Break the brakes



  • @Lorne_Kates said:

    Surely a billion dollar industry based around making people believe they need sugar water would lie or mislead people, right?

    Reminds me of this. Consumer Reports thoroughly debunked the idea that 4WD vehicles are categorically safer in the snow*, yet the radio is full of ads exclaiming "snow's coming, better buy an SUV".

    * Yes, some 4WD vehicles are safer than some FWD vehicles, but the generality is not true. For example, Subaru's entire AWD line was found to be very competent in the snow, but the 4WD Toyota RAV4 was found to be worse that almost everything regardless of driveline configuration.


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    @Jaime said:

    4WD Toyota RAV4 was found to be worse that almost everything regardless of driveline configuration.

    That's more of a car than a real SUV.



  • @Polygeekery said:

    That's more of a car than a real SUV.

    Sure, but it was worse in the snow than the cars the tested. Also, it wasn't the only 4WD that didn't do well. Only the Subarus and some Jeeps really stood out as winter performers. In all cases, poor snow vehicles with snow tires outperformed good snow vehicles with all-season tires.

    TLDR; four wheel drive might get you un-stuck, but it won't keep you on the road or keep you from slamming into the car in front of you. Getting un-stuck isn't a safety issue, but not hitting other cars is.


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    @Jaime said:

    keep you from slamming into the car in front of you.

    As it keeps all four wheels turning at relatively the same rate, it does help with braking.



  • @Polygeekery said:

    As it keeps all four wheels turning at relatively the same rate, it does help with braking.

    Nope. Consumer Reports did the test - it didn't help. ABS, however, does wonders in the snow.


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    Meh, I don't see Consumer Reports as the last word on this subject. I will trust myself and my driving in my 4WD SUV and drive more confidently and capably than FWD cars. No worries.



  • ????

    By what mechanism do you propose that 4WD helps in poor-traction braking? Not only does it go against the test data, there is no rational reason to believe that 4WD vehicles would brake better. I sense brain-washing.


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    @Jaime said:

    By what mechanism do you propose that 4WD helps in poor-traction braking?

    By distributing the braking load in the exact same way that it distributes accelerative forces.



  • Are you suggesting that there are cars on the road (built in the past 50 years) that don't distribute braking to all four wheels?


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    No, but I am suggesting that all the wheels are braked independently and that their rotation is not tied together by the driveline.



  • On a FWD car, the rear wheels are free-spinning and have their own brakes. What makes them not independent of the other wheels? In other words, being braked independently is not an advantage of 4WD, it's an advantage of 1960's and later vehicle design.



  • If the rotation of the wheels was tied all together as you seem to think, it would cause significant wear on the tires during turns, when the right and left sides of your vehicle are moving at different speeds.

    This is actually more of a problem in 4WD. They have to use special gearing to allow for some slippage so that the outside tires can rotate faster when you turn.



  • I'm sure the added weight of the AWD drive train really helps the stopping distance too.



  • Extra weight increases the friction between the tires and the road, which allows you to apply more stopping power without losing traction. I'd expect it to more than make up for its weight.



  • Nope. A Coefficient of Friction exists because the two pretty much exactly offset each other.


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    @Jaime said:

    On a FWD car, the rear wheels are free-spinning and have their own brakes. What makes them not independent of the other wheels? In other words, being braked independently is not an advantage of 4WD, it's an advantage of 1960's and later vehicle design.

    OK, I am beginning to think that I need to talk more slowly or something, but this is typing, so read slower.

    On a 4WD vehicle, the wheels all turn at roughly equivalent rates. Whether accelerating, or braking. You know how when you accelerate in poor traction in a 4WD vehicle, and all the wheels pull you forward? Same shit happens during braking, except it is braking forces.

    So yes, 4WD does help distribute braking forces, despite your incessant yammering about Consumer Reports because they are such a respected authority on physics and automobiles or something. Not quite sure :wtf: you are getting on about with that.


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    @loopback0 said:

    I'm sure the added weight of the AWD drive train really helps the stopping distance too.

    You can fix that problem by allowing for adequate stopping distances.



  • @Polygeekery said:

    On a 4WD vehicle, the wheels all turn at roughly equivalent rates. Whether accelerating, or braking. You know how when you accelerate in poor traction in a 4WD vehicle, and all the wheels pull you forward? Same shit happens during braking, except it is braking forces.

