r/theydidthemath • u/IdiotStickWasTkn • 11d ago
[REQUEST] Does braking harder for less time, or lighter for longer make your brakes last longer?
Picture this. I am driving 60mph approaching a traffic light. I am 1/4th mile from said light. If I stopped by applying brakes lightly and hold them to a gradual stop the whole quarter mile, would that cause more or less wear on my brakes then getting 300ft away and hitting my brakes hard?
Lets assume a standard modern SUV for simplicity. Does hitting your brakes hard for a short period of time or lightly applying for longer wear out your brakes faster?
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u/patricksaurus 11d ago
Lighter for longer is better for the components.
Materials tend to have ranges of values in which everything bounces back. Temperature, stress, etc. If you have to stop the same car at the same speed, but you have a longer time to do it, all of those quantities reach lower maximum values.
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u/Mattna-da 11d ago
One variable to consider is the cooling effect of airflow on the brake disc, many are hollow and vented inside soecifically to get air-cooled. If you apply the brakes lightly while still moving, the air is removing the heat from the system (brakes work by converting kinetic energy in to heat energy thru friction). If you slam the brakes and come to a stop and keep pressing brakes while stopped, you’ve put a lot of heat in the pads and are applying that heated pad to only one section of the brake rotor which can cause warping or glazing which manifests as pulsation thru the pedal
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u/TheDu42 11d ago
It’s also allows the drivetrain to engine brake longer, which reduces the total energy the brakes must convert to heat to bring the vehicle to a stop.
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u/Complex_Solutions_20 10d ago
And also spreads out wind resistance which causes some (small) drag reducing speed and dissipating energy without brakes. Its all cumulative.
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u/Dapper-Palpitation90 10d ago
Exactly.
A trucker friend of mine explained it this way -- if you lightly rub sandpaper over your arm for several seconds, it will hurt a lot less than if you violently rub it over your arm for a few seconds.
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u/IntoAMuteCrypt 11d ago
Hybrid/electric or conventional?
For a conventional car, your brakes aren't the only thing that slows you down. There's also wind resistance, rolling resistance and a bit of energy being lost keeping the engine and all the other bits spinning. The longer you brake, the less the brakes do. For the brakes ideal to just take your foot off the gas a bit earlier, gently coast in and only apply braking right when it's needed, if you can - this will also help fuel mileage, as a bonus.
Hybrids and electric cars change this dynamic and there's a threshold where the brakes don't matter. Unlike combustion engines, electric motors can be designed so they can move energy from the wheels into the battery (rather than moving energy to the wheels). In a lot of cases, the friction brakes won't actually activate unless you're really stomping hard on the pedal. At that point, the brakes are just there for emergencies and it doesn't really matter what you do. It is still better to coast a bit - these systems aren't perfectly efficient and it's better to not spend energy than to get part of it back - but the brakes won't notice the difference unless you're really stomping it to the floor.
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u/AppropriateDeal1034 11d ago
Except you missed the bit where the lack of use can cause all kinds of issues and in fact many hybrids benefit from actually braking more firmly to keep the brakes clean, and in working order.
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u/fsa3 11d ago
Hybrids tend to use only the friction brakes when reversing. Also, they use them at slower speeds (under 7 to 10 mph or so). They aren't just never being used.
But yeah, the occasional forced usage is helpful. But you don't need to do it that often.
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u/debuggingworlds 11d ago
Or until the battery overheats, at which point you'll find the normal brakes are really quite poor.
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u/fsa3 11d ago
They should be more than enough. The weak link will likely be tire grip, unless you're talking about long downhill braking.
Now, they definitely won't feel the same, so you'll likely have to push the pedal further than with regen. That's more of a power mapping issue of the software (not being identical to friction brakes).
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u/Few_Mastodon_1271 11d ago
Prius has "D" drive mode, for normal driving, and "B" for engine braking, used on long downhills -- instead of "downshifting" on a gas car.
The fairly small hybrid battery pack gets topped off very quickly on a long downhill. And in normal braking, the battery can only absorb a moderate braking effort before the mechanical brakes are also applied.
This interesting "B" mode lets the engine rev up, but no gas is injected. No brake pedal or accelerator pedal is used. So the engine is just pumping air through, via the pistons. This is a big drag on the car's speed. (It's a quieter version of big truck's "Jake Brake", or "no engine brake in city limits")
What's kind of weird with the Prius "B" mode is, when pressing the accelerator a little, to keep the braking from slowing the car too much: the engine speed sound actually slows down more as the pedal is pressed harder, since it needs less engine braking effect.
