4WD vs. AWD. What's the Difference?
outsideonline.com
outsideonline.com
"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. This provides great traction, but a vehicle locked in 4WD cannot safely be operated on dry pavement because its front and rear axles are forced to rotate at the same speeds."
I remember being scared when I bought my first 4wd vehicle, a '91 Toyota pickup. I knew I shouldn't have the wheels locked in 4wd on dry pavement, and I was worried about going from snowy roads to dry pavement. When I moved on to an Outback (AWD, not 4wd) I wondered why I didn't have to worry about transitioning between slippery roads and dry pavement. Now I understand.
There's also a great video from 1937(!) that shows why we need differentials, and how they work. Here's a direct link to the video if you're going to skip the article:
So, a well engineered AWD system actually sends power away from the slipping wheel(s) and towards the wheels with traction, which makes more sense.
When it goes to the wheels with traction, it helps get the car un-stuck. This is usually what you want when you're doing serious off-roading, stuck in snow, etc... Also, in this scenario you're usually going very slowly/not moving, so the sudden change of direction won't be a problem.
Modern AWD does a bit of both - it'll send power to wheels with traction, but will also compensate by sending power to a wheel on the other side of the car. For example, if there's no traction on your left front tire, it'll send power to your right front tire and left rear tire, and reduce power on your right rear tire to keep you going strait.
At one point I remember there was a workaround mentioned by the Hummer engineers (I believe) to apply the brake simultaneously with the gas if you ran into the "one wheel spinning in the air" problem. This is a simply a manual application of the same physical solution of creating at least some resistance everywhere.
Hummers have torsen diffs, which have a pair of worm gears where a conventional open diff would have single star gear. As a result, the torsen diff sends some fixed multiple (changes with model) of the torque used by the easier to spin side, to the harder to spin side.
Of course if one wheel is spinning in air, the easy side uses effectively 0 torque, and 0 times anything is still 0, hence the brakes.
Applying the brakes increases the torque requirement of all wheels by a fixed amount, but the hard to spin wheels get More than that amount of extra torque.
I had an old Toyota Tercel 4WD station wagon and it was very difficult to steer sharply at low speed with 4WD engaged because of the lack of a differential. It would lurch and I had to press harder of the accelerator to get straightened out and pop it back into 2WD. It was my first car so I didn't know any better. Fun fact: In 4WD, you got an 'extra low' gear below 1st and that car could very nearly climb trees. The gear ratio was insane. No ground clearance but I used to scare the crap out of friends by going up very steep inclines in extra low gear with 4WD engaged. They would have to park their pickup trucks at the bottom of the hill and ride with me, and this caused them some embarrassment. Good times.
One time in college I was turning too sharply while parking and the axle actually popped out of the right front. Loose axle would just spin in 2WD but switching to 4WD enabled me to make it to the nearby Mech E. building and get help. :)
One day, I think, even 80's and 90's Toyotas will be collectors items. I know some of them are collectors vechicles now, but I feel the 80's to 90's vechicles will see a huge boost in interest.
I think you just said the opposite of the guys above you while saying you agree. 4WD should not have a problem on slippery terrain.
An AWD car has _3_ diffs, one in the middle that splits power F/R, and then one at each axle that splits it L/R.
Highend 4wd setups can lock the diffs so all wheels spin at the same speed all the time - essentially on slippery ground, but very counterproductive when you actually have traction.
The term the are talking about when traction is applied to the wheel with the most binding to the ground and away from the one that has the least traction is called a Limited Slip Differential and by virtue of the slipping of the low traction wheel it will spool the high traction wheel and bind it transferring torque to it. This can also be done by what is called a locker, either electronic or manual but manual lockers can be dangerous at high speed if you are not used to their manor. You generally only see manual lockers and welded differentials in offroad trucks. Limited Slip differentials and electronic lockers are available in both 4WD and AWD vehicles.
The term they are talking about to apply power to both the front and rear wheels is not a differential but rather a transfer case and it is not always at a 50:50 ration man times they run 60:40 or 70:30. Both 4WD and AWD have a transfer case, 4WD's allow you to select whether they are engaged or not.
