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CR-V::Performance Pursuits What we do to our CR-V in the name of performance.

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Old 02-02-2004, 08:44 AM
wakehamc wakehamc is offline
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Default How does 4wd work on CRV

Ive brought a second hand CRV and thought it was a two wheel drive until i looked underneath to see it has a rear diff, How does the CRV engage the Rear wheel drive, does it do this auto, as i dont have any option to select it
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Old 02-02-2004, 08:47 AM
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http://www.hondasuv.com/crv/viewtopic.php?t=179
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Old 02-03-2004, 08:43 AM
Varmint Varmint is offline
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Default How much do you want to know?

History - The CR-V is not the first Honda to use the moniker Real Time 4 Wheel Drive (RT4WD) to describe it's AWD system. From 1987 through 1991, Honda offered a Civic Wagon with RT4WD. However, that car used a slightly different system. The RT4WD in the Civic functioned in a similar manner, but it used a viscous coupling to mate the engine to the rear wheels. The newer design in the CR-V uses a multi-plate clutch pack to accomplish the same goal.

RT4WD Operation - I'll try to answer quite a few common questions in this first section. Bear with me as I ramble a bit.

The AWD system in the CR-V is probably best described as an automatically engaging four wheel drive system. For the most part, it routes power from the front to the back whenever a wheel loses traction. There is no side to side distribution of power. As soon as traction is restored, the system disengages and the vehicle returns to a purely FWD mode. The amount of time it takes to engage is minimal. I've watched it on slow-motion video and in the real world. It takes little more than a quarter turn of the tire for RT4WD to engage the rear wheels. The more slippage you have, the more power it sends to the other wheels. RT4WD will work when driving in reverse. There is no indicator light to show when the rear wheels have been engaged. There is no way to lock the system as with Honda's other AWD system, VTM-4. If you were to force RT4WD to work all the time you'd break it or damage the entire drivetrain. The AWD CR-V may be towed "flat" as might be done behind a motor home. See the owner's manual for instructions. RT4WD will disengage when the brakes are applied. This allows the ABS to brake the front and rear wheels at different speeds as is sometimes necessary.

Technical Description - The RT4WD system is comprised of five basic components. There is a transfer case attached to the transmission, a propeller shaft running from the front to the rear, a dual pump and clutch system at the end of that shaft, a rear differential between the rear wheels, and two half-shafts which direct power to the rear wheels.

Little is known about the transfer case up in the front of the CR-V. Most of Honda's documentation focuses on the pump system built into the rear differential. What we do know is that it allows power to be routed from the transmission to the rear wheels through the propeller shaft. Think of it as the first step on the road to powering the rear wheels.

The second step is the propeller shaft that stretches from the transfer case to the rear differential. This propeller shaft is constantly spinning. It never stops. Well... unless you put the car in park and shut it off. Even though the shaft is constantly spinning, it does not send power anywhere. The connection at the transfer case spins the propeller shaft, but the propeller shaft isn't turning anything at the other end. Simply put, the shaft isn't long enough to reach the rear differential without help.

Step three for this distribution of power is where things get interesting. The pump and clutch system ahead of the rear differential makes the decision as to whether or not the differential deserves to get any of the power that the prop shaft is constantly offering. I wrote above that the propeller shaft isn't long enough to reach the rear differential. Well, the pump system allows the shaft to "grow" just enough to reach and make the connection. Here's how the pumps make the decision to send power, and how the clutches make the necessary connection.

The differential housing contains two hydraulic pumps and two wet clutch packs. Both are positioned just ahead of the differential. The propeller shaft powers one of the hydraulic fluid pumps. The rear wheels power the other hydraulic pump via the half-shafts and differential. Both pumps are responsible for circulating hydraulic fluid between one another. As one pump pushes, the other pulls. It's sort of a ying and yang thing. By the way, this is the same fluid that needs to be changed every now and then (see the earlier article). When the propeller shaft and the rear wheels are spinning at the same rate, the pumps are pumping at the same rate. Under those conditions, the pressure of the fluid is stable. If either the prop shaft or the rear wheels start spinning at a different rate, the fluid builds pressure within the pump system. This pressure pushes open a valve in the system and fees that pressure to the two multi-plate, wet clutch packs. This pushes to the two clutch packs together and they form a handshake between propeller shaft and the rear differential. The propeller shaft has "grown" long enough to reach the differential.

All our ducks are lined up, so let's recap for a second. Now we have power running from the transmission, through the transfer case, and along the propeller shaft. The propeller shaft is now linked to the rear differential via the connection made by the two clutch packs.

So what does the differential do? The differential is step four. It routes the power down each of the half-shafts to the rear wheels. It's a big Y joint. Normally, the differential is just spinning freely. It is powered only by the rear wheels, which are, in effect, being dragged by the front of the vehicle. When the clutch packs connect, the engine becomes the source of power. Now the engine is driving both the front wheels and the rear wheels, through this long series of mechanical connections. Welcome to four wheel drive.

All this happens very quickly. As I wrote above, the connection and power distribution process can be completed in the time it takes for a wheel to make one quarter rotation. The system also disengages just as fast. As soon as the clutch packs make their connection, the propeller shaft and rear differential start working in unison. That restores equality between the two pumps, which means no more pressure, which means nothing to keep the clutch packs engaged and they disconnect.