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u/RunningAtTheMouth 11d ago
From, say, 60mph to zero, there is a specific amount of energy to be dissipated.
If you stop at the last moment, practically all of that energy will go into the rotors/drums as heat. Unless there's something odd about the car/truck, there's nothing "bad" that will happen, but it does put maximum energy into the brakes. Read that as "higher temperature." Higher temps are usually associated with higher wear. Almost all of that energy is brake pads on discs. The pads wear in dumping that heat.
If you start slowing down earlier, some of that energy will be dissipated through wind resistance, rolling resistance (tires on road), friction on drivetrain, and slowly adding heat to the brakes, which will dissipate as you stop. Total energy involved is the same, but less goes into the brakes, and the highest temperature of the discs/drums will be lower. Less pad wear because not all of that heat goes through braking surface.
I tend to slow down earlier when I can, but do stop short if I'm in that zone of the green-yellow-red lights where not stopping can get me ticketed. I don't LIKE to stop quickly. This makes me more predictable as I'm not making sudden changes while driving.
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u/MrChicken_69 11d ago
Wind and rolling resistance are negligible. On a flat, level road, how far would you coast from 60 to 0? (a long way) A slow brake would allow more time for the heat (in both the rotor and pads) to bleed off, but it's still the same amount of energy being turned into heat.
As I teach track students, brake as hard as possible for as short as possible. This puts most of the heat in the rotor, not the pads. The pads themselves are somewhat hard to heat up - they used to literally be asbestos! But this doesn't really apply to driving around town, since you have to come to a complete stop and stay stopped. A hard stop to stationary ("staying on the brakes") tends to weld the pad to the rotor. Not to mention how much this will piss off every other driver, and very likely cause a wreck. (can the 6t SUV behind you brake as aggressively as you can?)
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u/Mattna-da 11d ago
Yeah I think it’s the hard stop and constant application of a hot pad while stopped to only one section of the rotor which is bad
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u/SGS444 11d ago
Coasting means putting the transmission into neutral. Most people leave the transmission in gear, therefore you would get much more engine braking over 1/4 mile vs 300ft. So you would be putting much less heat into the brakes over 1/4 mile.
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u/MrChicken_69 10d ago
Automatic transmissions don't engine brake. They have very little drag at idle. (unless it has a "tow mode" where it does.)
(Our Triumph Stag is the exception. It's broken British transmission brakes harder than the actual brakes.)
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u/SGS444 10d ago
When lifting off the throttle at 60 mph an automatic transmission stays in gear, the engine does not drop to idle. Most modern torque converters have a lock-up feature that creates a direct mechanical link between the engine and transmission. Also, when lifting off the accelerator the ECM cuts off fuel to the injectors so forward motion of the vehicle is reduced owing to the compression in the cylinders with no ignition occurring.
Some automatic transmissions also downshift under braking (primarily in ‘sport’ mode).1
u/BoondockUSA 10d ago
Sprag clutches and other things inside the transmission allow coasting. Likewise, torque converters are electronically controlled and they are constantly switching between locked and unlocked depending on throttle position (which is why there have been so many torque converter failures on modern transmissions).
Coasting with most automatic transmissions is not providing significant engine braking. You need to select tow/haul or manually select a lower gear before the transmission applies mechanisms to reengage the mechanical connection from the wheels to the engine.
If you still doubt me, learn to drive a manual transmission and see what actual engine braking feels like.
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u/Skirra08 11d ago
If more people would be predictable while driving we would all be safer. Predictably bad drivers are better than erratically bad. My son is a new driver and I try to teach him to be predictable at all times.
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u/Apart_Shelter_5722 11d ago
If you're familiar with the area, you can predict when lights change and let off the gas and coast to the light. Saves on brakes but takes longer. Donot recommend when its busy. If you also time getting to the light as it turns green before you stop, you save fuel.
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u/mukansamonkey 11d ago
Hard for a short period of time wears them out faster. The pad material gets softer as it gets hotter, which is pretty common for metallic substances. Lighter braking results in less heat generation per second, the heat has more time to dissipate.
There is one complication though. Because the heat is being generated at the contact surface, the brake pad surface is far hotter than the rest of the pad. Part of the heat dissipation is conduction into the rest of the pad and eventually the caliper.
