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Post by the light works on Nov 11, 2013 15:33:40 GMT
here, the reputable tire centers pull the label before the tire goes on the spin balancer. (or usually when they first set it on the balancer, because it puts it easy to reach.)
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Post by c64 on Mar 22, 2014 14:25:29 GMT
ever since traffic collision analysis was invented, analysts have used skid marks as evidence in investigating the crashes; but that all changed when the proliferation of ABS systems made skid marks all but disappear. Actually, even without ABS the skidmarks could only give you a "minimum speed" estimate. You could have started to brake without skid marks and the brake balance of the car makes a huge differnce as does the road condition and tire condition. Also busted shock absorbers can increase the braking distance by up to 40%! And with ABS, you still have skid marks! They are much fainter because the tire is still grinding while rotating when exposed to high forces. Also the ABS waits until a wheel locks before it frees it so you always have a telltale of dots when the ABS was active. The "mile long" skidmarks you can sometimes see on highways are caused by locked trailer brakes caused by a defect. Ordinary drivers don't notice such things, they just depress the accelerator deeper without thinking. In light rain when there is not much moisture on the road, the tracks of a vehicle appear visually dry. The task of the tread on a tire is to displace the water to have a solid contact between the road and the tire. Part of this water is kicked to the side and most of it is kicked up. That's why your windshield turns all dirty out of a sudden when a large truck passes you. All that dirt you get on your car is missing in the track of that vehicle. |
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Post by the light works on Mar 22, 2014 14:40:15 GMT
ever since traffic collision analysis was invented, analysts have used skid marks as evidence in investigating the crashes; but that all changed when the proliferation of ABS systems made skid marks all but disappear. Actually, even without ABS the skidmarks could only give you a "minimum speed" estimate. You could have started to brake without skid marks and the brake balance of the car makes a huge differnce as does the road condition and tire condition. Also busted shock absorbers can increase the braking distance by up to 40%! And with ABS, you still have skid marks! They are much fainter because the tire is still grinding while rotating when exposed to high forces. Also the ABS waits until a wheel locks before it frees it so you always have a telltale of dots when the ABS was active. The "mile long" skidmarks you can sometimes see on highways are caused by locked trailer brakes caused by a defect. Ordinary drivers don't notice such things, they just depress the accelerator deeper without thinking. In light rain when there is not much moisture on the road, the tracks of a vehicle appear visually dry. The task of the tread on a tire is to displace the water to have a solid contact between the road and the tire. Part of this water is kicked to the side and most of it is kicked up. That's why your windshield turns all dirty out of a sudden when a large truck passes you. All that dirt you get on your car is missing in the track of that vehicle. you would be amazed at how much you can learn from skid marks. and yes, we had a great "mile long" yesterday in town. it looked like a tractor trailer with underinflated tires had a spring brake stick on one wheel - but yeah - no. with ABS you have to know where to look and look extremely close to see any kind of marks from a vehicle in a full panic stop with modern ABS working properly. which is why it was noteworthy that we looked and saw a clear set of "skid marks" that were much more persistent than simple water displacement. of course, we look at things as minute as gravel displacement to get clues. |
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Post by c64 on Mar 22, 2014 14:57:46 GMT
I never said you can't make use of skid marks. But you can only tell a minimum speed so it's good enough to tell that the driver must have been faster than the speed limit but you can't tell by how much. For example, the mechanical brake balance regulator of my car can stop the car much faster than any car without this system and without ABS. In special circumstances, I can stop even faster than any car with ABS. So the speed you would figure out on my skid marks can be a lot lower than I was driving - if I was alert enough to brake just right  |
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Post by the light works on Mar 22, 2014 15:18:16 GMT
I never said you can't make use of skid marks. But you can only tell a minimum speed so it's good enough to tell that the driver must have been faster than the speed limit but you can't tell by how much. For example, the mechanical brake balance regulator of my car can stop the car much faster than any car without this system and without ABS. In special circumstances, I can stop even faster than any car with ABS. So the speed you would figure out on my skid marks can be a lot lower than I was driving - if I was alert enough to brake just right but I can tell if you were on the brakes, I can tell what path you followed, I can tell if you had lateral bias on your skid, I can get an idea if you were trying to turn, I can tell if your car was tilting. I can even get an idea how HARD you were on the brakes. remember: my investigation is what happened. who is at fault is for lawyers. |
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Post by silverdragon on Apr 2, 2014 8:49:03 GMT
But can you tell what caused the skid.... or how the skid started....