The term they are talking about when they talk about adjusting power front to back is called a Traction Control Unit and is handled by a Body Control Module that may or may not also communicate with the Engine Control Unit or the Transmission Control Unit to defuel or detorque the powertrain, as well as adjust torque via electronic lockers in the differential to the wheels that need it and away from the ones that do not. As well as engage or disengage front and back axles via the transfer case. Many newer AWD systems have this but it is also available in most newer 4WD vehicles.
TLDR is 4WD and AWD can be virtually identical depending on the subsystems but the article confuses a lot of those subsystems as being unique to one or the other. Though the upper end of 4wd's have more "hardcore" options not generally found or offered in AWD vehicles.
4wd without lockers means you can still get stuck (been there) because front and rear each have their own diffs. One wheel in each diff will be spinning and then you need to get a tug from a friend. The rule of thumb is always have 3 points of contact if you don't have a locker -- then one diff can spin while the other pulls you along.
I have a Dodge 2500 4x4 diesel and even with all that torque (about ~600 ft-lbs) it still bucks if I turn too tight even on ice.
I'm guessing it's due to the transfer case my truck doesn't have a center differential.
Here I am in snow no buck I guess no tight turns https://www.youtube.com/watch?v=zdK75UmoRMY
LSD's front and rear makes this worse, but center is the most important when it comes to on-road maneuverability.
Exactly this. The best ones are entirely mechanical, without any fancy computers or even viscous couplings. Audi sold (sells?) IMHO the most sophisticated AWD system on the market, which uses a system of crossed helical gears to continuously and instantaneously redistribute torque proportionally to wheel traction, up to a roughly 80-20 split. There are no computers and the whole thing is mechanical. The differential is a minor mechanical miracle sold under the trade name Torsen, short for "torque sensing." Despite Audi's German-engineering themed marketing, the differential was actually designed by an American.
In the 80's, the station wagons even had a manually-actuated center locker, which was eventually replaced by a servomoter driven locker. I don't know what the new ones have.
However, I don't think you should underestimate electric systems. I mean, I hate car software with a passion, but when they work, they actually have really good characteristics. Things like LSD's where the pump is electrically enabled give quite smooth transitions, and can simulate the response of both on and offroad mechanic systems. Response time is also nearly instant. If car computers weren't bloody black boxes (or at least leave only important things to the black box!), these systems often end up being much simpler than their purely mechanic black voodoo counterparts.
Front axle locker is similar, but with that you feel how difficult it is to steer as soon as you lock it. (In my Wrangler if I'm finding that I need the fronts we are in some serious shit anyway.)
I'd have to find my paperwork but I think such maneuvers are cause to void drivetrain warranty. Chaining the tires is, too, for unrelated reasons.
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.
I think the issue was with the auto locking hubs; when it malfunctioned it would lock the front hubs together and in turn the transfer case would lock the front and rear diffs together, causing the wheel hop. This defect probably contributed to the death of the original transfer case, now that I think about it.
Some of these can be locked manually/physically, but more often these days they are controlled electronically. Of course this defeats any traction control systems, normally leading to a warning light telling you that you are on your own in that regard. Antilocks also often disabled in this mode.
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 documented, but Tesla cars aren't intended for off-roading.
The most advanced systems are seen on locomotives. Modern heavy locomotives (GE Evolution series) use three-phase AC synchronous motors driven by IGBT inverters from power from a Diesel generator. Each axle has its own motor. All axles are normally locked together electronically by the control software, so no wheel can slip ahead of the others. Locomotives can be cabled together so that the axles on multiple locomotives synchronize. Now that's all-wheel drive. This is a huge win when starting a heavy freight train. Locomotive axles that are slipping provide little pulling power and damage the tires and rails.
Is that different than the traction control used by other automakers?
> Tesla's all wheel drive system has a separate motor for the front and rear wheels.
That is very cool. And it sounds much more complex - I wonder if the complexity is worth the benefit.
For AWD, you need a driveshaft and a differential. The differential has a lot of parts, there are a lot of bearings that sap power, and it adds a lot of weight.
Using separate electric motors for front and rear instead increases the available power, enables unlimited torque ratios from all-forward to all-rear, and has many fewer moving parts.