    What does the driveline have to do with the mechanics of braking? The brakes are mounted directly on the hubs.

    Here's a thought experiment - Imagine a car with no drivetrain at all coasting down a hill. You apply the brakes. How is the braking force distributed to the four wheels? BTW, the answer is that the braking force is applied at the wheels themselves, there is no need to distribute it.


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    @Jaime said:

    Here's a thought experiment - Imagine a car with no drivetrain at all coasting down a hill. You apply the brakes. How is the braking force distributed to the four wheels? BTW, the answer is that the braking force is applied at the wheels themselves, there is no need to distribute it.

    Here's a thought experiment: A 4WD vehicle rolling down a hill with one brake. Does one wheel lock up and skid so that it rotates about an axis, or does it stop straight?



  • @Jaime said:

    Nope. A Coefficient of Friction exists because the two pretty much exactly offset each other.

    They do offset each other. More weight means you have to hit the brakes harder. However, if you need to stop quickly then your initial reaction is to hit the brakes quickly, and if you lose traction and go directly into a skid you'll burn up a lot of stopping distance recovering from it. It's better to avoid skidding; increase pressure on the brakes in a quick but controlled manner, to the point at which you're applying as much force to the brakes as possible without starting to skid.



  • @Polygeekery said:

    Here's a thought experiment: A 4WD vehicle rolling down a hill with one brake. Does one wheel lock up and skid so that it rotates about an axis, or does it stop straight?

    The vehicle you're talking about doesn't exist in the real world, where vehicles have 4 brakes, 1 on each wheel. If all 4 are working properly, your argument is completely irrelevant.


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    @anotherusername said:

    The vehicle you're talking about doesn't exist in the real world,

    Yes it does.



  • What 4WD vehicle with working brakes only applies braking force from 1 brake?



  • @Polygeekery said:

    Here's a thought experiment: A 4WD vehicle rolling down a hill with one brake. Does one wheel lock up and skid so that it rotates about an axis, or does it stop straight?

    It stops straight, but not because the 4WD is taking the braking force from one wheel and applying it to the others. The same would happen in a FWD vehicle with one brake. Or a RWD car.



  • @anotherusername said:

    What 4WD vehicle with working brakes only applies braking force from 1 brake?

    INB4: an example which applies the braking force to the drive train itself.


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    @loopback0 said:

    an example which applies the braking force to the drive train itself.

    There ya go. ;)

    Let me get back to a desktop.


  • Discourse touched me in a no-no place

    @loopback0 said:

    an example which applies the braking force to the drive train itself.

    That's a pretty recent innovation, and those cars will still have conventional braking systems as well because those are exactly what you need when an “ohshit” moment strikes.


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    @anotherusername said:

    The vehicle you're talking about doesn't exist in the real world, where vehicles have 4 brakes, 1 on each wheel. If all 4 are working properly, your argument is completely irrelevant.

    Well, you are at least partially correct, sort of. The argument is irrelevant, but this is a thought experiment. Also, due to @loopback0's pendantry, I need to refine it a little:

    @Polygeekery said:

    Here's a thought experiment: A 4WD vehicle rolling down a hill with one brake. That one brake has enough force to stop the car, but the wheel it is attached to does not have enough traction to stop the vehicle. Does one wheel lock up and skid so that it rotates about an axis, or does it stop straight?

    That should get my point across.

    On a standard 2WD vehicle, it is possible, and very easy to, lock up one or two wheels and pivot about an axis when driving on snowy or icy roads. With 4WD, you are less likely to lock up one wheel and spin, due to all of the wheels being forced to rotate at roughly the same rate.

    A RAV4, which is basically a car with a pseudo-4WD system tacked on, would not get that benefit because it is still a FWD vehicle at heart.



  • @Polygeekery said:

    On a standard 2WD vehicle, it is possible, and very easy to, lock up one or two wheels and pivot about an axis when driving on snowy or icy roads.

    That is exactly one scenario that ABS is designed to prevent.


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    @dkf said:

    That's a pretty recent innovation,

    Oh it is?

    Maybe if we are talking about little cars, but such devices have been around for a long time. The first diesel exhaust brake was introduced around 1905 by Rover in England. They are a common device on heavy machinery and heavy trucking.

