ABS and Emergency Stop

[skilledhandz]

As i driving instructor and instructors trainer ive come up with a problem that no one in our trade seems to be able to answer.During the emergency stop on test with cars fitted with ABS when should the clutch be depressed?Before ABS came out it was always clutch before stopping in order to maximise engine braking.Thoughts now seem to be along the lines on ABS cars of clutch and brake together to maximise the ABS system.Now its swaying the other way of brake and then clutch.I need solid facts on this one please guys with lots of WHYs. Cheers.

 

[kebo57]

As i driving instructor and instructors trainer ive come up with a problem that no one in our trade seems to be able to answer.During the emergency stop on test with cars fitted with ABS when should the clutch be depressed?Before ABS came out it was always clutch before stopping in order to maximise engine braking.Thoughts now seem to be along the lines on ABS cars of clutch and brake together to maximise the ABS system.Now its swaying the other way of brake and then clutch.I need solid facts on this one please guys with lots of WHYs. Cheers.

In a real emergency stop you wont have enough time to press the clutch
Kevin

 

[beemerjay]

The clutch in all reality should have nothing to do with the ABS except that the Canbus or ECU are informed the clutch pedal is depressed, Poor thing

The ABS monitors the rate of deceleration of the wheel and if the rate of deceleration exceeds a max value it will disengage the brake effort on the affected wheel only in milliseconds until that wheel is back inside approved rates of deceleration, should another wheel cross the max rate line it will be be dealt with in the same way

 

[skilledhandz]

In a real emergency stop you wont have enough time to press the clutch
Kevin

Kev,the vehicle has to be stopped under full control so not pressing the clutch is not an option.The question is when should it be pressed to give maximum braking effect.

Beemerjay,your imformative post seems to say it doesnt matter when its pressed as it wont affect braking in any way.So my question now is will the vehicle stop quicker using engine braking or not?Thanks guys.

 

[star]

Kev,the vehicle has to be stopped under full control so not pressing the clutch is not an option.The question is when should it be pressed to give maximum braking effect.

Beemerjay,your imformative post seems to say it doesnt matter when its pressed as it wont affect braking in any way.So my question now is will the vehicle stop quicker using engine braking or not?Thanks guys.

I wouldn't think it would, as the tyres are at their max in terms of grip just from the brake supposing the abs has cut in.

 

[jberks]

IMO
I'd have said, press the clutch immediately.
In the ancient days of drum brakes etc, the engine braking effect was of value, adding to the overall braking power. Today, modern brakes are vastly more powerful and more than capable of exceeding the braking force needed to break traction on any surface from any speed. They also don't suffer from as much fade as they did in the old days. In other words, you just don't need the engine braking force in an emergency stop.

It is more likely that in an emergency stop, by leaving the clutch out too long, you would use up braking force, fighting the engine's unwillingness to stall (especially since you could be in a much taller gear than was available back then) hence I suspect keeping the clutch out is more likely to actually extend braking distance.

The best policy IMO is clutch and brake hard down and let the technology do its work.

 

[television]

The answer must be no, the rear brakes only receive 28% of the total braking force both FWD and RWD, this is to prevent the back of the car catching up with the front. The brakes can stall an engine running at any speed and the engine coupled via the clutch is only in the over run as the foot is off the gas, if the one driving wheel locked through the engine braking effect, the ABS would take care of the other others (as in driving in snow), it must be more efficient to press both pedals, I am sure that was a natural reaction when I drove manuals and Let the ABS-ESP take care of things

 

[Myros]

Sorry Malcolm, I tend to the other opinion

Clutch down just before stopping, as in a non- abs car. The engine braking effect may be minimal, but it also has the effect of stopping the more lightly loaded rears locking up.

An ABS-equipped car can become a non-ABS car with the burning of a fuse, or the blowing of an ECU. or any other electro-mechanical failure. You could then be running a real risk of locking the rears if you use both feet together.

It is therefore best to keep to the one approved technique, brake first, clutch just before stopping.

 

[television]

Clutch down just before stopping, as in a non- abs car. The engine braking effect may be minimal, but it also has the effect of stopping the more lightly loaded rears locking up.

An ABS-equipped car can become a non-ABS car with the burning of a fuse, or the blowing of an ECU. or any other electro-mechanical failure. You could then be running a real risk of locking the rears if you use both feet together.

It is therefore best to keep to the one approved technique, brake first, clutch just before stopping.