It's not like trying to coordinate rowers in crew; you have full, absolute, instantaneously-responding control over the motors, as well as information on exactly how fast they're going, how much force they're applying, how fast each individual wheel is turning, what the steering angle is, etc. And then what's controlling it is a computer, not a human.
Stability control, which already exists in every car, deals with all of those variables and is able to correct for driving with judicious braking (or release of the brake) on individual wheels, even when some or all may be slipping or locked up, the car going sideways, on varied surfaces.
Adding more things for a computer to control makes it easier, if anything.
"If the power delivered to the wheels is out of sync for a moment, I would imagine it could be catastrophic" The control software is probably updating every 3-15 milliseconds, so even if the software had a massive hiccup, it would have to last for multiple frames for it to actually get translated to the pavement. The drivetrains of cars are actually pretty mushy. Everything's mounted on bushings, there's backlash in the gears, and tires are rubber. All of this adds together to buffer out any kind of spikes. Much like a capacitor and resistor can be used to buffer electricity. So if, say, the front wheel was 5% underpowered for a frame, then all that would happen is the gears in the front differential would go slack for a few milliseconds. If on the next frame, the power was corrected, the differential would go back to being taut. If it was just 5% underpowered, you likely wouldn't even feel this. If the front motor totally locked up for one frame, you'd still probably only feel a little jitter.
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.
That is, I assume that no matter how fancy you made your differentials, two motors is better than one and four motors (one for each wheel) is better than two (all else being equal).
Is that correct ? Or is there some scenario(s) wherein mechanical distribution of torque (with differentials) is superior to (again, all else being equal) a dedicated motor on that wheel ?
...
If I try to answer my own question, all I can come up with is:
1. 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 ?
2. Sending power to a specific wheel via the path of least resistance takes zero time - it's instantaneous - whereas deciding what to do with each of the four wheels (in software, presumably) might have a lag ... although that sort of breaks my "all else being equal" tag, above ...
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 harder than just adding an extra gas can.
Maybe in that case the complexity of AWD/4WD/Traction control makes sense in that case.
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.
hmmm ... I wasn't thinking about motors in the wheel, although I am familiar with that concept.
I was thinking about a more pedestrian motor per wheel configuration wherein the motor is just behind the spring ... and is thus, sprung weight ... is there even space for that ?
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.
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.
You have 4WD if you have 4 driven wheels (and unless you count the spare, that's all my wheels). Then, you can have a multitude of differential configurations on the 3 differentials (open, lsd, locker, diff-less viscous clutch, diff-less locker).
A Jeep WK uses a locking differential center, and electric engaging LSD's front and rear. By that article, the car is AWD on road, and sorta 4WD when off. The Jeep WJ is either controlled by a viscous clutch (Quadra-Drive), or has manually selectable 2WD, 4WD open ("AWD") and 4WD locked ("4WD") modes. A 98 Ford Explorer pulses an electric clutch to go between 2WD and 4WD locked. These cars all behave very differently both on and off road, and they don't fit well in the authors categories.
If you're buying or owning a car that drives all wheels, the only useful information is the diffs. Labels like "4x4", "4WD", "AWD", "Quadra-Trac", and "Quattro" mean nothing.
Most new 4WD's use open diffs with electrically controlled lockers or LSD's, as that gives the best combined on/offroad experience. Permanent LSD's/viscous clutches (Jeep WJ, for example) is a bit more responsive in surprise low-traction conditions, but it eats your tires when you make tight u-turns, as the turn will progressively engage a complete diff lock. Center lock without diff works like a regular 2WD until you lock, but remember that if you have ANY type of locker on ANY axle (not LSD), then engaging in high traction conditions might snap your axels in a very loud and explosive manner, even if you think you're driving straight.
This is a sales pitch video, but it is interesting to illustrate the differences in AWD approaches, with different vehicles on rollers: https://www.youtube.com/watch?v=9cuZYTQLfA0
I spent a lot of time looking at this several years ago when my family was planning on taking a vacation to the Outer Banks of North Carolina where you have to drive on the sand. We ended up renting a Ford Explorer that had a "sand mode" in it's electronically controlled awd setup. Worked great.
