Brake failure - shold you change down the gears ?

[Nat]

In a recent conversation I was told that - in the event of a brake failure you should change down a gear to increase engine braking. I disputed this on the basis that whilst in gear you achieve some engine braking but if the vehicle is taken out of gear there is potential that you may not get it into the lower gear and therefore have less control.
I there a definitive answer to this ? Is it circumstantial ?

[Kevin]

In a recent conversation I was told that - in the event of a brake failure you should change down a gear to increase engine braking. I disputed this on the basis that whilst in gear you achieve some engine braking but if the vehicle is taken out of gear there is potential that you may not get it into the lower gear and therefore have less control.
I there a definitive answer to this ? Is it circumstantial ?

Hi Nat,

I've never experienced brake failure, so my reply is based on just my opinion and theory, but he's my two-penneth worth. Actually, that's not entirely accurate. I used to have a Morris Marina which had a hydraulic master cylinder on the brakes. When that failed I would need to pump the brake pedal to get the brakes to work .

With a modern syncromesh gearbox I would have thought it possible to engage at least the next gear down, which might provide enough braking to then select a lower gear once the speed has dropped sufficiently. I guess it will depend on how steep the hill is you're decending as to whether only engaging the next gear down will have much effect and how much time you have to play with. If you're running out of space you might as well try to bang it into as low a gear as possible.

Engine braking in my little diesel is not very significant, so I would probably apply the handbrake first and hope that slowed me down more than just changing down the gears.

If changing down and applying the handbrake didn't have much effect, I'd be looking for something soft to hit (not a pedestrian, even though they're soft!).

PS. Just had a thought (unually for me). I can try and engage a low gear at a high speed and see what happens. As long as I don't lift off the clutch pedal it shouldn't do any harm, I hope. I'll let you know how I get on.

[Horse]

In a recent conversation I was told that - in the event of a brake failure you should change down a gear to increase engine braking. I disputed this on the basis that whilst in gear you achieve some engine braking but if the vehicle is taken out of gear there is potential that you may not get it into the lower gear and therefore have less control.
I there a definitive answer to this ? Is it circumstantial ?

Definitive answer? No idea . . .

But if I were in that situation, mechanical sympathy would go the way of the brakes. It WOULD go into gear

[exportmanuk]

Having been driving for some time, when I started driving cars were more prone to component failure, brakes and clutch were not as reliable as today's vehicle plus cars did not have servo assisted systems so good planning end engine braking was normal driving.

From my experience you use engine braking through the gear box sequentially slowing down a little in each gear changing down as the revs become in range. Dropping into two load a gear is not such a good idea as the sudden braking will make the car unstable and the braking forces may not be as even due to the differential unit. Obviously if you are in an emergency situation you need as much as possible, but if you ever jack up your car so both driven wheels are off the ground and un braked if you turn one wheel and you will see the other go in the opposite direction. If your brakes fail and you use the gears to slow should one wheel lose traction the the differential will have a greater effect on the other wheel.
Same with the hand brake apply this but don't yank it on or you may lock the wheels and you lose control as the back tries to pass the front.

[Gareth]

there is potential that you may not get it into the lower gear

This part of your question struck me as interesting ... if revs are matched I don't see a good reason for not being able to select a lower gear, (normally the next lower gear). I can imagine an inexperienced driver panicking and not being able to change gear, but there the problem is the panic response.

Perhaps it behoves all drivers to practice changing down a gear such that the engine speed is at least 3/4 of maximum, for all possible gear changes. The reason to do this is to maximise engine braking; I think I haven't driven a car which doesn't have an appreciable amount of engine braking at high engine speeds.

With a bit of practice it will soon be found it is even possible to change 2nd to 1st, at the end of which you are likely to be going slow enough for the accident damage to be relatively minor.

[Astraist]

Lately, a horrible crash occured in my country where a driver was said to have lost his brakes by causing them to fade driving downhill. The subject was brought up to public awareness and drivers went to some advanced driving instructors to train for this situation.

Changing down the gears is the single most important step in the event of brake failure or brake fade. It can really help in slowing down the car, provided that you don't just go right into first if the speed is too high. Instead, going through the gears in sequence and at peak torque is the right approach.

Applying the handbrake is another measure, again, provided that it's applied gradually: One "step" at a time, with the ratchet always pressed in and ready to bring it back down if you need to do so (maybe resorting to some kind of "intermittent" braking with the handbrake). Anyhow, the guidance is to rely more on engine braking and less on the handbrake.

The third and last step is to slow down the car by gently scrubbing it against a solid surface like a guardrail, or even a hillside or the divider in between the carriageways. When done at a sloped angle, it can reduce speed nicely, even though it takes time to stop the car that way.

