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Questions on braking


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#51 just me again

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Posted 09 March 2015 - 15:24

One time long ago I Stupidly drove to quick on a road in a parking area. While beginning overtaking a car he turned left. I braked hard until the last 3 meters where I panicbraked.

 

After it was clear to see the color difference in the skidmarks. at first they were brown or sort light black. The last 3 meters where they were Black, like very Black.

 

I have never seen skidmarks from a ABS system.

 

Bjørn



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#52 Ross Stonefeld

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Posted 09 March 2015 - 16:25

Sure the skidmarks were from the ABS? :cool:



#53 Kelpiecross

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Posted 10 March 2015 - 02:56


I had the same problem of locking the brakes while coming to a stop on a wet road - slithered the last 20 feet or so.
I was told ABS doesn't operate below 10mph or thereabouts. Very disconcerting if you are relying on ABS to bring you to a complete stop.

#54 Robin Fairservice

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Posted 10 March 2015 - 04:21

A friend of mine owned a Chevrolet Suburban which had a selectable four wheel drive system ( few years ago).  She was driving along a snow covered winter road in four wheel drive, when she had to stop in a hurry.  All four wheels locked up and she slid into another car.  Afterwards she discovered that when this vehicle was in four wheel drive, the ABS didn't work!



#55 carlt

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Posted 10 March 2015 - 09:55

A friend of mine owned a Chevrolet Suburban which had a selectable four wheel drive system ( few years ago).  She was driving along a snow covered winter road in four wheel drive, when she had to stop in a hurry.  All four wheels locked up and she slid into another car.  Afterwards she discovered that when this vehicle was in four wheel drive, the ABS didn't work!

 

I guess the moral of the tale is "if you buy a vehicle with electronic aids - read the manual/small print"



#56 Greg Locock

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Posted 10 March 2015 - 10:55

Nice story, wrong conclusion. The best thing to do in snow (or gravel) is t lock the wheels and build up a wedge ahead of the wheels. Therefore ABS tends to do the wrong thing on those surfaces, and the reason the cycle time was so long on Ford Australia ABS initially was in recognition of that, in that it gave time for a wedge to form. Otherwise you'd be driving the Lexus LS400 towards a T junction on gravel, hit the brakes at the usual braking point, and merrily slide straight over the road and broadside into the ditch. BTDT got the damn T shirt.



#57 rdyn

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Posted 10 March 2015 - 19:08

Vehicle speed is not measured and needs to be estimated. If you select 4x4: all wheels are driven and front and rear axle show completly identical speeds - so how fast is the vehicle (based on wheel speed signals) ?

Additionally: with front- and rear axle locked together it is no longer possible to brake one single wheel without influencing another wheel.

 

The result could be a rather disappointing performance. For this reason Chevrolet probably decided not to put effort into a special ABS mode.


Edited by rdyn, 10 March 2015 - 19:09.


#58 Greg Locock

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Posted 10 March 2015 - 21:32

Again wrong conclusion. Before ABS was introduced owners of 4wd on 4wd driving courses were encouraged to use 4wd in slippery conditions partly because it distributed the braking loads equally among the 4 wheels, aka poor man's ABS. This was on the basis of even blind freddy realising that all 4 wheels were locked, Also if you were in a manual like any proper 4wd, if the clutch was out and the engine was going round then the wheels were going round. If you are in low range you can stamp on the brakes as hard as you like, the engine won't stall.



#59 bigleagueslider

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Posted 11 March 2015 - 05:50

On a paved road, ABS is most effective when you apply max force on the brake pedal as early as possible and let the ABS system do the work. By letting the ABS brake the vehicle at the limit of its ability from as high a speed as possible, you will minimize the braking distance required.



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#60 cedarsf1

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Posted 15 July 2015 - 04:45

I'm reviving this old threat that I started because I think there are a few things I still don't understand.

 

So the consensus here is that with most street-car ABS systems, you can stop quicker by not activating the ABS. If I understand correctly what you guys say, the reason that the ABS doesn't work as well as a skilled driver is because it is too sensitive in how it reads imminent lock up, meaning that it begins reducing brake pressure to a wheel when that wheel is not rotating as quickly as it ought to. A good driver, on the other hand, would continue increasing brake pressure closer to the point where the wheel actually locks up.

 

What I don't understand is if you are driving an ABS car, how can you not activate the ABS yet brake closer to lock up?



