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?
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.
- 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.
There are only two other alternatives when you're coming to a stop (per comments: in a manual car :) ) :
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.
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.
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.
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).
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.
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:
- (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.
- Being in the habit of first depressing the clutch prevents damage/death in the event of accidentally pressing the accelerator instead of the brakes.
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.