This article falsely assumes certain differential setups based on 4WD or AWD.
4WD vs. AWD. What's the Difference?
141–150 of 209 posts
Re: 4WD vs. AWD. What's the Difference?
#1424WD/AWD are almost never necessary in USA and Europe today, to the extent that, if you buy 4WD/AWD you're likely suboptimizing and even taking a risk (for your drivers who aren't familiar with the quirks of 4WD/AWD). As Saab has shown for decades, FWD will get you almost almost anywhere a 4WD/AWD will in bad weather. But sales of 4WD and AWD cars have been a boondoggle for the auto manufacturers. Cost, complexity and…
This is absolutely false. I live in Utah where we regularly get snow measuring feet deep. Driving up the canyon to go skiing, or doing anything in the state during winter or in all the wilderness, it is required by law to have 4WD. All over the west there are roads that are impassable without 4WD. I use it all the time.
Odd, here in Washington, 4WD/AWD get exemptionss[0]. However, my dad, who drives a 4WD, bought chains because he was told 4WD do not get an exemption, so there seems to be some confusion. The WSDOT site clearly says both are exempt though.
Re: 4WD vs. AWD. What's the Difference?
#143Here are the key takeaways for me: "Thanks to that differential between your axles, an AWD car will send your engine’s power down the path of least resistance—the wheel with the least grip. Where a two-wheel drive car can only choose between two wheels, an AWD system looks for that least resistance across all four wheels." "4WD works by locking the front and rear axles together, splitting torque 50:50 between them. T…
This isn't always true either. 4WD can mean that you send equal amounts of power to each axle, but at that point there are things that can change the amount of power going to each wheel within the center differential casing. If you have a limited slip differential (why some 4WD vehicles can drive perfectly fine on pavement) it'll basically send a 50/50 split (lock the axles together when going straight) but allow the…
I once had a Jeep Grand Cherokee that had a "full time 4WD" system. Basically you could choose between 4WD low (traditional 4WD with front and rear diffs locked to each other, in a low gearing for massive torque), 4WD high (traditional 4WD as above, in normal gearing), 2WD (front diff unlocked and spinning freely as if it were a regular 2WD car) and full time 4WD (front wheels would automatically lock together only if the system detected slippage in the rear, and would receive about 35% of the engine's total power output).
The funny thing was, even though you were supposedly able to drive in full time 4WD on dry pavement, it would sometimes engage the front diff in parking lots and when taking tight turns at low speed, causing the front wheels to hop. I ended up leaving it in 2WD unless I was on a dirt road or it was raining, figuring full time 4WD was a marketing gimmick to compete with Subarus.
Re: 4WD vs. AWD. What's the Difference?
#144It'll be interesting to see what happens when off-road vehicles start becoming electrified. Separate front/rear motors eliminate the need for that middle differential or transfer case, and if we eventually transition to a separate motor for each wheel, we'll be rid of all of the differentials and have much better ground clearance as well as better traction. For now, I suppose electric trucks/jeeps are more affected b…
The army gets interested in electric vehicles for troop transport because of their range. It happens that 1. A hybrid can basically provide the same range, 2. A hybrid can regenerate energy when the driver slows down, providing much better range, and 3. They can approach a target in silence for the last 15 minutes, thanks to the electric motor.
The drawback is, they struggle with on-field repairs.
Re: 4WD vs. AWD. What's the Difference?
#145There are lots of variations on this theme. There are off-road differentials which limit the speed difference between the output shafts to a fixed maximum ratio, usually in the 2:1 to 4:1 range. It's possible to do some or all of this in software. Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Tesla's all wheel drive system has a separate motor for the front and rear wheels…
"It's possible to do some or all of this in software. Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Tesla's all wheel drive system has a separate motor for the front and rear wheels. Power distribution between the two during acceleration is mostly equal, but once speed stabilizes, the rear powertrain takes most of the load. Off-road slip handling doesn't seem to be well do…
> I don't think about this a lot, but I assume that all of the interesting ways to distribute power from a single engine to 1-4 wheels are, all of them, inferior to having a dedicated motor for each.
While off-roading it's not uncommon to completely loose traction on two wheels (search for cross-axle articulation for a way to mitigate this). I've also lost traction on three wheels during steep hill climbs, rutted, muddy traverses, and on sand.
In situations like that locked differentials are vastly preferred as you can transfer the majority of the car's torque to 1-2 wheels. A motor for each wheel (or each set of wheels) would not be preferred as 1/4 or 1/2 of the vehicle's torque is often not enough to maintain forward momentum.
The challenge is that the axles of many stock vehicles are not designed to withstand all that torque, so breaking your vehicle's axle when all the torque is transferred to one wheel is a real risk.
Re: 4WD vs. AWD. What's the Difference?
#146Earlier quoted context omitted.