    As for one brake disk or drum actuating multiple wheels, articulated trucks usually have an emergency/parking brake that is situated on the driveshaft at the articulation point that applies its forces through the drivetrain to all of the wheels. They also have a diesel exhaust brake that does the same.

    4WD backhoes have no front brakes unless 4WD is engaged. In 2WD, the front wheels turn freely regardless of how you actuate the pedals (one brake for each back wheel to aid in turning).

    There are countless other examples once you move away from passenger vehicles, but I do not recall that being a prerequisite for that statement.


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    @anotherusername said:

    That is exactly one scenario that ABS is designed to prevent.

    Designed to help prevent. 4WD is still superior, as you cannot lock just one wheel.



  • @Polygeekery said:

    @anotherusername said:
    That is exactly one scenario that ABS is designed to prevent.

    Designed to help prevent. 4WD is still superior, as you cannot lock just one wheel.

    In a vehicle with properly working ABS, how can you lock just one wheel? If ABS is doing its job it'll reducing braking force to that wheel and it won't be locked.


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    @anotherusername said:

    If ABS is doing its job it'll reducing braking force to that wheel and it won't be locked.

    I don't want it to reduce braking force to the wheel, I want the wheel to brake harder. I could keep wheels from locking up just by never using the brakes.

    I would need a bigger bumper though...



  • @dkf said:

    That's a pretty recent innovation

    It wasn't a serious suggestion.

    @Polygeekery said:

    With 4WD, you are less likely to lock up one wheel and spin, due to all of the wheels being forced to rotate at roughly the same rate.

    They're not forced to rotate at the same rate. The diff allows them to rotate at different rates.

    @Polygeekery said:

    I don't want it to reduce braking force to the wheel, I want the wheel to brake harder.

    Brake harder and grip how? If the ABS is kicking in, you've got no grip to use to brake harder.


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    @loopback0 said:

    They're not forced to rotate at the same rate. The diff allows them to rotate at different rates.

    Really? Ever try making a hard turn with 4WD engaged in a proper 4WD vehicle on asphalt? The jumping and screeching might beg to differ.

    @loopback0 said:

    Brake harder and grip how?

    By making sure that all the wheels turn at the same rate.

    @loopback0 said:

    If the ABS is kicking in, you've got no grip to use to brake harder.

    That excess braking force gets distributed to the other wheels, unless or until you lock all of them. At which point ABS is useless anyway. ABS only works if you still have wheels turning.



  • @Polygeekery said:

    ABS only works if you still have wheels turning.

    ABS keeps the wheels turning.

    @Polygeekery said:

    Really?

    Yes, that's the whole point of the differential, allowing the wheels to turn at different speeds.

    @Polygeekery said:

    screeching

    I can make the front wheels in my car screech in a hard turn. It proves nothing.



  • @Polygeekery said:

    I don't want it to reduce braking force to the wheel, I want the wheel to brake harder.

    :facepalm:

    The wheel can't brake harder. You were already trying to brake harder than that wheel's coefficient of (static) friction could allow. It went into a skid, drastically lowering the braking ability of the wheel (its coefficient of kinetic friction is much lower), and that's why it locked. And it's why ABS has to reduce braking force to unlock the wheel to allow it to start rotating again.

    @Polygeekery said:

    @loopback0 said:
    They're not forced to rotate at the same rate. The diff allows them to rotate at different rates.

    Really? Ever try making a hard turn with 4WD engaged in a proper 4WD vehicle on asphalt? The jumping and screeching might beg to differ.

    The diff allows them to rotate at slightly different rates, which might not be enough for a hard turn, and plus it only works in 4WD-HI. If you're in 4WD-LO, it won't allow them to rotate at different rates at all, and the jumping and screeching will tell you that.

    @Polygeekery said:

    That excess braking force gets distributed to the other wheels, unless or until you lock all of them. At which point ABS is useless anyway. ABS only works if you still have wheels turning.

    As loopback0 said, ABS keeps the wheels turning. It ensures that you cannot lock the wheels. Locking the wheels will result in a much longer stopping distance. Preventing you from doing that is not useless.


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    @loopback0 said:

    ABS keeps the wheels turning.

