ABS brakes
in [Drive]
|
From: Matthew Russotto on 8 May 2010 20:11 In article <rs8vt5d3522s43sircri0i60a10hndfh9p(a)4ax.com>, Ashton Crusher <demi(a)moore.net> wrote: >On Wed, 28 Apr 2010 19:12:05 -0500, russotto(a)grace.speakeasy.net >(Matthew Russotto) wrote: > >>In article <slrnhthhs7.er2.aznomad.3(a)ip70-176-155-130.ph.ph.cox.net>, >>AZ Nomad <aznomad.3(a)PremoveOBthisOX.COM> wrote: >>> >>>I have too. You're not driving; merely a passenger when you're >>>sliding like that. You also run the risk of turning sideways, >>>backwards, or rolling the vehicle if you encounter sudden friction >>>while turned sideways. >> >>You're not a passenger when sliding with all four wheels locked; >>you've reached the limits of your control and will have to back off >>the brakes if you want to do anything else, but you're not out of >>control. > >You are on low friction wet pavement. Coefficient of friction of rubber on wet asphalt is lower than on dry, but it's far from zero. >>You're not going to turn sideways > >If there is lateral differential friction, which occurs often enough >it can be an issue, you will spin anywhere from 10's of degrees to >360+ degrees, all the while going "straight" ahead. With locked wheels on wet pavement, there's essentially never going to be significant lateral differential friction. Where's it going to come from? If you're off the road or partially off the road, that's a different story. -- The problem with socialism is there's always someone with less ability and more need.
From: Ashton Crusher on 9 May 2010 01:18 On Sat, 08 May 2010 19:11:47 -0500, russotto(a)grace.speakeasy.net (Matthew Russotto) wrote: >In article <rs8vt5d3522s43sircri0i60a10hndfh9p(a)4ax.com>, >Ashton Crusher <demi(a)moore.net> wrote: >>On Wed, 28 Apr 2010 19:12:05 -0500, russotto(a)grace.speakeasy.net >>(Matthew Russotto) wrote: >> >>>In article <slrnhthhs7.er2.aznomad.3(a)ip70-176-155-130.ph.ph.cox.net>, >>>AZ Nomad <aznomad.3(a)PremoveOBthisOX.COM> wrote: >>>> >>>>I have too. You're not driving; merely a passenger when you're >>>>sliding like that. You also run the risk of turning sideways, >>>>backwards, or rolling the vehicle if you encounter sudden friction >>>>while turned sideways. >>> >>>You're not a passenger when sliding with all four wheels locked; >>>you've reached the limits of your control and will have to back off >>>the brakes if you want to do anything else, but you're not out of >>>control. >> >>You are on low friction wet pavement. > >Coefficient of friction of rubber on wet asphalt is lower than on dry, >but it's far from zero. > >>>You're not going to turn sideways >> >>If there is lateral differential friction, which occurs often enough >>it can be an issue, you will spin anywhere from 10's of degrees to >>360+ degrees, all the while going "straight" ahead. > >With locked wheels on wet pavement, there's essentially never going to >be significant lateral differential friction. Where's it going to >come from? If you're off the road or partially off the road, that's a different story. The amount of friction is determined by the combination of road surface and tire. Given the same tire, different road surfaces with that tire can vary in friction when wet from as high as 0.7 on a good open textured asphalt or chip seal surface to as low as 0.2 on a dense, poorly textured, over-asphalted or polished chip seal surface. On a worn surface it would not be unusual to have a variation laterally as well as longitudinally of anywhere from 0.1 to 0.3 around the mean value. The lateral variation is why cars rotate and the lateral and longitudinal variation is why a good ABS does better then a human at threshold braking. On dry pavement the differences in friction variation are typically far less because texture depth is far less important as there is no water film that must be displaced. Almost all dry pavements will have friction values of 0.8 or higher but with locked wheels it's debatable if you should even call it friction since you are likely to be shedding rubber.
