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I suppose that’s good advice because many people overdrive their vehicle’s AWD in winter but it’s not explicitly correct. Real life as usual is more complicate

I suppose that’s good advice because many people overdrive their vehicle’s AWD in winter but it’s not explicitly correct. Real life as usual is more complicated. Braking in AWD scenario and traction are not same as your example and depends on the power split between the front and rear and how the 2 are coupled together. In 4WD with the 50/50 split, the drive shafts are locked together through the differential and braking is much more effective in part due to the fact that the braking is more evenly distributed among the 4 wheels since the drive shafts are locked. Also when you lock up the brakes in 4WD usually 2 wheels on the diagonal keep turning, again due to how the differentials work. This is not the case in FWD.

emilecantin 6y ago
> braking is more evenly distributed among the 4 wheels since the drive shafts are locked. There is something seriously wrong about either my understanding of how cars work, or yours. I have brake disks and brake pads on all 4 wheels of my (FWD) car; pretty much every car does. How would a drive-shaft help "distribute" braking power? The limiting factor here isn't the force each brake can exert on their wheel (they're pretty much always able to lock up), it's the traction between the wheel and the ground (the tire). In my mind, an "ideal" emergency braking scenario would be all 4 wheels braking at the maximum force they can without slipping, and that can be done much better by the ABS system when they're all disconnected from each other. A driveshaft connecting them would be irrelevant at best, but more likely it's a nuisance.
- sliken 6y ago
  4wd/awd is pretty complicated. Some depend mostly on hardware (like subaru), others try to use sensors and torque control and/or brake control to put the torque where it's needed. AWD is complicated with 3 differentials that can be a mix of open, limited slip, or actively controlled (by torque split or brake based system). It's especially tricky since it's not the actual traction that matters as much as the driver perceived traction and predictability. I've drivin AWD systems with snow tires that seemed to have more traction than my subaru with all seasons, but ended up driving more slowly because I couldn't predict what it was going to do. Subaru seems to communicate well with the driver, and making sliding just not a big deal. Other cars make it harder to tell what a given throttle/braking input is going to result in and can trigger heavy front or rear bias based on what the sensors/software tell it to do. It drives me nuts when a car acts completely differently in high traction and low traction conditions, especially when it changes one second to the next. Some Subaru folks make fun of other systems that they call "slip and grip", where there's a heavy FWD bias (minimal torque to the rear), until AFTER you start slipping. My WRX had a limited slip in the rear, center diff was a viscous coupling (100% mechanical/no software), and a open front diff. It was reassuring, never surprised me, and even when significant slipping I could always put the car exactly where I wanted. Even when crab crawling sideways in a parking lot at 2 mph it never surprised me. Numerous Toyota, Ford, Honda, and BMW cars I tried would try to outsmart me. Limiting throttle, moving the torque around, limiting braking forces, etc. Last thing I want when climbing a hill is the car to cut the throttle because one of 4 wheels slipped for 50ms. Consumer reports tested a Subaru forester, Toyota RAV 4, and Honda CRV in heavy snow conditions and concluded the Subaru was a clear winner despite them all having similar dry road performance and AWD. I think for normal snowy roads Subaru does it best with a mostly manual system, the ABS and traction control has a light touch that can be mostly avoided. But for heavy offroad the best thing to do (in certain messy conditions) is lock all differentials, which is something the Subaru can't do.