Was it down to the driver not seeing the hazard, was it down to the driver ignoring the hazard, or was it down to driver incompetence.
That is what the lawyers argue.
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Post by the light works on Apr 2, 2014 14:47:19 GMT
But can you tell what caused the skid.... or how the skid started.... Was it down to the driver not seeing the hazard, was it down to the driver ignoring the hazard, or was it down to driver incompetence. That is what the lawyers argue. to a degree we can draw conclusions about what mechanical factors were causing the car to skid: I.E. if there is a malfunction that causes a wheel to lock up. is that what you are referring to? but yes - the police only determine if there is reason to give a traffic citation. beyond that, it is just a question of what happened in what order. |
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Post by silverdragon on Apr 3, 2014 9:42:24 GMT
I ask because I was witness to an accident, where the accident could have been avoided.
I had a few questions afterwards, like why didnt "she" brake earlier.....
What I saw was a complete twit just pull out of a junction and get hit in the side by the car that they should have waited to pass before they pulled out... but that passing car appeared to either not notice, or ignore the idiot, until it was much to late, then emergency brake with all wheels leaving black stripes down the road.
On hearing the statement from the lady, she saw the car emerging, and panicked, she didnt believe what she was seeing, and panicked some more, then braked, HARD...
I have sympathy for her, heck even I didnt believe that the car pulled out THAT late right in front of another vehicle?... my statement echoed the disbelief.
However..... I do believe that if she had been trained to expect the unexpected, in the way of Hazard perception taught to us Truck drivers, she would have been watching that car and almost expecting it to pull out at the last moment, and been prepared to brake.
If so, I think maybe she would have stopped with a much lighter collision... or missed that car completely by using the other side of the road....
By what was on the road in Skid marks, I dont think it would have shown the full picture?...
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Post by the light works on Apr 3, 2014 15:21:08 GMT
I ask because I was witness to an accident, where the accident could have been avoided. I had a few questions afterwards, like why didnt "she" brake earlier..... What I saw was a complete twit just pull out of a junction and get hit in the side by the car that they should have waited to pass before they pulled out... but that passing car appeared to either not notice, or ignore the idiot, until it was much to late, then emergency brake with all wheels leaving black stripes down the road. On hearing the statement from the lady, she saw the car emerging, and panicked, she didnt believe what she was seeing, and panicked some more, then braked, HARD... I have sympathy for her, heck even I didnt believe that the car pulled out THAT late right in front of another vehicle?... my statement echoed the disbelief. However..... I do believe that if she had been trained to expect the unexpected, in the way of Hazard perception taught to us Truck drivers, she would have been watching that car and almost expecting it to pull out at the last moment, and been prepared to brake. If so, I think maybe she would have stopped with a much lighter collision... or missed that car completely by using the other side of the road.... By what was on the road in Skid marks, I dont think it would have shown the full picture?... yes, if people were to be in more comprehensive driving schools, they would have a better time of it when it all hits the pot. but yes, that is a very common phenomenon - and why people are beginning to say that a two second following distance is a little bit too close for the average modern driver. first they have to recognize that something is going wrong - then that they are going to have to be the ones to do something about it. but if you don't believe that, then consider one I went to. the driver coming from the side RAN OVER the hood of the car he pulled out in front of. one of my "investigation support" tasks was teaching the lead police officer how to bull the bulbs from the headlights to check the filaments for deformation. (if the headlights are on, the hot filament will stretch before it breaks) |
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Post by silverdragon on Apr 4, 2014 6:04:49 GMT
I know that one...
Or DID know that one...
Good luck doing that to my car.?... or many modern cars...
