Engagement of Clutch during Emergency Braking

[dms664]

Aoa I think it has been discussed alot of times but still there are two school of thoughts regarding this issue although many traffic warnings issues tell it's much better to brake without pressing the clutch pedal. For instance, we drive a car at 120 km/h and suddenly we have to brake in the shortest distance as possible due to some obstacle on the road. What is better? 1) Should we press the clutch pedal at the same time we are braking? 2) or should we press brake pedal only?.

In favour of 1)they argue: yes, it will be better to decouple wheel movement from engine movement. Why? because if we decouple it, the effective inertia the brakes must stop is only the car kinetic energy. If we don't decouple it, we must brake the car inertia in addition to the proper rotating inertia of the whole engine, which is rotating at let's say 3000rpm.

In favour of 2) I would argue: yes, I think it will be better to not decouple wheel movement from engine movement. Why? because the engine internal will collaborate to brake the car when we leave it coupled to the wheel shaft and releasing the accelerator pedal.

What do you think it is the correct answer? Please give logical reasons.

Please also intimate if same rules applies to both ABS and non ABS vehicles or there is different procedure for the two and manual vs auto

Thanks in advance.

[dms664]

before coming here I have searched the internet thoroughly but there are two groups still adamant on their pt of view. I ll post some excerpts from different forums

Here is the first one:

[I]Hi everyone, rencently I have a discussion with my friends about: car(normal family car/sedan manual transmission with ABS) during an emergency brake, shall we press the clutch pedal with brake pedal simultaneously?

Will this help the car to stop in a shorter distance?

My opinion is yes, because I saw some demo video from ABS supplier, the driver do press the brake+clutch pedal simultaneously

Some of my friends opinion is no, because he think if press the clutch pedal during emergency brake, you will probaly lose control of the car, and the engine brake will also help car to stop more quickly

So now we divide in 2 group and we cannot convince each other, we cannot run a test too, so I hope can find an definite answer here?[/I]

[dms664]

[I]The only correct way of emergency braking is full step on brakes only leaving engine and clutch alone, and only when speed is down enough that engine stalling is developing can you press clutch.

Reasons are many.
1) engine braking helps to decrease speed, additional braking force.
2) engine inertia avoids wheel lockup
3) you are faster when dealing with 1 pedal instead of 2
4) car retains better stability avoiding loosing control over the car
5) keeping differential cage connected to engine reduces risk of single wheel lockup and helps to distribute braking forces evenly between wheels.

Declutching under heavy braking force can cause the worst - single wheel lockup and spin with severe loss of braking force. Its dangerous.[/I]

[dms664]

[I]The friction from the transmission and engine when you have your foot off the gas will help slow down the car. I think truckers use this technique when going down long grade hills and when coming to a stop.

Yeah, it's called "engine braking". Since brakes are a lot cheaper than transmissions and engines in terms of wear, you should only use engine braking for long decents when there's a risk of brake fade. Otherwise, use the brakes for their designed purpose.[/I]

[dms664]

[I]It doesn't much matter. The limiting factor on how quickly you can stop the car should be the friction of the tires on the road.

Depending on the gear ratios and the RPM, the engine can both speed up and slow down the car so there's not enough information in the question to answer whether the engine would speed up or slow down the car.

In practice, stomping on the brakes while driving will usually stall the car (stop the engine running) so stepping on the clutch is the way to go.[/I]

[aamirgillani]

I think . Irrespective of any driving manual /skill, school of thought ,in emergency you will do whatever your reflex action dictates you to do. Therefore,
the issue in question is more of psychological in nature than of any driving concept.

How? see below:-

Emergency Braking

In emergency, your reflex action only permits you to press brakes as hard as the situation may demand. In such an emergent situation your mind is fully committed to stop the car as quickly as possible to avoid the impending collision or disaster leaving every thing beside, including, the very next action of pressing the clutch.

While your mind is busy in handling emergency in a time compressed situation you press the clutch only when engine diverts your attention by producing knocking voices at near stall. The engine will stall if your reflexes are slow or the time lapse of coming to complete halt is too short to press the clutch.

[dms664]

I think . Irrespective of any driving manual /skill, school of thought ,in emergency you will do whatever your reflex action dictates you to do. Therefore,
the issue in question is more of psychological in nature than of any driving concept.

How? see below:-

Emergency Braking

In emergency, your reflex action only permits you to press brakes as hard as the situation may demand. In such an emergent situation your mind is fully committed to stop the car as quickly as possible leaving every thing beside including the very next action of pressing the clutch.

At such a moment while your mind is busy in handling emergency. You press the clutch only when engine diverts your attention by producing knocking voices at near stall . The engine will stall if your reflexes are slow or the time lapse of coming to complete halt is too short to press the clutch.

Thanks for the kind reply but sir I have met some accidents personally and after the collision and getting back into senses when I tried to move the key in the ignition the vehicle was already running hence producing a krrrrrrrrrr sound which is known to all as almost all vehicles produce such sound incase u try to start a car with a running engine.
What it concludes that I pressed the clutch as well during emergency braking and it was not just any other street accident , had I been not wearing seat belt I would not be typing this right now so what do u say, I want technical answer.

@xulfiqar , levin, capsat etc might come up with a good reply
fingers crossed

[aamirgillani]

Thanks for the kind reply but sir I have met some accidents personally and after the collision and getting back into senses when I tried to move the key in the ignition the vehicle was already running hence producing a krrrrrrrrrr sound which is known to all as almost all vehicles produce such sound incase u try to start a car with a running engine.
What it concludes that I pressed the clutch as well during emergency braking and it was not just any other street accident , had I been not wearing seat belt I would not be typing this right now so what do u say, I want technical answer.

@xulfiqar , levin, capsat etc might come up with a good reply
fingers crossed

I would say that you met some near fatal accidents because you concentrated more on pressing the clutch than to avoiding the occurrence

[2c]

what about auto trany>

[deadonice]

Pressing the clutch just makes car more Free allowing it to become a bit faster so its not safe if u try hitting the Brake with Clutch on a 120 that even all of a sudden it would be quite risky ... Ive experienced it while driving my car if i apply brakes while clutch a can actually feel it goin a bit faster at times its a bit hard to stop the car so for these conditions i just hit brakes without clutch =) Nice topic by the way

[aamirgillani]

I am of the view that in emergency the brakes should be applied straight away without clutch because the compression stroke of the piston also acts as brake which thus augment the braking system. In emergency its much more important to stop the vehicle than to keep engine running after the collision. See an example above.

[deadonice]

i think im going to Subscribe to this topic best one ive seen yet hehe =)

[aamirgillani]

@dms664
[I]
"In practice, stomping on the brakes while driving will usually stall the car (stop the engine running) [U]so stepping on the clutch is the way to go."

[/U][/I]I beg to disagree Sir.

Its not, stepping on the clutch is the way to go. Its stopping of the car is the way to go about in emergency.

You are mixing up normal anticipated braking with emergency braking.

I may suggest to stick with your own subject matter i-e Engagement of Clutch during Emergency Braking.

[uiv]

In emergency braking situation it is never advised to free up the transmission, your RPMs would drop and you'll have no engine braking. Same rule for ABS systems.

Have you ever tried to put the car from a bigger gear to smaller without slowing down? Like going at 130 in 5th try to engage 3rd? The car will try to slow right down.
Do not try this coz there are chances of ending up with a broken shift fork if you do it without double clutching.
Nevertheless, what you really what to do is to engage a lower gear if its possible and try not to lock your brakes. Pumping them brakes is the best idea if your brakes are locking and car is skidding.

P.S. The krrrrrrrrrrr sound is the starter motor being grinded by the flywheel and you seriously don't want that.

[Abdussamad]

Years ago number 1 cost me a lot of money so it is a bad bad idea. Yes I crashed my car because I pressed the clutch first and the break second. That extra split second can make a huge difference. You have to slam the break without hesitation. So number 2 it is.

[NakedTruth]

Ha. Discuss what you might, but the cold fact is us humans somehow have a built in reply.

We brake with clutch pressed. Sorry but that is the truth. You cant suddenly change it when you are about to get your az handed to you.

Tell me... how will you be able to think so fast and act that fast when at 120Kmph the object is just a few feet away from you?. Everyday we drive our car and brake with the clutch pedal pressed. How do you think we will suddenly change it in that situation?. Answer that first.

and as for ABS situation, lol clutch doesn't even matter here. How is lockup and ABS related?. lmao.

Now that point of locking up one tire with clutch pressed. Wrong. A week or two ago i pressed the brake pedal without engaging clutch at high speed and only the right tire locked up. So i call BS ON THAT!.

Now the feeling on both:

At that speed and without a car with ABS, in both situations the sudden full force brake would lock all your wheels if not the front two (depends if rear are drum or disc). Either you will spin and be safe or will stop safely with sad tires.

[Abdussamad]

Ha. Discuss what you might, but the cold fact is us humans somehow have a built in reply.

We brake with clutch pressed.

Sorry but speak for yourself. Since that accident I modified my behaviour so now I always press the break first and then the clutch. Actually it's the fault of our car training schools that they teach you to first engage the clutch before you hit the breaks. That is the wrong habit! You first press the break and, if you have to, you use the clutch.

[Xulfiqar]

Ha. Discuss what you might, but the cold fact is us humans somehow have a built in reply.

We brake with clutch pressed. Sorry but that is the truth. You cant suddenly change it when you are about to get your az handed to you.

Tell me... how will you be able to think so fast and act that fast when at 120Kmph the object is just a few feet away from you?. Everyday we drive our car and brake with the clutch pedal pressed. How do you think we will suddenly change it in that situation?. Answer that first.

and as for ABS situation, lol clutch doesn't even matter here. How is lockup and ABS related?. lmao.

Now that point of locking up one tire with clutch pressed. Wrong. A week or two ago i pressed the brake pedal without engaging clutch at high speed and only the right tire locked up. So i call BS ON THAT!.

Now the feeling on both:

At that speed and without a car with ABS, in both situations the sudden full force brake would lock all your wheels if not the front two (depends if rear are drum or disc). Either you will spin and be safe or will stop safely with sad tires.

your driving technique is flawed - the correct way to drive is to rest your left foot away from the clutch and even when coasting you are supposed to disengage (press) the clutch only when the engine comes near to idle speed. With this in mind you will never have to face dual pedal pressing. And your clutch will live happier too.

now to the OP - engine braking is needed to stop the car effectively and quickly - they help the friction brakes on the car. Nowadays manufacturers are utilising coupling action in the auto transmission to assist the wheel brakes - so you have a ferocious brake action, as the engine compression slows down the axles.

Whoever gave thought that the engine is spinning at 3000 rpm and then when hitting brakes the inertia would be too great to overcome, is plain insane, when you hit the brakes you release the accelerator pedal, this causes the engine to immediately slow down and the action of the wheels slowing down by the brakes along with engine compression soaking up the rotation of the driveline all helps in stopping the car.

it would work even in auto as its a speed based system, as soon as the vehicle speed slows down the trans goes into lower gear.

[NakedTruth]

Sorry but speak for yourself. Since that accident I modified my behaviour so now I always press the break first and then the clutch. Actually it's the fault of our car training schools that they teach you to first engage the clutch before you hit the breaks. That is the wrong habit! You first press the break and, if you have to, you use the clutch.

God forbid, have you been in an accident after that?. Don't deviate from the topic. and i never even bothered learning from these so called schools. I learned watching my father, everyone i rode with and last but not least, driving myself.

your driving technique is flawed - the correct way to drive is to rest your left foot away from the clutch and even when coasting you are supposed to disengage (press) the clutch only when the engine comes near to idle speed. With this in mind you will never have to face dual pedal pressing. And your clutch will live happier too.

Oh look. Now people would even bash someones driving style. Once there was a person like you who blamed me of driving with his foot on brake (bike) when i went to him with dead rear brakes in under a month. Soon he took his words back and changed the brakes.

So as i said above, my father drives more or less the same like me. I haven't, yet, seen anything going 'wrong' with the clutch on my 2005 Cultus. Please backup your claim. The only 'thing' that has been changed are actually the brake pads than anything else.

I have backed up my claim. Please, i say again, back your up. I can go out now and make a video using both the techniques at 120Kmph. What about you?. I am not even talking about casual driving. How i brake at those times is totally different.

During an emergency the time it takes one to press the brake and clutch is probably half a second. and of course the brake is always pressed first and just a millisecond after it clutch... at least i do that. Heck who wont press the brake first?.

[Xulfiqar]

God forbid, have you been in an accident after that?. Don't deviate from the topic. and i never even bothered learning from these so called schools. I learned watching my father, everyone i rode with and last but not least, driving myself.

Oh look. Now people would even bash someones driving style. Once there was a person like you who blamed me of driving with his foot on brake (bike) when i went to him with dead rear brakes in under a month. Soon he took his words back and changed the brakes.

So as i said above, my father drives more or less the same like me. I haven't, yet, seen anything going 'wrong' with the clutch on my 2005 Cultus. Please backup your claim. The only 'thing' that has been changed are actually the brake pads than anything else.

I have backed up my claim. Please, i say again, back your up. I can go out now and make a video using both the techniques at 120Kmph. What about you?. I am not even talking about casual driving. How i brake at those times is totally different.

During an emergency the time it takes one to press the brake and clutch is probably half a second. and of course the brake is always pressed first and just a millisecond after it clutch... at least i do that. Heck who wont press the brake first?.

Yes why not?? If your style is incorrect and you actually tell everyone it is correct does not make it correct does it? Ive got licenses of 3 different countries outside of Pakistan and if you tried to pull that while giving your test in the 2 european ones - you get kicked out. The instructor watches your feet for this action. Similarly on a motorcycle you dont pull in the clutch when you hit the brakes - or you hit the tarmac as the bike frees up.

So please rather than being offended and putting up what would one do - get your driving technique right. Your argument shows that if I gave you a Heavy GW truck full loaded with 45 tons of weight on a trailer - you would actually burn the friction brakes downhill, crash into a house and complain that what could have been done. Its the driver's job to work the brake systems. The license is a certificate showing that you know all this.