From: Ashton Crusher on 9 May 2010 01:25 On Sat, 08 May 2010 19:07:59 -0500, russotto(a)grace.speakeasy.net (Matthew Russotto) wrote: >In article <e18vt5t8qcr4cjcibu7qkmveb455gbdqfk(a)4ax.com>, >Ashton Crusher <demi(a)moore.net> wrote: >>On Wed, 28 Apr 2010 18:01:52 -0500, russotto(a)grace.speakeasy.net >>(Matthew Russotto) wrote: >> >>>In article <bbeft5hfniv2tfhknmpm1oh71jpbp3m65n(a)4ax.com>, >>>Ashton Crusher <demi(a)moore.net> wrote: >>>> >>>>I'm not too keen on the idea of "effective" being that when the >>>>pavement is wet is to go sliding out of control. >>> >>>If you lock up all four wheels, the car does not go "sliding out of >>>control". It behaves in a quite predictable manner, sliding >>>straight in the direction it was going when the wheels locked up if on >>>a flat surface. >> >> >>If by predictable you mean your ONLY choice is to *hope* you don't hit >>whatever winds up in front of you since you have no control over your >>trajectory then sure. > >No, that's not your only choice. You can always release the brakes >and try something else. > >>If by predictable you mean that you know you'll >>be aimed somewhere but you don't know if it will be aimed forward, 45 >>degrees, sideways, or backwards, or god knows which way, then yeah, >>it's real predictable. > >IME, doesn't happen. The car rotates only slightly, unless you were >spinning to begin with. What you're describing sounds more like >driving on wet ice than merely locking the brakes. It may be that your experience does not encompass the full range of what can happen. As I said before, I've done hundreds of skid tests and on low friction surfaces with differential friction it is not at all unusual to experience significant rotation. Some low friction roads are barely better then ice when wet. Many of my tests were done where accidents had happened or where roadway conditions were suspect, IOW the selected areas would be expected to have poor surfaces. I suspect the places you have been on would be expected to have good surfaces and if that's the case it would not be surprising if you didn't notice problems.
From: Matthew Russotto on 9 May 2010 14:25 In article <qfgcu5lre44kf9n7odkgg877r84k9d4v7q(a)4ax.com>, Ashton Crusher <demi(a)moore.net> wrote: >On Sat, 08 May 2010 19:11:47 -0500, russotto(a)grace.speakeasy.net >(Matthew Russotto) wrote: >> >>With locked wheels on wet pavement, there's essentially never going to >>be significant lateral differential friction. Where's it going to >>come from? If you're off the road or partially off the road, that's a different story. > >The amount of friction is determined by the combination of road >surface and tire. Given the same tire, different road surfaces with >that tire can vary in friction when wet from as high as 0.7 on a good >open textured asphalt or chip seal surface to as low as 0.2 on a >dense, poorly textured, over-asphalted or polished chip seal surface. While I've been on roads with different surfaces between the left side of the lane and the right, they're not common even in Pennsylvania. I'll agree that significant rotation would be likely when locking the wheels on such a surface. >On a worn surface it would not be unusual to have a variation >laterally as well as longitudinally of anywhere from 0.1 to 0.3 around >the mean value. The lateral variation is why cars rotate and the >lateral and longitudinal variation is why a good ABS does better then >a human at threshold braking. The surface tends to wear in the tire tracks, so while total variation might be high, variation between braking surfaces is lower. Further, when the surface is very worn, you get ruts, which results in a slot-car effect limiting rotation. -- The problem with socialism is there's always someone with less ability and more need.
From: Nate Nagel on 9 May 2010 17:37
On 05/09/2010 02:25 PM, Matthew Russotto wrote: > In article<qfgcu5lre44kf9n7odkgg877r84k9d4v7q(a)4ax.com>, > Ashton Crusher<demi(a)moore.net> wrote: >> On Sat, 08 May 2010 19:11:47 -0500, russotto(a)grace.speakeasy.net >> (Matthew Russotto) wrote: >>> >>> With locked wheels on wet pavement, there's essentially never going to >>> be significant lateral differential friction. Where's it going to >>> come from? If you're off the road or partially off the road, that's a different story. >> >> The amount of friction is determined by the combination of road >> surface and tire. Given the same tire, different road surfaces with >> that tire can vary in friction when wet from as high as 0.7 on a good >> open textured asphalt or chip seal surface to as low as 0.2 on a >> dense, poorly textured, over-asphalted or polished chip seal surface. > > While I've been on roads with different surfaces between the left side of > the lane and the right, they're not common even in Pennsylvania. I'll > agree that significant rotation would be likely when locking the > wheels on such a surface. > >> On a worn surface it would not be unusual to have a variation >> laterally as well as longitudinally of anywhere from 0.1 to 0.3 around >> the mean value. The lateral variation is why cars rotate and the >> lateral and longitudinal variation is why a good ABS does better then >> a human at threshold braking. > > The surface tends to wear in the tire tracks, so while total variation > might be high, variation between braking surfaces is lower. Further, > when the surface is very worn, you get ruts, which results in a > slot-car effect limiting rotation. > ....but also the possibility of water building up in one rut but not the other due to differences in drainage, causing the possibility of *significant* split-mu in icy conditions. I've seen this also where for whatever reason the shoulder was not graded properly; the right tire track can be glare ice while the left tire track may be nearly dry pavement once the sun's been out for a few hours. nate -- replace "roosters" with "cox" to reply. http://members.cox.net/njnagel |