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

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

#121

Post by beachy »

anexprogrammer wrote: post #27
tunap wrote: post #15

Earlier quoted context omitted.

Normally I would 2nd this, but it does not apply on this particular 4x4 road. It really sucks when opposing traffic appears as it's barely wide enough for 1 vehicle in most parts: 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 t…

That doesn't seem that bad for normal traffic, conditions permitting - unless the steeper gradients are markedly so. I'm sure they have their reasons for 4wd only though - picking too many tourists out of the ditches or whatever. 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 l…

Here in New Zealand we lay claim to the steepest street in the world at 1:2.86 gradient https://en.wikipedia.org/wiki/Baldwin_Street.

There's an annual charity event that involves the rolling of over 30,000 Jaffas (spherical confectionery-coated chocolate confectionery) down the hill.

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

#122

Post by TylerE »

tiglionabbit wrote: post #103
tombrossman wrote: post #48

Earlier quoted context omitted.

That's right. A poorly-engineered 4WD system means you are stuck on a hill because one wheel is on slippery wet leaves. AWD should mean you can get up that hill with three wheels on slippery wet leaves, and one with grip. That's the simplest explanation. 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…

> That's right. A poorly-engineered 4WD system means you are stuck on a hill because one wheel is on slippery wet leaves. AWD should mean you can get up that hill with three wheels on slippery wet leaves, and one with grip. That's the simplest explanation. 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.

A typical 4WD without locking diffs will still spin wheels. You have to have differentials at each end to go around corners on pavement.

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.

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

#123

Post by rsync »

dsfyu404ed wrote: post #40

The performance of either system is completely dominated by the traction aid at the axles. The article is otherwise good because most AWD vs 4WD stuff completely igores the axles (because it's a heck of a lot to explain if you really want to compare the various systems) but kinda misses the main point. Open, bad traction control (cuts throttle, applies a little brake), good traction control (all brake no throttle), t…

Thank you for introducing me to the concept of lincoln-locking a differential. Appreciated.

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

#124

Post by Gracana »

mikeash wrote: post #86
4ad wrote: post #65

Earlier quoted context omitted.

> 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 moto…

And don't forget, you have generator efficiency to deal with as well. If you're 90% on both sides, the system is only 81% efficient. Ouch!

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

#125

Post by mindslight »

brbsix wrote: post #99
mindslight wrote: post #79

Earlier quoted context omitted.

FTA: > 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.)

It's not just "better acceleration", it's more consistent and predictable acceleration. I find it makes driving backroads on a dark stormy night quite a pleasant experience.

Well we can't really compare subjective experiences. Driving backroads on dark stormy night can be fun regardless.

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.

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

#126

Post by CarVac »

hackuser wrote: post #114
CarVac wrote: post #110

Earlier quoted context omitted.

Separate motors for front and rear wheels is far less complex than adding more moving parts. 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 fe…

Thanks. But isn't coordinating the motors complex? If the power delivered to the wheels is out of sync for a moment, I would imagine it could be catastrophic, and the system has to deal with variables of turns, traction, acceleration/braking, etc.

Why would it be?

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.

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

#127

Post by landryraccoon »

giardini wrote: post #95
landryraccoon wrote: post #72

Earlier quoted context omitted.

In California you are legally required to either have chains equipped or a 4WD vehicle with snow tires under R1 and R2 snow conditions: 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.

AFAICT you're required to have the chains available, so they're usually in the trunk, unless you're in a situation where they're required. Of course if you buy chains you should immediately put them on your vehicle, if only to ensure that later, when you need them, they will fit. Once you've done that, then you can put them in the trunk. I disagree with your interpretation of the law: dot.ca.gov simply state what the…

The cost of chains is negligible compared to the cost of the car. The advantage of not having to get out of your car in the snow and put on chains at a chain control checkpoint is huge though.

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

#128

Post by rpcope1 »

refurb wrote: post #69
brianwawok wrote: post #22

Earlier quoted context omitted.

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.

More often than not, even more so than slip, the outer wheel will "hop". This is especially true, if you've done something potentially unintelligent like welding the spider gears together.

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

#129

Post by magicbuzz »

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

Jet engines are not "efficient". The SFC is worse than piston engines and in fact, the smaller the turbine the worse the efficiency:

https://en.wikipedia.org/wiki/Brake_specific_fuel_consumptio...

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

#130

Post by elihu »

It'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 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.

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