Why Do Cars Have Brakes?
devjoy.com
devjoy.com
If all we were concerned about was going fast then why ever slow down? Cars need to regulate their speed in order to provide safety, and also so they are usable, if I can't stop then I can't get out at my destination. Many elements of a car help in it's overall function, which is to go fast, but you may as well say the purpose of a fan belt is to go fast. Brakes are a part of a car and saying that their purpose is the same as the whole car is useless and confusing. They have a specific purpose within the machine.
From the linked article: "[the] answer feels paradoxical which usually means there's a deep truth". Indeed. The simpler explanation is that it's pure twaddle.
if I can't stop then I can't get out at my destination.
While this isn't consistent with Richard Dalton's blog post (if you coast to a halt, you wouldn't need to throw your children out of a moving vehicle) but it isn't inconsistent with Jon Jagger's blog post, which Dalton credited as his inspiration.
[1] http://en.wikipedia.org/wiki/Energy-efficient_driving#Accele... [2] http://en.wikipedia.org/wiki/Velodrome#Bicycles_and_track_de...
...and that is why we change our motor oil.
Brakes are when I need to slow down fast.
I use my engine to brake about as much as I use my brakes. It provides for a smoother ride overall.
That being said, engine braking tends to only work really well with stick shifts...
I understand why: it's most efficient to drive in the highest gear even at speeds as low as 45 mph in a manual. I would imagine it's similar for automatics too.
Ford trucks with a Tow/Haul button enabled will do exactly what you describe though, especially if you tap the brake for an instant while going downhill.
Engine braking works well when you rev-match as you downshift, then when in the lower gear simply let off the gas completely. You will slow down, and at that point your drive/axle is pretty much directly connected to the engine which is simply turning without gas being injected into it, thus using the compression cycle to slow you down.
In a manual car you don't want to shift to 1st gear when you are driving 100. That's bloody insane (Whether that is 100 km/h or 100 mph).
I think you think I was saying that you switch to 1st and let the clutch come up until you stand still, but that's not what I meant.
When you are in 5th, you downshift, while the clutch is still disengaged you blip the throttle to get it up to the required RPM's (in my car 3k in 5th is about 5k in fourth), then let the clutch up (no slipping, RPM (rev) match it), at this point you are no longer giving the engine gas. It will start to slow you down, from 5k back to 2k, disengage the clutch, blip the throttle to 4.5k, 3rd gear, let the clutch up, slow down further.
The same way that you have to rev-match when down shifting to go pass a truck on the highway quickly.
The way you are doing it is slipping the clutch and is absolutely terrible for the clutch. You are now using the clutch like your brake pads/rotors and it is definitely not designed for it.
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No, I was thinking you were suggesting that you could put the manual transmission into first gear while driving 100, that is absolutely insane. The mismatch between speed from the axle and the engine is going to be too great and it will damage your transmission. Most transmissions even have protection against this so that you can't even push it in to first gear while going faster than 40 Mph unless you really force it.
Do I suggest engine braking in an automatic in the general case? No, generally because the torque converter isn't too happy unless it was specifically designed for it. However if you are going down a steep mountain you better downshift even in an automatic and use the engine, or when you get to the bottom your rotors will be glowing red, your brake pads will be disintegrated, your brake fluid will be boiling and your car will be smelled from miles away.
Yes, I have seen people brake all the way down the I-70 coming over the vail pass in Colorado, and it isn't pretty... if it doesn't break everything your rotors are going to be warped completely...
When I was newbie, I didn't downshift and was instead keeping the brakes lightly pressed. After a few mins of this, when I pressed them a little harder to slow down more (at a turn), the pedals sickeningly went way more than they were supposed to. Remembered in a flash what had happend. Thankfully, I was able to pull over and let the brakes cool a bit.
The brake fluid had overheated and turned into a compressible liquid (if you remember your Pascal's Law (iirc), hydraulics can only work if the fluid is nearly incompressible).
Then comes the weird feeling while braking because the rotors are warped...
Also you have to use manual in driving schools including at the exam or you will get a second class license that only counts for automatic cars.
I always do a combination of down-shifting and braking as this saves fuel vs purely engine braking and I'm using my brakes what they were designed for; and you don't ever want to try emergency braking with cold brakes.
On a slightly different note, if your rev-matching isn't smooth when you downshift to engine brake, you're just using your clutch to slow the car, which is a much more expensive part to replace than brake pads/rotors.
As for rev matching, yes, completely agreed, if you can't rev match your down shifts then engine braking is indeed going to use your clutch and that is more expensive to replace. However, if you are driving a stick shift car and you don't know how to properly rev-match on downshifts how will you easily get up the mountain where shifting from fifth to fourth to third is sometimes required simply to keep up with traffic properly? I can go from fifth at 3k to fourth at 5k without issues. Rev-matching in a stick shift is very important outside of the engine braking use case.
