Break the brakes
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That's not exactly a "better" scenario - that's a "least common denominator" scenario which reduces deceleration.
It wasn't meant to be, it was meant as a counter argument to a silly thought experiment of a car with no drivetrain. Jesus fuck, I see why blakey goes off the handle sometimes.
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And yet somehow a bunch of little guys in funny hats and black pyjamas had you shitting your pants and fleeing off the roof of the Saigon embassy.
That's a funny way to describe Democrats.
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You're still wrong, though. This comes from 20+ years of experience doing real offroad and heavy snow conditions in real offroad vehicles.
Well whoopty shit.
If you do not have your diffs locked, your wheels can spin at different rates.
No arguments there.
To the point where you can have one wheel spinning forwards, another backwards, and another 2 static.
I agree.
It's called unwinding the diff, and it happens just as much on 4wd as 2wd.
Of course it does.
If you have your diffs locked, your wheels all spin at exactly the same rate.
Yes they do.
It's really bad idea to do this on tarmac, as you will eat your tyres.
Yes it is.
That screeching and jumping - that's the sound of expensive drivetrain work and tyre imminent replacement.
Yep, which is why I don't do that, normally. The only time I run in to that is when I go from an unplowed street to a plowed and salted parking lot and momentarily forget to unlock the diffs.
There is exactly one use for this mode - to get out of a situation where you've bogged one or more wheels (or are likely to).
Or, deep snow in order to keep from getting stuck, and to make you brake more easily and straighter. ;)
Locked diffs will not, and do not, help braking.
They will not make you stop any better than the coefficient of friction will allow. They are not magic. I never said that they were.
All they will do is ensure all your wheels are turning at the same rate, regardless of whether they are sliding.
That is what I have been saying all along! What the hell are you arguing with??
All that screeching and jumping that happens on tarmac when you've got your diff locks engaged? That's what's happening when you're driving on snow with your diff locks engaged, except the screeching and jumping is replaced by sliding.
Seriously, why are you just repeating what I am saying and then posing it as an argument?
If you're stupid enough to try and rely on locked diffs to somehow improve your braking
Because it does.
But be aware that you've probably got one or more wheels sliding, not braking
Meh, not really. Not unless you lock all 4 of them. You cannot lock just one or two wheels. Literally cannot. By the very nature of how 4WD works, with diffs locked, if you have wheels turning then they are all turning and if they are turning at the same rate as the road is moving underneath of them then they are not locked or sliding.
when you get to 3 wheels sliding (and sometimes just 2), your super 4x4 starts doing pirouettes like a ballerina...
So will a car with ABS. ;)
ABS, on the other hand, keep your wheels turning, and not sliding, thus braking.
Lock all 4 wheels on a car with ABS on ice. Without wheels turning, your ABS is useless. And yes, it can happen. If the computer does not register a differential in speed, it assumes you are stopped and then there is no need to pulse the brakes. It is ABS, not magic.
That is why they tell you that if your brakes lock with ABS, you need to let off and re-apply the brakes. It needs a speed differential.
Jesus fuck people. I never said that 4WD is better than ABS in all situations. My original response was to the idiot statement that:
Nope. Consumer Reports did the test - it didn't help. ABS, however, does wonders in the snow.
He is categorically saying that 4WD does not help at all with braking. That is what I object to. But it does. It does help. It does help prevent wheels locking and it does help distribute braking load.
Am I suggesting that you run with locked diffs on summer days on the interstate at 85mph because it is better than ABS? Hell no, you would eat your diffs and make them very unlocked (shattered) in very short order.
I am not even suggesting that it is better than ABS when Atlanta gets 1/8" of snow and they shut the entire town down and it starts looking like The Walking Dead around there with all of the abandoned cars.
But, here where I live, we regularly get snowfalls over 12" in the wintertime. We also get ice. And I can say with absolute certainty that my vehicle is categorically better at driving in those conditions than a FWD car. I don't give a shit what Consumer Reports says. And I also know that my vehicle stops straighter and more quickly in those conditions with the diffs locked in 4WD.
So Consumer Reports and anyone who wants to parrot their statements as though they are some authority and make absolute statements can both piss off. Goddamn people, we deal with edge cases all the time.