  - emilecantin 6y ago
    I can certainly appreciate the "doesn't surprise me" aspect, but I still don't see how AWD / 4WD will help you when braking. It's obvious that it'll help when accelerating, and perhaps even when turning, with a (more) even application of torque on all wheels. My main gripe with it is that it often gives drivers a false sense of security (as they got going easily), and they'll drive too fast for the conditions. Just look at some parts of this sub-thread, where people say "I don't need winter tires, I have AWD / 4WD". If you need to brake in an emergency on snow / slush / ice, nothing is going to help you except your tires (and perhaps some smart ABS).
    - sliken 6y ago
      Well, good ABS is hard. Having locked differentials can make the most of a really messy situation. Thus the reason for locked differentials (for going or stopping). There's a wide variety of software solutions as well, some work better than others. It's a non trivial problem. For instance. In normal conditions less braking when any wheel slips is best. In slippery conditions for stopping or going some slipping is best. In really messy conditions it can be best to not keep applying torque (in the positive or negative direction) without any "smarts". For emergency braking in the snow it can be best to just lock them up and have snow build up in front of the tires. To see an example of how big a difference this makes, check out https://www.consumerreports.org/video/view/cars/auto-test-track/4503044972001/awd-showdown-subaru-forester-vs-honda-cr-v-vs-toyota-rav4/ https://www.consumerreports.org/video/view/cars/auto-test-tr... I agree that AWD leads to overconfidence to some, I've been driving at 35 mph on the highway (with a 65 limit) in my Subaru (with good tires) in bad conditions and had giant SUV blast past me at 10-20 mph over what I was doing ... only to find them in a snow bank later. Doesn't mean that AWD wasn't a huge help. My first AWD was a result of me spinning out in a FWD around lake tahoe in poor conditions, in heavy traffic, at 10 mph or so. My front right tire lost traction, the open front differential put all power to the front right tire, and it spun me into a snow bank (just 18" away") in a blink. I was quite impressed how much rotational acceleration could happen in a blink of an eye. The open differential meant I was basically 1 wheel drive (when slippery), the car I replaced it with (Subaru WRX) had 3 wheel drive (when slippery) and was MUCH more predictable. In fact sliding in the snow would bring a smile to my face... and I changed my weekly commute (6 7200 foot passes) to maximize the snow involved. And yes I'd MUCH rather have all seasons and AWD instead of snow tires and FWD. It's not the ultimate traction as much as being predictable. Sure all seasons have less traction, but it's way more predictable and gives much better control when sliding. FWD, even with snow tires can be very unpredictable, and can easily spin a car when the traction is low. Subaru + snow tires seemed overkill for the roads I was on. Sure Subaru + all seasons might be safe in some conditions at 35 mph, snow tires might raise that to 40 or 45 mph, but FWD regardless of tires would mean much less, and Caltrans would require the pain of installing and uninstalling chains and worries about chain failures and/or damage to the car. Even with all seasons, my main fear with a Subaru (I've had two) is hitting enough snow to life the tires off the ground.

