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

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

#151

Post by tapeman »

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.

Keeping the front and rear motors in sync is a cake walk compared to controlling the brakes for traction control.

"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.

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

#152

Post by sctb »

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.

These kinds of comments are not OK on Hacker News. Please stop.

https://news.ycombinator.com/newsguidelines.html

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

#153

Post by post_break »

bluedino wrote: post #66
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

There is a trick that seems idiotic but if you have a mechanical hand brake you can stop the wheel from free spinning. I'm not sure how well it would work for a front wheel, maybe you could hold the brake slightly while on the throttle. Again, seems ass backwards but if you think about what you're doing you're basically "locking" the open diff.

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

#154

Post by repler »

The newer Jeeps actually have both! It's an improvement over AWD and they call it "Active Drive".

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.

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

#155

Post by taneq »

sunstone wrote: post #33
japhyr wrote: post #3

Here are the key takeaways for me: "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. T…

And in the next paragraph "To counteract this [sending of power to the wheel of least resistance] , the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel. " So, a well engineered AWD system actually sends power away from the slipping wheel(s) and towards the wheels with traction, which makes m…

Everyone's focusing on the technology involved, but that's kind of pointless because AWD isn't a technical term. It's a term to distinguish an on-road car with four driven wheels from an all-terrain car, because when the first WRXes and such came out, people saw 4WD and thought that meant they could be used to go bush bashing.

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

#156

Post by to3m »

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…

This distinction isn't really understood in the UK either, at least not as far as I've ever seen. If your car drives all 4 wheels, it has 4 wheel drive - no, seriously! - and you just have to know what its limitations might be. AWD vs 4WD seems to be one of those distinctions that Americans have, like drink vs beverage, that British English lacks completely.

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

#157

Post by garyrichardson »

tombrossman wrote: post #48
sunstone wrote: post #33

Earlier quoted context omitted.

And in the next paragraph "To counteract this [sending of power to the wheel of least resistance] , the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel. " So, a well engineered AWD system actually sends power away from the slipping wheel(s) and towards the wheels with traction, which makes m…

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 the nature of 4wd -- take a modern jeep, but it in 4wd on dry pavement and feel it lurch and moan because it can't slip. Put it in 4lo and on dry pavement and it'll lurch so hard your face will go through the windshield.

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.

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

#158

Post by refurb »

rpcope1 wrote: post #128
refurb wrote: post #69

Earlier quoted context omitted.

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.

Not sure why you're getting downvoted. On high traction surfaces like dry pavement, the outer wheel does tend to hop during turns.

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

#159

Post by icantdrive55 »

alanning wrote: post #97
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…

I loved my old Tercel wagon. Didn't use that low gear all that often but fun to have. CV joints were always my problem, ended up having to replace them quite often. 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. :)

I gave a 89 Toyota that kinda took care of, with 240,000 miles. She still has 125 psi in the cylinders. Changed the oil when I felt like it. Changed the oil filter only sometimes. Never changed the air filter. Never repaired a tapping valve. In other words, didn't take care of her like other vechicles I owned.

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.

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

#160

Post by lathiat »

slr555 wrote: post #134

All this talk of traction and moving forward is great but one of the most important things to remember about 4WD or AWD especially in snow and ice is that is doesn't enhance your ability to brake. It is easy to feel fearless being able to accelerate on a slippery road but a bummer to discover you can't stop (yes, I get that engine braking plays a role...).

which is precisely why tyre choice is more or less equally important on your little buzz box as it is your high end sports car.
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