Why this is so, I'm not entirely sure; it does mean that you can change from manual to auto transmission without thinking (and potentially without making a fatal mistake at speed if you are pursuing a suspect). However, it just shifts the cognitive effort of making sure you're always in the correct gear (which I think is also standard advice in eg. the roadcraft system).
From a professional owner / driver perspective (eg. a taxi), the answer is simple; brakes are cheap to replace, clutches aren't.
What they really don't tell you about are things like left foot braking (which I don't do) and heel-and-toe (which I do, especially when leaving motorways where we tend to have a junction followed by an uphill slip road).
Yeah, I was used to driving my Subaru Impreza with a stick shift, then I drove my dads BMW 5-series, got into a sticky situation in traffic and had to come to a quick and sudden stop, there was no thinking involved, push the clutch and the brake was my immediate reaction... let me tell you, that BMW stopped on a dime, luckily there was no-one behind me or I'd have to explain to my dad how instinct and sheer muscle memory on my part caused someone to rear-end me and ruin his car, all because my left foot went for the clutch and caught the edge of the brake pedal...
Under overrun conditions (e.g. engine braking - driveshaft turning the engine, essentially), a modern engine totally shuts off fueling completely, so engine braking alone is always a MPG win, as zero fuel consumption is less than idle fuel consumption with the clutch in.
Also, brakes these days are so good that you really NEED anti-lock. The limiting factor in stopping is the grip of the tire.
In the real world, just brake.
Personally, I wouldn't downshift without a rev match because of the clutch slip factor, but I was always under the impression that with the throttle closed and the engine being driven by the wheels (not by the throttle via idle), you were using virtually no fuel.
This is not how modern fuel injected cars work. Fuel consumption is determined by the ECU and is directly proportional (but not linear) to the load required to fulfill the accelerator pedal or maintain idle RPM. When the wheels drive the engine, no fuel is consumed regardless of the RPM.
Also if there is no power demand (e.g. engine RPM > transmission RPM, but RPM > idle), again, no fueling.
This is good advice for the common case of city and suburban driving, but very poor advice for mountainous terrain.
YMMV
(... sigh, couldn't resist that one)
If you need to slow down quickly enough that you're not just letting the car coast in the gear you were driving in then using the brakes to bleed some speed off isn't going to be less fuel efficient than down-shifting to a gear that will stop you in time. The engine is engaged for almost exactly the same period of time.
Assuming your car is reasonably recent, engine braking doesn't use significant fuel. See http://en.wikipedia.org/wiki/Engine_braking#Legal_implicatio....
Most cars now have automatic transmissions, and in many such cars the habit of slowing down by downshifting will destroy the transmission's bands and wear down the rear differential, neither of which is designed for the stress caused by downshifting.
I'm not speaking hypothetically -- I mostly drive manual transmissions and I'm accustomed to downshifting to slow down. I then acquired a car with an automatic transmission, but didn't change my behavior. Within 20,000 miles I had destroyed the transmission and the rear differential.
The clunky software analogy would be to do with lightweight simplicity.
The alternative is to prove that your program won't crash. So you need to write in a very restrictive style. This is more like you need to be able to compute a path your car will take ahead of time given all the other obstacles that could get in its way. That works too but it is highly impractical.
When you need to go from 50mph to 30mph you dont need brakes at all.
Engine braking wears transmission components which are considerably more fragile and expensive than replacing brakes. Engine braking only stops you using the driven wheels, brake discs apply to all wheels.
And, as long as the car remains in gear there's not much extra wear on the transmission itself (certainly not compared to the torque from the acceleration the preceded it, at least).
For what it's worth, it does makes complete sense to leave the car in gear while braking and not to change to neutral.
No it doesn't. Synchronized, rev-matched downshifts put negligible load on the drivetrain. Coasting to a stop with the gear already selected, putting it in neutral just before stall does that even less. The stresses produced by acceleration are ballpark estimate around an order of magnitude greater. This can be inferred by the fact that usually the rate of deceleration by engine braking is smaller than the rate of acceleration for a given gear ratio, ergo, the drivetrain is strained less.
Just saying / rant over / other clichéd sign off.
I think it's called Deceleration Fuel Cut-Off (DFCO).
As others have pointed out, lots of fast things don't have brakes, because they don't need to stop. As I don't think others have pointed out, slow things have brakes too, such as the space shuttle transporter.
The book Switch[1] goes into more detail about what actually works when you're trying to get people to change. I highly recommend it if you're interested in Psychology or Marketing.
[1]: http://www.amazon.com/Switch-Change-Things-When-Hard/dp/0385...