Which would wear auto brake pads more quickly within a normal range at, lets say, 35 mph, Stopping quickly or stopping slowly?
Normal would be, I guess, not slamming on your brakes and not riding them for a block.
Peace,
mangeorge
The least brake pad wear as well as fuel consumption is by letting off the throttle completely and roll without using the brakes. So if you see the light turning red ahead, let off and roll towards it. It always amazes me how many people will go full speed ahead to a red light and brake hard right before it.
As for your hypothetical example, wear would be virtually the same. Braking late would generate a higher peak temperature in the pads and rotors, but at 35 mph it’s not enough to do any kind of damage.
I know that good drivers do not rely on the other cars brake lights, but many drivers do just that. Cars with manual transmissions, and some with automatics (like my Audi with the multitronic cvt) will slow quite quickly if you let off the gas completely. So will a six speed paddle shift. That’s a good thing.
Your pad wear answer is as I assumed, but would a higher speed make much difference? Quick or slow stop, you’re still transferring pretty much the same energy to the pads, no?
I think it would be best to ignore engine braking. I am interested in an answer to this question too.
Specifically, I would like to know how the pad wear w (measured as an amount or thickness or pad material removed, or whatever metric is used in the industry) changes with:
- Initial vehicle speed v.
- Duration of stopping s, assuming constant pedal pressure and/or deceleration force.
Meaning, I want to know the formula w = w(v, s).
I do know that brake pad wear is correlated with temperature - but it is not clear whether a rapid spike to high temperatures followed by cooling is better or worse than a sustained period at moderate temperatures.
Do you mean “easier”? 
You can’t ignore engine braking unless you first determine whether or not it’s significant.
There are two reasons racers downshift into a curve, to brake and, more important, to be in the best gear coming out of the curve.
Either extreme of slamming on the brakes (heats the pads and rotor possibly leading to premature wear) or riding the brakes (pads get glazed and lose stopping power) is bad. In the normal range of application, I would imagine that pad wear is proportional to the energy dissipated. The brakes convert kinetic energy of the vehicle to heat, slowing it down. Kinetic energy is 1/2 mass * velocity squared, so twice the velocity would wear the pads four times as quickly.
As to the original question, within the normal range I can’t imagine there’s much difference – if there is it would be so tiny that nobody would ever bother to measure it.
Mangeorge, I just wanted to add that I’ve also noticed the rapid deceleration when the accelerator is disengaged with my CVT (a 2007 Altima 3.5). I wonder if this an intentional thing in the design of the tranny, or a side-effect. On the one hand, it’s useful on the highway for planning ahead for slower traffic (argh! slower traffic!) because it helps to conserve braking (which is just a waste of gas’n’pads. OTOH, it means you’re burning more gas (I presume) in lower speed conditions.
So really, I’m not answering a damned thing… just making an observation that seems to match yours.
I really like the effect. I feel more in control of the car. I don’t like the “floating” feeling when I let off the gas in my Honda Ridgeline.
When you notice the rapid deceleration it’s what is called “decel cut-off”. The engine computer takes advantage of the car rolling and shuts off the fuel completely. The engine can’t stall as it is being driven by the wheels. That is why rolling towards a stop light gives the best fuel economy, even better than putting the transmission into neutral. You’re also reducing the vehicles kinetic energy so there’s less energy for the brakes to dissipate = less wear on the brakes.
Giantrat: Why do you think it uses more gas at low speeds?
Still, the brake lights don’t light up. Maybe they should. It would be a simple job for the computer. Blaming the surprised other driver isn’t going to fix a ding in your rear bumper.
Anyway, releasing the gas at low speeds in many cars with automatic transmissions doesn’t decel very quickly at all.
That’s interesting. It sounds counter-intuitive to me, but I deal with little bits of data rather than guzzlers of gas for a living. Are there implications to not using the ICE on a long slow downgrade in regard to allowing the alternator to keep on truckin’? [Apologies, Mangeorge if this is a bit of a hijack, but it seems like you’d be curious, too]
I guess the reason it seems like slower speeds burn more gas in the only CVT I’ve driven is that I decelerate quickly if I’m not using the gas at, say, 25 mph (in a traditional tranny car, I’d coast along much longer). Also, at least with my Nissan, the MPG indicator actually indicates greatest fuel economy between ~70-80 MPH.
