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Old 15th December 2010, 22:46   #1
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Default ABS - How Failsafe

Hi,
A recent thread on brake failures of a luxury marque, and some ranting on another forum about the ABS on the Honda CBR250 brought to the forefront a thought which has been there at the back of my mind for a long time:-
How failsafe is the braking system in a ABS equipped car? Esp. from point of view of single component failure.

Sutripta

Last edited by Sutripta : 15th December 2010 at 22:49.
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Old 16th December 2010, 07:53   #2
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Red face Re: ABS - How Failsafe

ABS in a car is your first line of defence as far as safety is concerned. I have experienced the ABS coming into action a couple of times when I had bring my Fiesta to a sudden halt. I had enquired with the Ford service centre in my place about the regular inspection of the ABS components and found that no such tests/servicing are carried out regularly. Its kind of use & throw components which build the ABS system.

I remember a fellow Fiestian Capt_rocker had a major issue with the brake booster due to which the ABS was not effective. This was finally replaced in warranty.
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Old 16th December 2010, 20:25   #3
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Default Re: ABS - How Failsafe

^^^^
No, was not talking of the ABS system malfunctioning, but rather of a single point failure in the ABS subsystem (whatever be the cause) critically affecting the WHOLE braking system.

It can range from
no effect,
reduced efficacy (hopefully graceful degradation),
catastrophic failure (system crash, pun intended).

In the MB SBS system (brake by wire, rather than ABS) MB said that the problem was in the software. But affecting only high usage cases!

This point (failure of a single component critically affecting total system) I don't think has been discussed on this forum, because I guess no one is really interested!

Regards
Sutripta

Last edited by Sutripta : 16th December 2010 at 20:26.
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Old 16th December 2010, 21:06   #4
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Default Re: ABS - How Failsafe

Please note if the ABS system fails/malfunctions in a car the braking system continues to function just like the non-ABS equipped car. ABS failure will not lead to complete brake failure at all. Owner's manual of most ABS equipped cars have this clearly mentioned.
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Old 16th December 2010, 21:14   #5
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Default Re: ABS - How Failsafe

Also, in most ABS equipment cars, the MID throws up a warning light/tone if there is an ABS failure, so you will very much know if your ABS is in tact or not. If it is not, as .anshuman pointed above, the braking system would function in stand alone.
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Old 16th December 2010, 21:25   #6
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Default Re: ABS - How Failsafe

Quote:
Originally Posted by .anshuman View Post
Please note if the ABS system fails/malfunctions in a car the braking system continues to function just like the non-ABS equipped car. ABS failure will not lead to complete brake failure at all. Owner's manual of most ABS equipped cars have this clearly mentioned.
Hi,
How about a scenario where a release valve is stuck open. Or the pump has failed. And we are not even going into the realm of software bugs.

Anyway, this is what I wanted discussed! Discussion is not on malfunction of the ABS, but its effect on the rest of the system.

Regards
Sutripta
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Old 26th December 2010, 21:37   #7
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Default Re: ABS - How Failsafe

^^ ABS cars have been successfully selling western markets since a long time. No such concerns have been reported till date. As GTO said in the other thread, Manufacturers take braking and other crucial safety features very seriously. If ABS had such defect the manufacturers would have been fighting their cases in western courts.

Mercedes introduced Brake by Wire in last gen previous gen E-Class. It was withdrawn soon just because of the security concerns. All the cars continue to function like Non-ABS equipped cars if ABS fails, the servo Assistance fails the braking system will continue to function like non power brakes, only additional force will be required to stop the car. I am not saying brakes cannot fail at all, but i intend to say that the brakes are designed to be fail safe as possible.

I am surprised to see posts on the forum like 'What if Airbags don't pop out?' or 'What if ABS fails?' We have already had such discussion on several threads some time back, please search for the discussion.

Last edited by .anshuman : 26th December 2010 at 21:39.
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Old 26th December 2010, 21:43   #8
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Default Re: ABS - How Failsafe

Might sound stupid but how do we get to know if ABS is working properly in our car or not?
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Old 26th December 2010, 22:04   #9
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Default Re: ABS - How Failsafe

Quote:
Originally Posted by harry10 View Post
Might sound stupid but how do we get to know if ABS is working properly in our car or not?
Brake hard while running @ 40-50 Km/hr on an open road and keep steering as straight as possible, you should feel knocking sound "tak-tak" sound while braking and car should stop in straight line.

Yes ?? Your ABS is working fine
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Old 26th December 2010, 22:23   #10
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Default Re: ABS - How Failsafe

Quote:
Originally Posted by .anshuman View Post
...
All the cars continue to function like Non-ABS equipped cars if ABS fails, the servo Assistance fails the braking system will continue to function like non power brakes, only additional force will be required to stop the car. I am not saying brakes cannot fail at all, but i intend to say that the brakes are designed to be fail safe as possible.
Hi,
Agree with you on that brakes are meant to be as failsafe as possible. But not with a blanket statement like 'continue to function like Non-ABS equipped cars if ABS fails'. To take your example of a servo: if it is working on the pushrod, which is a must with tandem master cylinders, its failure will not affect the rest of the system. But if it is a hydraulic one, a leakage in it would affect the entire system. A critical failure.

