The premise is surely nonsense! The full logic ought to go: Why do we have cars? To go faster. Why do cars have brakes? In order to slow down again.
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.
Different people seem to have different interpretations of this question. My interpretation was: Why have brakes instead of coasting to a stop? Coasting to a stop is popular with drivers who are 'Hypermiling' [1] - and bicycles used in velodromes don't have brakes [2].
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.
I wonder if car technicians go around making weak analogies with software development to justify their industry practices.
Something like: "Why do software developers have compilers? blah blah blah, and that's why we need to keep your tools organized"
Why do computers have virus scanners? They only serve to slow down the computer and to block some programs from running. This is contrary to the goal of a computer, which is to run programs! But by blocking evil programs, the good ones can run faster, with less disruption.
My expeerience is they mostly wants to know as little as possible about computers. Computers are necessary evils to most craftsmen I can think of. And I kind of understand them.
If this was written by an American, yes. I don't know many people who know what "engine braking" is. A very slightly higher number of people I know are capable of driving a standard shift vehicle. Most people know what a manual transmission is, but not everyone.
I live in a state that has a few mountains (not a mountainous state, though). A lot of people I know just have shitty brakes now. I have no idea why knowledge of downshifting is not getting out. Even an automatic transmission will support it.
Each time you use the brakes, the energy has to be dissipated somehow, and that is done by heating up the brake pads and rotors. I know people that will use brakes the entire way down a mountain, even in a stick shift but even more so in an automatic... and then wonder why it smells like overheating brakes and why their rotors are glowing red.
Then comes the weird feeling while braking because the rotors are warped...
It was completely frowned upon in my driving education (around 7 years ago) - and my driving school was/is the most popular in the UK.
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.
You're more or less totally wrong, in a modern car.
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.
But when driving a manual car the driver doesn't put the clutch in while braking until almost completely stopped.
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.
I'm not sure I understand why your revs would need to be perfectly matched to avoid fuel consumption. If you're slipping the clutch, you're wearing the clutch, but what does that have to do with fuel consumption?
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.
If you don't rev match then when you down shift your revs will be higher and as such you'll be fuelling again without having to touch the throttle with your foot.
If you don't rev match then when you down shift your revs will be higher and as such you'll be fuelling again without having to touch the throttle with your foot.
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.
The only way there won't be an increased load on the powertrain is if the revs are matched. If the revs are mis-matched then the transmission will be at a different point in the powerband (likely out of the powerband) and will result in increased loads experienced.
Right. Perhaps jamesjguthrie isn't seeing the difference between engine noise (mechanical strain), and engine load. "Engine load" is the additional force requested, which will be negative when the wheels are driving the engine. He's also including rev matching in a conversation that is not about clutch or torque converter wear, so there's definitely some confusion.
I must be missing something here. Your revs will go up regardless due to the nature of downshifting. It's just a question of whether you get there immediately or if you slowly glide there at the expense of your clutch. It seems to me you'd actually use less fuel if you didn't rev match, since rev matching requires you to blip the throttle. That said, I'll take the throttle blip over the clutch wear any day.
It's pretty simple to see that traditional braking triggers brake lights, while engine braking does not. Brake lights are there for safely, to prevent rear-ends. I'd be shocked if any driving education program encouraged engine braking.
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.
In most cases when it is appropriate to use engine braking the person behind you has to already start matching your speed. In situations where you have to brake suddenly and quickly brake lights can warn the other driver, but in most cases when you are downshifting to engine brake you are slowing down slowly enough that the person behind you can make proper adjustments simply based upon the fact that they are now getting closer to you. It's a human PID loop!
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.
During my driving course they always told me to apply the brakes, if only very lightly, while downshifting. This way the brake lights are there to warn people behind you. I always do it (even on motorbikes with the rear brake). I thought it was pretty standard.
It's standard dogma, including in the advanced driving courses (eg. police). "Gears for go, brakes for slow"
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).
Moving from an manual to an automatic can still be quite jarring, even if you don't have engine braking. Ever been in an automatic after driving a stick shift for years? The brake pedal is twice as wide and is in approximately the same location as where the clutch is found ... get into an emergency situation? Your mind will automatically want to disengage the clutch and hit the brakes, so you go for where the clutch pedal would be normally and instead hit the brake pedal ...
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...
Some automatic cars even put the parking brake on a pedal close to where the clutch is on manual. It's quite... surprising when you're used to drive stick.
In most cases though if you pay attention to the road and the conditions around you, engine braking can be used quite effectively, and it is not as jarring as braking using the brakes. This helps when you've got friends/family that easily get car sick, mainly due to the jerky motions from people hitting the gas too hard, and slamming on their brakes.
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...
And every single last automatic. Besides D there is always a couple others, like 1, 2, 3 that are used for getting out of sticky situation or when you need a lot of torque... those also apply for engine braking.
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).
My auto Mazda 3 has 'manumatic' shifting, and you can downshift to slow down in it. I guess it's not really the same though as the computer does a lot of shifting and timing for you even in manumatic mode.
