I have an F-150 4X4 truck. I can select rear-wheel drive. I can select four-wheel drive. But I can’t select front-wheel drive. This perplexes me, since I would think front-wheel drive would be the best mode for normal street driving (and when not carrying or towing a heavy a load). Especially when the road is a little bit slippery due to rain or light snow.
I suppose I could just put it in 4WD when the road is a little bit slippery. But I’ve heard this isn’t good on the drivetrain for extended periods of time.
Couldn’t a simple modification be made to the transmission or transfer case to allow for front-wheel drive (in addition to rear-wheel and 4WD)? If so, why don’t they do it?
I think the basic reason is that the power goes to the rear of the truck first.
The way the transfer case is designed, the front wheel power is extracted from the shaft that drives the rear wheels (through the rear differential, of course). So, there’s no way to disconnect the rear wheels, without installing an additional gearbox or clutch.
It’s definitely possible. The NP205 and 203 can be twin sticked to allow for independent operation/engagement of each axle. Though that might prove problematic on a production vehicle.
Heh, rebuilt an NP205 twice. If I remember, there are 157 needles bearings/rollers in it. This should not be done in your dorm room. The smell of 90wt gear oil does not go away.
it’s that way because the transfer case has no means to disconnect the driveshaft going to the rear axle from the transmission. here’s a pic of the guts of a transfer case. on the right side is the part which connects to the transmission and rear driveshaft. there is always a firm, permanent mechanical connection from the transmission output shaft and the rear driveshaft. when you engage 4WD, a dog clutch moves and allows the sprocket to begin applying power to the front driveshaft via that chain.
it isn’t so much about extended periods of time, it’s that you don’t want to put it in 4WD on dry pavement. the “gotcha” in part-time systems is that they don’t have any way to allow the front and rear axles to spin at different speeds, they’re mechanically locked together. so when you try to turn around a corner, all four wheels are trying to turn at different speeds, and you stand a chance of binding stuff up or even breaking it because the dry pavement won’t let any of the wheels slip.
full-time or “all wheel drive” systems incorporate something like a viscous coupling or center differential on the front driveshaft to allow the axles to turn at different speeds.
It wouldn’t be a simple modification, it would be a different transfer case design. Which does exist, as K2500 mentions. But they don’t do it because nobody is really asking for it.
I think most 4WD vehicles sold globally now are based on FWD vehicles - in that case, wouldn’t drive be transferred from the front to the rear (i.e. transfer case at the front)?
Hmmm. It looks like the Toyota RAV4 is now the best-selling 4WD in the world (and #7 overall!) and it’s based on the FWD MC floorplan, so I’m guessing it has the transfer case at the front.
Since the question has already been answer to GQ standards, I’ll add that in many ways light day trucks are a victim of their own success. If you put a bunch of stuff in the back of a pick-up, the front wheels aren’t going to have any traction. The idea that the front wheels would therefore be the default drive wheels is counter to the whole concept of having a truck bed. But trucks became so good that people starting using them as cars, and now you have owners wondering why they’re not actually cars
They didn’t become good. They just became popular. Body-on-frame pickups make bad cars. They ride, handle, stop and go worse than an equivalent-sized unibody vehicle, and they typically ride much higher than necessary.
Meh, I’m not going to argue that there’s not drawbacks but I maintain that they’re good. The gap has narrowed considerably, and consumers responded. You can daily drive an F150 to your office job in reasonable comfort. 30 years ago only masochists would have tried that.