While driving on slippery conditions in the real world, it is likely the system will be cycling back and forth between engaged and disengaged frequently. The process is pretty much seamless. Ordinarily, people cannot feel the power transfer happening. Because the difference in rotational speeds controls the pressure in the system, it also controls how much power is sent to the rear wheels. If there is a big difference in pumps speeds, more power is sent rearward. If there is only a slight difference in speed, then only a little pressure is applied to the clutch packs and only a little power is sent through the rear differential. If you were to slam your foot down on the accelerator and suddenly create a good deal of wheel spin, the system will slam the rear differential with power from the engine. The sudden onslaught of torque at the rear wheels may cause them to break traction and send you spinning. That is one reason why experienced CR-Vers will tell you to drive your CR-V like it is front wheel drive and trust that the system is working.

Another characteristic of the design is its ability to overheat. As the clutch packs engage over and over again, they build up heat. If this generates enough heat, the system opens a venting valve and shuts down to cool off. The CR-V will still function in FWD mode. RT4WD resumes as soon as the heat is dissipated. This does not damage the system. The process prevents damage. Furthermore, in the five years I've been reading about the CR-V, I've only heard of two or three reports describing this. One was during testing by a magazine, so you can bet they'd been pushing the vehicle to test its limits. Personally, I have driven for hours on snowmobile trails without overheating the RT4WD unit.

Honda claims that the RT4WD system has several advantages over full-time AWD or a traditional 4X4 system. They claim that RT4WD allows the vehicle to achieve higher fuel economy, generate less vibration or noise, reduce weight, and offers less difficulty adapting it for use with ABS. Is that just marketing hype? Yes and no. The truth is, there is no real way to compare most of these because there are many factors that add up to the end results. For example, we cannot compare NVH levels because the vehicle's tires, sound insulation, and chassis rigidity also play a part in the overall NVH of the car. We all know that the CR-V is not the best equipped vehicle when it comes to sound insulation. That said, there are a few ways we can compare at least two of these claims.

We'll start with weight. MotorTrend once published that the RT4WD system weighed only 15 lbs. I think they meant the transfer case mounted on the transmission. Comparing the 2WD LX with the 4WD LX reveals a more realistic difference of 117 lbs. Doing the same with the first generation CR-V shows a difference of 110 lbs. These figures include the transfer case, the propeller shaft, the rear differential, and the half shafts that deliver the power to the wheels.

If we compare the CR-Vs competition in the same manner, we find that the Escape's AWD system weighs about 165 lbs. The VUE's is between 121 and 152 lbs (depending on whether we compare the I4 CVT models or the V6 5AT models). Both of those designs are reactive systems, like RT4WD. Looking at full-time AWD systems, we find more significant differences. AWD adds 222 lbs to the RAV4 and between 209 and 222 lbs to the Santa Fe. I cannot compare the Forester as they do not offer a FWD model. These numbers do not necessarily reflect the exact weight of the AWD unit as they may include additional hardware like a limited slip differential. Still, it appears the CR-V leads the pack and Honda speaks the truth.

On fuel economy, we cannot compare vehicle to vehicle. All of them have different engines, curb weights, and aerodynamics. But if we take each AWD model and a similar FWD model of the same car, we can make a general comparison. It turns out that the CR-V, Escape, and VUE (the reactive systems) all lose 1 mpg in city driving and 2 mpg on the EPA highway cycle. This amounts to an average loss of 1.5 mpg with the addition of AWD. The RAV4 and Santa Fe both average a loss of 2 mpg. Depending on whether you drive more highway or city miles, you may see no difference at all or as much as 5%. For reference, 8% is about the same difference manufacturers expect to get from designs like GM's "displacement on demand" engines, or the 7-10% gain for a variable valve system like Honda's VTEC.

It appears that reactive systems are generally more fuel efficient than full-time systems, but the RT4WD system shows no advantage over the other reactive AWD designs. Furthermore, we don't know if this advantage is a result of the reactive nature of the design, or simply the fact that the hardware adds less weight than full-time AWD.
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Old 02-03-2004, 09:06 AM
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Amen. This is The FAQ article, I hope it finds its way to the relevant section.
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Old 02-03-2004, 10:06 AM
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Can we bring back the Post Of The Day award? Now there's one hard-working Varmint!
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Old 02-03-2004, 01:16 PM
N_Jay N_Jay is offline
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Default Re: How much do you want to know?

Quote:
Originally Posted by Varmint
. . . Little is known about the transfer case up in the front of the CR-V. Most of Honda's documentation focuses on the pump system built into the rear differential. What we do know is that it allows power to be routed from the transmission to the rear wheels through the propeller shaft. Think of it as the first step on the road to powering the rear wheels.
. . . . .
Great article except for this one paragraph.

Anyone with access to the service manual can know all they want about the transfer case.

Yopu make a few gears sound way to misterious! :wink:
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Old 02-03-2004, 03:10 PM
Varmint Varmint is offline
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Vince - It's amazing what you can do with cut and paste. :-)

N_jay - It was written with non-tech heads in mind. That said... Post a simple description of the transfer case and I'll see about getting all of it added to the articles section.
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Old 02-04-2004, 06:12 AM
wakehamc wakehamc is offline
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Well thats cleared that up alright. Thanks for the essay, do you know anything on how a woman thinks
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Old 02-04-2004, 06:25 AM
jackson jackson is offline
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Hi,

I may undertand how it works, but I really could not know whether the 4WD in my V is still functioning or not. How could I detect the RT4WD is still working properly in the town ?

Cheers,
Jackson
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Old 02-04-2004, 08:18 AM
N_Jay N_Jay is offline
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Quote:
Originally Posted by jackson
Hi,

I may undertand how it works, but I really could not know whether the 4WD in my V is still functioning or not. How could I detect the RT4WD is still working properly in the town ?

Cheers,
Jackson
Get on a soft surface like sand or snow and give it enough gas to spin a wheel (or two).
Go back and look at the tire marks.
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