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 or right wheel movements. Also try to keep a safe distance from the vehicle in front of you, and, before you venture on a trip outside of the city, just check the weather forecasts: if there's a "red code" storm announced you'd better postpone your journey, only a tank can get you out from something like this: https://www.youtube.com/watch?v=Frf2lV77fl0
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.
I was puzzled by this as I was stopped by a ranger coming out of Waipio Valley on Hawai'i, where there was a sign "4WD only" and my rental car had AWD. I thought they were the same, just different names, so I argued with the ranger that I do in fact have 4WD, which is in my case just called AWD and in other cases 4x4 as well. Well, he was still pretty upset but the "I am a European and we have different cars" finally disarmed him...
The annoyed ranger was probably just tired of pulling stuck tourists out.
However, in the general case throughout most of the US West, "4WD roads" do indeed tend to be as much about having high clearance as about having 4WD per se. It's also just a way of saying that, if you just have a random rental car and aren't used to driving very rough and often narrow/exposed roads, this is probably not a good idea.
The road gains 800 vertical feet (243.84 m) in 0.6 miles (0.9 km) at a 25% average grade, with steeper grades in sections. This is a paved public road but it is open only to 4 wheel drive vehicles. It is the steepest road of its length in the United States.
There's quite a few roads in the UK with 1:4 gradient, and one just down the road from me at 1:3 whose only restriction is no trucks and vans. Going downhill on the motorbike makes it feel like you're falling off! :)
There's an annual charity event that involves the rolling of over 30,000 Jaffas (spherical confectionery-coated chocolate confectionery) down the hill.
The video posted in another comment doesn't look that bad.
I am not sure about the grade, but one of the Bangalore's weekend gateway has some steep sections(a low average grade but some sections are steep).
https://www.youtube.com/watch?v=C0_MyaPzM88
That footage is from a rally but every weekend, fwd cars with 800/1000cc engines and a peak torque of about 70 Nm do it just fine. Most of the cars have 2-5 occupants decreasing the already low torque/kg, and almost the whole climb is done in a bumper to bumper traffic(no momentum for the climb).
https://dc-cdn.s3-ap-southeast-1.amazonaws.com/dc-Cover-nj6b...
The Subaru Outback (AWD) has 8.7" of ground clearance. The Ford Explorer ("Intelligent" 4WD) has a ground clearance of 7.8".
If "Intelligent 4WD" means some kind of differential, then it really is just a marketing term at this point, though I learned about the distinction between 4WD and AWD when I bought a Subaru.
4WD is meant for off road conditions. You get more torque so you'll be able to get over rocks and out of ditches. The downside is you shouldn't use it over 10mph or so.
I can't think of any recent car that is 4WD. It doesn't make sense since you'll probably never be "off road". Cars don't have proper ground clearance.
That's not strictly true, and varies between implementations/transfer case designs. In Jeep Wranglers, for example, which have both high and low gearing options in 4WD, the owner's manual will indicate that you can drive at any speed (on loose surfaces such as sand or dirt) in 4HI and 0–25 MPH in 4LO.
A friend of mine had a old truck that would randomly switch to 4LO on the highway. It's pretty scary driving 60 and having 4LO switch on.
The inside wheel would spin if the outside doesn't slip.
Source? Center locking diffs are more or less standard, rear lockers are options in LR3/4/post 2004 Disco's, Sports and Range Rovers (although standard in some, I think the Supercharged and possibly the SVRs, but I'd need to check) Front lockers are not factory options and I don't ever recall seeing after market lockers for these models advertised.
Defenders, Discos upto 2004, Range Rover classics (and probably P38s) and all series vehicles have the standard after market options for front and rear lockers.
http://www.ebay.co.uk/itm/like/182246825443?lpid=122&chn=ps&...
Center diffs are for the most part (in Land Rovers) locking differentials, though some Discovery IIs had open center diffs and things like Freelanders use a viscous coupling in place of a mechanical diff lock.
Newer models have an electronically controlled locking diff in the gearbox (center) and an optional electronically controlled locking differential at the rear.
But front diffs on modern Land Rovers are open.
Edit: Here is an image showing the locking diffs on an LR3/Discovery 3: http://www.landroversonly.com/forums/attachments/f41/37257d1...
Note the center diff is unlocked (or open) and the rear is locked, there is no option to lock the front, which will always be open.
