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:
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.
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.