Of course, provided that the failure is hydraulic and not a result of brake fade - pressing the brakes intermittently for a few times can usually help in squeezing a decent amount of braking force from the brakes. It can also help to dry the brakes if the "failure" is a result of water or frost in the pads or due to pad knock-back.

The steps which we technically decline are: Turning off the engine, going down the gears too quickly, applying the handbrake aggressively or trying to jump out (!) of the car.

[Horse]

So, worst case you managed to change to and fully engage a 'too low' gear, will the engine go bang? If so, will it stop you? I suppose the options could be the top end turning to shrapnel, a seize (which would lock the driving wheels).

Presumably a diseasal over-revving has differnet outcomes from a petrol engine?

Also, albeit you'll [most of your] lose steering control, would turning off the ignition help or hinder engine braking?

[Gareth]

going through the gears in sequence and at peak torque is the right approach.

Are you sure about that? Peak torque in one car I drive is at 1900 rpm, but the braking effect there feels much less than at, say, 4500 rpm.

[Kevin]

PS. Just had a thought (unually for me). I can try and engage a low gear at a high speed and see what happens. As long as I don't lift off the clutch pedal it shouldn't do any harm, I hope. I'll let you know how I get on.

Excuse me quoting myself, but I gave it a go. I tried to select 1st at 60mph, but the gearbox was having none of it I did manage 2nd at 60mph without any problems, without having to increase the revs. Tried 2nd at 70mph and again, no problems, even without increasing the revs.

Dropped down to 40mph and could select 1st, without increasing the revs, but when I tried 1st at 50mph I had to increase the revs to 4,500 (where the red starts).

I also checked out at what speed 1st, 2nd and 3rd gear each red-lined at (4,500rpm). 1st red lines at 25mph, 2nd at 45mph and 3rd at 68mph. (These were 'true' speeds using a sat nav, not speedometer indicated speeds).

I don't know what would have actually happened to the engine or the effect it would have on the stability of the car if I'd actually lifted off the clutch in 1st at 50mph, twice the red-line speed for that gear. I can imagine the outcome would not have been good. If you've got time to come down through the gears gradually, then that would probably be the best option. If time is not on your side, then you may be forced to select as low a gear as possible and hope for the best.

It seems, for my 1.6 diesel Fiesta, there is a limit to how low a gear that you can reasonably select at high speeds, with 1st being difficult to select at 50mph and perhaps impossible at higher speeds.

Hopefully my little experiment didn't do any mechanical damage.

PS: I think I'd try to avoid blowing up the engine by going for too low a gear as I would lose power steering. Losing brakes would be bad enough, but to make steering harder at the same time would, I submit, be undesirable.

[7db]

Hopefully my little experiment didn't do any mechanical damage.

You will have caused wear to the synchro rings, unless you spun the layshaft up (and if you did, then you will have probably been able to engage the lower gear).

The synchro rings only cost a quid or two to replace. Sadly you need to take the gearbox out and apart to find them and that costs proper money.

[TripleS]

going through the gears in sequence and at peak torque is the right approach.

Are you sure about that? Peak torque in one car I drive is at 1900 rpm, but the braking effect there feels much less than at, say, 4500 rpm.

I agree; but is Astraist talking about revs at peak torque before or after the gearchange. Either way, the higher the revs the higher the rate of retardation, I would say.

Best wishes all,
Dave.

[Kevin]

Hopefully my little experiment didn't do any mechanical damage.

You will have caused wear to the synchro rings, unless you spun the layshaft up (and if you did, then you will have probably been able to engage the lower gear).

The synchro rings only cost a quid or two to replace. Sadly you need to take the gearbox out and apart to find them and that costs proper money.

Doh! Better not tell the lease company.

[fungus]

I seem to remember Tom Topper recommending grazing walls or barriers as well as changing down through the gears.

[Horse]

I seem to remember Tom Topper recommending grazing walls or barriers as well as changing down through the gears.

The motorcycle equivalent is advise on how to slide: 'scuff a lot of leather a little bit, rather than a little bit a lot' ie because it will wear through. Then it's flesh . . . The idea is to make a 'starfish' shape

[Kevin]

I seem to remember Tom Topper recommending grazing walls or barriers as well as changing down through the gears.

Topper certainly recommends using the handbrake and to also "'smash' gears down - straight to 2nd, then 1st."

John 'Lofty' Wiseman suggests 'scraping the edges of your wheels along kerb will reduce your speed, as will brushing along a hedge or wall.'

Topper's advice about using 2nd gear first seems to agree with the outcome of my little experiment where attempting to get 1st at high speed wasn't easy and may not be possible, whereas 2nd gear could be selected without difficulty.