There are many types of SUVs and trucks with 4wd in the US. I guess in Europe you only have Land Rovers or G-wagons. I don't think GLs or MLs are common in EU but you can get beefed up off road packages in the US.
Open (just gears, sends power the the least resistant path)
Locked (a spindle connects the input directly to the output and all three spin proportionally)
Posi (multiple ways of actuating this, can be viscous clutch, electric or air powered actuation, or mecanical geared torsion style - regardless the point is to keep power at all wheels, some with variable speeds allowing for turns - others act like a spindle)
Most "awd" cars have a open gear in all three diffs. My jeep has a spindle in the front and center and an air locker in the rear (not the stock design). My old BMW 325ix has a viscous clutch in the center and in the back, and open in the front. My dad's audi has open gears all around and uses the brakes to actuate were the power is delivered. At the end of the day tho, its three differentials.
For now, I suppose electric trucks/jeeps are more affected by "range anxiety" than daily-driver sedans, since people tend to imagine themselves driving to some remote location when they buy them. But as technology improves, I would love to see something like a modern reincarnation of a 1960's era CJ-5 (but with better safety features) -- simple, easy to modify, nothing that doesn't need to be there, and reasonably priced.
The drawback is, they struggle with on-field repairs.
Open, bad traction control (cuts throttle, applies a little brake), good traction control (all brake no throttle), the many varieties of LSDs, automatic lockers, manual lockers, Lincoln lockers and spools all have a huge affect on traction.
Lincoln lock the diffs of any AWD crossover, screw with some wiring so it's full time and you'll have something that can out wheel a $50k Jeep but trades off a tiny bit of grocery getting capability.
But sales of 4WD and AWD cars have been a boondoggle for the auto manufacturers. Cost, complexity and maintenance are significantly higher: often you must "fiddle" with them, woe to you should you damage them due to bad judgement. They reduce mileage, increase car weight, and reduce reliability.
I prefer a RWD car for most driving, since it handles better and I'm never climbing bare hills or plunging through swamps, but I was raised on oversteering go-karts.
"All Wheel Drive Does Not Make You Safer": http://www.mrmoneymustache.com/2014/12/01/all-wheel-drive-do...
"The Myth of the All-Powerful All-Wheel Drive:" http://www.popularmechanics.com/cars/a3091/the-myth-of-the-a...
4WD/AWD safety - Is 4WD/AWD safer on snow and ice? http://www.4x4abc.com/jeep101/safe.html
http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm
It doesn't matter what the studies say, 4WD is legally recognized as safer than 2WD in California.
I disagree with your interpretation of the law: dot.ca.gov simply state what they require and say nothing about which is superior. And note that 4WD/FWD vehicles _also_ must carry chains in the R2 & R3 areas (from the URL you posted http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm):
"(NOTE: Four wheel/all wheel drive vehicles must carry traction devices in chain control areas.)"
Unfortunately for an AWD/4WD vehicle, following this provision costs twice as much as for a FWD car and will take twice the time to install (not an insignificant factor in freezing rain) and, should you fail to install on both front and rear properly, or should they suffer mechanical failure, may put one in a bad situation.
Sometimes it's better to do less.
> That’s why AWD ... helps you safely navigate both inclement weather
This is certainly how AWD/4WD etc is marketed to consumers, but it's worth remembering that every car has four wheel stop. Better acceleration really just helps you hit that snow bank first.
(Also this article loved to misuse the word "torque" when they really meant rotation or energy. Sigh.)
If you're driving in weather, acceleration should be a low priority (barring edge cases, like steep driveways). Contrary to Fast and Furious movies, accidents aren't generally avoided via acceleration. I'd rather be reminded that traction is spotty when attempting to accelerate on two wheels than abruptly finding out when I need to brake on all four.
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.
I go trout fishing a lot in North Georgia and many places I go would be accessible in a small fwd car. In fact if people saw you driving an incapable vehicle they would stop and warm you about proceeding further.
Anecdotal evidence makes me claim that electronic traction control systems are excellent, but still no match for good ol' manual systems - probably in part because the automagic systems are easily taken for granted, making drivers forget what icy conditions are like - and using the electronic systems to drive faster, not safer.
