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SUVs 4WD & AWD systems explained
1376 messages, Last post on Nov 23, 2009 at 9:11 AM
You are in the SUVs Forum. Your Hosts are steve_ & tidester
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Replying to: ateixeira (Mar 30, 2009 11:37 am) AWD means there IS A CENTER DIFFERENTIAL, PERIOD. 4WD has transfer case which has two output shafts. One goes to front axle differential and the other to rear axle differential. When both output shafts are coupled to work it means that front and rear axles are turning at the same speed (if tires are designed to be same diameter as usually is the case) and this means that both front and rear tires should be exactly the same diameter under any driving conditions. Well, front and rear tires practically never are the same diameter and that is the problem. Different tire radius on road side of the axle means tire speed against road surface is different. This makes one of the axles pull the car and the other to slow it down. If Honda CR-V has a rotary blade coupling in the drive shaft going to rear axle it means it is a 4WD system with an automatically or manually controlled coupling that engages rear wheels when it is necessary. It is not an AWD system as what AWD is originally meant to be. Arrie
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Replying to: arrie (Mar 31, 2009 4:56 pm) Bob |
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Replying to: arrie (Mar 31, 2009 4:56 pm) 4WD has transfer case What about a hybrid with electric motors at each drive wheel? Now that's real four/all wheel drive, complete with lots of torque at any speed.. And you know they are coming.
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Replying to: steve_ (Mar 31, 2009 7:08 pm) separate electric motor on each wheel would be the best, just like it is already done using hydraulic motors in some earth moving heavy machines. That system would be a really good solution as electric motors waste very little power compared to mechanical gears that lose 1 - 2 % of input power at each gear mesh. This lost power of course is the reason why transmissions get so hot. Problem with electric motors is though that for slow speed motor diameter gets very big if it is connected directly to the wheel hub. This means that to keep electric motor physical size small enough a gear reducer has to be fitted in and the mechanical power loss is back. AWD name probably originally came from All time 4WD as differential between axles makes it a true 4WD but as that name was already in use something else had to me made to differentiate between the two. Arrie
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Replying to: arrie (Apr 01, 2009 8:06 am) There's always a gotcha. 
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Replying to: arrie (Mar 31, 2009 4:56 pm) Now there are many system that do not meet either of your definitions. The RX400H is one. There is no center diff, so it's not AWD. There is no transfer case, so it's not 4WD. What is it, then?  We can no longer adhere to traditional definitions for AWD and 4WD when they just do not fit modern technology. A separate electric motor is used for the rear axle. There is no 2nd driveshaft connecting the rear axle to the front axle, hence no center diff is needed. This is what I would call a "through the road" AWD system. The only link from the front axle to the rear axle is the fact that the tires are on the ground. That's still AWD, though, so your "no center diff" rule fails.
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Replying to: ateixeira (Apr 03, 2009 10:55 am) First of all, I was talking about mechanically drive axles. Secondly, you could think of the electrical power output as a differential between axles, i.e. electric control allows for the speed difference between axles without danger of breaking drive system parts when driving on dry pavement. What power rating does the electric motor for the rear axle have? Arrie
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Replying to: arrie (Apr 03, 2009 5:02 pm) I'm gonna guess the rear axle gets about 40hp or the equivalent nowadays, probably less when the original Highlander Hybrid came out. Torque is better and peaks just off idle, though. Funny thing is it was based on a FWD Highlander, not the AWD model. I saw a cut-away at an auto show and studied it closely, it was just .... odd. |
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Replying to: steve_ (Mar 31, 2009 7:08 pm)
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Replying to: kdshapiro (Apr 19, 2009 12:22 pm) I'm thinking a fun application would be to reroute the brake lines so you could brake the right side wheels while goosing the left side wheels. Be a bit of tire scrub but think of the turning circle.
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