luvflyin
Touchdown! Greaser!
Stop shorter?? I can see it assisting in slowing down using engine compression. But stopping ‘shorter’ I’m not getting. What is overdrive?
Taxiway closed by notam due to snow/ice
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EdFred
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Highway gear. Low gear ratio. Better gas mileage. Low torque.
GaryM
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Gary M
I'm skeptical as well.
luvflyin
Touchdown! Greaser!
Figured that. You got any idea about the stopping shorter thing?
EdFred
Taxi to Parking
Pre-ABS: Engine braking. Gotta be careful though, too low of a gear and you can get in trouble. I still do my own old school ABS, and don't slam the pedal to the floor and hope.
luvflyin
Touchdown! Greaser!
I’m still skeptical. Pretty sure wheels locked will always give you the shortest stopping distance. Dry, wet or ice. I ain’t sayin do it. Stopping under control is almost always better than stopping a little shorter outta control.
midwestpa24
Final Approach
Not always true on ice. When testing breaking action on an icy runway, I've experienced the difference between rolling friction and locked tires. Once the tires lock, it almost feels like you speed up because the braking effect decreases so drastically.
*Note this is done in a vehicle with ABS disabled. When attempted with ABS, the ABS kicks in way before you achieve maximum braking ability. But in a controlled stop on an icy runway, there is not the sudden panic braking that ABS helps mitigate.
EdFred
Taxi to Parking
Rolling slower will stop faster than locked wheels on snow or ice, and once the wheels are locked, you aren't in control and since most roads around here are crowned, you're most likely going off roadin'.
Gary Ward
En-Route
My 2014 F250 has overdrive that you use normally. When towing or hauling a heavy load you turn the overdrive off and the trans will downshift to offer considerably better braking. Very helpful when hauling a 1000# snow plow on the front or towing down hill.
Trans operates cooler with the overdrive off during heavy work loads also.
All vehicles will stop shorter with less braking with overdrive turned off 2 or 4 wheel drive.
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luvflyin
Touchdown! Greaser!
I've had that feels faster feeling to with wheels locked. I'm talking about stopping distance though. Distance. But I'm rethinking the on ice thing. Maybe the friction from locking them up can heat things up, melting the ice, and adding hydroplaning to the equation.
luvflyin
Touchdown! Greaser!
Yeah. No argument there about more compression allowing less braking to achieve the same deceleration rate. That doesn't equate to less 'distance' though. If you need stop in the shortest possible distance, like there's a 50 ton semi jackknifed across the road ahead of you and you want the shortest possible stopping 'distance', locking the wheels is what gives that. Unless maybe on a sheet of ice where there's no chance of breaking through it and getting some rubber on the road. Maybe that hydroplaning thing will have an affect. I dunno.My 2014 F250 has overdrive that you use normally. When towing or hauling a heavy load you turn the overdrive off and the trans will downshift to offer considerably better braking. Very helpful when hauling a 1000# snow plow on the front or towing down hill.
Trans operates cooler with the overdrive off during heavy work loads also.
All vehicles will stop shorter with less braking with overdrive turned off 2 or 4 wheel drive.
luvflyin
Touchdown! Greaser!
If you go off the side of the road, that could considerably shorten your stopping distance depending on what's there.
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EdFred
Taxi to Parking
Mtns2Skies
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Completely wrong. Your friction coefficient between brake pads & rotor is significantly higher than the DYNAMIC coefficient of friction between tires and road. When the tire is spinning you are using the STATIC coefficicent of friction between rubber and asphalt/ice/whatever which has much more stopping power. Static coefficients of friction are higher than dynamic.
Skidding is 100% going to consume more distance than maximum effort rolling stop, every single time.
Mtns2Skies
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So much drama over a little bit of snow. It'll be fine.
luvflyin
Touchdown! Greaser!
