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4WD vs. AWD. What's the Difference?

outsideonline.com

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Re: 4WD vs. AWD. What's the Difference?

#81

Post by tzs »

The problem of the outer wheel going farther during a turn also occurs with trains, and trains do not have differentials to let the wheels have different angular velocity. On a train, the inner and outer wheels have the same angular velocity regardless of turns.

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.

Re: 4WD vs. AWD. What's the Difference?

#82

Post by spacerace »

spacerace wrote: post #64
rwmurrayVT wrote: post #16

Earlier quoted context omitted.

No it doesn't. I think you're just confused on fundamental mechanics.

That's rich. The lack of understanding of the fundamental mechanics is exactly what makes this article shit and your comment along with it. But do go ahead a school me on this PLEASE.

To user: phpnode that replied to me. I can't reply directly to you but that's a fair enough point.

I was avoiding doing so before because I didn't (and still don't) think this article is even deserving of being picked apart.

I would be happy to do so for the sole benefit of users here though. Just give me a little time to get back from doing some errands and I promise I'll clarify my grievances with the information in the article.

Re: 4WD vs. AWD. What's the Difference?

#83

Post by lqdc13 »

giardini wrote: post #68

4WD/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…

4WD/AWD definitely helps in some day-to-day driving cases. One example is parking in a foot of snow in NYC. Otherwise you have to spend an hour digging. Another example is getting out of a driveway in the morning when you are late after a snowstorm.

Re: 4WD vs. AWD. What's the Difference?

#84

Post by abstrakraft »

ledriveby wrote: post #55

Subaru's higher end AWD system has separate output shafts from the engine for the front and the rear, with electronically-controlled clutches modifying the torque. Default is 60/40 front biased, but it can send 100% of the torque to either end of the car. Excellent performance with no manual mode switching needed, also manages good fuel economy at the same time. Best system in the business.

Not quite. All AWD Subarus (i.e. except the BRZ) have one output shaft out of the engine, 3 out of the transmission/transaxle (2 to the front tires, 1 to the rear diff), and 2 out of the rear diff. Torque split varies between models and years with most near the 60/40 you quote, but the only one (in the US, anyway) that can send 100% torque to either end is the STI with DCCD on full lock.

Re: 4WD vs. AWD. What's the Difference?

#85

Post by maxerickson »

mschuster91 wrote: post #70
samwillis wrote: post #41

Earlier quoted context omitted.

The Jaguar C-X75 [1] concept car uses exactly this idea, it uses a tiny and very efficient jet engine that is charging the batteries. Very neat idea. 1: https://en.wikipedia.org/wiki/Jaguar_C-X75

What the f..k. Thanks for that link, a jet engine?! That's crazy. But it's the "awesome" kind of crazy. And the CO2 values... wow, how is that possible? Burning a defined amount of fuel should always produce the same amount of CO2?

I assume the CO2 values factor in the electric only range.

Re: 4WD vs. AWD. What's the Difference?

#86

Post by mikeash »

4ad wrote: post #65
mikeash wrote: post #62

Earlier quoted context omitted.

Mechanical transmissions are still more efficient, at least for cars, than spinning a generator that then powers an electric motor. 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 t…

> Mechanical transmissions are still more efficient, at least for cars, than spinning a generator that then powers an electric motor. 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.

Do you? I can't dig up anything particularly good, but what I do find indicates in the mid-90% for practical electric motor efficiencies.

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!

Re: 4WD vs. AWD. What's the Difference?

#87

Post by nappy »

giardini wrote: post #68

4WD/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…

AWD absolutely gives you more control of your vehicle in icy and snowy conditions. No, they don't help you stop more quickly, but they do aid in preventing fishtailing, and in being able to accelerate when you choose to, and maintain control of your car. Dedicated snow tires are a bigger factor, but the best option is to have both AWD and snow tires if you will be driving in snowy conditions on challenging roads. All of this make you more able to safely control and operate your car. Also you're far less likely to get stuck or slide from a stop.

Re: 4WD vs. AWD. What's the Difference?

#88

Post by shiven »

giardini wrote: post #68

4WD/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…

I don't know what your definition of USA is, but here in eastern Massachusetts I have had spin outs on regular roads while driving FWD cars in snow. Ever since I started driving an AWD vehicle, I had zero skids/spin outs in similar conditions.

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.

Re: 4WD vs. AWD. What's the Difference?

#89

Post by wil421 »

gambiting wrote: post #24
wil421 wrote: post #11

Earlier quoted context omitted.

AWD is meant for adverse conditions on a road and will probably help on a unpaved road. Think rain, ice, snow. It will keep you on the road. 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".…

Any new Land Rover or Range Rover? They come with 2 locking differentials, with 3rd being an option that can be added extra. Same with Mercedes G-class - they come with 3 locks as standard. And I've no idea what you mean by "cars don't have proper ground clearance" - just buy a one that does?

I don't think you understand what I mean. When I say car it doesn't include SUVs/trucks. The difference used to be cars (unibody) vs SUVs/trucks (body on frame). A lot of SUVs are now unibody including my Grand Cherokee.

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.

Re: 4WD vs. AWD. What's the Difference?

#90

Post by lathiat »

kinofcain wrote: post #25
johngalt wrote: post #21

Earlier quoted context omitted.

Mechanically a drivetrain has no means of differentiating between a tire that is slipping vs a tire at that's on the outside of a turn. A 4wd car with three open differentials will absolutely send all of the engine power to one slipping wheel. 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 i…

Even a torsen diff, or similar helical/gear type limited slip diffs are "torque multiplying", they need a load on one wheel in order to send the "multiplied" load to the wheel with traction. Modern stability/traction systems equipped with helical diffs will apply the brake to the "free" wheel in order to provide a load to the mechanical diff. You can also apply the brakes yourself if you have a torsen diff but no ele…

There is at least one mechanical design which (claims to?) solve this problem, Wavetrac.

Don't want to embarrass myself trying to explain it, so click "how it works" on this page: http://www.wavetrac.net/technical.htm

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