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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?

#61

Post by tdicola »

blackguardx wrote: post #5
bitL wrote: post #2

Great! Finally somebody explained this to me, I thought it was just a marketing term related to different patents of similar ideas. 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 cal…

Generally a sign saying you need 4wd in the US really means you need a lot of ground clearance. Your car probably would have high-centered, that is stuck resting on the middle of the car with the tires in the air. The annoyed ranger was probably just tired of pulling stuck tourists out.

Also 4WD cars generally have a super low gear for crawling at low speeds & high torque whereas AWD cars usually don't have a similar gear. Low gear is critical on steep grades, particularly descending them so you can go down safely and slowly without riding your brakes and overheating them.

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

#62

Post by mikeash »

mschuster91 wrote: post #26

I wonder why car manufacturers still use direct force transmission with hugely complex (and in case of ignoring the instructions, outright dangerous) like 4WD/AWD instead of diesel-electric or diesel-hydraulic systems like on locomotives. 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 trail…

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 the car to transition between parallel and serial hybrid modes.

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

#63

Post by bitL »

blackguardx wrote: post #5
bitL wrote: post #2

Great! Finally somebody explained this to me, I thought it was just a marketing term related to different patents of similar ideas. 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 cal…

Generally a sign saying you need 4wd in the US really means you need a lot of ground clearance. Your car probably would have high-centered, that is stuck resting on the middle of the car with the tires in the air. The annoyed ranger was probably just tired of pulling stuck tourists out.

It was some US-made SUV (Ford or GM, can't recall which one) with fairly high clearance - I specifically booked it so that I can go everywhere (Waipio, Mauna Kea etc.). The road to valley was fine, it was also dry and the only possible issue could have been crossing a few streams at the bottom of the valley, but SUV had no problem there as well (clearance was sufficient). I experienced far worse roads on Tenerife while blindly following a route computed by then NOKIA maps that took me to super steep straight-up roads where my car ended up spinning all wheels unable to move up ;-)

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

#64

Post by spacerace »

rwmurrayVT wrote: post #16
spacerace wrote: post #14

Jesus what a SHIT article It gets so many things wrong I don't even know where to start. Why is this garbage spam on Hacker News?

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.

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

#65

Post by 4ad »

mikeash wrote: post #62
mschuster91 wrote: post #26

I wonder why car manufacturers still use direct force transmission with hugely complex (and in case of ignoring the instructions, outright dangerous) like 4WD/AWD instead of diesel-electric or diesel-hydraulic systems like on locomotives. 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 trail…

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.

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

#66

Post by bluedino »

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.

However, if you don't have locking differentials you can still get into the situation where one wheel slips and the wheel that grips doesn't get any power - as shown by the Subaru getting stuck on the hill in this video: https://youtu.be/hrdb_UVTa20?t=543

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

#67

Post by kinofcain »

4ad wrote: post #56
kinofcain wrote: post #30

Earlier quoted context omitted.

The diesel-electric/hydraulic systems are used in situations where the gearing ratio would have to be absurdly high, they're not used because they're efficient or because they have good torque vectoring capabilities. The electric motor can produce nearly 100% of torque at zero RPM, which helps locomotives get several million pounds of train moving. In your car, truck, or even semi, first gear plus some clutch/torque…

> a lockup torque converter or a fully engaged clutch is more efficient than the combination of an electric generator and a motor. 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…

> I doubt it, generators and motors operate at 99% efficiency. Do you have any reference?

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.

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

#68

Post by giardini »

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 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

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

#69

Post by refurb »

brianwawok wrote: post #22
wil421 wrote: post #20

Earlier quoted context omitted.

You right that's why I said 10 or so. Try taking a turn at 25 in 4LO. I'm not sure of the use case for 25 in LO. My Grand Cherokee can operate the same way but I doubt I would go into LO at those speeds. It also will switch to 4HI when it detects wheel slip. 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.

In dirt taking the turn at 25 is fine. The inside wheel will just spin a little.

Outside wheel wants to travel more than the drivetrain so it would slip, not spin correct?

The inside wheel would spin if the outside doesn't slip.

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

#70

Post by mschuster91 »

samwillis wrote: post #41
mschuster91 wrote: post #26

I wonder why car manufacturers still use direct force transmission with hugely complex (and in case of ignoring the instructions, outright dangerous) like 4WD/AWD instead of diesel-electric or diesel-hydraulic systems like on locomotives. 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 trail…

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?

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