Because of the woeful dim tock-h "Ran out of batteries" not-quite-a-glow, and I have seen brighter firefly's, I am serious about that, I have upgraded my side-lights to LED....
Lets see you inspect the filament from THEM.
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Post by c64 on Apr 4, 2014 12:59:56 GMT
teaching the lead police officer how to bull the bulbs from the headlights to check the filaments for deformation. (if the headlights are on, the hot filament will stretch before it breaks) That's correct. The trouble is that during a hard crash, the filaments may be lost. Also the turn signal has a 50:50 chance to become deformed during a crash so you can't really tell if it was set and didn't "record" the crash or not on. In classic EU compliant cars, the dashboard indicator is connected in parallel to the relay contact ¹) so the indicator and turn signal bulbs flash alternately. So after a crash, either the turn signals or the indicator must be deformed when the turn signal was set. When my first car I owned was introduced to the market, there were lots of discussions if it should have a "black box" like an airplane due to its LED indicators ¹). The latest cars with their LED lights and/or Xenon do record a lot of things while driving so at least the manufacturer can figure out what had happened.
¹) This is the classic EU turn signal circuit:
VCC
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.-o-o--------.
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| o |Indicator
_|_ '\ .-.
|_/_|- \ ( X )
| o \ '-'
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'-o-o--------'
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o Turn signal switch
\
\
o \o
F/R left | | F/R right
.---o' 'o----.
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.-. .-. .-. .-.
( X ) X ) ( X )( X )
'-' '-' '-' '-'
| | | |
'---o--o--o----'
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===
GND
The relay is a resistor which heats up a bimetal switch. When the turn signal switch selects one side of the turn signals (or both for the hazard lights), the circuit is completed. The resistor inside the relay is too big to make the lights glow but heats them up somewhat. Almost the full voltage of the system is dropped over the relay which makes the turn signal indicator light up. When the relay becomes hot enough, it short circuits it self. The relay starts to cool down while the turn signal lights are switched on. The turn signal indicator however is short circuited so it's off. After cooling down, the relay opens its contact so the turn signals turn off but again, a small heater current remains. The beauty of this circuit is that the resistance of the turn signal bulbs are part of the circuit. If a bulb is dead, the heater current is reduced and this affects the hysteresis of the heater plus thermal switch. So if you have a broken turn signal, the clicking speed of the relay (needs to be audible by EU rules) and the flashing of the dashboard indicator speeds up. This is done by isolating the contact somewhat from the heater causing a thermal delay. The faster the heater heats up, the longer it takes until the relay clicks off again. If the relay is dead, you either get a constant indicator light and the turn signals are dead, too or you get no indicator and the turn signals are glowing permanently. Either way, you can see that something is wrong and the garage will instantly know if its the relay or the turn signal switch or the wiring. The heating current prevents that the filaments cool down much so they are not cold started every time the relay clicks on which increases the lifetime of the bulbs dramatically. If you attach a trailer, you have 3 bulbs so you can't tell if you are missing one of them, no matter if from the vehicle or from the trailer. If your car has a trailer hook, it needs a special relay which has a second thermal switch inside which is directly coupled to the heater. It clicks on when the heater becomes hotter than normal which indicates that you have more than 2 bulbs in the circuit. This is reported by a second turn signal indicator, the "trailer hook indicator". So if you have a trailer, you must look at this indicator and you know that all turn signals are working. You can also see if the trailer has a bad connection so you can stop and fix the problem. ²) Since blue LEDs were not invented back then (even Orange was unheard of) and the high beams indicator must be blue by EU rules, the car came with a certificate of exemption that the high beam indicator may be "white". And a yellow LED emits just a "dirty shade of white", doesn't it?  Attachment DeletedLater, blue LEDs became available (around 1990) but they didn't last long enough for cars and were highly expensive (>$30). Since for the customer there was no obvious reason not to have a blue LED, they switched to an alternative which became available, a "10,000 hour bulb" in a 5mm LED casing. So the cars with LED dashboard had blue high beam indicators in the 1990s. Real blue LEDs which last long enough came up around the year 2000. |
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Post by c64 on Apr 4, 2014 13:14:15 GMT
Here's an interesting experiment about deforming a filament:
Take a work lamp with bulb and cage (to prevent injury in case you smash the bulb). Test it and unplug it again. Then knock it against a hard surface. When you plug it back on, it will work normally.