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Meh, not really. Not unless you lock all 4 of them. You cannot lock just one or two wheels. Literally cannot. By the very nature of how 4WD works, with diffs locked, if you have wheels turning then they are all turning and if they are turning at the same rate as the road is moving underneath of them then they are not locked or sliding.
This is incorrect. A spinning wheel can still be sliding and in many scenarios a still spinning wheel will actually slide further than a locked wheel will.
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This is incorrect.
Not it isn't, because:
if you have wheels turning then they are all turning and if they are turning at the same rate as the road is moving underneath of them then they are not locked or sliding.
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And I also know that my vehicle stops straighter and more quickly in those conditions with the diffs locked in 4WD.
So basically, you're saying that your anecdote trumps a proper study?
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if you have wheels turning then they are all turning and if they are turning at the same rate as the road is moving underneath of them then they are not locked or sliding.
That is not strictly true:
- the coefficient of friction is not guaranteed to be the same underneath all of the tires
- the angle of momentum is not necessarily in alignment with the direction of rotation
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That is not strictly true:1) the coefficient of friction is not guaranteed to be the same underneath all of the tires2) the angle of momentum is not necessarily in alignment with the direction of rotation
You have out pendanted me. You are correct, but in my example those were assumed to be true.
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I never assume either of those to be true when driving on icey roads. But perhaps that is what I have only ever lost control once in my FWD car.
But really, in the end it comes down to how you drive your vehicle. Living in Colorado, it amazes me how many people fail at driving in any sort of adverse weather condition no matter the vehicle.
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I don't know anything about Consumer Reports. I assumed they're basically like Which?, which is quite well respected
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I don't know anything about Consumer Reports. I assumed they're basically like Which?
Basically.
which is quite well respected
Consumer Reports generally is also. But we are getting a third hand account hearsay of what they actually said, so it should be taken with a truckload of salt.
Also, their summation may have been spot-on. For most people a FWD car is probably as good as 4WD. But in my case, where we spend a good chunk of our winters driving through shitloads of snow, I can categorically say that 4WD makes a huge difference.
I also think that if what was said (third hand hearsay) is true, then they missed the point in one huge way. We drive through road conditions that a FWD car is probably safer, because it won't leave your garage all winter because it won't fucking move.
It is pretty hard to get in an accident in your car when you cannot drive it. ;)
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If the snow was that bad here (and it never is) I'd not be giving two shits about driving anywhere. I'd be out making snowmen.
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So will a car with ABS.
Indeed it will. However, locking your diffs forces you to have a wheel or wheels sliding wrt the road surface at one point or another.Let's take the obvious example. You have your diffs locked on snow, driving on the flat. Not braking. If you turn your steering wheel, thus entering a curve, you are making your exterior wheels move further than the interior wheels, whilst they are constrained to turning at the same rate. This is what causes screeching and jumping on tarmac, but on snow it forces (at least) two of your wheels to slide on the snow. They aren't locked, but they aren't tracking the surface either.
Now do the same when braking (regardless of how stupid it is to brake in a corner on snow). Again, you're forcing unequal travel, and at least 2 of your tyres must slide.
As you're probably aware, as soon as a tyre starts sliding, the physics become radically different, as it is floating on a layer of water; the coefficient of friction is massively reduced, and you can neither brake nor accelerate effectively with it. So, taking a corner on snow with your diffs locked effectively removes any useful traction or braking ability from at least 2 of your wheels, pushes the braking load onto the others, and may well cause them to slide as well. This is the usual cause of 4wd vehicles going "offroad" in an unanticipated manner on snow - brake, turn the wheels to avoid a hazard, lose all traction. Been there, done that, pulled the Landrover out of the river (on a muddy slope, but the same logic applies)
But what about straight line? Again, diff lock fucks you up, as traction is not equal on a slippery surface. In the proper use case for diff lock, we're saying
I don't care if one or more of my wheels are sliding, I'm using the others to get traction and pull myself out of this hole.
Unfortunately, in a braking situation, that doesn't hold. You need all your tyres to keep traction. But by forcing your tyres to the same speed you will probably end up with one or more wheels in a situation here they have lost traction, but are still tracking the surface due to the rotational constraint - this pushes the braking load onto the others, perhaps overloading them in turn, until all your wheels let go "at once".
