ABS brakes
in [Cars]
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From: Dave__67 on 27 Apr 2010 07:39 On Apr 26, 2:41 pm, Alan Baker <alangba...(a)telus.net> wrote: > In article > <9bd36917-1c4c-48ba-bb9b-7061a9d6a...(a)z7g2000yqb.googlegroups.com>, > > > > Dave__67 <spamTHIS...(a)yahoo.com> wrote: > > On Apr 22, 9:04 pm, Ashton Crusher <d...(a)moore.net> wrote: > > > On Thu, 22 Apr 2010 05:08:07 -0700 (PDT), Dave__67 > > > > <spamTHIS...(a)yahoo.com> wrote: > > > >On Apr 21, 11:55 pm, "Nick Naim" <orb...(a)ix.netcom.com> wrote: > > > >> I pegged her at 70 years old plus minus. > > > >> She pulled out of a fast food on my left in a heart beat > > > >> I,m in the left lane of thru traffic. > > > >> I,m on the binders on ABS and they are working on overtime. > > > >> I come to a stop 3 feet from her right front fender. > > > > >If your ABS is kicking on on dry clean pavement you need better tires- > > > >you're throwing away stopping distance! > > > > >Dave > > > > How do y9ou figure that? ABS works, wet or dry, to maximize the > > > amount of time the tire is at or near peak friction level. > > > No, it does not. > > > ABS is designed to allow you to still steer while mindlessly whomping > > on the brakes. To do so it has to leave a little traction on the > > plate. > > Again, not quite. ABS will reduce the amount of force applied to the > brakes whenever conditions result in the tire starting to lock up. > > One of those conditions could be a change in the friction of the > surface, but one could also be that you've turned the wheel. > > So there is no traction "left on the plate" in the way you meant. > > > > > Anyway, on dry pavement, if you can lock up the brakes, your brakes > > are better than your tires. If you improve the grip of the tires > > you'll stop faster. > > Not just on dry pavement. :-) > > -- > Alan Baker > Vancouver, British Columbia > <http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg> If the algorithms and sensors were perfect, and there were no control loop delay, sure. Dave
From: Alan Baker on 27 Apr 2010 13:37 In article <e00930b2-999a-4467-adec-85b62b9093e5(a)v14g2000yqb.googlegroups.com>, Dave__67 <spamTHISbrp(a)yahoo.com> wrote: > On Apr 26, 2:41�pm, Alan Baker <alangba...(a)telus.net> wrote: > > In article > > <9bd36917-1c4c-48ba-bb9b-7061a9d6a...(a)z7g2000yqb.googlegroups.com>, > > > > > > > > �Dave__67 <spamTHIS...(a)yahoo.com> wrote: > > > On Apr 22, 9:04�pm, Ashton Crusher <d...(a)moore.net> wrote: > > > > On Thu, 22 Apr 2010 05:08:07 -0700 (PDT), Dave__67 > > > > > > <spamTHIS...(a)yahoo.com> wrote: > > > > >On Apr 21, 11:55�pm, "Nick Naim" <orb...(a)ix.netcom.com> wrote: > > > > >> I pegged her at 70 years old plus minus. > > > > >> She pulled �out of a fast food on my left in a heart beat > > > > >> I,m in the left lane of thru traffic. > > > > >> I,m on the binders on ABS and they are working on overtime. > > > > >> I come to a stop 3 feet from her right front fender. > > > > > > >If your ABS is kicking on on dry clean pavement you need better tires- > > > > >you're throwing away stopping distance! > > > > > > >Dave > > > > > > How do y9ou figure that? �ABS works, wet or dry, to maximize the > > > > amount of time the tire is at or near peak friction level. � > > > > > No, it does not. > > > > > ABS is designed to allow you to still steer while mindlessly whomping > > > on the brakes. To do so it has to leave a little traction on the > > > plate. > > > > Again, not quite. ABS will reduce the amount of force applied to the > > brakes whenever conditions result in the tire starting to lock up. > > > > One of those conditions could be a change in the friction of the > > surface, but one could also be that you've turned the wheel. > > > > So there is no traction "left on the plate" in the way you meant. > > > > > > > > > Anyway, on dry pavement, if you can lock up the brakes, your brakes > > > are better than your tires. If you improve the grip of the tires > > > you'll stop faster. > > > > Not just on dry pavement. :-) > > > > -- > > Alan Baker > > Vancouver, British Columbia > > <http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg> > > If the algorithms and sensors were perfect, and there were no control > loop delay, sure. Sorry, but they don't have to be perfect. Just better than human. And give the speed of human reactions, that's not that hard. -- Alan Baker Vancouver, British Columbia <http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>
From: Matthew Russotto on 27 Apr 2010 22:29 In article <rda9t5d7puuaqspomilea9hlpe8gpe3t9f(a)4ax.com>, Ashton Crusher <demi(a)moore.net> wrote: > >I don't think the design goal is to be "oscillating around" peak >friction, that would be highly unstable. It is unstable. That's why you need an active control system to maintain it. -- The problem with socialism is there's always someone with less ability and more need.
From: Matthew Russotto on 27 Apr 2010 22:38 In article <57q9t5lr4jhb5hervqhb2q51b3poff1eig(a)4ax.com>, Ashton Crusher <demi(a)moore.net> wrote: >>No. ABS is very close to threshold braking, but it is not. It is a very >>rapid sequence of going slightly beyond the threshold and then back to >>less than the threshold. > >Where you are getting that definition from? It makes no sense to me >and is contrary to what people have claimed in the past. What's your >definition of the "threshold" point? Take a chart of brake pressure (at the caliper or shoe) versus braking force applied to the road by that tire. It'll look basically like a line going from 0 upwards fairly linearly, leveling off slightly as it nears a peak, followed by a sharp dropoff to a horizontal line. Perfect threshold braking would be holding braking pressure right at that peak. -- The problem with socialism is there's always someone with less ability and more need.