So, my personal experience is quite contrary to your opinion. In fact, we are buying a second vehicle now and one of the critical requirements is 4WD/AWD, esp since we don't know how much snow the coming winter will bring.
I'd have disagreed a few years ago, then I moved to Norway. Snow is normal.
And the average car is simply a FWD. Most folks tend to have cars or station wagons instead of trucks and SUV's, and most of those are FWD.
The biggest things that help in snow?
Snow tires and snow socks for the tires, and chains when necessary. My spouse thought it was nuts that people didn't change tires in the winter, and average travel speeds on roads is slightly higher on snow and ice. Most folks will take their cars lightly offroad without incident (prinmitive vacation houses are often on dirt roads with weedy grass parking). In cities, at least, it is also pretty common to have bins with grit and I think folks need to keep safety items (a shovel) in their cars.
The snow socks are neat, and I wished they were more common in the states. They weren't in Indiana, anyway. http://www.autosock.co.uk/http://www.autosock.co.uk/
This is true.
I live in the US and am guilty as charged with over-provisioning vehicles. It's what we do.
Meanwhile, last time I was in Denmark, I was amused to see a decent sized horse trailer being towed by ... an Audi A8.
Or going to the SO's cousin's for a family thing, parking in the yard like everyone else, getting rained on, and needing their tractor to pull you out.
Now, I'm fine with my 2WD truck. But if I had to pull those kind of antics more than once a year, I would seriously consider 4WD.
How trains accomplish this is interesting: http://jalopnik.com/5820296/how-trains-make-turns-without-di...
I'm linking to Jalopnik, which embeds Richard Feynman's video on train turning, rather than directly to the video because there are some interesting comments on the site. In particular, see the comment from railroad employee "Gonemad" about how they have a system that automatically greases the wheels before turns, which seems rather counterintuitive at first.
I've owned cars with every type of AWD system (Audi, Subaru and currently a Honda) and day to day they all work just as well. Given that, I suppose the manufacturers are ultimately right to go with the cheaper and weight saving system.
There was an episode of Dirt Every Day where Fred and co. took a Jeep Renegade(?), or whichever the small one is, off-roading there were some great scenes of the traction control in action. Basically by spinning the dial to rocks the traction control knows to use ABS to approximate how one would work the brakes with a torsen.
Even during good weather on dry roads, the system has benefits by engaging the clutches when accelerating from a stop, so all four wheels are driven when you need to get going. The clutches are disengaged after about 20 mph so you get the fuel economy of a FWD vehicle.
It engages/disengages the center differential (4WD) as needed automatically, but can also transfer power to be greater in the front or rear as desired - with presets for 60/40 and 40/60 rear/front power. (Sport mode, Mud/Snow)
It's also got traction control w/ ABS, and it uses the brake locking differential technique on each axle as the article describes in advanced AWD systems.
Of course they have different variations, one of which also has locking differentials on top of everything else.
It's amazing to drive. It's all fully automatic too.
http://www.eaton.com/Eaton/ProductsServices/Vehicle/Differen...
Having driven a "4WD" with all three differentials of this type, I now wonder if it would be classified as 4WD or AWD.
Really, from what I've seen, current usage is AWD is a 4WD system marketed for on road use, and 4WD is an AWD system targeted to off road use. The biggest indicator seems to be if a vehicle has a low range transfer case, it will likely be marketed as 4WD. Also, if it is marketed as 4WD, HOPEFULLY it will have under body armor.
Remember, not all AWD/4WD systems are equal. Last winter our neighbor's Subaru Impreza with Symmetric AWD (which Subaru claims is super good) couldn't make it up a small incline by our houses. Both my Audi and Land Cruiser on all season tires had no problem.
Generally it's more helpful to think of 4WD/AWD as fitting into three categories:
1: A system with a transfer case that predominantly drives one axle (almost always the rear, and almost always labeled "4WD"). This is your typical pickup truck 4wd system. When it is engaged, the two axles are locked together and may as well be welded and they'll turn at the same speed.
