse: My brother has a '98 Accord EXV6, and I don't find its leather of particularly good quality.
Bob
What say you on Acura manuals...??
You mention that your wife is a realtor, meaning that she will have clients spending time in the back seat. I recently drove a Hyundai Azera and it was quiet, had plenty of room (great leg room) and power. If you took the Hyundai name badge off you might think you were in a Lexi or Infiniti. You can pick up a fully loaded Azera for a little over $28K providing the dealer with $1K over invoice. With a 10 year warranty your wife might be more inclined to make this purchase.
The downside of the Azera is that it does not retain its value. The TL that MSRP's at $37 would be worth $21.8K in three years, where as the Azera that MSRP's at $30K would be worth only $12.6. Even if you can purchase the Azera for $5K less, you still are coming up short at the end of three years by at least $2K. The Azera only makes sense if you are going to run it for the full 10 years - you drive it for 4 and then give it to your kids and the warranty sticks.
I'm 6'3" and if your wife is driving me around in the back seat of a TL for more than a couple of miles I am not happy. The Murano and Forrester offer the most leg room at 43.4 and 43.6 which is almost 2" more than the RDX.
I'm not talking about the theoretical ability, I mean the actual feature. With VTM-4, you can press a button on the dash and lock the power split 50/50 fore/aft as well as lock the rear halfshafts together so there is no left/right slippage. So far, SH-AWD does not have this feature. That button does not exist in the RL or RDX.
Somewhere around the turn of the century, Honda started to offer chairs with "leather seating surfaces", which mean leather on the cushion and backrest, but the sides and headrests were pleather. Sometimes, the door accents and such are real leather, as well.
This has been demonstrated effectively with vehicles like the original X5 and M-Class, which used the brakes to modulate torque. They get so busy rerouting torque, none of the wheels gets enough to move the vehicle.
That's why the MDX, Pilot, and Ridgeline have this locking feature. With the driveline "locked up" slipping tires do not leak all the torque. Every wheel gets equal amounts. Even though a wheel doesn't have perfect traction, it may still have some traction. So, you can rooster-tail your way out of trouble.
Whether you'd consider the ability to get unstuck more or less important than the benefits of systems which constantly adjust torque is another issue. But there are advantages to the locking option.