From: Ashton Crusher on 28 Apr 2010 00:36
On Mon, 26 Apr 2010 23:19:32 -0700, Alan Baker <alangbaker(a)telus.net> wrote: >In article <r1uct5hggm69oh0jhjpv7gdceodkjadlvp(a)4ax.com>, > Ashton Crusher <demi(a)moore.net> wrote: > >> On Mon, 26 Apr 2010 03:01:28 -0700, Alan Baker <alangbaker(a)telus.net> >> wrote: >> >> >In article <57q9t5lr4jhb5hervqhb2q51b3poff1eig(a)4ax.com>, >> > Ashton Crusher <demi(a)moore.net> wrote: >> > >> >> On Sun, 25 Apr 2010 14:32:51 -0700, Alan Baker <alangbaker(a)telus.net> >> >> wrote: >> >> >> >> >In article <dp67t55v3elj8vebi9ti24rlrqtr2hhl8b(a)4ax.com>, >> >> > Ashton Crusher <demi(a)moore.net> wrote: >> >> > >> >> >> On Thu, 22 Apr 2010 21:48:20 -0700, Scott in SoCal >> >> >> <scottenaztlan(a)yahoo.com> wrote: >> >> >> >> >> >> >Last time on rec.autos.driving, Ashton Crusher <demi(a)moore.net> said: >> >> >> > >> >> >> >>On Thu, 22 Apr 2010 21:47:45 +0000, Alexander Rogge >> >> >> >><a_rogge(a)yahoo.com> wrote: >> >> >> >> >> >> >> >>>Nick Naim wrote: >> >> >> >>>> I pegged her at 70 years old plus minus. >> >> >> >>>> She pulled out of a fast food on my left in a heart beat >> >> >> >>>> I,m in the left lane of thru traffic. >> >> >> >>>> I,m on the binders on ABS and they are working on overtime. >> >> >> >>> >> >> >> >>>The ABS should not be activating on a dry surface, as this increases >> >> >> >>>your stopping distance. >> >> >> >>> >> >> >> >> >> >> >> >>ABS will activate on a dry surface the same as on a wet surface >> >> >> >>whenever the tire is about to lock. And in the process it shortens >> >> >> >>your stopping distance, not increases it. >> >> >> > >> >> >> >It shortens your stopping distance vs. screeching to a stop with >> >> >> >locked brakes, sure. >> >> >> > >> >> >> >But true threshold braking is the shortest stopping distance of all. >> >> >> >> >> >> ABS is threshold braking. And any decent ABS is able to do it far >> >> >> better then a human can, particularly on wet pavement. >> >> > >> >> >No. ABS is very close to threshold braking, but it is not. It is a very >> >> >rapid sequence of going slightly beyond the threshold and then back to >> >> >less than the threshold. >> >> >> >> Where you are getting that definition from? It makes no sense to me >> >> and is contrary to what people have claimed in the past. What's your >> >> definition of the "threshold" point? >> > >> >The point of maximum deceleration, of course. Isn't that obvious? >> >> >> If that's your definition I don't see how human can successful >> threshold brake for more then a very very short period before they >> lock their tires or lose the peak. Critical slip for dry is about 26% >> slip and wet about 13%. That's the point of peak deceleration and it >> drops off on either side. If you put the tires there and the pavement >> changes slightly the wheels will either lock or you will no longer be >> at your defined threshold. I'm sure people call it threshold braking >> but I'm also fairly certain it's not actually where they're at since >> the peak is not likely to be a point a human can maintain, it's not a >> stable point on the chart. More likely, when people think they are >> "threshold braking" they are slightly off peak so they can feel >> changing surface friction and react to it before a tire locks. > >Yup. > >That's the challenge of threshold braking. > >But although I do think that threshold braking is sufficiently difficult >that there are very few who can actually better a good ABS system's >stopping distances, you must remember that unless graph of slip vs >friction is very different from most physical phenomenon, almost all of >the benefit can be achieved by getting *close* to the value of maximum >slip. Typically, such curves only have a very modest slope near their >maxima and thus as you approach the peak you get smaller and smaller >increments of increase in the y-axis value (in this case: friction) for >the constant increases in the x-value (slip). > >IOW, you get almost all the benefit of the maximum friction from nearly >reaching the maximum slip. > >And you have your two figures backwards, BTW. It's 13% for dry pavement >and 26% for wet. > The results vary with researcher. http://www.itk.ntnu.no/ansatte/Johansen_Tor.Arne/thesisIdarPetersen.pdf shows the more common result where the wet pavement slip % is lower then dry. Similarly for the shapes of the curve, different researchers get different shapes depending on the tires, water film thickness, speed, etc. ><http://www.mm.bme.hu/~stepan/mm/journal2003/olson_shaw_stepan_vsd.pdf> > >Note the shape of the curves, BTW, and you'll see that particularly for >wet braking, you can have considerably less slip than the amount for >maximum possible friction and still be doing pretty well. > >You'll also see why in a car without ABS it can be more effective to >simply kick the brake pedal to lock all four wheels as close to >instantaneously as possible (and getting more than 90% of the maximum >possible braking) rather than try and find the threshold point (and >moving very quickly while trying to find it). I'm not too keen on the idea of "effective" being that when the pavement is wet is to go sliding out of control. |