Hang on, the question was emergency braking, thats the pedal hard down to the MAX

 

[Myros]

You're right Malcolm

but that is the accepted technique for an emergency stop as recommended by the DSA. I was a driving instructor as well, some years ago.
The instructor's test then included an emergency stop from speed, above 40mph. Believe me, I practised long and hard before my test, and the car is definitely more controlled and less likely to lock the driving wheels if the clutch is left alone until just before stopping.
Skid avoidance is always better than skid control.
I used to point out to those pupils who showed more than average interest that the Police Roadcraft manual advocated a lock-all-the-wheels, stomp on the pedals type emergency stop, with steering input to correct off-target sliding. I hope they have changed that bit of advice now.

Neither here nor there on an automatic, unless you're thinking of knocking it into N as you brake!

 

[Rory]

Mercedes drivers generally don't have to worry about such things as clutches!

In a manual car, braking in an emergency situation, would say that the clutch should be depressed immediately - if the car is braking to such an extent that the ABS is operating then there must be a possibility that the engine will stall, which isn't what you want for all sorts of reasons.

Many modern cars have very little engine braking effect in top gear, but even if it was considerable, it add nothing to the braking effect of a vehicle that's already got its ABS operating.

 

[jberks]

I suspect the answer may vary slightly depending on what car you're talking about. Most cars are FWD so the engine stopping the rears from locking becomes irrelevant. The ABS, even with 28%, will be working on the rears almost immediately. Ok it could stop the front wheels from locking but the question is "with abs" - and with abs, wheel locking doesn't happen (ok - I know yes it does, but only for a very short moment).

In fact, in an emergency stop in a modern merc you couldn't cadensce brake anyway as the BAS will have kicked in and taken you effectively out of the loop.

I am still a firm believer that people should be taught how to handle a car if all the clever trickery didn't work (skid pan training etc) as they don't have the advantage of learning on a Viva or other poorly handling car. But lets be honest, ABS is pretty reliable and the chances of it dying at that exact moment you needed it is miniscule. You have an equal possibility of the clutch cable braking or the servo packing up and you can't legislate for that.

I stick to my original point that the only advantage of keeping the clutch out is to have the added benefit of an engine brake and in a modern abs equipped car you really don't need that minimal extra help and while i agree that you can stall any manual car with the brake, my impression is that modern ECUs will try to fight the stall within certain parameters, effectively fighting the stopping action if the clutch doesn't go in at the right time - the last thing you need in an emergency.

With no real advantage to keeping the clutch out and a possible downside, why would you keep it out?

 

[Bolide]

Going back to the original question: As I recall, when Autocar do their 0-100-0 runs the drivers hit the clutch & brake together

Nick Froome
www.w124.co.uk

 

[Rory]

..my impression is that modern ECUs will try to fight the stall within certain parameters, effectively fighting the stopping action if the clutch doesn't go in at the right time - the last thing you need in an emergency.

That thought occurs to me too. Imagine the ABS kicking in when stopping on an a slippery/icy surface; with the clutch engaged you'd be stalling and releasing the engine with every pulse of the ABS. It's hard to imagine that that is desirable, especially if the ECU is doing its best to keep the engine running.

 

[Myros]

make your own minds up chaps

http://www.smartdriving.co.uk/Driving/CarControl/Emergency_braking.htm

http://www.2pass.co.uk/brake.htm

http://www.confidence-som.co.uk/emergencystop.html

 

[television]

The ABS runs on the engine and not the battery, so if your engine stops you have no ABS

 

[Myros]

this is a good thread

I remember teaching my pupils not to stall, as to do so in an emergency stop would be to lose control of the car.

All you have to imagine is doing an e-stop in lane one of the m/way, stalling, everyone ahead of you driving on again, and you left there trying to restart your motor with a 40-tonner bearing down you...........

 

[skilledhandz]

Some great answers here im starting to lean towards brake and clutch together.

 

[television]

Some great answers here im starting to lean towards brake and clutch together.

Though I have not driven a manual as an everyday car for a long time, I feel sure that I would have pressed both in this situation by instinct , after driving an auto where one can use any foot, I think that i would forget to do this now in an emergency, not easy is it

 

[johnmc]

I'm 100% with Myros on this one. The ABS on my last car stopped working, and the warning lamp stayed off. I only noticed when braking on ice. I'd rather not have to remember two techniques for the split second decision making. Myros's approach is a good balance, easy to remember.

Here's hoping I won't need to use it anytime soon.

Bye!
John