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I've driven manual transmission for over 2 decades, and I've been braking by downshifting with rev matching for about one decade. I recently heard a radio show host advise people to drive carefully in snowy/icy conditions, and to brake by downshifting. To me, it seems to be risky to do this because sudden deceleration (or sudden forces in any direction) can cause tires to slip. If one does not downshift smoothly, the momentary jerk can contribute to loss of traction. Apart from preventing acceleration in long descents, is there any reason why downshifting would be advantageous in snowy/icy/slushy conditions?

And manual transmission aside, it seems that even for an automatic, it would be preferable to go into neutral and use the brakes to minimize the risk of losing traction. Can more experienced winter drivers comment on this?

Finally, I often wondered about the wisdom of downshifting a manual transmission combined with braking. If you just downshift (with rev matching), the car slows down at its own rate. Since no fuel is being injected, I visualize this as the dissipation of momentum in the engine, though I'm sure it's more complicated (webpages have described the kinetic energy of the car as being consumed in the compression cycle of the piston). Regardless of the exact details, if you apply brakes after downshifting, you're "fighting" the natural rate of deceleration, i.e., you're changing the deceleration because the natural deceleration rate is insufficient. Obviously, the brakes wear if you impart this additional slowdown, but I do not know whether it wears more than if you shift into neutral and apply the brakes.

Before the days of fuel injection, I would very uncertain about which wears the brakes more because (for example) maybe fuel is drawn in to the combustion chamber even though you aren't pressing the accelerator. I don't know if this really happened back in the day, I'm just describing things that I don't know, and these things make it hard to visualize what happens when you brake after downshifting. However, if that picture is accurate, then there may still be combustion occurring, albeit perhaps degraded; it is then quite conceivable for braking in-gear to be less effective than braking in neutral. Note that this does not mean that car accelerates due to the degraded combustion (if there is any); the overall acceleration/deceleration depends on other contributors and detractors. e.g., (i) degraded combustion, if any, (ii) momentum in the moving engine parts prior to the clutch plate, (iii) momentum due to the physical displacement of the vehicle, (iv) friction due to movement of the vehicle, of which air resistance is one component, (v) dissipation of kinetic energy in the compression stroke, and (vi) braking. It is this confluence of factors that complicates my mental model of what happens, and makes it hard to conclude whether it is more effective to brake in-gear or in neutral. If it is in fact less effective to brake in-gear (and I didn't know whether it is when I originally posted this question), that would imply that (i)-(iii) result in large forces, so that its better to eliminate them by going into neutral before apply (vi).

More than likely, this would depend on the natural rates of deceleration after downshifting versus going in to neutral, and on what deceleration you actually want. For example, if you wanted only slightly more deceleration than the natural rate of deceleration after downshifting, then braking in-gear might be the obvious best choice because you would only use the brakes a tiny bit, regardless of whether the natural rate of deceleration in neutral is greater or less. I say "might be" because, in modelling this situation, one cannot get away from the dependence on multiple factors, such as the 5 listed above So if (for example) the natural rates of deceleration after downshifting and in neutral are the same, that doesn't mean it would take the same force of stepping on the brakes in both cases to further change the deceleration by the required amount.

That is my best guess at what happens for pre-fuel-inject days; these days, however, the fuel injectors turn off when you aren't pressing the accelerator. Of the 6 factors listed above, this means that factor (i) is absent, so I'm tempted to conclude that when you brake after downshifting, you aren't fighting any process that is adding to the kinetic energy of the car. If that's the case, then it seems that braking in-gear can only be better than braking in neutral (for the same deceleration rate). Unless factor (ii) is a significant contributor to the overall kinetic energy (the brakes will be fighting this component). Can anyone from a automotive mechanical engineering background confirm or deny this, or shed insight into this in any way?

This is a long post, and there were a number of requests buried throughout. I specifically elaborated on my mental models of what could be happening so that respondents could clarify where they were wrong or muddy. However, the questions need to be summarized:

  1. To avoid loss of traction on slippery roads, is it better to brake in neutral rather than downshifting and braking in-gear? I asked this about both standard and automatic transmission.

  2. On normal roads, is it more effective (and/or less wear on the brakes) to brake in-gear after downshifting, or to brake in neutral?