emilecantin 6y ago

> braking is more evenly distributed among the 4 wheels since the drive shafts are locked. There is something seriously wrong about either my understanding of how cars work, or yours. I have brake disks and brake pads on all 4 wheels of my (FWD) car; pretty much every car does. How would a drive-shaft help "distribute" braking power? The limiting factor here isn't the force each brake can exert on their wheel (they're pretty much always able to lock up), it's the traction between the wheel and the ground (the tire). In my mind, an "ideal" emergency braking scenario would be all 4 wheels braking at the maximum force they can without slipping, and that can be done much better by the ABS system when they're all disconnected from each other. A driveshaft connecting them would be irrelevant at best, but more likely it's a nuisance.

sliken 6y ago
4wd/awd is pretty complicated. Some depend mostly on hardware (like subaru), others try to use sensors and torque control and/or brake control to put the torque where it's needed. AWD is complicated with 3 differentials that can be a mix of open, limited slip, or actively controlled (by torque split or brake based system). It's especially tricky since it's not the actual traction that matters as much as the driver perceived traction and predictability. I've drivin AWD systems with snow tires that seemed to have more traction than my subaru with all seasons, but ended up driving more slowly because I couldn't predict what it was going to do. Subaru seems to communicate well with the driver, and making sliding just not a big deal. Other cars make it harder to tell what a given throttle/braking input is going to result in and can trigger heavy front or rear bias based on what the sensors/software tell it to do. It drives me nuts when a car acts completely differently in high traction and low traction conditions, especially when it changes one second to the next. Some Subaru folks make fun of other systems that they call "slip and grip", where there's a heavy FWD bias (minimal torque to the rear), until AFTER you start slipping. My WRX had a limited slip in the rear, center diff was a viscous coupling (100% mechanical/no software), and a open front diff. It was reassuring, never surprised me, and even when significant slipping I could always put the car exactly where I wanted. Even when crab crawling sideways in a parking lot at 2 mph it never surprised me. Numerous Toyota, Ford, Honda, and BMW cars I tried would try to outsmart me. Limiting throttle, moving the torque around, limiting braking forces, etc. Last thing I want when climbing a hill is the car to cut the throttle because one of 4 wheels slipped for 50ms. Consumer reports tested a Subaru forester, Toyota RAV 4, and Honda CRV in heavy snow conditions and concluded the Subaru was a clear winner despite them all having similar dry road performance and AWD. I think for normal snowy roads Subaru does it best with a mostly manual system, the ABS and traction control has a light touch that can be mostly avoided. But for heavy offroad the best thing to do (in certain messy conditions) is lock all differentials, which is something the Subaru can't do.
- emilecantin 6y ago
  I can certainly appreciate the "doesn't surprise me" aspect, but I still don't see how AWD / 4WD will help you when braking. It's obvious that it'll help when accelerating, and perhaps even when turning, with a (more) even application of torque on all wheels. My main gripe with it is that it often gives drivers a false sense of security (as they got going easily), and they'll drive too fast for the conditions. Just look at some parts of this sub-thread, where people say "I don't need winter tires, I have AWD / 4WD". If you need to brake in an emergency on snow / slush / ice, nothing is going to help you except your tires (and perhaps some smart ABS).
  - sliken 6y ago
    Well, good ABS is hard. Having locked differentials can make the most of a really messy situation. Thus the reason for locked differentials (for going or stopping). There's a wide variety of software solutions as well, some work better than others. It's a non trivial problem. For instance. In normal conditions less braking when any wheel slips is best. In slippery conditions for stopping or going some slipping is best. In really messy conditions it can be best to not keep applying torque (in the positive or negative direction) without any "smarts". For emergency braking in the snow it can be best to just lock them up and have snow build up in front of the tires. To see an example of how big a difference this makes, check out https://www.consumerreports.org/video/view/cars/auto-test-track/4503044972001/awd-showdown-subaru-forester-vs-honda-cr-v-vs-toyota-rav4/ https://www.consumerreports.org/video/view/cars/auto-test-tr... I agree that AWD leads to overconfidence to some, I've been driving at 35 mph on the highway (with a 65 limit) in my Subaru (with good tires) in bad conditions and had giant SUV blast past me at 10-20 mph over what I was doing ... only to find them in a snow bank later. Doesn't mean that AWD wasn't a huge help. My first AWD was a result of me spinning out in a FWD around lake tahoe in poor conditions, in heavy traffic, at 10 mph or so. My front right tire lost traction, the open front differential put all power to the front right tire, and it spun me into a snow bank (just 18" away") in a blink. I was quite impressed how much rotational acceleration could happen in a blink of an eye. The open differential meant I was basically 1 wheel drive (when slippery), the car I replaced it with (Subaru WRX) had 3 wheel drive (when slippery) and was MUCH more predictable. In fact sliding in the snow would bring a smile to my face... and I changed my weekly commute (6 7200 foot passes) to maximize the snow involved. And yes I'd MUCH rather have all seasons and AWD instead of snow tires and FWD. It's not the ultimate traction as much as being predictable. Sure all seasons have less traction, but it's way more predictable and gives much better control when sliding. FWD, even with snow tires can be very unpredictable, and can easily spin a car when the traction is low. Subaru + snow tires seemed overkill for the roads I was on. Sure Subaru + all seasons might be safe in some conditions at 35 mph, snow tires might raise that to 40 or 45 mph, but FWD regardless of tires would mean much less, and Caltrans would require the pain of installing and uninstalling chains and worries about chain failures and/or damage to the car. Even with all seasons, my main fear with a Subaru (I've had two) is hitting enough snow to life the tires off the ground.