2: A system with a center differential, in which both axles are driven but may turn at different speeds (Almost always labeled AWD). The differential may allow full or progressive locking and/or torque vectoring. It's rare to find one of these that can fully lock (Subaru WRX STI is the only one I can think of), but it's common to see ones that favor one axle (Audi, BMW, Mercedes) or that can partially lock in response to slipping of one axle (often with the brand name Torsen). These are almost always found on passenger cars, and with few exceptions they're found on cars with longitudinally mounted engines (Mitsubishi EVO being the only transverse layout car with a center diff I can think of), that means they're usually found on higher end cars that are based on front-engine rear-wheel drive models.
3: A system with a power-take-off unit (often brand name Haldex), in which one axle is driven and a clutch pack can partially or fully engage the other axle. (this is often labeled AWD when put on a passenger car, and 4WD when placed on a "truck") The axles can operate at different speeds but can also fully lock in the way a transfer case can. Typically you find these on "AWD" cars that are based on transverse-mounted front-drive models as it's easier to add such a system and get partial engagement of the rear axle, but you'll also find them on performance AWD applications based on rear-drive mid/rear-engine models like Porsche 911s or the Buggatti Veyron. You'll even find it labeled as a 4WD system on "trucks" like Honda Ridgeline or Pilot, or on the new Ford Raptor. These tend to operate more like an automatic transfer case, as the only way to partially engage the secondary axle is to slip the clutch, which you don't want to do 100% of the time. (Ford actually calls this system a transfer case on the Raptor).
You'll see all sorts of spurious claims about systems "sending 100% of the torque to the rear axle" (often just means that the system can fully lock, and 100% torque to the rears means the fronts are in the air). You'll also see things like "rear biased", which sometimes means you have a center differential that overdrives the rear axle by default, which is great, but is often misleadingly applied to PTO/Haldex systems where the rears are also overdriven, but only during the rare times when the system is engaged.
Basically if you're looking for traction in low speed settings where you might not have grip at all on one axle, like offroading or being stuck in the snow, you want as many of your axles locked as possible, which technically you can get with any of these systems, but is usually seen on transfer cases or haldex/PTO systems.
If you're driving quickly on pavement with some traction, you usually want something more like a center differential, preferably one that can partially or entirely lock when you lose some grip at one end.
A computer could instantly shift torque with both of the hybrid solutions, you wouldn't need a clutch or a gearbox any more, not to mention cheap all-wheel-drive including the trailer in trucks. And a fuel-electric drivetrain could easily plug in a battery or supercaps for braking energy recuperation, and the fuel engine could always run at peak efficiency/exhaust gas condition...
There's also the matter of power distribution, if you have four independent electric motors, you can only ever send 25% of available torque to one wheel, if you have front, rear, and center lockers, you can send 100% of torque to one wheel.
I doubt it, generators and motors operate at 99% efficiency. Do you have any reference?
> if you have four independent electric motors, you can only ever send 25% of available torque to one wheel, if you have front, rear, and center lockers, you can send 100% of torque to one wheel.
That assumes that engines are so small that they operate at 100% under normal conditions. This doesn't have to be the case, electric engines are so small, you can have bigger-sized engines where one single engine can use all the power. The nature of electric engine is such that the loss of efficiency by doing this is minimal. Plus you need to do this only fractions of the time, so the engine doesn't have to be designed to operate in high power mode at full duty cycle.
A locked up torque converter or an engaged clutch is basically a solid steel bar. You're the one that is going to need to provide sources that show an electric generator and motor, operating in series, are 99% efficient across a wide operating range. (you won't find that that's true).
>That assumes that engines are so small that they operate at 100% under normal conditions. This doesn't have to be the case, electric engines are so small, you can have bigger-sized engines where one single engine can use all the power. The nature of electric engine is such that the loss of efficiency by doing this is minimal. Plus you need to do this only fractions of the time, so the engine doesn't have to be designed to operate in high power mode at full duty cycle.
Look up the efficiency of electric motors running outside of their optimum power/rpm ranges. Also, you basically just said that you can get around a percentage problem by increasing the coefficient, which kind of misses the point. Also, if you're going to put four motors, you're probably doing that because you want to put them in the hubs, (if you're putting them inboard, you'd just use one or two motors and put a differential in there, like Tesla does), if you're putting motors in your hubs you want the smallest motors you can get away with, because larger motors increase unsprung weight.
