[sudev]

The genesis for this thread arose from a customer demonstration event that I was involved with last year. There were two SUV's from one manufacturer being displayed and our "job" was to engage with the visitors and demonstrate various features of the vehicles. Both SUV's had been introduced in last couple of years.

We were really not surprised to find that the sales teams of their dealers were most knowledgeable about the features - but only as far as rattling off jargon was concerned. They had absolutely no idea about what these features actually meant for the customers or their safety or how they operated.

For instance ABS

ABS - Anti Skid Braking System - is much talked about selling point in all cars. Ideally cars without ABS ought not be sold as this is one important safety feature.

The problem was that :
One: They do not know what ABS actually is
Two: No one teaches you how to use ABS.

About ABS basic information can be gleaned from http://en.wikipedia.org/wiki/Anti-lock_braking_system and also various threads here itself. The readers here knows ABS but does the average Joe walking in to the showroom know?

Quite a lot of time was taken up by "taxi" rides for the sales staff - right form top - and explaining to them what and how of ABS.

For this purpose we had a large flat ground at our disposal and here we built a setup of a two straight parallel run up's with a flex board depicting a wall at the end of one. There were cone placed to indicate "braking" point at which we would jam the brakes with full force.

First run was done in the open lane and by switching off ABS (pulling out the fuse) and braking with full force with steering movement to left while wheels were locked. Then in the "walled" lane we would approach the end at a speed and at last moment jam the brakes causing ABS to kick in while steering to the side to avoid the "wall". . Diagram:


In the first instance the car would skid straight (red arrow) even with steering input as the wheels were locked. In the second we would be able to avoid the "wall". It also demonstrated that the braking distance would not vary much in either case but with ABS we were able to avoid the wall. Most of the sales staff were dismayed that ABS did not cause the vehicles to stop quicker - as they had been telling customers!!

Then we would talk to them about what wall could represent in real life city driving scenarios - a child jumping in front from between two parked cars in city roads or a careless scooterist coming in from side lane even on a rainy day - and how even for city driving ABS is a life saver. Only then they could relate to this essential safety feature.

Some of the VIP invitees were also allowed to test drive after we had finished the explaining. Here I noticed that most would freeze on steering inputs when I asked them to do ABS braking on the open lane assuming a " sudden intrusion". That is normal expected reaction but if your car has ABS then you need to learn not to "freeze" on steering control while braking heavily. Being VIP invitees they all had high end cars but no one had explained till then how ABS works and how to use it.

One such customer returned in the afternoon with his driver in tow. He wanted me to demonstrate and explain ABS to his driver. His reasoning was that a few months back he had had a crash on highway - thanks god and airbags saved him - which ended up totalling a car worth more than 50lacs!! Seeing my demo he realised that how proper use of ABS could have saved them from getting in to crash in the first place.

AFAIK no drive teaching school has this sort of training imparted. Nor do almost most of the test drive reps in dealership know this.

That said there is another part of ABS story from the same demo. The two SUV's had far different responsiveness of the ABS system. In fact the cheaper of the two had much better ABS system with much faster pulsations. And no one talks about these issues - yet!! Even for the knowledgeable it is hard to find information about type of ABS being used - brand / generation - in vehicles being sold.

[amolbh]

Can anyone please explain why have they pressed the clutch this early in the Bosch videos? Wouldn't that cause the wheels to lock up easier/faster?

https://www.youtube.com/watch?v=mKiTAcXK6M4

[Jeroen]

Originally Posted by amolbh

Can anyone please explain why have they pressed the clutch this early in the Bosch videos? Wouldn't that cause the wheels to lock up easier/faster?

The whole point of ABS is that the wheels don't lock up, or in practice and as shown in this video, very little to maintain steerage.

Although there might be different schools of thought, in an emergency stop it's advisabel to de-clutch as well. You want to transfer as much of the weight as quickly to the front wheels as possible, get maximum braking action and you don't want anything upsetting the balance on the car, hence press the clutch!

Jeroen

[IshaanIan]

I didn't know this. In case of emergencies on the road like a bunch of cows crossing an empty smooth 6 lane highway (only in India ), I have never pressed the clutch unless I had to shift down. Even once I have made the shift to the lower gear, I release the clutch because I always imagined that the engine braking would aid me and only press the clutch again, once I have come close to a safe stop to prevent the car from stalling. Am I doing things wrong? Should I actually be pressing the clutch all the way till I stop? Would the car not take longer to stop while pressing the clutch?