This situation is arguably little worse than a non-ABS situation with unlocked diffs, except that it all happens at once. You don't know you've got an issue until it's too late - with unlocked diffs you at least have the chance of feeling one wheel sliding and letting off / pulsing the brakes until you get traction back.
If you touch the steering wheel, though, all bets are off. Locked diffs are actively hurting your roadholding ability.
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If the snow was that bad here (and it never is) I'd not be giving two shits about driving anywhere.
Well, you generally don't. But it is still nice to be able to get to the grocery. Or occasionally go out to stave off cabin fever.
I'd be out making snowmen.
At -15F, it is not really possible to make a snow man. The snow is at too low of a temperature to pack together.
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-15F
We don't measure temperature in moon units.
I'm sure as shit not going outside for anything in that though.
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Unfortunately, in a braking situation, that doesn't hold. You need all your tyres to keep traction. But by forcing your tyres to the same speed you will probably end up with one or more wheels in a situation here they have lost traction, but are still tracking the surface due to the rotational constraint - this pushes the braking load onto the others, perhaps overloading them in turn, until all your wheels let go "at once".
I get what you are saying, and I don't necessarily disagree with you. But would you say in an absolute manner that 4WD does not help braking at all.
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But it is still nice to be able to get to the grocery.
Bit of a wasted journey for a single grocery.
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We don't measure temperature in moon units.
-26C ;)
I'm sure as shit not going outside for anything in that though.
Well, you don't sunbathe in that weather, that is for certain. But life has to go on.
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Sorry, we don't understand Australian.
Pity you didn't understand Vietnamese either.
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UK diesel has probably frozen by that point anyway.
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it should be taken with a truckload of salt.
Now that does help in snowy conditions.
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But would you say in an absolute manner that 4WD does not help braking at all.
Absolutely. It's a godsend in terms of traction when driving ("proper" 4wd is at least, as opposed to the limited slip shite fitted to most cars), but it doesn't help with braking at all and may, if you're driving with diffs locked, actively harm your braking ability.Mostly you'll get away with it because proper 4wd vehicles have great big fat chunky snow tyres. But you're pushing your luck.
For what it's worth, my drive to work for 5 months of the year involves 1/2 mile vertical on (at best) packed snow, and often 12-15 inches of fresh. I do this in a 2wd "consumer" car with snow tyres, I have to chain once or twice a year, and that's usually because some idiot tourist is stuck with their non-equipped vehicle (often a 4wd car following the mantra of "I've got a 4x4, thus I don't need snow tyres or chains"). I have to use the chainsaw I keep in the boot more often than the chains.
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Which in the UK goes down to -15C.
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Which in the UK goes down to -15C.
True shit? Here where we are they will switch over to winter mix soon that is good to well below that. Although, if you do not have an engine heater, good luck getting the diesel to fire.
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True shit?
It says so on the page you just linked
It's a complete non-issue considering it's rarely that cold (unless you're in bits of Scotland but no-one cares about them anyway) and if it was the whole country would grind to a halt anyway.
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but it doesn't help with braking at all
I would disagree. I can seriously tell a difference in braking on my vehicle between standard "permanent 4WD" and when I lock the diffs. It certainly stops straighter and if you drive like a sane person you are unlikely to ever lock the wheels. With the diffs unlocked, the fucker is always beeping at me.
may, if you're driving with diffs locked, actively harm your braking ability.
I would not disagree with that, since you added the qualifier of may.
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It says so on the page you just linked
I learned from the idiot fox, I just post shit without reading it. ;)
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-26C is fine for sun bathing provided the sun is out and there is no wind. Hell, it can feel damn nice outside when its perfectly still and the sun is out at those temps.
Now, once that sun goes down, its fucking cold.
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-26C is fine for sun bathing provided the sun is out and there is no wind. Hell, it can feel damn nice outside when its perfectly still and the sun is out at those temps.
Now, once that sun goes down, its fucking cold.
On the internet, no one knows you are a polar bear.
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When the sun is out and its dead calm, it isn't cold. It is only once the air starts moving or the sun goes down that you can really start to feel the chill in the air.
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When the sun is out and its dead calm, it isn't cold.
