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Old 22nd May 2014, 11:04   #16
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Default ABS - Learning and experiencing

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Originally Posted by SS-Traveller View Post
Interesting discussion.

Engine braking - how would one harness that in an automatic transmission-equipped car? So without engine braking, is an AT vehicle inherently less safe than a manual-transmission car?

ECU-equipped car = anti-stall programming (in most cases). ABS brings wheels to the point of almost stopping, and anti-stall fiercely tries to rev up the engine as soon as ABS releases the brake (even for that millisecond that it can). In neutral / with clutch disengaged, less braking effort and distance required to stop the car. This applies to slower speeds (below 25-30 km/h) - not for braking in from 140 km/h.

Quick experiment, on a traffic-free road or private path - engage 1st gear and rev up to 2000+ rpm. In first gear, without touching clutch, try to bring the car to a halt. Next try the same thing after shifting to neutral or pressing in the clutch. Let's hear what you find out, esp. if your car is a heavy diesel and has anti-stall programming running the engine.

On the engine braking on AT cars, I really don't think we can call any combination inherently unsafe. Its partly preference, practice and some theory thrown in.

On your second point I don't think that putting the gear in neutral / pressing the clutch will influence the braking distance at all, see my earlier point. once ABS kicks in you get as near as maximum braking power as you can get. Im not sure the ECU will let the engine behave as you suggest. I've certainly never noticed the engine trying to rev up with the ABS kicking in. Would be difficult too, what with the ABS pulsing several times a second. So I think the ECU knows when the ABS kicks in and behaves accordingly.

What is relevant, or at least interesting to note, is that diesels have a very different engine breaking characteristics compared to petrol engines. Old, non turbo diesels, with no EGR and all today modern fancy stuff bolted on have notoriously little engine braking power to start with.

Jeroen
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Old 22nd May 2014, 14:11   #17
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Default Re: ABS - Learning and experiencing

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Originally Posted by sudev View Post
@Rehaan at risk of going OT engine braking will kick in only if the rate of de-acceleration needed is within the ambit of engine braking curve.
Thanks Sudev! That really brought an interesting new point to this age-old discussion.

Only one thing i think needs correction is : >>

Quote:
Originally Posted by sudev View Post
....engine braking curve. (To experience just how much : cruise at a mid rev speed in each gear and then leave accelerator)
This would not give you the true engine braking curve that we are referring to, as it would also have the forward interia of the vehicle fighting against it.

I think the correct test would be : Press the clutch pedal and revv your engine up till 4,000 rpm (or whatever rpm you would in theory be at). Then, leave the accelerator and see how fast the revs drop to idle.

This will give you the true braking curve for just the engine component of the equation that we are talking about.


Quote:
Originally Posted by Jeroen View Post
Unless, you're thinking of the engine turning. In that case it is hardly relevant, because its only at the very last moment before the engine starts to stall, the brakes would have to overcome the "engine inertia".
Hi Jeroen,

Yes, he's talking about the rotational inertia of the engine.

Breaking it up, the brakes have to do two things when the clutch (and gears) are still engaged :
1) Reduce the forward inertia of the vehicle
2) (Possibly) Reduce the rotational inertia of the engine

As Sudev says, if the engine's own braking curve is sharper than the vehicles braking curve (with rpm to km/h conversion accounted for) - then the engine braking is working to your advantage. If its not, then its actually putting some additional load on the brakes!


cya
R

Last edited by Rehaan : 22nd May 2014 at 14:17.
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Old 22nd May 2014, 14:41   #18
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Default Re: ABS - Learning and experiencing

Quote:
Originally Posted by Rehaan View Post
I think the correct test would be : Press the clutch pedal and revv your engine up till 4,000 rpm (or whatever rpm you would in theory be at). Then, leave the accelerator and see how fast the revs drop to idle.

This will give you the true braking curve for just the engine component of the equation that we are talking about.
This is incorrect. Engine braking is always measured 'under load'. In your method described above - the engine is not under load.
What Sudev has explained earlier is pretty much what it is and how it should be done.
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Old 22nd May 2014, 19:09   #19
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Default Re: ABS - Learning and experiencing

@Rehaan - rotational inertia - yes that is the word I should have used in the first place.
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Old 22nd May 2014, 19:28   #20
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Default Re: ABS - Learning and experiencing

Quote:
Originally Posted by Kaumad View Post
This is incorrect. Engine braking is always measured 'under load'.
What we are talking about here is splitting the braking effort into two parts (to help understand what we are discussing).

One part is the vehicles interia. And the other is the engines rotational inertia. If you want to isolate the two, you need to disengage the clutch to measure the 'engine's braking' over time - without the vehicles inertia adding to it.

The test would be as such:
Test A: At 100 km/h, clutch disengaged (pedal pressed), slam the brakes and see the time taken to reach 10 km/h. Note that time (A).

