After the car hits around 5-10 MPH it shifts to front wheel drive mode . Depending on the car you might be able to see this happen by looking at the dashboard. My Nissan Rogue can show me which wheels are getting power and at the start it’s 50/50 front and back wheels. I can lock it in 4WD mode if I want which is also true of many AWD cars.
I have a Subaru AWD and I don’t think I have a choice.
Yes Subaru is unique since all their cars have full time AWD that cannot be turned off. Don’t know of any other brand that has that same capability for all their cars
What is the significance of this point? Does it affect performance in some way?
I think it’s done because you might start out in a slippery situation such as sand, water , snow , ice ,etc. I don’t think its going to impact performance
It might impact performance in a small way but I’d wager the reason is that shifting out of AWD mode save wear and tear on components. Going around a corner in AWD mode on clean, dry pavement forces all the wheels to turn at different speeds. The mechanicals are designed to handle this but that doesn’t mean they need to be doing it all the time when there’s no real advantage (slow speeds on clean, dry pavement). Disengaging the center differential accomplishes this.
I do not know about the newer Subaru s, but the 1996 ones will automatically shift to Front Wheel Drive at about 30 MPH.
The way I know this is that one day while away from home, the right front CV joint failed with a mighty CRACK! It really shook the car & created a major grinding sound.
Being used to Willys Jeeps, I figured that if I removed the offending axle shaft, I could limp it home on three wheel drive. After removing the axle it drove fine with no vibration or noise. However, at any speed above 30 MPH, It would no longer get power from the engine. It would just coast until the speed dropped to 25 MPH, at which speed it would get power to the tires.
On the way home, I stopped at the Subaru dealer & was informed that my plan to limp home with three wheel drive would work at speeds below 30 MPH, but not at any higher speed as the rig automatically switched to Front Wheel Drive at about 30 MPH. With the axle removed, the front differential would not put power to just one axle, so no drive.
Once I got it home I stole an axle from my parts car, & all was well again.
Thus, while I cannot select Front Wheel Drive, the car automatically selects it for me at speeds above 30 MPH. This has not created any issues here in the Colorado Rockies. No slips, nor slides.
What does “start out” mean? Does this mean every time I launch from a dead stop? Or just when I fire up the engine and switch from “Park” to “Drive?”
yes it means every time you launch the car from a dead stop at a stoplight , stop sign, right after the car is started, etc.
Assuming this behavior (switching from AWD at launch to FWD post-launch) is as ubiquitous as you claim, I think it’s perhaps not so much about questionable traction, as that can happen anywhere. But when launching, that’s the time when the driver might ask for the maximum possible traction force from the wheels, because the transmission will be in its lowest gear. If the driveline isn’t ready for that (e.g. by being in front-only mode instead of AWD), then that’s the most likely time for slippage. This situation would be especially true for cars with more powerful engines.
I believe it also helps with fuel efficiency.
yes. sending power to all four wheels all the time increases driveline losses. so most “AWD” systems out there are reactive; the engine typically sends all of the power to the “primary” drive wheels, and if one or both lose traction the AWD system will start sending power to the other wheels.
As of now Subaru has 1 vehicle without AWD , the BRZ model, it is rear wheel drive. All the rest are AWD.
GM AWD vehicles are now selectable AWD. The cars default to FWD upon start and require a rotation of a control knob on the console to engage AWD. Saves gas and wear and tear on the rear diff for 95% of your normal driving.
In Automatic mode, my Expedition will send power to all four wheels during acelleration, at least during the times I’m actually checking it. In general, for maintaining existing speed, power only goes to the rears.
Of course I can select rear-only, or true 4H and 4L.
My Rogue Sport’s (Qashqai in the rest of the world) AWD monitor screen usually does *not *show 50/50 drive at takeoff. If merely pulling out of a parking spot it does show a little light line in rear wheels bar graph indicating that the AWD is engaged but barely that. If needing to accelerate from a stop to catch up to or get out of the way of traffic, or I’m starting to go uphill and hit the throttle, then it *will *signal significant power to the rear wheels, varying depending on the power demand and sometiems oscillating until I rev down.
So I believe Machine Elf’s explanation is probably likeliest here, as those are the times where there would be high demand on a low gear, or when you’d downshift in motion except this is a CVT – of course in this case it would involve a laughably liberal definition of “powerful engine”, it would be more a matter of getting the best out of a light-duty drivetrain without beating it too hard. This like many such Primary-Front AWDs can be manually locked *into *AWD for bad terrain conditions, but not the other way around.