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

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

#111

Post by chiph »

tunap wrote: post #15
blackguardx wrote: post #5

Earlier quoted context omitted.

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.

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…

They have a sign now - no AWD allowed. Super steep road.

https://www.youtube.com/watch?v=2xGp5anlXSA

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

#112

Post by dredmorbius »

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?

Gas turbines operate efficiently at constant output, which a hybrid-drive system with storage can manage. They only need to kick out enough power, on average, to keep the storage system fully charged. Batteries and eletric motors can handle both high-demand accelleration, and regenerative braking. Since the turbines are running at a very constant range, and can be rated to average out power load inclusive of idle times, they can be designed for optimum efficiency within that range. Or that's the theory.

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.

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

#113

Post by rsync »

Animats wrote: post #38

There 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 documented, but Tesla cars aren't intended for off-roading."

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 ...

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

#114

Post by hackuser »

CarVac wrote: post #110
hackuser wrote: post #107

Earlier quoted context omitted.

> Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. 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.

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.

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

#115

Post by examancer »

Kind of an odd post to see on hacker news. As somewhat of a car buff my take away is that this article gives an adequate high level explanation but leaves out a lot of useful details, especially to anyone evaluating 4WD/AWD vehicles, like the tremendous differences between modern AWD systems.

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

#116

Post by mikeash »

dredmorbius wrote: post #112
mschuster91 wrote: post #70

Earlier quoted context omitted.

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?

Gas turbines operate efficiently at constant output, which a hybrid-drive system with storage can manage. They only need to kick out enough power, on average, to keep the storage system fully charged. Batteries and eletric motors can handle both high-demand accelleration, and regenerative braking. Since the turbines are running at a very constant range, and can be rated to average out power load inclusive of idle tim…

For anyone interested in turbines and cars, there's a delightful episode of Jay Leno's Garage about a Chrysler turbine car:

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.

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

#117

Post by bitL »

rconti wrote: post #34
dsr_ wrote: post #23

Earlier quoted context omitted.

And in particular, the Hawaiian rental car counters will tell you when you rent which roads your car is not allowed to go on.

I've always wondered why this matters. I've got car insurance, I've got additional rental coverage on my credit card, and there are plenty of other vendors from whom to rent cars if you want to blacklist me.

Having violated this all the time I've been to Hawai'i I can understand why do they do that - for example, if you continue on the road to Hāna/Maui beyond, down to the southern side of Haleakalā the road becomes really dodgy, later turns into gravel and there is no water/gas for xy miles available, so for casual American tourists it might be pretty scary. Also, driving the north-western part of Maui is on a very narrow road over cliffs and fallen rocks (not as bad as parts of Tenerife though). I actually had an encounter on a pretty tall cliff where I was driving Subaru Forrester and some US female celebrity named Kim was driving the opposite way in even a larger car; we drove past each other with smiles in about 1mph to avoid collision and she must have been right on the edge with her car while I was almost scratching the right side rocks. Even Saddle Road between Maunas on the Big Island could be pretty demanding to casual tourists in fog and really slippery when descending down to Hilo in strong rain, which is like always (saw a crash once). There are also some scary roads on Kaua'i and O'ahu. In US it's so much easier to get a driving license than in EU, so your casual drivers are not used to fast and safe driving like in Germany, many off-road segments like in Spain or navigating chaotic situations as in 3rd world countries, and your society is generally on the safe side (even excessively so).

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

#118

Post by Gracana »

shiven wrote: post #88
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 kn…

That's probably a difference in suspension/tires/balance rather than drivetrain. With front biased AWD (very common setup) you have to apply power to maintain attitude in a skid, just like FWD.

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

#119

Post by pps43 »

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.

To add to confusion, Subaru sells several very different systems and calls all of them the same "all time all-wheel drive". There is viscous coupling center differential that goes with manual transmissions and nearly always splits torque 50/50 between front and rear, computer-controlled limited-slip clutch pack that comes with most automatics and changes torque split with speed, and some newer systems that do even more.

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

#120

Post by rsync »

Broken_Hippo wrote: post #106
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 are almost never necessary in USA and europe today... 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…

"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."

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.

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