I have a newer Mitsubishi Montero, and an older Jeep Grand Cherokee. My brother has a GMC Jimmy (IIRC), and my father has a Chevy Suburban.
All of these vehicles are RWD “by default”, and have a 4WD shifter. There are a variety of 4WD options in them. With the popularity of front-wheel-drive vehicles, I got to wondering:
Why are they rear-wheel instead of front-wheel drive by default?
Why is there no FWD option on the shifter?
I’m guessing the second one is too much $$, not enough demand, but thought I’d ask.
It’s because 4 wheel drive vehicles don’t have a front differential. That is, when going around turns, both front tires will spin at the same rate, causing a very uncomfortable ride. This isn’t much of an issue when towing, because you’re usually going slow and taking wide turns. It’s also not much of an issue on snow and ice because the sliding of the tires compensates for the difference in rotation.
To add a front differential would be expensive. Vehicles like Subarus are all-time all-wheel drive and have the front differential. There’s no reason to make “off” an option for those types of cars.
RWD is mechanically simpler powertrain set-up in a vehicle. Mount the engine and transmission longitundinally in the car, extend an axle to the rear wheels, and you’re done. To add four-wheel drive, a transfer case is installed to split the engine power between the front and rear drive axles. (Although, 2WD->4WD conversion is not as simple as I’ve just described.)
To make such a vehicle front-wheel drive, you’d have to substantially beef up the front axle and transfer case to take all the torque generated by the engine. You’d lose a tremendous amount of power, as Keweenaw said, by essentially re-directing the power around 180 degrees to the front axle. Adapting one’s driving technique from a front-wheel drive to rear-wheel drive vehicle is much less difficult, even in slippery conditions.
What happens is that in 4 wheel drive mode, a center differential is locked (between front and rear axles). When going around a corner, the front of the vehicle travles a different distance than the rear. This is what causes the wheel skip.
They aren’t RWD by “default,” they’re RWD by design. Applying power to the front wheels is an "add-on,’’ so to speak.
FWD is a popular design because it allows a significant space and weight savings. And in vehicles designed to be FWD, “adding on” drive power to the rear wheels (almost always, if not always, in an AWD set-up) offers advantages similar to having 4WD or AWD in a RWD-by-design vehicle. But in a vehicle designed to be RWD, there is really no advantage nor point in having a FWD-only option. FWD is not inherently better than RWD, in fact many cars are designed to be RWD (e.g. Mercedes, Volvo, most race cars) because RWD is considered the better overall approach. FWD exists mainly due to desire for smaller, lighter cars, largely for fuel mileage reasons. In the vehicles you’re talking about, there’s essentially nothing to be gained by a FWD-only option. Considering the trouble and expense that would be involved, it would be counterproductive to offer it.
I don’t know of any four/front drive vehicle that doesn’t have a front differential but that doesn’t mean they must have one to drive the axle. The front axle is driven by a pinion and ring gear. The differential connects the ring gear to the axle shafts but no reason a ring gear can’t drive a single axle to both wheels.
Of course having a front diff would be foolish if the vehicle was ever driven on a good traction surface like dry pavement as the wheels would chatter and skid whenever turning. I’ve seen some 4WD vehicles where the rear and center differentials could be locked and I’m sure some can even lock the front for severe conditions.
The reason there’s no front-wheel-only drive option on 4wd vehicles is because the front axle drive system suffers more from friction and wears more with use. SO if you don’t need 4-wd, it makes sense to just use the rears.
Also–most relatively-modern 4wd vehicles most certainly do have front differentials, and by “relatively modern” I mean any of the big-3 US companies, from at least the 1970’s. Most newer ones have limited-slip rear ends, open or limited centers and open fronts. It is true that older 4wd vehicles may “chatter” if you switch them into 4WD and then drive around corners on pavement, but these vehicles are very old. These older vehicles are not supposed to be operated in 4-WD at all on pavement–only on loose ground or other low-traction situations.
There is also an accessory called an “air locker” that some 4-WD enthusiasts install on purpose–which allows running open or limited-slip differentials normally but locks them together when you want. These can be installed on the front , rear and center diff. Not a lot of people outside of rock-crawlers bother with them though. Viscous limited-slip differentials used to be very expensive–I don’t know why exactly–but most current manufacture vehicles would probably soon suffer damage if you installed air-lockers anyway. You’d really want at least 3/4-ton running gear to mess with these kinds of things.