The engine (and alternator) is still turning - it’s still mechanically connected to the wheels through the transmission. It’s just that it’s not burning any fuel to keep turning, since there’s no need - you’re coasting downhill.
I guess what I should have asked was “not burning any fuel?” That just doesn’t sound right to me. Again, I’m not a mechanic. There’s gotta’ be something to “keep you motor runnin’” as you coast down on the highway. Or at least that’s what I’ve always thought.
When the car is in gear, the engine and the wheels are mechanically connected. Meaning, if the engine turns, the wheels turn. If the wheels turn, the engine turns.
If you are coasting downhill and take your foot off the gas, gravity keeps pulling the car along and thus keeps the motor turning (through the transmission). If you are on a highway doing 80 mph and you take your foot off the gas, the same things happens - the computer stops injecting fuel into the engine, and the car’s momentum alone keeps the engine turning. Obviously, the car then slows down due to wind resistance, and friction in the engine (this is what people mean by engine braking).
If you were to put the car in neutral in either of these situations, you disconnect the engine from the wheels, and it would drop to idle speed. At that point, the computer would have to start injecting fuel again to keep it running, since the wheels are not turning it anymore.
Have you ever ridden a single-speed bicycle? Whenever the bike is moving, the pedals are turning. If your feet are on the pedals and the bike is moving, your feet are going to move too, regardless of whether you’re trying to move them (“burning fuel”, so to speak).
I think you mean fixed gear bicycle, and most people have never ridden one of these beasts. But a unicycle or front wheel of a child’s tricycle works the same way - when the wheel turns, the pedals move.
I understand that (I’m not the nitwit that my posts in this thread make me out to be) and appreciate your explanations. But, for example, why do we still see exhaust fumes coming from trucks going down a slope while not accelerating? Also, if I’m coasting down a steep slope and use the gas, the tachometer indicates no significant change in RPM; it remains at the idle level (unless I use the fake CVT shifting mechanism).
I’m still inclined to think that there’s a quantity of fuel going through the engine to maintain operational capacities.
Mangeorge, again, I’m not trying to hijack. This is neat info/opinion/perspective that we’re getting, IMHO. Call me off if you like.
With an electronically fuel injected engine, the quantity of fuel injected is determined by the throttle pedal position and the idle speed control valve (ISCV). If your foot is not on the throttle and the engine is not under load (ie: coasting) then no fuel is being injected. As has been stated, whilst the car is in gear, the wheels turn the transmission, which turns the engine. If the computer kept injecting fuel when you let off the gas, there would be no engine braking, since the engine would keep itself turning, rather than using up the kinetic energy from the car’s movement to overcome the engine’s compression.
The ‘operational capacities’ of the engine consist mainly of the engine turning, which drives most or all of the ancillaries. Anything not driven directly by the engine is powered by electricity generated by the alternator, which is belt driven from the engine. If the engine is being driven by the wheels, there is no need to inject fuel to maintain any operations, as they are driven by the motion of the vehicle.
The ‘fumes’ you see from truck’s exhaust when going downhill are most likely carbon being blown out of the exhaust while exhaust braking (which is different to engine braking, and normally only found on large, heavy vehicles).
Could you clarify, when you go down a steep slope and use the gas, does your speed increase while the engine speed remains the same? If so it’ll be your gearbox computer altering ratios to increase the speed, as it will be aware of the downslope, and that there is no need to increase engine revs. I’d bet if you floored the throttle in that situation (not that I’m suggesting you should, of course) that the engine would rev up.
No problem. It’s all interesting, and related.
My fuel burning ignorance is cured by y’all. While it still feels counter-intuitive, you folks have provided sound explanations. Many thanks.
It also makes hybrid vehicle technology seem much more mundane.
If you were to ask me what mileage my last several vehicles got, I’d have to go to their website to find out. And you know They’d never lie.
Time to go for a couple carnita tacos at my local taco truck. Mmmm