My question was not on 'what if ABS fails' but if a component added because of the ABS fails. Take the case of the three position valve. If it fails open (whatever be the cause, no matter how low the probability, it is not 0) one looses braking on that wheel.

What will happen if the pump fails will, I guess, depend on how the software is written. How is it written?

What all is checked, and how, during a power on self test?

Pls. note that I'm not bashing ABS systems. Would love to have it on all my cars. But would also like to know what exactly I'm getting.

Regards
Sutripta
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Old 27th December 2010, 03:43   #11
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Default Re: ABS - How Failsafe

i recall a horror story about a scorpio with repeated ABS failures which even M&M were not able to diagnose. the vehicle would simply lose braking entirely now and then. understandably, the owner was last heard trying to sell it off
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Old 27th December 2010, 21:27   #12
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Default Re: ABS - How Failsafe

^^^^
Hi,
Any links?

Regards
Sutripta
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Old 28th December 2010, 03:07   #13
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Default Re: ABS - How Failsafe

sorry, no proof. in fact i believe the whole thing was being hushed up until an unsuspecting sucker was found
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Old 28th December 2010, 12:52   #14
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Default Re: ABS - How Failsafe

@ sutripta

What you are talking is about the design of the braking system. During design a detailed FMEA is carried out which analyses the chance of failure, chance of detecting failure before it affects performance and the severity (the effect of the failure). The questions you pose are addressed at that stage and a detailed discussion on such issues are highly technical and may be beyond the scope of the forum.

All safety critical systems are generally designed for fail safe mode. In case of a failure of a part, the system will continue to work with reduced efficiency. For example the brake master cylinders have a tandem arrangement and if there is a hydraulic leak in one circuit, the other will work providing you braking on 2 wheels . If the servo fails still you can stop with a higher pedal force.

Above all, any new design gets validated before they are approved for production and production parts are routinely subjected to validation tests (endurance, etc) to ensure they meet the required norms.
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Old 28th December 2010, 14:17   #15
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Default Re: ABS - How Failsafe

Good question there, Sutripta, for a system we've taken practically for granted. Before we jump into analysing how failsafe they are, let's look at what can fail (this is not for you to read, Sutripta, but to enlighten folks in general ).

Broadly speaking, the components of ABS that are not part of a traditional hydraulic braking system are:
  1. The wheel sensors (which sense the relative speed of one wheel to another, and the actual speed as well);
  2. The ABS valves for the wheels (assuming this is a 4-channel system) (when open, they allow the brake fluid pushed out by the master cylinder to reach the wheel cylinder - they remain open, unless an electronic signal asks them to close, and release braking force on that wheel);
  3. The pump (once the valves cut the pressure to the wheel and unbrake them, someone has to put the pressure back in the next few milliseconds to apply the brake to a particular wheel again - the pump does it). Some systems may have an accumulator to store the brake fluid under high pressure - which is one of the reasons why, even with the engine turned off (brake booster off) some cars still get decent braking for a few pushes on the pedal;
  4. The controller/ECU (not the ECU that runs the engine). Does all the work of opening and closing valves, running the pump etc.
So what can go wrong? Let's see when we can end up with no brakes!
  • IF the ABS valves stick in open position - sure, the brakes will continue to function - the hydraulic pressure still reaches the wheel cylinders.
  • IF the ABS valves stick in a closed position - no brake to that wheel. Highly unlikely that more than one (or two) valves will stick closed, so reduced braking efficiency is what we can look for (unless someone poured Elfy into where the brake fluid goes - then all valves can stick permanently closed all at the same time. )
  • Pump failure - before ABS kicks in, brakes would work fine. Once ABS takes control, there is no pressure to bring the brakes back to boost for a particular wheel that is skidding. In extremis, pump the pedal to get braking power back to wheel cylinders again.
  • ECU failure - the ABS valves and pump don't work. But the valves remain open in their resting state anyway, so the normal hydraulic system continues to function.
  • And finally, the wheel sensors. The biggest culprits in getting the ABS light to stay on. It's easy to damage those delicate wheel speed sensors while doing brake or suspension work, or when water/mud gets into them (ABS-equipped OTR cars!), or even when metallic filings stick to them - the sensors are magnetic, remember, so it's nicer not to let your brake pads wear down to metal before changing them). This can put the system into default (so ok, the hydraulics still work, and you can brake like a non-ABS-equipped car), or make it go permanently into anti-lock mode (called "false modulation"). Any which way you hit the brakes, the pedal starts juddering, and braking efficiency is much reduced. Got that handbrake working??? Use it... and head for the nearest service centre.
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