[Hypothetical calculation] If you were in 2nd gear, you'd be at 6,000 rpm at 100 km/h, and 1000 rpm at 10 km/h.

Test B: Press the clutch pedal, revv the engine till 6,000 rpm, leave the accelerator - measure time taken to drop from 6k to 1k rpm. Note that time (B).
Results:
If the time taken for (A) is less than (B), then engine braking might slow down your overall braking.*

If the time taken for (A) is more than (B), then engine braking would improve your overall braking.*
Read the earlier few posts again, and if there's still confusion I'll expand upon this.

cya
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* = Of course there's other intricacies too.

Last edited by Rehaan : 22nd May 2014 at 19:31.
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Old 23rd May 2014, 08:22   #21
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Default Re: ABS - Learning and experiencing

LOL : As usual the thread goes extreme OT when detailed analysis and differing view points come in.

Let me restore sanity by saying brake whichever way you need or want but make sure you understand beneficial use of ABS and how it can save you (and others) under emergency heavy braking.
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Old 23rd May 2014, 09:01   #22
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Post Re: ABS - Learning and experiencing

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Originally Posted by sudev View Post
Make sure you understand beneficial use of ABS and how it can save you (and others) under emergency heavy braking.
It's high time ABS is made mandatory on our cars here, just like seat-belts. Example of a real-life situation on our roads which could've been avoided by ABS.



Cross posting from here (Rants on Bangalore's traffic situation)...
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Old 24th May 2014, 07:23   #23
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Default Re: ABS - Learning and experiencing

Excellent example how wet patches can effect car dynamic on braking.

Though I think here it was more about aqua planning due to highly worn out tyres cabies are notorious in using. See how his front tyres grip the road the moment they are off the wet surface but rears keep sliding.

When aqua planning occurs ABS will not be able to help as there is no steering left and tyres are skidding on top of a layer of water with no contact patch to road.
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Old 24th May 2014, 09:02   #24
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Default Re: ABS - Learning and experiencing

Quote:
Originally Posted by im_srini View Post
It's high time ABS is made mandatory on our cars here, just like seat-belts. Example of a real-life situation on our roads which could've been avoided by ABS.
Quote:
Originally Posted by sudev View Post
Though I think here it was more about aqua planning due to highly worn out tyres cabies are notorious in using. See how his front tyres grip the road the moment they are off the wet surface but rears keep sliding.
I agree with you Sudev but the video does show the carelessness and a warning to all drivers who tail gate cars so that they can overtake the F1 style.

Apart the wet surface that was against the cabbie + low thread tyres + NO view of the front to see the speed breaker nor any heed paid to road signs by the side caused that spin-off. Again I see the lack of anticipation here with just blind driving. Nothing much can be said but even the whole lot of safety features provided such drivers will NEVER learn.

The driver should be

Anurag.
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Old 27th May 2014, 07:51   #25
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Default Re: ABS - Learning and experiencing

One excellent resource to know ABS benefits and its use. Especially watch the part where the car is half on road and half on ice. And the mantra "S S S" repeated through the video


Last edited by sudev : 27th May 2014 at 07:52.
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Old 27th May 2014, 21:27   #26
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Default Re: ABS - Learning and experiencing

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Originally Posted by sudev View Post
One excellent resource to know ABS benefits and its use. Especially watch the part where the car is half on road and half on ice. And the mantra "S S S" repeated through the video
Being in the tech section, I think someone should mention the term 'split mu surface'!

Regards
Sutripta

Last edited by n_aditya : 27th May 2014 at 21:48. Reason: pls avoid quoting videos as it inconveniences mobile users
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Old 28th May 2014, 05:56   #27
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Default Re: ABS - Learning and experiencing

Thanks Sutprita....important input in concise words
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Old 28th May 2014, 20:02   #28
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Default Re: ABS - Learning and experiencing

^^^
Also applies to the video in post #22.

Regards
Sutripta
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Old 1st July 2014, 20:49   #29
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Question Re: ABS - Learning and experiencing

Quote:
Originally Posted by Sutripta View Post
Being in the tech section, I think someone should mention the term 'split mu surface'!
Quote:
Originally Posted by sudev View Post
Watch the part where the car is half on road and half on ice.
In a Split-µ situation, the ABS eliminates wheel lock on the wheel on the Low-µ side, thereby preventing a spin out.
So what happens on the High-µ side ?
The straight track of the ABS equipped car implies that the braking force on the High-µ side has been reduced also.
Wondering how the ABS is able to judge how much brake modulation is required given that it has to match friction available at the wheels - from just the wheel RPM data ?
Or, is that all it takes ( ? ), matching wheel RPM ( or preventing wheel lock ) is enough to prevent a spin - regardless of the surface underneath each wheel.
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Old 1st July 2014, 22:33   #30
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Default Re: ABS - Learning and experiencing

^^^
Steering control is still with you. You have to use it to maintain directional control. ABS (not ESP) will not steer for you.

Regards
Sutripta
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