[Rehaan]

Originally Posted by IshaanIan

I didn't know this. In case of emergencies on the road .... I have never pressed the clutch unless I had to shift down. Even once I have made the shift to the lower gear, I release the clutch because I always imagined that the engine braking would aid me and only press the clutch again, once I have come close to a safe stop to prevent the car from stalling. Am I doing things wrong?

I think you're doing things right.

As Jeroen has mentioned, there's different schools of thought >

Originally Posted by Jeroen

Although there might be different schools of thought, in an emergency stop it's advisabel to de-clutch as well.

Personally, I think its better to NOT press the clutch until it is required to prevent the engine from stalling / just before the ECUs anti-stall kicks. This way you get to harness engine braking.

Naturally, i also think this is the best way to do it, and so i assume a debate will follow

Originally Posted by Jeroen

You want to transfer as much of the weight as quickly to the front wheels as possible, get maximum braking action and you don't want anything upsetting the balance on the car, hence press the clutch!

Regarding the reasoning above, there's pretty much nothing that is more upsetting to the balance of a car than slamming on the brakes - therefore engine braking isn't going to make things any worse. Also, with engine braking, you'll be transferring weight to the front of the car more quickly (as soon as you release the throttle).

Keen to hear your other reasoning for preferring this method. (I assume you're saying if you're braking from 140 km/h - press the clutch right away?).

cya
R

[saket77]

Originally Posted by Rehaan

Personally, I think its better to NOT press the clutch until it is required to prevent the engine from stalling / just before the ECUs anti-stall kicks. This way you get to harness engine braking.

Naturally, i also think this is the best way to do it, and so i assume a debate will follow

No arguments from my side. I too think it is the best way to brake. Disengage clutch only to prevent the engine from stalling.

Also, agree with the opening post that one needs to 'learn' to brake in tandem with ABS so as to able to steer while braking hard - which is the main benefit offered by ABS.

[nishantbhatia84]

IMO the general perception of most car buyers / owners in India is that ABS is not really required in Indian conditions!
I would partially blame the dealers / manufacturers for this as they arent keen on selling high-end variants and focus mostly on mid-variants - which sell more - and hence they do not advise customer's about the advantages of ABS.
Most people i know who're active advocates of ABS are those who drive / have driven a vehicle with ABS. People do not realize that ABS not only helps during emergency braking scenarios but also works during normal driving conditions, without the driver realizing it! (my personal opinion)
Personally, even i did not realize the importance of ABS until i purchased one with ABS. Now the situation is that i feel scared when driving any vehicle without ABS as i feel the brakes are grossly inadequate!

[sudev]

@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. (To experience just how much : cruise at a mid rev speed in each gear and then leave accelerator)

If you want really quick stop engine mass adds to the work to be done by brakes. So use of clutch actually helps but not at the risk of loosing time to use clutch before braking hard!!
But as I said this is going OT.

For learning and testing the setup I described can be replicated by all on a open road with no traffic or in a parking lot by imagining sudden lane change manoeuvre while braking hard. Or by using a cone or such visual but non harmful aid.

[anachronix]

Thanks sudev for bringing up this important point about ABS and how to use it.

We do external trainings on safe driving etiquette as part of a CSR initiative in my organization, similar demonstrations and face similar feedback/questions from the participants about braking efficiency of ABS vs non-ABS. But we never had the luxury of having another car without ABS to prove our point though. But they get the importance of ABS that aids in steering away from an obstacle while full braking is engaged during an emergency.

Originally Posted by sudev

For instance ABS

[Jeroen]

Originally Posted by IshaanIan

Am I doing things wrong? Should I actually be pressing the clutch all the way till I stop? Would the car not take longer to stop while pressing the clutch?

You are doing fine. As I said there are different thoughts. What is really relevant to distinguish between an emergency stop and normal braking action. The latter could be still quite force full. Under all circumstances it (nearly) always advisable to keep the car in the appropiate gear and only de-clutch when you get near to stalling speeds.

But an emergency stop is somewhat different. Its really where the ABS kicks in, (or would have if you don't have one). And although there are the odd high speed emergency stops, in fact most of the times when emergency stops are made are at pretty moderate speeds, say 35-80 km/h. Try for yourself, a car will stop in seconds so stomping on the brakes needs to be followed up, or more or less in parallel by declutching.