"It's possible to do some or all of this in software. Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Tesla's all wheel drive system has a separate motor for the front and rear wheels. Power distribution between the two during acceleration is mostly equal, but once speed stabilizes, the rear powertrain takes most of the load. Off-road slip handling doesn't seem to be well do…
One more thing to think about, on this line of questioning, is efficiency in "normal" driving situations. If you're driving in a straight line, with good traction on all wheels, will a single larger motor be more efficient than two smaller ones on each wheel? For many people, I think that's 99% of their driving, so it makes sense to optimize. Especially on electric vehicles, where increasing battery capacity is much…
One motor per wheel, in the wheel, is an old idea. It's been used most notably on LeTourneau heavy equipment. The usual problem is too much unspring weight, not a problem for giant earthmovers but bad for fast cars.
Michelin was pushing it for cars, with their "Active Wheel" concept, from about 2003 to 2012.[1] That seems to have disappeared. Siemens has demoed a motor-in-wheel unit, and Volvo and Nissan have fooled around with this. Protean, in Shanghai, is trying to sell their wheel motor. There are others. Nobody has shipped production cars yet, though.
Re: 4WD vs. AWD. What's the Difference?
#147There are lots of variations on this theme. There are off-road differentials which limit the speed difference between the output shafts to a fixed maximum ratio, usually in the 2:1 to 4:1 range. It's possible to do some or all of this in software. Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Tesla's all wheel drive system has a separate motor for the front and rear wheels…
"It's possible to do some or all of this in software. Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Tesla's all wheel drive system has a separate motor for the front and rear wheels. Power distribution between the two during acceleration is mostly equal, but once speed stabilizes, the rear powertrain takes most of the load. Off-road slip handling doesn't seem to be well do…
> If you have a motor for each wheel, the max output on that wheel is the max output of that motor, and theoretically, you can distribute more than 1/4 of the single engine to that wheel with differentials, so ... maybe that's a very big deal ? Do 4WD vehicles often send 70-80-90% of output to one wheel ?
The situation that leaps to mind would be an especially muddy, low-speed, off-road terrain where the differential is keeping three wheels from spinning. The neat thing about that situation is that, in spite of having at most 1/4 of the vehicle's power available, electric motors are still probably a big win, even if you're towing something.
That's because with electric motors you've got almost all the motor's torque available at low RPM. With a gasoline or diesel you've got to get RPM up a bit before getting the power you need. Getting the vehicle moving and balancing RPM and wheelspin and everything is a goofy exercise that would be seem to be made a lot easier by just being able to just gradually bring up the throttle, which you can't always do with a ICE motor.
And more to the point, 1/4 of the total available torque is still more than you're likely to get with even a good diesel and a good automatic transmission that deals with everything gracefully at low-speed, low-RPM. Maybe I'm wrong about that last part: all the off-roading I did was in a primitive Jeep with a manual transmission.
Re: 4WD vs. AWD. What's the Difference?
#148Earlier quoted context omitted.
Something I've learned is that snow tires make a big difference during the winter, and I've read that they make a bigger difference than AWD.
Have been a driver for only 2 years but I can confirm that. I live in Eastern Europe where we tend to have at least one or two snow storms per winter, and, indeed, having winter tires and ABS makes a ton of difference, even on a 1.4-liter FWD vehicle like mine. And, even more important, you have to keep a level head when you see that there's snow on the road and not try to do anything crazy, like making sudden left o…
Bah, you can still see the road in that video. :-)Not to one-up you, but rather to point out what's possible, your video is a not-uncommon regular winter storm in the midwestern U. S. I grew up driving 2WD RWD vehicles in that stuff, usually got home just fine. I did land my parent's full-sized van in the ditch once. Should have put 25kg bags of salt in the back end to add weight, but might have used said for the water softener, don't recall.
So you don't need 4WD (let alone a tank) if you have some sense. Which is why a lot of people buy 4WD vehicles, I guess.
Re: 4WD vs. AWD. What's the Difference?
#149Earlier quoted context omitted.
This isn't always true either. 4WD can mean that you send equal amounts of power to each axle, but at that point there are things that can change the amount of power going to each wheel within the center differential casing. If you have a limited slip differential (why some 4WD vehicles can drive perfectly fine on pavement) it'll basically send a 50/50 split (lock the axles together when going straight) but allow the…
I once had a Jeep Grand Cherokee that had a "full time 4WD" system. Basically you could choose between 4WD low (traditional 4WD with front and rear diffs locked to each other, in a low gearing for massive torque), 4WD high (traditional 4WD as above, in normal gearing), 2WD (front diff unlocked and spinning freely as if it were a regular 2WD car) and full time 4WD (front wheels would automatically lock together only i…
sounds like maybe the locking clutch pads were malfunctioning (pretty sure they used clutch packs vs. gear type). the hopping is never fun.
Re: 4WD vs. AWD. What's the Difference?
#150Here are the key takeaways for me: "Thanks to that differential between your axles, an AWD car will send your engine’s power down the path of least resistance—the wheel with the least grip. Where a two-wheel drive car can only choose between two wheels, an AWD system looks for that least resistance across all four wheels." "4WD works by locking the front and rear axles together, splitting torque 50:50 between them. T…
And in the next paragraph "To counteract this [sending of power to the wheel of least resistance] , the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel. " So, a well engineered AWD system actually sends power away from the slipping wheel(s) and towards the wheels with traction, which makes m…
A well engineered differential [0].