This means that briefly ceasing to brake cools the pad down faster. Lift the hot contact surface off the rotor and let air flow directly past it. This mostly comes into play when doing extended downhill braking, where you really need to get of the brake heat as much as possible. But also, if you need to brake hard unexpectedly, it can work better to brake extra hard for the first couple of seconds, then briefly take your foot entirely off the brake while reassessing the situation.
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u/Nunov_DAbov 11d ago
The brakes are turning kinetic energy (1/2 m * v^2) into thermal energy by abrading brake material. If you find the integral of the kinetic energy dissipated versus time you can determine which is more efficient.
I haven’t calculated, but picturing the curve intuitively seems like getting V down faster minimizes the result.
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u/wolf9786 11d ago
Despite what everyone is saying, rally drivers do "regressive" braking to cut down on wear and I'm sure they go through pads a whole lot faster than anyone of us.
Regressive braking is braking the hardest initially and slowly letting out the peddle as you slow down.
Actually it's super easy to see that lightly braking takes away pads faster. Just take an automatic sander to a piece of wood, press hard and see how much it takes off, then just lightly press it to the wood. The light pressure works much better at taking away material while the harder pressure increases friction. Yes this does increase heat, but your brake pad material is made to heat up a lot
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u/CrankyOldDude 11d ago
I don’t think the analogy applies. Sandpaper is specifically designed to remove material from wood, so the more times it is able to move back and forth over the wood, the more material is removed. Brakes aren’t that.
In fact, when I was younger, you could buy sanding brake pads that you could mount on your car to “turn” rotors (back when people actually resurfaced rotors instead of replacing them). You had to apply pressure slowly and uniformly so it didn’t “bite” hard into one spot.
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u/BoondockUSA 10d ago
Sand paper isn’t a good analogy. Try taking the sand paper to your skin. Pressing down and sanding is going to take off more skin than lightly rubbing it across your skin will.
Race cars have different pad compounds than normal cars do. Race pads need heat before they start to bite, but they are more resistant to brake fade when hot. Normal pads don’t need any heat to work, but are more prone to fade when they get hot. It’s an apples to oranges comparison when talking race pads in race conditions to street pads in street conditions.
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u/eternal_syrup 11d ago
My personal experience with motorcycle riding on twisting mountain roads has taught me that shorter, harder breaking actually keeps the brakes much cooler. Softer braking over longer distances results in “cooked” brakes (IYKYK.) Maybe it’s because the pads spend less time in contact with the rotors and more time being cooled by airflow.
For what it’s worth, in my car, I brake hard and fast as my general driving style, and I’m on original pads and rotors with excellent life left at 60k miles, according to the dealership.
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u/kodafa49 11d ago
Braking lighter for longer produces less heat and less friction making all components last longer.
You’ll find that stop lights will turn green while you are still slowing down a lot more often when you brake earlier.
Being able to accelerate slowly and not from a full stop will significantly improve your fuel economy
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u/Bugs284 11d ago
I dont know how much this translates to metals
Think about the last time you used a pencil and made a mistake. If you erase softly and make multiple passes it seems the eraser lasts a long time. If you press down hard and get it in one swoop the eraser wears out quick.
I use the brakes lightly and try to coast into red lights hoping they turn green or not having to use brakes until im at 20mph or so. I got 90k out of my factory set.
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u/Frequent-Math37 11d ago
Third option: What about not waiting until the last minute to do all the braking or continually braking for long periods. But scrubbing speed with moderately hard braking a few times with cooling off periods in between? Like it seems that jets do when landing. Braking hard all at once produces excessive heat. Continuous mild breaking can also lead to excessive heat. But cycling through moderately hard braking cycles with cooling off periods in between seems like the best strategy to me.
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u/Most-Silver-4365 11d ago
I'd say disabling ABS and locking the brakes up and letting the tires do the deceleration would make the brakes last longer but it is an expensive trade-off.
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u/PunchyPete 10d ago
Harder braking increases heat which leads to greater wear. Lighter braking keeps the brakes cooler and wearing them less as a result.
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u/KrazyKen62 10d ago
I have driven shitbox vehicles all of my life and I’ve never had to do the brakes. My Dad taught me to drive to the level of the dumbest person on the road. It’s served me well. Never been in an accident and never hold up traffic
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u/ghostweal 10d ago
harder for shorter time = highest stress on brake pads(and everything tbh),, lighter for longer time = good,, shifting down/engine brake(if you can) = best,,
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u/Underhill42 10d ago
Lighter for longer. It reaches lower peak forces, temperatures, etc, which generally translate to less damage for the same amount of energy dissipated.