#61 cedarsf1

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Posted 15 July 2015 - 05:06

I guess another way of stating this is how can you beat ABS when it will activate before you want it to, just like you can't beat traction control (w/o turning it off) because most older systems will activate at the slightest hint of wheelspin.



#62 Joe Bosworth

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Posted 15 July 2015 - 05:41

Even the best of the road car systems can be beaten.

 

My oldest son is a Corvette afficiendo who regulalrly goes to Vette track days with one of his toys, (he has several).  Regulalry sets FTD sometimes with his Cadillac which really pisses some of the Vette drivers off.

 

Anyway, to make a long story shorter, he is often asked to do a lap or so with the latest toy of others as they then have a marker to try to beat.  He always beats there time by a considerable margin. and gives the car back to the owner to try to beat his time which doesn't happen.

 

His modus operendi is to turn off brake and traction control, do his laps and then turn the gadgets back on before giving the car back!!!  This is true for even the ZO6 derivatives which has the best and latest that GM can trick up.

 

Regars :clap:  



#63 cedarsf1

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Posted 16 July 2015 - 23:11

Even the best of the road car systems can be beaten.

 

My oldest son is a Corvette afficiendo who regulalrly goes to Vette track days with one of his toys, (he has several).  Regulalry sets FTD sometimes with his Cadillac which really pisses some of the Vette drivers off.

 

Anyway, to make a long story shorter, he is often asked to do a lap or so with the latest toy of others as they then have a marker to try to beat.  He always beats there time by a considerable margin. and gives the car back to the owner to try to beat his time which doesn't happen.

 

His modus operendi is to turn off brake and traction control, do his laps and then turn the gadgets back on before giving the car back!!!  This is true for even the ZO6 derivatives which has the best and latest that GM can trick up.

 

Regars :clap:  

You can't turn off the ABS on any car. I am not talking about stability control. But yes, on any road car the stability control is way too eager to intrude. I have never used it on my Z06.



#64 GreenMachine

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Posted 16 July 2015 - 23:41

You can't turn off the ABS on any car. I am not talking about stability control. But yes, on any road car the stability control is way too eager to intrude. I have never used it on my Z06.


But you can pull the fuse.

There may be other tricks too, I know that in my car pulling the handbrake on (just enough to get the handbrake light to come on) switches the ABS off.



#65 cedarsf1

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Posted 17 July 2015 - 00:53

But you can pull the fuse.

There may be other tricks too, I know that in my car pulling the handbrake on (just enough to get the handbrake light to come on) switches the ABS off.

My understanding is that it can be more complicated than that to disable ABS. I think on many cars you would also lose the wheel speed sensor by pulling that fuse. I think there are other issues as well.



#66 saudoso

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Posted 17 July 2015 - 23:56

There are really silly things  that will shut it all down. In a first gen A Class merc if you messed the brake light switch (under the pedal) all the ABS & Stability Controll would go offline.



#67 Joe Bosworth

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Posted 19 July 2015 - 04:11

Cedars F1 wrote above, "My understanding is that it can be more complicated than that to disable ABS. I think on many cars you would also lose the wheel speed sensor by pulling that fuse. I think there are other issues as well."

 

Well, exactly!  The wheel sensors are there precisely as inputs for ABS and stability control.  They have no other purpose and there are millions of vehicles on the road without wheel sensors.

 

It is a sad world where believe believe that we have to have bib brother types of things controlling our activities.  But then who would we have to blame when things go wrong.  Actually, the well trained human body contains more than enough to controll our actions.  Of course ABS and SC are great for those who don't have reasonably controlled bodies or manage to drive while half asleep at the wheel! 

 

Regards :drunk:



#68 desmo

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Posted 19 July 2015 - 04:20

If humans are better then why are these drivers assists which should provide no advantage generally illegal in racing?  ESC/ASC and ABS seem like they could benefit anyone if properly configured for what that person wants/needs. That's not to say that every driver's aid is better than an expert driver, but that they could be. 



#69 Greg Locock

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Posted 19 July 2015 - 07:29

Well let's see, launch assist was banned. Surely a well trained or skilled race driver could beat an electronic box at doing what every hoon knows how to do? Apparently not.



#70 saudoso

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Posted 19 July 2015 - 10:17

If humans are better then why are these drivers assists which should provide no advantage generally illegal in racing?  ESC/ASC and ABS seem like they could benefit anyone if properly configured for what that person wants/needs. That's not to say that every driver's aid is better than an expert driver, but that they could be.