Originally Posted by nishantbhatia84

People do not realize that ABS not only helps during emergency braking scenarios but also works during normal driving conditions, without the driver realizing it! (my personal opinion)

Personally, even i did not realize the importance of ABS until i purchased one with ABS. Now the situation is that i feel scared when driving any vehicle without ABS as i feel the brakes are grossly inadequate!

Could you please elaborate on that statement? What do you mean by "works during normal driving conditions".

Originally Posted by sudev

if you want really quick stop engine mass adds to the work to be done by brakes. So use of clutch actually helps

That is incorrect. Maximum braking power is achieved just before the wheels starts blocking. How that is achieved, by you pressing on the brake pedal, or assisted by ABS, or with a little help of engine braking. The max is the max, period. If you achieve maximum braking power with or without ABS it doesn't improve by "adding engine braking".


Jeroen

[D4D]

Originally Posted by amolbh

Can anyone please explain why have they pressed the clutch this early in the Bosch videos? Wouldn't that cause the wheels to lock up easier/faster?

I think disengaging the transmission doesn't matter at all. As a matter of fact, I think it's better to use engine braking(however little) along with brakes to slow your car down.

[sudev]

We do external trainings on safe driving etiquette as part of a CSR initiative in my organization, similar demonstrations and face similar feedback/questions from the participants about braking efficiency of ABS vs non-ABS. But we never had the luxury of having another car without ABS to prove our point though.

Disable ABS by removing fuse just for demo

[sudev]

Originally Posted by Jeroen

That is incorrect. Maximum braking power is achieved just before the wheels starts blocking.

Please read again...I did not say maximum braking is achieved...I said additional work has to be done by brakes also to over come engine mass - more correctly inertia unless clutch is used.

[SS-Traveller]

Originally Posted by Rehaan

Personally, I think its better to NOT press the clutch until it is required to prevent the engine from stalling / just before the ECUs anti-stall kicks. This way you get to harness engine braking.
Naturally, i also think this is the best way to do it, and so i assume a debate will follow
Keen to hear your other reasoning for preferring this method. (I assume you're saying if you're braking from 140 km/h - press the clutch right away?).

Originally Posted by Jeroen

...in an emergency stop it's advisabel to de-clutch as well.
...you don't want anything upsetting the balance on the car, hence press the clutch!

Originally Posted by sudev

@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.
If you want really quick stop engine mass adds to the work to be done by brakes. So use of clutch actually helps...

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.

[Jeroen]

Originally Posted by sudev

Please read again...I did not say maximum braking is achieved...I said additional work has to be done by brakes also to over come engine mass - more correctly inertia unless clutch is used.

Engine mass doesn't change, whether you brake or not. Inertia is a result of speed and mass. So unless you have found a way to get rid of the engine when you hit the brakes, the engine mass, and there for its inertia at any given speed is going to remain exactly the same.

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". Until that time, it is likely to provide braking action, but that is also the last possible moment you would de-clutch. Being in the lowest gear possible is of course relevant as well.

Here in some more detail what happens when you push the brakes and where the engine is also braking. As per my earlier post the maximum braking power doesn't change, what does change is how hard the brakes have to work. So, at least in theory, engine braking does reduce brake wear. Again, that only works if the engine is actually providing braking, see above

What effectively happens is that the wheel is slowed down by the brake pads onto the rotor, more braking power is provided to the wheels, through the driveshaft from the engine (engine braking). It means the brakes can work a fraction less, hence less wear.

Whether such brake wear reduction is measurable in real life is highly questionable. And usually, in order to get the maximum braking out of the engine you need to downshift more, thus more wear and tear on the clutch. So the nett effect is questionable.

As a matter of interest, on planes they do use engine braking mostly to reduce brake wear and break component temperature. Most planes are perfectly capable of stopping with their brakes only (and most have pretty advanced ABS as well). But by deploying thrust reverser, the brakes need to work less. A lot of commercial planes have auto brake systems which provide for a specific setting a specific deceleration. (e.g. 4 m/s2). Even when deploying the thrust reverser the auto brake system keeps this deceleration to its chosen setting, it just uses the brakes less, thus less wear and tear.

Jeroen