If I am wearing snow pants and a parka, I don't disagree with you. But if it is -26C outside, I am not going to be comfortable in swim trunks, no matter how still the air is.
I don't know if we have ever had those sorts of temperatures here that did not come with high winds also. And high winds coupled with those temperatures hurt. Painful to breathe sort of hurt.
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You should try it sometime, everybody needs a little sun in the winter.
Yeah, winds (especially high winds) really suck when its below zero outside.
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Listen very carefully: In deep snow, or on ice, 4WD keeps all of the wheels turning at the same rate.
That is counterproductive. In deep snow or on ice, is very likely that all 4 different wheels have different amounts of traction, so they need to be turning at different rates for all 4 wheels to provide the same amount of force. If they are all turning at the same rate, some of them are slipping. To achieve maximum braking force, none of the wheels can be slipping... a slipping wheel has significantly less traction (kinetic friction vs. static friction) than a wheel that isn't slipping, and therefore provides significantly less braking power.
That allows you to stop straight, and it helps to prevent wheels from locking by distributing excess braking force before a wheel has a chance to lock.
No. It doesn't. Some of your wheels are slipping, which is no better than them locking. They're providing unequal braking force to the wheels that aren't slipping, and actually it can result in the exact same sideways motion as your imaginary scenario where the ABS on a 2WD vehicle mysteriously lets one wheel go into a skid and it locks.
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To achieve maximum braking force, none of the wheels can be slipping... a slipping wheel has significantly less traction (kinetic friction vs. static friction) than a wheel that isn't slipping, and therefore provides significantly less braking power.
Goddamn, at least @tufty appears to know what he is talking about...
Why does a tire produce more traction when sliding slightly?
It is well known in racing that driving the car on the ideal "slip angle" of the tire where it is crabbing slightly from the pointed direction produces more cornering speed than a lower slip angle ...
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If you even think that applies to driving in a passenger vehicle on an ordinary road, then you're a fucking idiot who's probably going to be buying more tires soon. Deliberately deforming your tires CAN give you more traction, yes. It heats up and/or stretches the rubber, both of which can give a stickier, larger point of contact with the pavement. It also destroys the tires very quickly.
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A tire stops better when slipping slightly. If it is not slipping slightly, you are not getting maximum braking or cornering force.
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If you even think that applies to driving in a passenger vehicle on an ordinary road
You are the one who made the statement, not me. I try to not speak in absolutes, because there are almost always edge cases.
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It also destroys the tires very quickly.
If I need to stop, I am unlikely to think, "I could plow in to the back of this car and save a little bit of my tires".
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If I need to stop, I am unlikely to think, "I could plow in to the back of this car and save a little bit of my tires".
Won't someone please think of the tires!
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FWIW though, that was still an infinitely better post than:
In which universe are brakes connected to the axles or the driveshaft?
;)
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A tire stops better when slipping slightly sideways. If it is not slipping slightly sideways, you are not getting maximum braking or cornering force.
FTFY. Actually read the link you posted, it said that. Slipping in the direction your car is traveling is not helpful, in any amount.
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FTFY. Actually read the link you posted, it said that. Slipping in the direction your car is traveling is not helpful, in any amount.
Meh, the concept is much the same. I just picked one that was talking about slip angle in particular.
I am just being pendantic with you. You get maximum braking force when the tires are slipping slightly. You are correct though that this comes at the expense of tire wear. And no, it is not something that happens when I drive to the store, but I was being pendantic towards your absolute statement of:
To achieve maximum braking force, none of the wheels can be slipping... a slipping wheel has significantly less traction
Which is false.
If you had said "sliding", you would have been correct.
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You get maximum braking force on tarmac when the tires are slipping slightly
FTFY. Slipping slightly on tarmac means you're working against shearing of the rubber. Slipping slightly on ice or snow isn't the same at all.
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FTFY. Actually read the link you posted, it said that. Slipping in the direction your car is traveling is not helpful, in any amount.
Here is a better link:
See that little colored hump at the top? That is the sweet spot where you are getting the most braking force, but at the expense of tire wear of course. But that is where the pendantry lies.
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But that is where the pendantry lies
No, the pendantry is in the bit where it says "On a dry road". Woops.
Winter diesel fuel - Wikipedia
