18

When coming to a stop, will depressing both the clutch and brake pedals cause any damage to the engine or transmission? Is there anything mechanically wrong with doing so?

  • 3
    misleading or incomplete title – agent provocateur Mar 15 '17 at 17:29
  • @agentprovocateur Fixed. – Hari Ganti Mar 15 '17 at 18:42
  • 1
    This is off-topic for this site. See "Are driving questions on-topic?" on the 'meta' site for this site. – Kenny Evitt Mar 15 '17 at 19:01
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    I flagged this to be closed for being off-topic, but now I'm torn about whether it really should be closed. This is pertinent for potential future maintenance and repair of the relevant vehicles ... – Kenny Evitt Mar 15 '17 at 19:07
  • Have edited and am retracting close vote. I personally feel it can fit the site as it is now. – anonymous2 Mar 15 '17 at 20:50
10

No other problem than using slightly too much fuel.

The recommended way to brake is to use the current gear and keep the clutch engaged during braking until idling RPMs at what point one presses the clutch to avoid stalling the engine.

So:

  • Save some fuel and press the clutch only until idling RPMs, not immediately
  • Don't downshift, that'll wear the synchronizers of the gearbox and the clutch unless you double-declutch and rev-match
  • Don't disengage the clutch too late, that'll put stress on the engine

Edit: For long descents, you should select an appropriate gear which may result in quite high engine RPM. The engine's cooling system is much better at disposing of the braking heat than your braking system. This answer applies to braking when coming to a stop.

  • 10
    "Don't downshift" is actually pretty poor advice. Downshifting lets brake in a more regulated fashion because the engine provides less impedance than the brake pads, ergo you're less likely to lock the wheels. Also, even without rev-matching or double-clutching, you can downshift with minimal stress on the drivetrain. Synchro wear is very minimal because you only fight the rotating mass of the clutch plate, and slowly letting out the clutch to rev up the engine is similarly minimal wear. The engine is easier to rotate than your vehicle is to move. – Hari Ganti Mar 15 '17 at 17:50
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    @Hari: There is no right answer to this, and many people will argue both sides. I've come around to the argument of "wear down your brake pads, not your clutch", but I still do both. – user12176 Mar 15 '17 at 19:44
  • @FighterJet Sure, just like there's no right answer to shift timing (for normal driving, not racing). There are preferences, and because the wear on downshifting is minimal, I consider it to contribute negligibly (unless you are horrible at downshifting). FWIW, I typically heel-toe or double-clutch anyway. – Hari Ganti Mar 15 '17 at 20:07
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    The better argument for downshifting is safety. You retain the ability to accelerate out of trouble, if your engine rpm remains in its power band at all times, and you have some degree of engine breaking, should your brakes fail. The latter especially on a long descent with a heavy load, which may overheat your brakes. It also always feels to me that the (front-wheel drive) steering is more responsive when appropriately downshifted. Anyone know, is this real or psychological? – nigel222 Mar 16 '17 at 13:06
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    @nigel222 It is very real, and potentially dangerous (see my answer). – dasdingonesin Mar 16 '17 at 13:51
2

There are only two other alternatives when you're coming to a stop (per comments: in a manual car :) ) :

  1. Just press the brake. If you do this all the way to a stop, it will result in the engine stalling. Usually, not what you're wanting; produces more wear on the starter, etc. As per comments, it is what some people recommend until right before you stop, but there is no advantage mechanically.

  2. Just press the clutch. No mechanical problems with it, except that it could take you a long time to come to a stop - especially if you're rolling down hill.

Bottom line:

You have to press the brake and the clutch at the same time to come to a halt effectively most of the time. It produces no harm on the engine or the transmission that is not expected in your car.