The difference between medicine and poison is in the dose.



#71 Joe Bosworth

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Posted 19 July 2015 - 12:05

The technology of ABS and traction and stability control needs to kept in paerspective.

 

I tried to bring that perspective to this thread back in post #16 where I said (in part), "I also accept that in the electronic days of F1 that they developed systems that couldn't be beaten.  The real problem was that these systems cost far more than the cost of 99.9% of the total cost of cars bought today.

 

So, if you really are good enough and can prove, it switch off the factory electronics for best performance on your evey day driver.  Otherwise leave it on because even the factory settings will beat your performance. The proof only can come from lap time on the track."

 

The ultimate in such systems have a cost beyond the cost of most cars on the road today.  It is easy to understand why the book keepers will not allow the engineers a free hand in road car technology and why F1 and most racing bodies insist on drivers making the difference in lap times.

 

If you think F1 is dull now just let the boffins have a free hand and drivers will become redundant and the boffs will load their latest programmes into an enormous computer game that we tune our computers into to see who is going to win instead of turning our TVs on. 

 

Come to think of it, if Bernie lives long enough we won't have anybody at all watching from most of the tracks' stands.

 

Regards :clap:



#72 cedarsf1

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Posted 20 July 2015 - 16:25

I fundamentally still don't understand ABS braking. Does it activate when a wheel locks, or just when it is about to? If your car has ABS, can you someone brake better by not activating it? 



#73 GreenMachine

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Posted 20 July 2015 - 22:21

Have a search in here, it has been thrashed to death in a thread, probably more than one.



#74 gruntguru

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Posted 20 July 2015 - 22:35

I fundamentally still don't understand ABS braking. Does it activate when a wheel locks, or just when it is about to? If your car has ABS, can you someone brake better by not activating it? 

Not sure of the latest algorithms but early ABS would reduce pressure at a wheel when deceleration of that wheel exceeded either a set absolute limit or some other limit relative to the remaining wheels.



#75 Greg Locock

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Posted 21 July 2015 - 00:14

Here's some data http://sine.ni.com/c...c/p/id/cs-15303

 

a bad abs cal locks the wheel, a good one slows the wheel by 50% before cycling the pressure off. This is for a trailer, the algorithm for a car is more complex, the following diagram shows one wheel

 

https://d1wshrh2fwv7...75336773780.jpg

 

upper graph is vehicle speed, middle is wheel angular acceleration, lower is brake line pressure. As you can see unlike the trailer example the algorithm can decide on a safe braking pressure from the wheel acceleration, and defaults back to that before ramping up again. Knowing the lecturer involved this is probably a BMW system.


Edited by Greg Locock, 21 July 2015 - 00:23.


#76 Lee Nicolle

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Posted 21 July 2015 - 06:04

Well let's see, launch assist was banned. Surely a well trained or skilled race driver could beat an electronic box at doing what every hoon knows how to do? Apparently not.

They can, just not consistently.  The electronic gizmos really annoy me, at hillclimbs some expensive snot box road car or rallycar is popping and banging then leaves with no wheelspin. Driver just turns the steering wheel. Joke is they seem to crash regularly with all the electronics! Though too a degree the electronics can be trained to still semi act as traction control. The reason really for control ECUs



#77 Kelpiecross

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Posted 22 July 2015 - 05:04


Does ABS operate on each wheel separately - or on the lot as a whole? (Separately I would imagine it would have to be).

#78 gruntguru

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Posted 22 July 2015 - 05:35

Seperately.



#79 Greg Locock

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Posted 22 July 2015 - 07:34

Nowadays, each wheel. Originally 3 channel systems were popular, which (I think) treated the rear axle as one unit. Not perhaps a terrible idea, but the cost save was miniscule. Incidentally can other people see those plots?



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#80 saudoso

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Posted 22 July 2015 - 11:38

Nowadays, each wheel. Originally 3 channel systems were popular, which (I think) treated the rear axle as one unit. Not perhaps a terrible idea, but the cost save was miniscule. Incidentally can other people see those plots?

No, I guess you have to get rid of the httpto make it appear inline

 

secondary_70275336773780.jpg


Edited by saudoso, 22 July 2015 - 11:39.


#81 yawpower

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Posted 19 October 2015 - 04:22

Even the OE calibrations can be hard ot beat, unless you have 4 brake pedals and two extra feet.