Now knock it when on. If you didn't knock too hard, the bulb turns brighter without breaking because the filament is stretched. The next time you plug it back in, the filament will burn out due to the massive, now even increased starting current.
You can often see the deformation easily by visual inspection. The police sends it to a lab to be examined under a microscope to make sure.
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Post by the light works on Apr 4, 2014 14:41:37 GMT
yeah, HID headlamps don't let you inspect the filament, either.
as for blue LEDs, in the US, they deal with the problem that their indicator lamp isn't blue by putting a piece of blue plastic over it.
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Post by c64 on Apr 5, 2014 10:52:36 GMT
yeah, HID headlamps don't let you inspect the filament, either. as for blue LEDs, in the US, they deal with the problem that their indicator lamp isn't blue by putting a piece of blue plastic over it. LEDs are monochromatic, you can't change the colour by filtering light out. By the way, I wonder how much the kind of tire affects the skid mark speed calculation. In Germany, this winter was unusually hot. In my area, the temperatures hardly dropped below freezing. One day in January was real hot, I was driving with my snow tires when there were over +18°C (according to the cold air scope sensor) which is way too much for those kind of tires. I was hired to move more than half a ton in payload on a very tight schedule so I had to drive really "swift". I noticed that the wheels tended to lock real quickly when braking hard since the tires were real soft and couldn’t handle braking well. There was a crazy fox running on the road and I had to perform emergency braking to avoid hitting it. It was about 50 meters away when I started to brake and after running over it, I switched to 3th gear to continue driving which makes my speed at least 26kph. At least the fox seemed to be OK, I stopped braking right in front of it and I think it must have lied down because the "front skirt" of my car didn't seem to have hit it and I saw it running in a circle in my back mirror. Doing the mental calculation, the braking distance was immense due to driving with snow tires on this really hot day. I think I had read that those tires are not meant to be used at more than +7°C. With the summer tires, the braking distance shouldn't be much more than 40 meters at this speed. |
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Post by the light works on Apr 5, 2014 13:02:48 GMT
yeah, HID headlamps don't let you inspect the filament, either. as for blue LEDs, in the US, they deal with the problem that their indicator lamp isn't blue by putting a piece of blue plastic over it. LEDs are monochromatic, you can't change the colour by filtering light out. By the way, I wonder how much the kind of tire affects the skid mark speed calculation. In Germany, this winter was unusually hot. In my area, the temperatures hardly dropped below freezing. One day in January was real hot, I was driving with my snow tires when there were over +18°C (according to the cold air scope sensor) which is way too much for those kind of tires. I was hired to move more than half a ton in payload on a very tight schedule so I had to drive really "swift". I noticed that the wheels tended to lock real quickly when braking hard since the tires were real soft and couldn’t handle braking well. There was a crazy fox running on the road and I had to perform emergency braking to avoid hitting it. It was about 50 meters away when I started to brake and after running over it, I switched to 3th gear to continue driving which makes my speed at least 26kph. At least the fox seemed to be OK, I stopped braking right in front of it and I think it must have lied down because the "front skirt" of my car didn't seem to have hit it and I saw it running in a circle in my back mirror. Doing the mental calculation, the braking distance was immense due to driving with snow tires on this really hot day. I think I had read that those tires are not meant to be used at more than +7°C. With the summer tires, the braking distance shouldn't be much more than 40 meters at this speed. white LEDs use an ultraviolet LED with a fluorescent coating. |
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Post by c64 on Apr 6, 2014 10:56:42 GMT
white LEDs use an ultraviolet LED with a fluorescent coating. Actually the first generation used common blue LEDs, you can tell easily by the blueish center of the light cone. The modern ones use UV-near LEDs, they don't generate true UV-light like fluorescent tubes. There are true UV-LEDs available now, but there are not used for white LEDs (yet). |
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