ABS brakes
in [Cars]
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From: Ashton Crusher on 9 May 2010 21:13 On Sun, 09 May 2010 13:25:05 -0500, russotto(a)grace.speakeasy.net (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. Nate pretty much covered a large portion of the rut issue. I would add to his comments that the ruts may not be uniform in depth either from side to side or lengthwise, which will change the water depth, Changes in water depth can cause a variation in the effective friction level even if the underlying road surfaces are about the same. This will be particularly the case on crowned roads where the left rut is on a part of the pavement that has less cross slope then the right rut and therefore is likely to have a greater water depth in it in heavy rain.
From: Ashton Crusher on 10 May 2010 01:25
On Fri, 07 May 2010 22:17:14 -0700, Alan Baker <alangbaker(a)telus.net> wrote: >In article <hqo9u5dr0n0f6bd2624293pn3jftt3nl3v(a)4ax.com>, > Ashton Crusher <demi(a)moore.net> wrote: > >> On Wed, 05 May 2010 01:24:51 -0700, Alan Baker <alangbaker(a)telus.net> >> wrote: >> >> >In article <cpu1u5d24dcg73osel1vfuf3nrsvv028cp(a)4ax.com>, >> > Ashton Crusher <demi(a)moore.net> wrote: >> > >> >> On Mon, 03 May 2010 23:50:20 -0700, Alan Baker <alangbaker(a)telus.net> >> >> 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. >> >> > >> >> >Nope. Because you can release the brakes whenever you need to. >> >> > >> >> >> >> If the need is to stop, releasing the brakes is not "control". >> > >> >If the need is only to stop, there is no need for any other control. >> > >> >> >> >> >> >> >> >> >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. >> >> > >> >> >Nope. I've made a lot of test runs, and it doesn't happen. >> >> > >> >> >Empirical data trumps theory every time. >> >> >> >> >> >> I've made hundreds of test runs on different friction levels of both >> >> wet and dry pavement including both uniform and non-uniform lateral >> >> friction and I can assure you that from real world empirical data, not >> >> theory, that the car with four locked tires will spin as I previously >> >> described. If all you've done is some runs on dry pavement or wet >> >> pavement with good uniform friction you simply have not experienced >> >> the full range of what can happen. I have also seen many people try >> >> to do these tests and who think they have locked all the wheels but >> >> they have not, including experienced "expert" accident >> >> reconstructionists. >> > >> >Except I know how to lock up all four wheels and I deliberately did >> >tests in non-uniform lateral friction situations. >> > >> >> If all four wheels were locked and there was a friction differential >> between the left wheel path and the right wheel path the physics >> dictates that the car WILL rotate. If your car did not rotate either > >True: as far as it goes. > >The question is *how much*. > >> the wheels were not locked or there was no differential friction. >> Perhaps you are thinking I'm referring to a "spin" in the sense of a >> spinning top, that would be rare, it's a rotation that continues as >> long as the car is moving and as long as the friction differential >> exists. The greater the differential and the higher the starting >> speed (hence longer the stop) the greater the amount of rotation. Most >> of the time it's rarely more then 30 to 45 degrees. I have been on >> wet low friction surfaces sufficient to allow a full 360 degrees of >> rotation all the while going "straight" ahead. I've also been on >> uniform low friction surfaces where the car did not rotate but slid >> sideways "downhill" off the crown toward the side of the road. >> >> >> >Look up John Powell: >> > >> ><http://www.powellmotorsport.com/index.php?module=htmlpages&func=display& >> >pid=24> >> >> >> And?? I saw nothing applicable to this discussion there. > >Naturally. If you think there is something there, please point it out. All I saw was what amounted to an ad. |