I dunno. But I'm not giving up on what I think I know. The coefficient between brake pads and rotor has nothing to do with what I said. That is that wheels locked will give the shortest stopping distance. If the brakes lack the power to lock the wheels, that's another story. My rational is the friction between the tires and the road. A rolling tire is going to give less resistance to stopping than a locked up rubber screeching on road tire. Everything I've read about ABS says this. It does not reduce distance. All the little interruptions to braking increase it. As far as on a sheet of ice, I dunno. Maybe locked up tires kinda melting the ice could figure into it. I am talking stopping 'distance.' I have no argument that outta control, slippin' and a skiddin' stops are more of a problem than a little less 'distance.' At the end of the day it's just going to be the difference of a few MPH that you collide with something. And head on, while still on the road, in your lane, is much better than broadside all over the place. Hence, vehicles almost always have ABS nowadays. Even though it increases stopping 'distance.'
Mtns2Skies
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You're over thinking it. Static friction is stronger than dynamic friction. It's that simple. I highlighted a sentence you have provided that is flat out wrong. It may 'seem' shorter, but physics doesn't care about your feelings.
https://carsafetyphysics.weebly.com...eans that because the,a car with sliding ones.
Wagondriver
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375Taylor
Locked up sliding tires will take longer to stop. Why does everything now have antilock?
in 4 wheel drive, all the drive train is connected. Under heavy braking in 4x4, all the tires will be going the same speed. Most vehicles have the braking biased to the front, if in marginal traction, the front will lock up way before the rear. If the fronts are locked, you are not getting maximum speed reduction.
I know from a few decades of life in the mountains that I’d rather be in 4x4 going downhill. On flat icy highways it takes a lot more effort to get out of control with all 4 tires splitting up the transfer of power to the ground.
Don’t believe me, go try it.
in 4 wheel drive, all the drive train is connected. Under heavy braking in 4x4, all the tires will be going the same speed. Most vehicles have the braking biased to the front, if in marginal traction, the front will lock up way before the rear. If the fronts are locked, you are not getting maximum speed reduction.
I know from a few decades of life in the mountains that I’d rather be in 4x4 going downhill. On flat icy highways it takes a lot more effort to get out of control with all 4 tires splitting up the transfer of power to the ground.
Don’t believe me, go try it.
I think I heard somewhere that it preserves your ability to steer. Judging by an experience I had when a wrong-way driver almost hit me head-on on a highway one night, it seems to be true, at least on dry pavement. I was able to whip my car over into the other lane with the ABS actuated, missing the idiot by about a foot.
EdFred
Taxi to Parking
That was the rationale behind ABS. As drivers got dumber, the feds decided we needed countermeasures to allow idiots to keep contributing to the gene pool and mandated it.
Mxfarm
Line Up and Wait
Traction control is even worse than ABS on the slick stuff.
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Drake the Outlaw
No. ABS is an approximation of ideal braking. The part about Anti-Lock is the key. On ice or on dry pavement you do not want to lock the wheels for maximum braking. The coefficient of static friction (tire to road or ice) is greater than the coefficient of dynamic friction, and that gives you better stopping power. On gravel, snow, or sand, locked wheels *may* give you a shorter stop.
And 4WD doesn't help or hurt stopping distance. It helps in forward acceleration and in accelerating in curves.
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Hang 4
A rolling tire on ice has more traction than a not rolling one. The reason ice is slippery is because a layer of liquid water forms between the tire and the ice. The friction from a locked tire creates more heat, which creates more slippery.
pfarber
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pfarber
ABS is about CONTROL, not stopping distance.
Once you put on 4x4 you will slow down faster as you now how more friction in the drive train via the transfer case or whatever power splitting device the vehie uses.
A simple test. Drive up to 30mph and then coast in 2wd then 4wd. You will notice a difference.
Once you put on 4x4 you will slow down faster as you now how more friction in the drive train via the transfer case or whatever power splitting device the vehie uses.
A simple test. Drive up to 30mph and then coast in 2wd then 4wd. You will notice a difference.
luvflyin
Touchdown! Greaser!
Good article. Thx. I found a couple others that said pretty much the same thing. And some others that said ABS does increase stopping distance. All from what seem to be reputable sources. The road surface comes up, Pavement, gravel, ice, snow etc. I think I'm gonna have to put this up on the shelf with high wing or low wing, Foreflight or Garmin Pilot, slip or not with full flaps and other such subjects.
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luvflyin
Touchdown! Greaser!