    Maybe Limey snow is different than American Midwest snow? But it is very easy to lock all your wheels on snow or ice here. Nothing that cannot be avoided, except for the slickest of ice, but with 4WD it is remarkably easier to avoid, IME.

    @loopback0 said:

    Yes, that's the whole point of the differential, allowing the wheels to turn at different speeds.

    Even the transfer case on a 4WD vehicle?

    @loopback0 said:

    I can make the front wheels in my car screech in a hard turn. It proves nothing.

    It proves something when you are moving at a snail's pace and the wheels are screeching because they are not allowed to turn at the same rate.


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    @anotherusername said:

    The diff allows them to rotate at slightly different rates, which might not be enough for a hard turn, and plus it only works in 4WD-HI.

    You can take that up with the engineers at Toyota then. I don't really give a shit if you want to be wrong or not.

    @anotherusername said:

    As loopback0 said, ABS keeps the wheels turning. It ensures that you cannot lock the wheels. Locking the wheels will result in a much longer stopping distance. Preventing you from doing that is not useless.

    Or just make it harder for them to lock, by keeping the vehicle in 4WD.



  • @Polygeekery said:

    Designed to help prevent. 4WD is still superior, as you cannot lock just one wheel.

    A two wheel drive car with ABS already applies around 95% of the available traction as braking force. There's only a theoretical possible 5% improvement left. Unless you are claiming that there exists a 4WD braking system that somehow manages more braking force than the available traction, there is no room for anything more than a minor improvement.

    Your arguments are entirely based on straw-man scenarios of 2WD cars without four brakes and without ABS.



  • What chain restaurant serves-up the ABS?



  • @Polygeekery said:

    Even the transfer case on a 4WD vehicle?

    One designed for on-road use? It allows it to a degree - the front and rear wheels are unlikely to be turning at exactly the same rate.


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    @Jaime said:

    A two wheel drive car with ABS already applies around 95% of the available traction as braking force.

    On a wheel by wheel basis.

    @Jaime said:

    There's only a theoretical possible 5% improvement left.

    On a wheel by wheel basis.

    @Jaime said:

    Unless you are claiming that there exists a 4WD braking system that somehow manages more braking force than the available traction,

    Of course not, I am just saying that it makes better usage of that traction. If you have one or two wheels that are locking, but others that are not, the nature of the drivetrain will distribute the excess braking force to the wheels that still have traction.

    Holy fuck, what is so hard to understand about this simple concept?

    @Jaime said:

    Your arguments are entirely based on straw-man scenarios of 2WD cars without four brakes and without ABS.

    No it isn't, I simply followed your thought experiment of a vehicle with no drivetrain rolling down a hill. Or whomever made up that ridiculous one.


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    @loopback0 said:

    One designed for on-road use?

    Yes.

    @loopback0 said:

    It allows it to a degree - the front and rear wheels are unlikely to be turning at exactly the same rate.

    Not necessarily. That is why it says to only use 4WD on slippery road conditions, so that the wheels can slip if/when they turn at slightly different rates.



  • Jesus shut up about the fucking cars, who gives a fuck.


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    Obviously we do, that is why we are talking about it.


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    It is not your thread, why do you give a fuck?



  • @loopback0 said:

    unlikely to be turning at exactly the same rate

    @Polygeekery said:

    if/when they turn at slightly different rates.

    Did you disagree then say the same thing? :laughing:

    @Polygeekery said:

    That is why it says to only use 4WD on slippery road conditions

    Oh, right, yeah... part-time 4WD.

    Start discussing transaxles and we can combine this with another thread :laughing:


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    @Polygeekery said:

    Not necessarily. That is why it says to only use 4WD on slippery road conditions, so that the wheels can slip if/when they turn at slightly different rates.

    Though, in fairness, on my vehicle that is only when the center differential is locked, which is what I consider to be "true 4WD". With the diff unlocked, it does what you say, but you do not get near the traction in really slippery conditions. With the diff unlocked, it attempts to use the ABS to keep the wheels from spinning or locking and in deep snow or slick ice it works rather poorly.


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    @loopback0 said:

    Did you disagree then say the same thing? :laughing:

    Yes and no, I disagreed with the mechanism by which they accomodate the difference in wheel speed. ;)

    @loopback0 said:

    Oh, right, yeah... part-time 4WD.

    Not really. It is full-time, but you can lock the center diff.


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