I have this same car and exclusively use this technique in tunnel traffic. The computer has saved me a few times from accidentally shifting into 1st gear at 40mph ..
Some automatic transmissions allow the driver to force a lower gear to engine break. It is not used as often, but can be useful in long slopes to avoid overheating the breaks and reduce wear.
AFAIK mine does it automatically (i.e., when driving down a slope, it engine brakes to avoid acceleration). Then again we don't have that many slopes so maybe I misinterpreted what was happening the few times I drove down a parking garage :)
What car? Without cruise control? My car doesn't downshift unless I've set it on cruise control at an intermediate speed like 55 mph and it is a long downhill slope.
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.
2006 BMW X3, yes without cruise control on. I'm not sure if it downshifts - what I meant was that the transmission adjusts itself to brake on the engine when coasting downhill, so that you don't have to brake in order to not accelerate downhill.
Interesting, I first saw this automatic trans behavior as early as about Y2K in a midrange Mercedes but only with the cruise control on, and it did actually downshift to set up the engine braking effect. I've never encountered it with cruise control off though.
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.
I can vouch for this; I think this is a BMW thing. Our 2012 X3 does the same thing. If you let off the gas while going down hill, you can see the RPMs on the tach rise as the transmission downshifts. I was really pleased with the feature on a trip to Asheville last fall.
Not unconditionally - yes you can switch to a 'manual' mode, but it won't let you e.g. shift to 1st gear when you're driving 100. The manual shifting is useful when driving on steep slopes, mostly. Furthermore, the gear selector certainly won't let you slip the clutch a little bit, like what you need for engine braking.
You shouldn't be slipping the clutch while engine braking. At that point you are simply moving the braking from the brake pads/rotors which are designed for it to the clutch disk/friction material... which is definitely not designed for it.
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).
You're driving 100, in 5th. You want to decelerate, so you press down the clutch, put the stick into 4th gear, then slowly release the clutch until you feel it gripping, and you do this until the rpms are low enough to switch to the 3rd in the same way. I don't know much about cars, but that process seems to be what you mean by 'rev matching'. While doing that, the clutch is slipping (even if just a bit), that's what I meant.
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.
Maybe on fancy transmissions, but the Citroen I used to drive certainly didn't. Never tried at 100km/h, but I did sometimes accidentally switch to the wrong gear at lower speeds (like switching to 2 or 3 while driving 80) and it never stopped me.
You can always force it ... but in general there is some protection that at least makes a lot more difficult to move to 1st/2nd when doing 100 km/h. Third and fourth are no issues.
> Has the art of using the engine to slow down become so unknown that it is no longer considered a way to slow down a vehicle?
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.
Overextending the analogy, there's no brakes on track bikes but they go fast too. They don't need them, the fixed gear means applying counter pressure on the pedals stops the bike.
The clunky software analogy would be to do with lightweight simplicity.
The analogy between automobiles and software testing can be extended to consider the racing line - the combination of velocity and distance through a corner which results in the fastest time.
Which brings me to the point that the steering wheel is much more important than brakes. Being able to go faster because of brakes is not very useful if you are heading into a wall.
Well, you can still stop before hitting that wall. Steering might be of little use if you're going fast and approaching a dead end with walls on either side.
When I was on my company's R&D team, we were looking at possibly designing a car. One of our ideas was regenerative braking (nothing new, I admit). Basically, you can use several types of pump or motor as a generator when run in reverse. This allows you to collect that energy back into the system instead of wasting it all as heat. It also has the added benefit of turning the rotational kinetic energy into stored potential energy, thereby slowing down the vehicle. Our design still included disc brakes for safety, but would have used the regenerative braking most of the time.
To really add breaks where you can stop while running without blowing completely up you need to add fault isolation and tolerance in your system. You need to use actors and messages (you might get parallel execution as well depending on your algorithm as a bonus).
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.
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.
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.
It still wear components that are not as easily replacable as brakes. As for coasting to a stop whilst still in gear, I agree with that being a good idea but I don't think that's what digitalWestie was getting at.
Engine braking allows you to save fuel (by using stored energy of the drivetrain to handle compression of the air that normally would require work from combustion).
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).
I think we're talking about different things. digitalWestie's advice to use the gears to slow implied changing down through the gears to use the engine to brake, not simply leaving the car in gear while applying the brakes.
For what it's worth, it does makes complete sense to leave the car in gear while braking and not to change to neutral.
Fuel usage is 0 when you slow down without brakes... When you use brakes your engine needs fuel to idle. (Assuming you are using manual transmission, and on automatic you can force down the gear to do the same.)
This is ridiculous. The typical "stoner effect" -- a phrase that sounds in some way deep or metaphysical causing susceptible people's neurons to flare, producing only the word: "Woah..." I know this is probably controversial, but in my opinion it's the same effect that causes people to find deep meaning in a Jackson Pollock painting.
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.
You're certainly right in that there are logical flaws in the article. But the stoner effect is useful (and dangerous) because it's much better at creating changes in beliefs/behavior than rational argument. It's a pattern worth understanding because you can use your understanding to defend yourself, or use parts of the effect to communicate your ideas and convince others.
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.
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.