Yeah. I've had a couple cars with that that tell you to turn it off on ice and deep snow. And there's another button to push if your trying to get going from a stop. I think It effectively locks the differentials.
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Drake the Outlaw
All you're doing there is adding mechanical friction. So, yes, you'll coast down slower with 4wd engaged. If you need to stop (not coast to a stop), you use ABS, which maximizes the braking action of each wheel. And since none of the wheels lock up, yes, you get better control too.
Mxfarm
Line Up and Wait
it reduces engine power to stop wheel spin - compensates for the low skill level of knowing when to be easy on the gas vs gitty-up go.
luvflyin
Touchdown! Greaser!
Ah. Like the ol’ start out in second gear with stick shifts thing.
Weird thread! Check out the Airbus and Boeing manuals for stopping distance with ABS on and off. Note the landing distance tables for both conditions.
There is one situation where I read a test and anti skid off resulted in a shorter stopping distance. This was pure ice with several inches of snow on top. In that one case the locked front wheels would plow the snow creating a dam of snow ahead of the front wheels slowing the car a bit faster. That of course came with zero control and required a unique condition.
There is one situation where I read a test and anti skid off resulted in a shorter stopping distance. This was pure ice with several inches of snow on top. In that one case the locked front wheels would plow the snow creating a dam of snow ahead of the front wheels slowing the car a bit faster. That of course came with zero control and required a unique condition.
UngaWunga
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can you borrow a snowblower? Should take a few hours...
Matthew Johnson
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I might have some experience here: my day job is tuning ESC, ABS, TCS, and rollover stability control. We use metrics like ABS efficiency (think mean deviation from optimal slip %) as pass/fail criteria on snow, ice, and dry pavement - and do instrumented tests of stopping distance with vs. without ABS. The short version: ABS will always win on non-deformable surfaces (wet pavement, dry pavement, and ice) and locked tires can sometimes stop shorter on deformable surfaces (snow, dirt, and sand). We do some tricks to try and detect deformable surfaces and modify the longitudinal slip targets for ABS to build up that wedge of sand or snow in front of the tires as well on some vehicles.
AWD/4WD/locking differentials are a mixed bag, they generally help keep the wheels at the same speed as you might expect and often that helps acceleration and deceleration, but they also can create control problems in getting a wheel back up to speed once it slips (very few 4WD vehicles have 3 locking differentials), and they create all sorts of issues with knowing exactly how fast the vehicle is traveling along the road (reference velocity) and therefore exactly how much each tire is slipping vs. the target.
The other thing about ABS is that it is your brake proportioning valve now, and the best way to reduce stopping distance is to put more braking on the rear axle - but that reduces vehicle stability under braking. Usually, we only do things like that in track mode - and are getting better at building control algorithms that sense the vehicle getting unstable and re-adjusting the brake proportioning dynamically to keep the vehicle stable enough while in minimizing stopping distance in ABS.
AWD/4WD/locking differentials are a mixed bag, they generally help keep the wheels at the same speed as you might expect and often that helps acceleration and deceleration, but they also can create control problems in getting a wheel back up to speed once it slips (very few 4WD vehicles have 3 locking differentials), and they create all sorts of issues with knowing exactly how fast the vehicle is traveling along the road (reference velocity) and therefore exactly how much each tire is slipping vs. the target.
The other thing about ABS is that it is your brake proportioning valve now, and the best way to reduce stopping distance is to put more braking on the rear axle - but that reduces vehicle stability under braking. Usually, we only do things like that in track mode - and are getting better at building control algorithms that sense the vehicle getting unstable and re-adjusting the brake proportioning dynamically to keep the vehicle stable enough while in minimizing stopping distance in ABS.
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Drake the Outlaw
Matthew, I absolutely agree with you for most drivers, and almost all drivers in emergency situations where the immediate reaction is to hammer the brake pedal.
Have you had the opportunity to compare ABS performance against superhumans like F1 drivers who are absolute masters of non-ABS braking? That would be an interesting comparison.
