ADUK Archive

When carrying out an emergency stop, when should the clutch pedal be depressed, immediately the brake is applied or just before the car comes to a halt?

Also, does whether or not the car is fitted with ABS braking make any difference to when the clutch pedal is depressed?

Kevin wrote:When carrying out an emergency stop, when should the clutch pedal be depressed, immediately the brake is applied or just before the car comes to a halt?

Also, does whether or not the car is fitted with ABS braking make any difference to when the clutch pedal is depressed?

I vote for depressing both pedals together.

It seems to me the most straightforward technique, and while some people set great store by the engine braking factor, I'm not convinced that the contribution made by engine braking is significant in an emergency stop situation, and it might even be detrimental.

With regard to ABS systems, I'm not sure to what extent they affect the issue with regard to the timing of clutch disengagement. Probably the timing of the clutch disengagement makes little difference to the effective functioning of the ABS feature.

Best wishes all,
David.

The clutch is not connected to the ABS system

As said - I doubt if depressing the cluch early or late will make much difference in a real emergency braking situation

I was taught to do brake first (just).

There was a thread on here recently about this where the favoured choice seems to be both feet down from the start.

Chris

Brake then clutch (with the difference in timing it takes you to say it).

I teach brake, then clutch, more or less as Von said.

The reason the DSA say brake, then clutch just before you stop, is to take advantage of engine braking. However, if you leave it too long before pushing the clutch down, you risk a stall, or the engine management system forcing the revs up as they drop below idling speed, especially on a modern turbo diesel. If you do as the DSA say, when you depress the clutch, will depend on the gear you are in. How quickly you are loosing speed, and the idling speed of your engine. My advice would be, as long as you brake first, you can depress the clutch almost immediately.

The reason I mentioned ABS is because many years ago (about 10 yrs) I was on an advanced driving course for LGVs, and some of the training included driving a tractor unit along a skid pan. The units were fitted with a switch that could turn off the ABS. This was so we could practice emergency braking with and without the benefit of ABS.

I was told not to delay depressing the clutch pedal when braking (either with or without ABS being used), it being thought that engine braking on modern vehicles was not necessary given the efficiency of the brakes, but the main reason was that, when ABS was active, it is possible for one of the driven wheels to actually rotate backwards. Knowing that those of us on the course would find that hard to believe, we were shown a slow-motion video of an LGV braking hard with ABS fitted and the clutch not disengaged and, sure enough, one of the driven wheels began to rotate backwards. (The wheel was painted up black and white in four segments to make the effect clearly visible) Obviously this would upset the balance of the unit which would be undesirable.

It seems to me that brakes on all modern vehicles are now so efficient that the additional effect of engine braking during an emergency stop is negligible (especially the higher the gear) and might even be disadvantagous, especially in a diesel-driven vehicle, where the high torque of the engine might actually fight against the brakes. This is what caused one of the driven wheels to rotate backwards in the video I was shown. When the ABS slightly released braking on one of the wheels which had begun to lock up, the differential allowed the half-shaft to rotate backwards to dissipate the energy from the, still engaged, engine.

I'm surprised the DSA, in their publications, still advise delaying the clutch pedal until the car has nearly stopped.

In an emergency stop situation the priority is that maximum braking power should be applied as quickly as possible. That's what our instincts will lead us to do, and what we do about the clutch pedal is a secondary matter I would have thought.

If both pedals don't go down together, I can envisage a slight delay before the clutch pedal is depressed, but I can't visualise anybody consciously delaying the clutch disengagement until just before the vehicle comes to rest. In the heat of the moment I very much doubt if we're likely to think about it much at all.

Best wishes all,
Dave.

TrippleS said,

I can't visualise anybody consciously delaying the clutch disengagement until just before the vehicle comes to rest.

You'd think so wouldn't you, but the DSA think otherwise. The advice for stopping in an emergency in The OFFICIAL DSA GUIDE to DRIVING the essential skills is' "Don't touch the clutch pedal until just before you stop. This helps with your braking and stability." They do, however, go on to say, "Note This routine is not necessarily correct if you have ABS brakes. Refer to your manufacturer's handbook."

The handbook for my car, which does have ABS fitted, doesn't offer any advice. My daughter is nearly 17 and is keen to start learning to drive, so I was interested to see what is being taught today in view of the advice I was given about 10 years ago that the pressing the clutch pedal shouldn't be delayed (I hesitate to say that it should be pressed at the same time as the brake as I'm sure someone will come back and say there will be a split second difference between the operation of the two pedals ) whether ABS if fitted or not.

Would a learner, when on test, be penalised for not leaving the clutch pedal alone until just before stopping, as recommended by the DSA or would it not matter? Obviously, from the advice I was given and for other reasons too, I would like my daughter to do what I believe is the safest course of action and apply the clutch pedal without delay (assuming she's asked to perform the emergency stop as I understand that it's not done on all tests), but I don't want to jeopardise her prospects of passing the test.

I guess you could find a nice quiet road, and try it both ways from say 60, measuring the braking distance each time (get out and pace back to the point you picked to start braking at). Park the car up at the point it stopped and leave the hazards on.

I could try an experimental approach I suppose, although I'd prefer not to do it on public roads, but it wouldn't answer my question about whether pressing the clutch at the same time as the brake (there, I've said it and I'd said I wouldn't ) would be frowned on by a driving test examiner.

I'll try and remember to dig out Andy Walsh's DVD when I get home as there's definitely some experimentation with stopping distances and techniques, although I can't remember if he demonstrates clutch effects. What ISTR from my first carlimits day was being told to declutch as late as possible - declutching at the same time as braking lead to the car trying to slew around (this was doing an emergency stop from ~70mph on an airfield). This is a mid-engined RWD car with no ABS, so not sure how transferable that is to other cars.

I did a car skid pan course at Thruxton, some time in the mid/late-80s, and we were taught to declutch, but I can't remember that the actual timing relative to the brake pedal was even mentioned.

I'd hazard a guess that it probably happens that most drivers would swap from teh A - B pedals quicker than getting a foot to C, since braking is (should be?) mor einstinctive a reaction.

In the 90s I did the US bike training, where declutching was taught as part of e-stops, interestingly also to include a downshift (that's 'selecting a lower gear' this side of the pond), on a bike that means each hand & foot is doing something. I've used the technique once, and when mid-way through braking the obstruction cleared I was able to acclerate away in that lower gear.

The MSF's (US bike training) suggestion was that if you lock and then release with the clutch engaged then you will have stalled and bump-started the engine. This means that your 'engine braking' is not consistent.

Edit: Just realised I have the instructor manual here, so have a quote:

"Disengaging the clutch removes engine braking from the rear wheel, simplifying the task of controlling rear-brake application. Engine braking alone could be enough to lock a lightly loaded rear tire during a quick stop. If a rear wheel is inadvertantly locked, engine braking could be enough to keep the tire skidding even if all brake pressure were released"

Kevin wrote:I'd prefer not to do it on public roads

I just did it this morning with an associate. If you never do it on public roads, you are missing a chance to calibrate your perception of how far it takes to stop on a public road. Having done both, I think the stopping distances look different on airfields and tracks.