Are 4 wheel drive passenger vehicles wasting precious resources?

[Duffman]

Yeah there would be some benefits to having 3 wheels (less rolling resistance too) but 4 wheels gives so much better stability it no wonder that the car has evolved that way. Also 4 wheels can be an advantage over 3 when off road or in deep snow. The front wheels plow the track for the back wheels to follow.

The numbers I have seen for rear differential were 3-4% so I can accept 5 as a good number but not all of that is lost to changing direction of the rotational torque. A normal gear will loose 1-2% in its operation so the rear diff is only loosing maybe 2% changing the direction of rotation. Thats pretty acceptable for me given the other benefits that 4 wheels brings to stability and safety.

[trebuchet03]

Originally Posted by Frank Lee

In a straight line, diff gears aren't turning or doing anything at all!

I think what you meant is that there's no slip when going straight... That doesn't mean there's no rotation or no motion - there's still [at least] 3 points of gear meshing for a car diff - but the planet gear doesn't have any relative angular velocity as the side gears spin about it. That assumes 50/50 load distribution - which is likely not the case (especially with passengers, cargo and/or slightly uneven tire pressure). As duff said - 1-2% for each [helical] gear interface.

Then add the energy to move oil etc. etc. etc.

LSD can increase efficiency a bit - but not too many cars use them....


All that said - I'll take the loss over tire wear

[Big Dave]

I’d done pickups both ways and unless I get a major bite by the outdoors bug, I wouldn’t have another 4x4. If I lived in upstate NY, I might reconsider.

I had a 83 K2500 and after 305,000 miles I realized I had about 3,000 miles on the transfer case and if I hadn’t done some recreational four-wheeling in Colorado I would not have had more than 500 miles on it.

For a full-sized pickup 4x4 entails a half-ton of machinery with all that entails. You get stuck with a top-heavy poorly-handling vehicle.

Like Frank Lee, don’t get me started on the marketing department.

All this said, there are a handful of situations where 4x4 earns its keep. Deep snow. Here on the frozen steppes the ISP gets by just fine with Crown Vics, excellent tires and good training but when it really snows (once or twice a year) they rely on commandeered 4x4s. Boat ramps. My old GMC saved a guy who got his Caprice’s rear tires on the slimy part of the boat ramp and was sliding into the water.

I keep a foot in both the ecomod and pickup truck camps. Most of the truck guys I view as poseurs. My metric is simple: Is the truck dirty and is the paint scratched. If so, maybe you have a use for 4x4. If not you are a poseur. I’m no more shy there than here about airing my opinions so you can just imagine the reaction. They tell me my truck is “gay.”

I like the little Dutch three-wheel adventure in hydraulics. No doubt in my mind that really big MPG requires tandem seating.

[trebuchet03]

Originally Posted by Frank Lee

We'll see how long your youthful idealism holds up after you've actually had to deal with an idiotic marketing department!

I'll let you know how it goes after I hire one I can only do it myself for so long

[trebuchet03]

Originally Posted by Frank Lee

What I meant was, in a straight line the diff gears aren't turning or doing anything at all... so the losses would be from the final drive gears and bearings. Compare to a shaft drive motorcycle- probably about the same losses and it has no diff.

There are four points of gear mesh in a standard diff. and the only time they turn is during a turn or when one wheel slips. There is no planet gear.

Misinterpreted your earlier post

And for whatever reason, my textbooks call what most call a pinion gear (on a diff) a planet gear (sorry for that confusion).... It calls the input gear (that meshes with the ring gear) the pinion gear (it is, small gear driving a big one). I said at least three because only 1 planet/pinion gear is required - having a second is good for reliability. And technically - including the ring gear mesh - there's 6 mesh points or a diff with 2 planet/pinion gears (each as two meshes with the side gears. /end textbook dump

[tjts1]

The excuse I hear the most around here (bay area) from people that have a large 4wd is something along the lines of "Oh we need the 4wd to take the kids up to Tahoe in the winter."
Yeah, right.