Matthew Johnson
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I got to spend ~4 years between IMSA DP, IMSA GTLM, and WEC GTE Pro - those guys are good (and a couple are ex F1) and the teams would have used ABS had it been legal hands down, no questions asked. All it takes is one little flat spot from a momentary lockup and it can ruin an entire race. That would take precedence over stopping distance, but I doubt that the same drivers could beat ABS stopping distances. Also, if we are talking about cars like F1 or LMP1/Hypercar there is a pretty good chance that at high vehicle speeds the driver simply can't push the brake pedal hard enough to lock up any tires due to the aerodynamic downforce.
The fundamental things that give the ABS computer a dominant advantage over the human are: 1) precise wheelspeed (direct real time measurement vs. humans can only hear sliding tires and sense slight decrease in deceleration), 2) cycle time (digital controller and quick/accurate actuation vs. ~100ms delay to chemically transmit the message to your leg muscles to back off the brakes), and 3) ABS has the ability to control each brake individually rather than lumping them all together in a single control (i.e. the human's best choice it to back off the brake pressure as soon as the tire with the least grip starts to slide thus leaving stopping distance on the table).
To be fair, I do notice that there are some subtle differences in ABS cycle time, precision/accuracy in controlling slip targets, and thus ABS efficiency that the driver can cause by how hard they push on the brake pedal on vacuum boosted brake systems. My pet theory is that mashing on the pedal makes the precision of the timing and modeling of the valve openings vs. fluid flow just a little worse - so usually just tickling the ABS is about the best stopping distance achievable (and the good racing/development drivers tend to do exactly that in vehicles with ABS).
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Albany Tom
I used to rent a place with a steep driveway, that let out to a city street. I had an '88 Bronco at the time, part time 4wd, C6 transmission, snow tires on all four wheels. I was too lazy to run the snow blower, and it was safe enough to risk sliding into the street. By a good margin the most controllable way to get down that hill when it was icy was 4wd, and the transmission in neutral. I could creep that truck down the hill when it was too slippery to walk on.
ABS and traction control are awesome on intermittent problems on the highway. They will react to patchy ice spots on a road way better than any person could. In my current truck, Nissan, the traction control works great even in 4WD. The ABS works pretty well, too. But in snow, threshold breaking is usually more effective, and you can do that with the ABS engaged. At slow speed, with the front/rear diff locked, that's pretty easy to do. In deep snow, locking them up can be the quickest way, because as you slide you're kind of 'plowing' the snow with the tires.
In light snow, or patchy snow on mostly clear roads, I run in 2wd. Much easier on the drivetrain, and it works just fine.
ABS and traction control are awesome on intermittent problems on the highway. They will react to patchy ice spots on a road way better than any person could. In my current truck, Nissan, the traction control works great even in 4WD. The ABS works pretty well, too. But in snow, threshold breaking is usually more effective, and you can do that with the ABS engaged. At slow speed, with the front/rear diff locked, that's pretty easy to do. In deep snow, locking them up can be the quickest way, because as you slide you're kind of 'plowing' the snow with the tires.
In light snow, or patchy snow on mostly clear roads, I run in 2wd. Much easier on the drivetrain, and it works just fine.
thito01
Pre-takeoff checklist
Well, it looks like it's going to snow in TN again this weekend. I hope the OP is not down south again. His little rant has already created 4 pages. If he is gets 'notam grounded' again, we may get another 4 pages.
Rockymountain
Pre-takeoff checklist
Happened to me a month or so ago. Airport was open, but taxiways were nil braking and closed. Which was not on the ATIS btw. Called ground for my clearance, and they said they can’t clear me because the taxiways are closed. The airport manager heard that, and said I could taxi at my own risk, and then ground gave me my clearance. Only time that has happened to me. I fly a turboprop, so not much need for brakes. Little nervous about having all that on radio, if I did slide off, but all was fine. Tower asked me for a report, and I said braking was very poor, avoided the word nil, but added that I would not recommend any aircraft without reverse thrust taxi.
I definitely listen to NOTAMs on Alton Bay. Saw a picture of a truck that went through the ice.
PaulS
Touchdown! Greaser!
A truck went through the ice on Berry Pond. It was checking the ice thickness in preparation for an ice race. It had a snow plow on it. Apparently it hit a soft spot and the front end just dropped in. The driver managed to get out, his 70 year old friend did not.