I gave various reasons for why I think it's better to brake in neutral for #1, and why it is better to brake in-gear for #2. However, I was wondering how accurate my mental models were. I'm hoping that if my reasoning is wrong, the person who corrects me provides clear and convincing explanations of his/her answer.

closed as unclear what you're asking by Josh Caswell, Pᴀᴜʟsᴛᴇʀ2, Rory Alsop, Bob Cross Jan 3 '15 at 20:58

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • Even in carbureted vehicles, there isn't any process "adding to the kinetic energy" of the car in a lower gear. If it was adding to the kinetic energy of the car, then the car would be accelerating. The increased effective vacuum of the higher engine speed in a lower gear is what causes the engine braking effect. Note that in a carbureted vehicle during engine braking, while there is fuel drawn into the combustion chamber, there is very little air drawn in (since the throttle is closed), meaning little combustion --> little power produced. – Shamtam Dec 29 '14 at 4:46
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    What question are you actually asking? It's not clear from the paragraphs above. FYI, engine braking gets brought up with regularity. For example: mechanics.stackexchange.com/q/4340/57, mechanics.stackexchange.com/q/1758/57, mechanics.stackexchange.com/q/3600/57, mechanics.stackexchange.com/q/1210/57 – Bob Cross Dec 29 '14 at 14:43
  • My responses to the comments are too long, so please see the follow-on edits to the original post. – user2153235 Dec 29 '14 at 18:36
  • Please don't reply to comments and answers in your question. If someone brings up a point that needs refinement in your post, just integrate the necessary revisions with the rest of the text. All you did with the various @user segments was make this even harder to read and figure out what you're asking. – Josh Caswell Dec 29 '14 at 19:32
  • At a bare minimum, please rewrite the title as a specific question that people can answer. – Bob Cross Dec 29 '14 at 19:45
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The post is very long and contains several questions buried within, I'll try to answer the basic ones.

Let's assume we are always talking of braking downhill, since this is the most dangerous scenario. I'll cover only manual transmission vehicles, as the automatic case is much more complicated, as it depends on the kind of automatic transmission. In short, if the automatic is a clutch system (manually activated autos or dual clutch transmissions) which have a manual mode, it is pretty similar to having a manual transmission, except for (*). I'll assume the vehicle has not automatic speed-controlled descent mode, which most offroads have nowadays and which is very suited to our scenario, even if it is usually limited to a low speed. I'll also assume non-studded winter tyres are used, as I'll cover the most likely "slippery" case: snow.

Slippery roads

First, one must distinguish from front and rear wheel drive vehicles. In our downhill braking scenario, engine braking only will be much less efficient if the car is rear wheel drive, simple because the braking will not cause weight transfer on the braking rear wheels. With rear wheel drive cars, it is strongly advisable to apply both engine and "foot" braking when downhill. With front wheel drive cars, engine braking may be enough on most descents. Permanent all wheel drive vehicles provide the best engine braking of course.
Why is engine braking better than braking in neutral on slippery surfaces? The car's ABS system which prevents wheel lockup cannot work properly at low speeds. Even though ABS systems have actually seen a great improvement in the last decades, the probability of locking the wheels in a slippery (snowy - icy) descent is very high. Engine braking breaks the wheels proportionately to their actual rotation speed: the higher the speed, the higher the braking force. If a wheel locks up, the braking power supplied by the engine is suddenly zero for that wheel, as it will be the wheel with less resistance, and the wheel will start spinning again. So engine braking is actually a "perfect" ABS system when going downhill at low speeds. This does not mean you should not gently help the braking with foot braking. Using both methods with caution works best. Foot braking only will likely cause wheel lockup, as the ABS system has limited capabilities at low speeds and whithout it you are applying the same amount of braking force to all wheels. Friction between parts and other factors make engine braking less perfect. If the conditions are bad, you can have wheel lockup even when engine braking only. In this case, as any decent offroad driver will know, you should actually accelerate to restart the locked wheel (pressing the clutch pedal should have a similar effect), as counter-intuitive as it may seem.

(*) One should always try to downshift as smoothly as possible, gently possibly braking and/or rev-matching to avoid sudden speed decrease. In manual vehicles, the necessary smoothness can be easily provided simply dosing the amount of clutch travel.

Normal roads

Non slippery surfaces almost zero the risk of wheel locking (without stopping ), thus eliminating the main reason to prefer engine braking. However, engine braking still provides added control over foot braking. Engine braking will cause more wear on the clutch and flywheel assembly, normal braking will cause more wear to brake discs and pads. Depending on which component is more costly to replace and on the estimated added wear, one could decide which system to use. I personally use a mix of both, always using rev-matching on downshifting to drastically reduce clutch wear.

Also, engine braking has a huge advantage to "foot" braking: you don't risk losing your brakes. On long, steep descents, continuous "foot" braking may overheat the braking fluid (incompressible) and cause it to boil, creating vapour bubbles (compressible) which effectively remove any possibility to apply the brakes. This is also the reason why normal DOT4 brake fluid MUST be changed every two years: with age, the boiling point lowers. Despite using engine braking, this happened to me while driving a 1990 ill-mantained campervan down Vrsic Pass. That was a fun experience, but one I'd prefer not to repeat.