And for the record, I had a 1979 Dodge Ram for a while, and once off-roading I bent the rear driveshaft by striking it on a rock in the ground. I couldn’t afford to fix it for a few weeks, so I just took it off and switched the transfer case into 4-WD, giving me… front-wheel-drive. It worked fine–except for not having 4-wd, the truck ran just like normal. (mine had an open front-end)
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You shouldn’t run a brand new trucks in 4HI on dry pavement either.
With the 4HI button pressed, the transfer case has locked together the speeds of the front and rear driveshafts. When you go around a corner the front tires will speed up go faster than the rear. THe T-case dosen’t let the front and rear propshafts spin at different speeds, so there is a lot of torque on the t-case chain itself. Eventually the tires will overcome the friction and you’ll hear the scrubbing noise. It shouldn’t immediately cause damage, but its wearing out your driveline a lot faster than it should.
I got a 96 Ford Explorer and looked it up, and we’re both right, sort of. You may be more right than me: there’s three switch positions on this vehicle: 2WD, 4HI and 4LO.
The owner’s manual only says not to use 4LO on dry pavement.
The supplemental manual “4-Wheeling with Ford” says not to use either 4WD setting on dry pavement, but explains that 4HI supplies partial power to the front wheels, where 4LO supplies full power to the front wheels.
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But I know I have had 4HI on and driven onto pavement, and [probably] turned corners, and I never heard the front-wheel chattering sound. I have driven old IH trucks that did it easily, loudly, obviously (IH’s being basically 55MPH tractors) --so I had assumed the center diff in the Ford is a limited-slip that locks, and is only partially-locked up in 4HI. I don’t think I have ever used 4LO for anything.
Not that it matters much though, no real reason to use 4WD on pavement anyway…
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Well, that’s not really what it means despite what they say. 4HI just switches from RWD to 4WD, that is the transfer case merely engages the front axle.
4LO means that the transfer case also engages a lower final drive ratio to both the front & rear. Similar to the difference between fourth gear and first gear, 4LO trades speed for much, much greater power allowing you to climb steep hills or slog thru mud.
This is why you can switch between 2WD and 4HI while driving but you must stop to engage 4LO. You in fact are disengaging the transfer case completely (shifting it to N) then engaging the lower gear ratio (shifting it to 4LO).
Having a low range is the primary difference between a true 4WD off-road vehicle (like a Blazer, Bronco, Jeep, Pathfinder etc.) and just an AWD (all-wheel-drive) SUV.
Mercedes G-Wagens, for one, have both front and rear diff locks (at least the old ones I’ve driven.) Plus there’s plenty of aftermarket diff locks you can install on your Toyota Landcruiser or Landrover or whatever. They are phenomenal for extreme situations and I personally would not want to drive a 4x4 without differential lockers. They just give you so much more flexibility in low and mixed-traction situations. The way differentials work, they send more power to the wheel with least resistance (it’s a purely mechanical process. If you’ve ever seen the guts of a diff, it’s pretty ingenious in its simplicity). When you have one wheel on, say, snow and the other on solid ground, you will normally get tire spin as all the power will be transferred to the spinning wheel. With differential lockers you can send an equal amount of power to both wheels, and thus the wheel on dry ground will turn and allow your vehicle to be driven out of a rut.
An old thread, but a good one. Just some other add-ons.
There is a car (line of cars?) that can drop out of AWD mode to FWD only. Subaru has(had) this feature. By installing a fuse in the FWD slot (not in the normal fusebox), some of them would ‘lock out’ the rear wheels from getting power. I think this was ment for towing and different sized tires, but it does exist.
aother reason that the main choice is 4wd or rwd is that a main advantage of FWD is added traction, at the expense of handling. But for a truck you can put into 4wd mode the traction reason is negated.
Not always. In some Jeep transfer cases, at least, there was a limited-slip differential that allowed normal driving in 4H, and only locked in 4L. I’m pretty sure the higher-end new 4x4s still work that way (possibly with a seperate lock switch, for additional 4H locked/4L unlocked).
The original Jeep Quadratrac (from the '70s, not the totally different one with the same name in newer Jeeps) didn’t even have a 2wd option – it was 4wd limited-slip all the time (high and low), with an additional mechanism to lock the center differental with it in either mode. Fun fact: it used a whale-oil-based fluid (possibly the last machine that did).
My Jeep had the NP229 transfer case, which had 2wd, 4H, and 4L modes, with a “viscous coupling” (somewhat like a torque converter/liquid clutch) that allowed you to run in 4H on dry pavement. A lot of owners, however, didn’t put it in 4H at all, even for the recommended 10 miles a month to keep the lubricant circulated, causing it to dry up and eventually grenade at inopportune times when the second owner did finally put it in 4H after 15 years of nonuse. :mad: