Emergency stop - brake and clutch down together?

[g3org3y]

Stumbled across a couple of links that suggest this is the case.

I was under the impression if was BRAKE BRAKE BRAKE and once the car is coming to standstill you consider the clutch to prevent stalling. The reason being that with the clutch down (coasting) engine braking is reduced as is overall car control.

[Mr Will]

Depends what speed you are doing it from.

At a driving test < 30mph the stop will (should?) happen so quickly that the clutch will need to go down almost immediately regardless. On the motorway doing 70+, you have a lot more time between applying the brakes and getting slow enough that you need the clutch down.

[Dyl]

I was taught (4 years ago) to dip the clutch at the last second, though in practice it just depends on the situation, i.e. is my left foot on the clutch or foot rest.

Good point raised above re:stopping from 70, I'd wait til <30 before I dip the clutch - dependent on gearing though.

[kieranjholland]

depends what gear you're in, and how much you like your current brake pads.

I'd recommend only depressing the clutch when your nearing the bottom of the rev range, i.e. before it stalls so you use as much engine braking as possible.

...of course, how many of us would think about this in an actual emergency?

- edit -

doogz said:

This is how they used to teach it, back in the day where brakes weren't as effective as they are today. In a modern car, you should be able to stamp on the pedal, any time, at any speed, and lock the wheels/activate the ABS.

So engine braking isn't really a concern.

This is a fair point, I'd always use engine braking in an effort to save brake pad wear but not sure it would have an effect when you stamp on the brakes in this situation.

So, at the risk of contradicting myself it really doesn't effect your chances of stopping however I'd always depress at the last minute becuase I'm cheap and like my current brake pads

Edited by kieranjholland on Wednesday 16th February 16:31

Edited by kieranjholland on Wednesday 16th February 16:31

[Davi]

doogz said:

Back in the day, you needed all the assistance you could get, so you'd make full use of engine braking, basically to make up for the fact that car brakes were crap.

plus they tend to teach you to drive according to which bits of the learner vehicle are most expensive to replace now...

[Alfanatic]

In my day before ABS I was taught both pedals at the same time. If the wheels lock you'll stall the engine and have even less control.

Colin Chapman actively designed engine braking out of his later F1 cars. He believe that, because it's relative to engine revs and gear, its not a constant force and reduces braking accuracy. He designed a gearbox to freewheel when off the throttle, so that the brakes did all the braking, and the drivers thus had better control.

Personally, I believe engine braking is going to be of little use in an emergency stop, since you're on the brakes for such a short time and would be better advised to focus on your surroundings outside than downshifting through the gears to improve braking performance. Best to simply stand on the brake pedal as hard as you can and let the ABS do the work while you keep an eye out for escape routes to steer in to. If you really must somehow get just that little bit extra braking performance, you can possibly adjust the braking balance and make a bit more use of the brakes on the rear wheels with careful use of the handbrake, but that's more likely to end in tears too.

Finally, if you are hard on the brakes and have to do some dramatic steering, the car is going to be less prone to oversteer with no engine braking (clutch pedal down) than would be with engine braking. Partly for this reason, and also to give the driver one less thing to trip over in an emergency, a BMW instructor on a course I did taught all his pupils to put the clutch pedal in when correcting oversteer. It stopped the student doing the wrong thing with the power (RWD cars on the course) and meant that they focussed on steering to get them out of trouble.

Not the most efficient method, but one less thing to go wrong.

[The Black Flash]

"If in doubt, both feet out"

[anonymous-user]

I doubt it makes a lot of difference whether you push the clutch straight away or as the revs reach tickover point, in full-on emergency stop mode (which I only recently found out is quite violent, never needed to do a proper one before) the engine probably won't make any difference to how quickly you stop.

[fluffnik]

You'll get more vacuum assistance for the brakes with the clutch up which is likely far more significant than any engine braking...

The most important thing is to get on the brakes HARD ASAP.

[Alfanatic]

doogz said:

Alfanatic said:

Finally, if you are hard on the brakes and have to do some dramatic steering, the car is going to be less prone to oversteer with no engine braking (clutch pedal down) than would be with engine braking.

In a RWD car, maybe, slightly.

Could you explain your reasoning?

FWD cars can oversteer on a trailing throttle if the manoeuvre is violent enough or the braking effect strong enough. They are at least as prone to it as RWD cars are, not least if they are a hatchback with a nice short wheelbase. Most cars, FWD or RWD, IME tend to be most stable in cornering under a steady load, i.e. some throttle applied so that there is no acceleration or deceleration. Freewheeling with the clutch pedal depressed is a closer equivalent to this than cornering in gear with the throttle fully closed (what I mean by trailing throttle) which will move the balance towards oversteer.

[Bursar]

I was taking lessons towards the end of 2009 and my instructor told me that in cars with ABS you apply the brakes and use the clutch just before you stop to prevent a stall, and if you don't have ABS, you put both pedals in.

[Alfanatic]

doogz said:

Alfanatic said:

FWD cars can oversteer on a trailing throttle if the manoeuvre is violent enough or the braking effect strong enough. They are at least as prone to it as RWD cars are, not least if they are a hatchback with a nice short wheelbase. Most cars, FWD or RWD, IME tend to be most stable in cornering under a steady load, i.e. some throttle applied so that there is no acceleration or deceleration. Freewheeling with the clutch pedal depressed is a closer equivalent to this than cornering in gear with the throttle fully closed (what I mean by trailing throttle) which will move the balance towards oversteer.

When you're as hard on the brakes as you can be, i really don't think it'd make the blindest bit of difference.

I do see where you're coming from. I just don't think it'd make any difference. Whether or not the throttle is fully closed is irrelevant, as the actual torque at the wheels is going to be almost exactly the same, which is the amount that the tyres can deal with, without the wheels locking.

I think you're totally right, if you're hard on the brakes it won't make any difference. I tend to come off the brakes before I start steering because, I suppose, a) I'm old and in my day, before ABS and stability control, we were taught not to turn with the brakes on, old habits die hard, and b) in the few emergency braking situations I've had where steering has been a requirement I've had room to come off the brakes when I started steering and start braking again once I stopped steering - if needed. So I guess I was subconsciously thinking back to my experiences when making that point about engine braking.

[davepoth]

If your brakes are strong enough to lock the wheels at 70 then it's going to make little or no difference. In a rubbish car the engine braking can reduce the net braking effect a bit if the engine and flywheel want to slow down slower than the car.

[Monty Python]

davepoth said:

If your brakes are strong enough to lock the wheels at 70 then it's going to make little or no difference. In a rubbish car the engine braking can reduce the net braking effect a bit if the engine and flywheel want to slow down slower than the car.

Quite correct - once the brakes have reached the point of locking, there's no more braking force that can be obtained by this route. The only way to slow down any faster is with an airbrake or a parachute (or an immovable object)

[GAjon]

The Black Flash said:

"If in doubt, both feet out"

Also - 'In a spin both feet in.'

[Zad]

Don't forget that when the driven wheels lock, even for a fraction of a second, the engine has now stalled. Almost all ABS systems allow the wheels to fully lock before releasing brake pressure. Having sensed the engine has now stalled, the ECU may possibly (depending on car and ECU program) cut the spark and fuel, preventing it from automatically restarting. You would need to practise emergency stops, and faffing around depressing the clutch at whatever speed to know how your own car reacts. Personally, I would prefer to know that I still have drive when it is needed, to pull me away from the potential accident.

Another point is that the rotational inertia of the engine will slow down the wheels from reaching road surface speed again, and during this time there will be much less brake pressure. Disconnecting the engine means the ABS can modulate the driven wheels more rapidly, getting closer to maximum retardation and control.

What it boils down to is that in an emergency you need all the brainpower you can get to avoid or minimise an accident. Any mental effort used on delicately controlling the brake and clutch is attention that is being taken away from steering. Mash the pedals to the floor and let the computers do the work there, while your brain does the really tricky stuff.

[brillomaster]

isnt brake and clutch for emergency stops taught because its easier for a learner to remember? since most emergency stops are going to be around town where speeds are slow anyways, you might as well just do both? As another has said, you dont want to be faffing around thinking 'do i clutch yet?' just both feet hard down.

also, if you're eco driving in 4th doing 30, theres not a lot of leeway before you'd be in a stall situation anyways.

[kambites]

I'd be surprised if it makes a measurable difference to your stopping distance whether you floor the clutch at all. In an emergency stop, the limit to your stopping power is the interface between the tyre and the road, not between the brake pad and disc.

[Wills2]

davepoth said:

If your brakes are strong enough to lock the wheels at 70 then it's going to make little or no difference. In a rubbish car the engine braking can reduce the net braking effect a bit if the engine and flywheel want to slow down slower than the car.

I'm not sure there are many road cars that can lock their wheels at 70mph?

[kambites]

Wills2 said:

I'm not sure there are many road cars that can lock their wheels at 70mph?

I'd bloody well hope that they all can! I've certainly never owned one that can't, including my 70s MG with drums on the back.