[Duffman]

Originally Posted by trebuchet03

Misinterpreted your earlier post

And for whatever reason, my textbooks call what most call a pinion gear (on a diff) a planet gear (sorry for that confusion).... It calls the input gear (that meshes with the ring gear) the pinion gear (it is, small gear driving a big one). I said at least three because only 1 planet/pinion gear is required - having a second is good for reliability. And technically - including the ring gear mesh - there's 6 mesh points or a diff with 2 planet/pinion gears (each as two meshes with the side gears. /end textbook dump

If I'm not mistaken (took machine design back in 2000), the smaller gear is always the pinion by definition.

[LostCause]

I'm no engineer, but the differential pinions seem to be attached to two different objects in those pictures.

In the first picture, the "planet pinion" is directly attached to the ring gear, causing it to rotate in relation to the "sun gear." While it's not exactly an orbit, it is a rotation. In the second picture, the differential pinion is attached to the housing, which I assume is stationary. Maybe it is just a different implementation of the same thing, with labels adjusted.

Really, this thread is the equivalent of a man on welfare calling a hobo a drain on society...

- LostCause

[DifferentPointofView]

I don't know about you guys. but I definitely only see small cars and rwd trucks/suv's in ditches around here. I live in as rural an area as you can think of. Our roads don't get plowed. In fact we can't even afford real roads. 75 percent of our roads are gravel, and the rest of the roads aren't even paved right. We just dump rocks down when the tar starts bubbling up in the summer. we have a gas station that we got this year... and that's about it, otherwise, even if it's snowing 3 feet outside, you gotta go out of town to get food and other necessities. In the winter, we rely on the 4wd hickeroonies to make large tracks in the road to go through. And I'll tell ya one thing. 2wd in the winter, sucks. And our town, which is about 528 people, is 2 giant hills that come off of the "highway" where truckers normally are coming down. since the highway is state owned, it's the only thing plowed.

You see, the problem is, unless you somehow build enough speed to get to the top of the hill, your screwed. you get about half way, loose traction, and slide back into the highway. 4wd trucks and Jeeps around here just putt up the hill, and some whip out the winches and pull people up it. I remember one time before I had my drivers license when I watched one guy push another guy's car up the hill. I'm pretty sure that made my day. I've found one way to get around the big hills, but you have to turn a 90 degree turn that has a 6 foot ditch at the apex of it, and then climb an even longer, but a lot more shallow hill to the top. luckily that's where the Jeep guys live, and if you get stuck they'll push or pull you up the hill, or pull your ride out of that ditch.

If you live in a similar place, you know what I'm getting at. I'd love to have 4wd, but I'm stuck with 2wd. With the whole ditch thing, you guys haven't been around rednecks for 11 years. people around here that have 4wd can't really go over 60mph anyway cause their tires are so massive and they're lifted in the air so high aero and rolling resistance gets to them. They don't use their trucks and Jeeps on the highway anyway, mostly, they're worst worry is not being able to see past the 2" mud and snorkle intake that has a possum hanging off of it. Probably the main reason I don't see 4wd vehicles stuck in ditches. another reason may be they've been driving on the farm their whole lives.

I don't think that 4wd is necessary 90% of the time either, because you'll always have a guy that has 4wd as a necessity to pull you out or help you up the hill ". I totally agree that 4wd isn't for everyone. What do you need it for anyway? if getting traction to get that last moon pie from the gas station before the other dude in the SUV is what you need it for, you shouldn't need at all. If you live on a farm, work in the muddy construction yard, live in Timbuktu or you have to cross a river to get to your house cause it's been raining too much (happens here a lot) then you might wanna consider 4wd.

[zjrog]

While reading about the driveline losses and advantages/disadvantages of various drive systems, how many of you factor in loss due to wheelspin at speeds above 50 MPH? I realize this is a tiny fraction compared to Land Speed Record vehicles that can lose up to 10% to wheelspin at 150 MPH and higher.