As a side note, it is usually better, downhill on good friction surfaces, to apply firmer less frequent braking actions compared to a continuous braking action, which causes more wear to the brakes and more heat to be dumped to the braking fluid.

Effect of the kind of engine on braking

There is no difference whatsoever on the braking if you have an fuel-injected engine, carburated engine or whatever. Power to the wheels, if not zero, will always be extremely minimal when the accelerator is not pressed. Engine braking will always greatly compensate for that. When performing a very strong braking from medium-to-high speed on good surface, chances are that the limiting factor is not the ABS system nor the friction with the tarmac, but braking power. In this case, engine braking will help reduce stopping distances, if you have time to apply it. It will also provide smoother braking, thus reducing the risk of wheel lockup. This is why engine braking is fundamental in race driving. This however does not hold true for rear wheel drive vehicles which can have a strong weight transfer (high, soft-suspension cars, as most US vehicles), as the added braking force on the rear wheels without added (weight-transfer caused) friction could cause wheel lockup and possibly loss of control of the vehicle. Race cars have little weight transfer when braking thus reducing, but not eliminating, this risk.

  • Thanks, Adriano. I'm unable to square off some of the explanation with my basic understanding of physic laws, but these comments don't permit me to explain. The suggestion to fold my responses into the original post has made it much, much longer. I don't see that as being a practical way to communicate. – user2153235 Jan 1 '15 at 23:53
  • @user2153235, that's because this isn't a part of the site dedicated to dialogue. Feel free to chat in the chat room. – Bob Cross Jan 3 '15 at 20:55
  • @user2153235 That may also be because of my poor grasp of the English language. If you can point out what is unclear, I'll try to clarify. Basic physical laws (e.g. Friction = NormalForce * ConstantCoefficient), by the way, cannot describe correctly such a complex scenario as a great mass braking on an uneven surface, with tyres deforming, car pitching on the nose, vibrations from the ABS kicking in. Experimental tests and the derived experience are the only way to assess such a complex scenario. – AF7 Jan 4 '15 at 9:41
  • No, I don't recall anything particularly unclear about your answer. However, I'm unable to respond in a comment. Thanks to Bob for pointing out the chat room. It's been decades since I used one. Back then, they were real time. Time to get back into it, I guess (not real time chat here, more like a posting forum). – user2153235 Jan 4 '15 at 18:31
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Apart from preventing acceleration in long descents, is there any reason why downshifting would be advantageous in snowy/icy/slushy conditions? And manual transmission aside, it seems that even for an automatic, it would be preferable to go into neutral and use the brakes to minimize the risk of losing traction. Can more experienced winter drivers comment on this?

From my years of driving in snowy conditions (mountain regions of Montana state in the US), I can tell you that in a rear-wheel drive vehicle, shifting your vehicle into neutral to brake in snow packed road conditions can aid in the deceleration of your vehicle. This, I believe, is due to not having power going to the wheels, which would tend to fight the braking your are trying to accomplish. I'm not sure if this has the same effect in a front-wheel drive car, but have to assume it would be the same. The best bet, however, for winter driving, is to drive to the road conditions. Always give yourself more room to decelerate/accelerate. Never drive too fast. In doing so, leaving your vehicle in drive (whether manual/automatic) will give you the best chances to get out of a bad situation if the need arises.

As for your question about engine braking, please read the thread ... I believe it will give you most if not all of the answers you seek.

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    This should be flagged as : not an answer, but it is entirely too long for a comment. The beginning is basically opinion, which is off topic. The last part is the answer without any reference, other than the link itself. For completeness, please edit your answer with some of the points from that link or other sources. Otherwise this is a duplicate of that link. – HasH_BrowN Dec 29 '14 at 16:39
  • Greater certainty is always better, but I appreciate Paulster2 for sharing his experience. More authoritative/substantiated responses are welcome as well. I augmented the original question with 6 kinetic energy contributors/detractors in order to better understand the explanation of the advantage of braking in neutral. I actually read the link in the answer prior to posting. It doesn't really answer the questions, as I've now summarized in my original post. I appreciate it nevertheless. – user2153235 Dec 29 '14 at 23:13
  • Actually, since I'm still soliciting responses backed by engineering expertise, I hope you don't mind if I mark this as unanswered...how does one actually do this? – user2153235 Dec 30 '14 at 18:44
  • You just don't mark it as the answer. – Pᴀᴜʟsᴛᴇʀ2 Dec 31 '14 at 0:35
  • I don't recall doing that. But it does seem to be labelled as an answer, and I'm not sure how to undo that. – user2153235 Dec 31 '14 at 0:50

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