  • 3
    1 is only true when your engine RPM has reached the idle RPM. Until that point, it is recommended to keep the car in gear with the clutch engaged. This also helps in case emergency maneuvers (or acceleration) is needed. – Hari Ganti Mar 15 '17 at 18:43
  • @HariGanti Interesting, I had never heard that, even from my manual driving professor. Makes sense, though: I'll have to try that! (Can't believe I've been driving manual tractors, trucks, cars, etc. all these years without knowing anyone telling me!) :) – anonymous2 Mar 15 '17 at 18:46
  • 1
    Third alternative: "Clutch? What's a clutch?" Get an automatic and never have to worry about such details! ;-) – Mason Wheeler Mar 15 '17 at 18:57
  • @MasonWheeler, Lol. I'll edit to improve clarity. – anonymous2 Mar 15 '17 at 18:57
  • Comments are not for extended discussion; this conversation has been moved to chat. – Pᴀᴜʟsᴛᴇʀ2 Mar 15 '17 at 23:00
2

I agree with the other answers saying that it doesn't make a huge difference for your engine and transmission whether you press the clutch or not while braking normally.

However, depending on the age of your car, it does make a huge difference in an emergency braking situation. If you drive an older car without anti-lock brakes/electronic stability control, not pressing the clutch may cause the car to become uncontrollable and spin much earlier than with the clutch depressed.

The reason is that the braking moment exerted on the live axle by the engine introduces a moment in the opposite direction on the chassis, which results in an upward force on the rear axle (FWD) or a downward force on the front axle (RWD).

Effects of engine braking

In both cases, the overall effect is that the front wheels get lots of grip while the rear end becomes light. If you then try to do an evasive manoeuvre, you'll find that the rear end of your car is going to overtake you at much lower speeds and steering angles than if you would have depressed the clutch. In cars without electronic aids, I would therefore strongly recommend to always press the clutch pedal in an emergency braking situation. cristiancrc's answer is badly wrong and very dangerous advice for people driving old cars.

For modern cars, it doesn't really matter. Just hit the brake pedal as hard as you can and point the steering wheel in the direction you want to go, and the computer will try and take care of the rest, using all the grip there is to keep the car going in the right direction. Just be aware that the computer can't defeat the laws of physics – if you are too fast, you're inevitably going to crash. Electronic driving aids are a massive improvement in driving safety, but they do not perform miracles.

  • I just realized that this answer may be off-topic. If you don't think it belongs here, I will delete it. I just felt like I really needed to correct cristiancrc's answer. – dasdingonesin Mar 16 '17 at 13:48
  • A benefit of 4-wheel drive, even if I never stray off the tarmac? Or does the modern computer wizardry mean that it only used to be a reason? – nigel222 Mar 16 '17 at 14:45
  • I don't overly disagree with you, but I feel there is a feel more nuance to this. The effect of engine braking is far less than your brakes. It's a matter of impedance. Your brakes are very high impedance (takes a lot of force to get any rotation) while the engine is relatively low. As a result, engine braking is less likely to lock your wheels, which is a huge boost to stability. Also, I find oversteer generally easier to correct (I drive a RWD). Lastly, you're never supposed to hard turn while braking. aaafoundation.org/faqs-anti-lock-braking-system-abs#steer – Hari Ganti Mar 16 '17 at 17:05
  • @nigel222 4WD unfortunately doesn't help, it just combines the two mechanisms described in the sketch. – dasdingonesin Mar 16 '17 at 17:14
  • @HariGanti To drive an evasive manoeuvre in a non-ESC car, there should be no braking moment at all. Even if the engine braking moment is quite small compared to the wheel brakes, it can cause dramatic oversteer, especially on wet/slippery surfaces. This can be used e.g. in Rallye driving ("scandinavian flick"), but it is virtually impossible to control for an average driver in an emergency situation. – dasdingonesin Mar 16 '17 at 17:28
1

Being in the habit of depressing the clutch and then a split second later (or simultaneously) pressing the brakes would actually be good for two reasons:

  1. (as anonymous2 already mentioned) Sometimes, when braking, you are suddenly slowing down, and don't have time to adjust the gears. So depressing the clutch helps prevent the engine from stalling.
  2. Being in the habit of first depressing the clutch prevents damage/death in the event of accidentally pressing the accelerator instead of the brakes.
0

Only press on the clutch to avoid the engine stalling or when the changing gears.

Should you need to perform an emergency maneuver to also avoid an obstacle during breaking, you need your body to be stable, pushed against the back of the seat. So it is better to keep your foot on the footrest, because it acts as support for your body as it keeps you stable and allows you to move the steering wheel without also using it as support at the same time.

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