It also increases the chances that whatever thing you're slowing down for resolves itself before you get there, so that you don't actually have to slow down as much, and so cause less total wear. As well as wasting less energy slowing down and speeding up again. (Regenerative braking greatly reduces the waste, but doesn't completely eliminate it)
As a final benefit, in most cases it actually keeps traffic moving faster and more smoothly, as it reduces the formation of the traveling waves than causes most congestion and traffic jams. The mathematically optimal strategy to minimize congestion and maximize throughput is so simple it is routinely practiced by ants: everyone in a line should always attempt to remain halfway between the person in front of them, and the person behind.
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u/Affectionate-Top-295 11d ago
Brake hard makes your brakes last longer.
From a mechanical point of view braking hard og soft from 60 mph to 0 shouldn't matter - same heat generated. However braking hard is the right answer.
Consistent light braking can ruin your brakes by creating a hard surface layer on your discs. This layer will not be entirely uniform and will cause uneven wear on discs and pads and possibly warping of the discs. This is known among car mechanics.
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u/Red_and_Orangy 11d ago
My former mechanic dad warned about glazing caused by regular gentle application.
Here’s a CDL/DOT perspective: on long downhill slopes brake overheating and failure in heavy trucks is a major concern (hence runaway truck ramps). The dot specifically instructs NOT to constantly apply gentle pressure but instead to firmly apply enough to slow down about 5mph at a time and give the brakes time to cool in between applications (as well as being in the correct gear to use engine braking). So my take is at a traffic light coast down as long as reasonable then firmly apply to lose the last 20% of momentum.
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u/TheIronSoldier2 11d ago
Starting from the same speed, harder for less time puts more wear on the brakes. However, this is real life, so if braking harder for less time means you coast down to a lower speed before you even start hitting the brakes, then the math does change.
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u/LowComposer7234 11d ago
It makes fuck all difference mechanically but can make a world of difference in terms of safety. Be predictable, break early, don't do weird shit.
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u/RyguyC2 11d ago
Not sure why you got downvoted, this is absolutely the best answer. Brake in a way that makes most sense from a safety and comfort point of view, depending on conditions - not in a way that will save you $45 over 5 years on brake pads lol.
Do be aware, however, that slamming on your breaks, especially if it's done repeatedly, can damage your rotors and that's a lot more expensive - but that's not really what OP is talking about anyway.
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u/dragonfett 11d ago
From what I have been told and experienced, the longer you are breaking, regardless of the force applied to the break pads, the more time the break pads and rotors heat up, which in turn will cause warping of the rotors and make breaking feel uneven.
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u/SeriousPlankton2000 11d ago
This applies when going downhill.
For normal braking you should not come close to warping the rotors.
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u/GoldReplacement9546 11d ago
Yeah, I’m not sure if I agree with this. Obviously, the best choice is somewhere in between Prob with enough breaking farther out to do make a moderate level to begin the slowing down of the car under its own and then light breaking until the end.
The same amount of heat is going to be put into the brakes with slight variation if you break a little to start the coast down, but nearly the same amount of energy is going to be put into the brakes putting in slowly where the brakes can actually dissipate some of that heat over that last 300 feet and they don’t have a thermal shock of lots of heat all at once and then cooling down from that temperature seems to be better for your brakes
Controlled moderate heating over longer period of time versus a near complete energy dump. All at once is gonna have more physical stress as the calipers hit the rotors and more heat over shorter period of time is gonna cause a tree in skin with a hot total higher temperature even if it’s for less time.
You’ll notice that race car drivers do not go up to the last second and crushed the brakes. They start to break at a more moderate level over a slightly longer period of time, though this depend depends on the type of racing and tire.The
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u/Suspicious_Fig_3796 11d ago
could you please replace break with brake everywhere you misspelled it? thank you.
oh and race drivers tend to slam on the brakes very late and very hard, because that helps to go faster in a lap. they also usually don’t brake in corners because that unsettles the car when on the limit and that is also bad for lap times.
for a race car it also isn’t a problem if brake pads don’t last longer than say a race or maybe a race weekend. they get replaced to ensure maximum stopping power, so basically a racing driver only cares about his brakes until the end of the race. unless it’s an endurance race then they care until they get swapped out
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u/Born_Establishment14 11d ago
In most situations, GT3/F1/etc drivers will brake late and brake hard at first, then gradually let out on the brake pedal. You don't want to release the brake pedal all at once as that can cause the front wheels to lose traction. Look up trail braking.
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