And the CO2 values... wow, how is that possible? Burning a defined amount of fuel should always produce the same amount of CO2?
There've been other gas-turbine automobiles, including some prototypes built in the 1960s. A friend of mine test drove one, claimed it could lay a patch (spin the tires) at highway speeds. Though my understanding is that in direct-drive applications (such as that), turbine lag is an issue.
The exhaust also runs quite hot, and turbines typically emit a lot of NOx (nitrous oxides), what you get when you run atmospheric nitrogen through a high-temperature field.
Much anecdata here, apply salt liberally.
https://www.youtube.com/watch?v=b2A5ijU3Ivs
He talks extensively about how it works, what it's like to drive, and the various difficulties that resulted in it never entering mass production.
https://en.wikipedia.org/wiki/Brake_specific_fuel_consumptio...
In all cases, you can increase efficiency somewhat by going to the complication of compound cycle, where you harness some of the wasted heat in the exhaust by making steam.
Large, high bypass turbofan jet engines are very efficient from an overall propulsive efficiency standpoint, but you aren't measuring the same thing (shaft power times time, divided by energy consumption).
2) Gas turbines scaled down to small size (let alone "micro" size) generally have dismal thermal efficiency. All the most efficient gas turbines are gigantic (tens of thousands of hp).
Even hybrids usually have a way to get the engine power directly to the wheel. In the Prius's system, it uses a planetary gear with a pair of electric motor/generators which allow it to split power between the mechanical and electrical paths. In the Volt, there are clutches which allow the car to transition between parallel and serial hybrid modes.
Do you have a reference?
Since both electric generators and motors well exceed 99% efficiency at this power level, I don't see how can this be the case.
This says 85-95% efficiency for the motor in an EV: https://matter2energy.wordpress.com/2013/02/22/wells-to-whee...
This says 93% for a brushless DC motor: http://electronics.stackexchange.com/questions/58236/why-doe...
I found a bunch more of similar quality searching for "ev motor efficiency" and similar.
I'm not particularly satisfied with those, so if you have something better, I'd love to see it!
Cars generally suffer quite a lot if you add significant weight: both performance and fuel economy will be hit. That is why locomotive solutions are not very suitable for cars.
Clearly, with AWD the car is designed to allow wheels to rotate at different rates (hence the diffs). Otherwise you would't be able to make a turn. Once you dispel that myth, they then tell you to replace the pair ...
Why do dealerships, repair shops and tire places keep insisting on replacing all 4? ... good for tire sales I guess.
(According to the cool old video linked in the article.)
Not sure if by design or some kind of failure...
That should be the tire with the most grip. If torque goes to the wheel where the tire on it has the least grip the tire still has no traction torque doesn't make grip appear out of nowhere.
In four-wheel or all-wheel drive one wheel where a tire has no grip the other three wheel with tire that have grip move the vehicle isn't that the entire point of 4WD and AWD?
The differential contains a nifty little clutch. When the vehicle detects no traction, it engages and sends the torque to the correct location. Otherwise you'd end up in a situation similar to engaged cruise control with no traction. Limited slip differentials and locked differentials cause much confusion.
The author is describing the tricks used to get around that fact. Namely locking differentials or limited slip differentials. The end result is a fraction of the torque makes it to other Wheels depending on the design of the drivetrain. The author spent a lot of time on locking differentials when those are rare. Limited slip differentials are more common. Most awd cars will have a LSD in the center diff.
There are torsen diffs which do what you describe but they are rare.
A clutch-based diff or some other type of locking diff would otherwise be required to solve the "wheel in the air" problem without applying an artificial load to the free wheel.
Don't want to embarrass myself trying to explain it, so click "how it works" on this page: http://www.wavetrac.net/technical.htm
[0] Assuming ideal naive differentials.
Why is this garbage spam on Hacker News?
I'd suggest you start with the thing you believe they got "most wrong" and go from there. Otherwise this comment seems to lack substance.
I just don't see why I should be giving free editorial services and in effect encouraging what I consider to be blog spam infecting a site I love.
This is a discussion site, meaning we discuss things, telling us a potential problem with the source is a great discussion topic. Taking unqualified potshots at the article doesn't provide a jump-off point to any discuss worthwhile.