ELI5: What is the fastest way to stop a car with a manual gearbox ?

Submitted by navenarf t3_11da1qa in explainlikeimfive

I recently went into a debate about the fastest way to stop a car (with a manual gearbox), in an emergency situation. Is simply hitting the brake as fast, as hard as possible, assuming ABS is doing its job, the fastest way to slow down a car? Or will the car slow down faster in combination with rev matching and engine braking?

My thought is most modern vehicle have efficient brakes and that its braking force will simply override the braking force from engine braking, how does it work mathematically?

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jaa101 t1_ja7fnx6 wrote

Car brakes are good enough these days to apply maximum braking force short of locking up in almost any situation. ABS then means you can just stomp on the brakes for the best result. The exception would be if your brakes were failing due to overuse, say on a long steep descent. In that case you should have been changing down gears to use engine braking to prevent the brakes from overheating in the first place. You can do that even with an automatic gearbox. In an emergency stopping situation, even the best driver is going to struggle to shift down fast enough to help, short of real flappy paddle gear changing and/or extremely high speeds.

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racecarthedestroyer t1_ja7x7vu wrote

can you explain how you can engine brake with an automatic?

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RhynoD t1_ja7ynr0 wrote

Pretty much all automatic cars have a "gear" or setting for engine breaking. Even my Prius with a CVT has it: a separate button for Park, then you "shift" to put the car in Drive, Neutral, Reverse, or B for engine Braking. I don't think I've ever needed to use it, but it's there.

Automatics that don't have a CVT may not have a dedicated Engine Braking setting but they should have a forced 1st, 2nd, and/or 3rd gear (or Low and High which is 2nd or 3rd). The computer is locked so that it won't shift higher, which you can use to engine brake.

Finally, there are the "semiautomatic" cars or automatic with a semi setting, where you can "shift" like you're in a manual but the computer and automatic transmission does the actual physical shifting of the gears; they just do the shifting when you say to rather than when the computer wants to.

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Decent_Response_904 t1_ja96iu0 wrote

The flip side of the “sport mode” is if the RPMS stay increased, the car will shift up for you.

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Calcori t1_ja8glec wrote

Something people haven't mentioned is you can also take advantage of cruise control to make your automatic downshift. Turn it on and then use the buttons to decrease the set speed and the car will downshift to help slow down. But this isn't something you'd do to normally come to a stop, more just on long descents to use some engine breaking.

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disruptioncoin t1_jaa1kd5 wrote

I engine brake all the time with my automatic. My honda accord has slappy flaps (paddle shifters). Turns out my transmission is considered an "automated manual" meaning that it's basically a manual but with electro-hydraulically controlled clutches on each gear, controlled by a computer (unless put in sport mode).

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phiwong t1_ja7o5id wrote

In pretty much any modern vehicle, the limiting factor in slowing down a car (other than crashing into something) is the traction between the tires and the road. The brakes in a modern car can always reach this limit and engine braking cannot help.

In any sort of real situation manually downshifting in an emergency braking situation is not recommended. Unless the road situation is hopeless (eg ice), the driver should be concentrating on steering and avoidance.

99.9% of drivers cannot heel-toe and rev match a manual transmission while doing 100% braking. That is not a skill anyone other than track or race drivers develop over many hours of practice.

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pneurotic t1_ja7rxn3 wrote

This is the answer. Time used to downshift and rev match is time lost braking.

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Frazeur t1_ja8loaa wrote

Good answer. Also 99.9% of manual drivers cannot heel-toe or rev match, period.

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Soccerfanatic18 t1_ja8trfa wrote

Accurate for the heel toe portion as most race cars have their pedals positioned for the driver to properly heel toe, but I feel like a large percentage of stick drivers can rev match. At least in my circle of manual drivers that seems to be the case

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Frazeur t1_ja97xdn wrote

If you live in the states, then that is probably true since most manual drivers tend to be somewhat enthusiastic about cars over there. But a majority of all manual drivers don't live in the US.

Here in Europe, a vast majority of cars were manual still a few years back (still are I think, but almost all new cars are auto nowadays), which meant that everyone including your wife and granny (literally) were driving manuals. Which means that most manual drivers were the type of people who see cars as a mandatory but boring appliance. These people don't know how to rev match, let alone heel-toe.

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Any-Growth8158 t1_ja9a1bg wrote

Very true. Many Americans are shocked at the cost of renting a car in Europe. They don't know how to drive a manual and automatics cost about twice as much to rent.

Manual transmissions are also a fairly effective anti-theft device here (US) as well. There have been cases where a carjacker tried to steal a car, found out it was manual, and left the car looking for something they could actually drive.

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alnyland t1_ja8y6j0 wrote

> and engine braking cannot help.

Can you expand on this? I’ve been driving a manual daily for 7-8 years and I’ve found the opposite of what you say to be the case. I’ve successfully used engine breaking to avoid skidding in some situations.

Engine breaking transfers the energy of you moving into engine rotations instead of via the wheels to the road, so this makes sense. But your statement confidently says the opposite.

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phiwong t1_ja97xgb wrote

Once the brake locks the wheel (or ABS comes on), that is the limit of traction between tire and ground. Engine braking has to go through the tires (it doesn't brake by increasing air drag or anything). Since any modern car brake system has enough power to lock the wheels, engine braking does not slow down the car any faster than simply slamming on the brakes (with ABS) or threshold braking (non ABS).

If you are noticeably slowing down the car more through engine braking while applying FULL pressure on the brakes - your brakes are likely defective.

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alnyland t1_ja9nsex wrote

Right, but engine braking reduces the excess energy leftover once the tires would start skidding, and in my experience can decrease it enough to be below the point where skidding would begin. I drive in snow a lot and use this frequently to decrease speed, not to stop.

In the instances I mentioned, I started with engine breaking then switched to brakes once at a slower speed - my brakes were fine.

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Bensemus t1_ja9pvqs wrote

> engine braking reduces the excess energy leftover once the tires would start skidding,

This sentence makes no sense. The wheels have a certain amount of grip. That doesn't' change depending on how the car is being slowed down. If you are skidding you've exceeded the wheel's grip.

To control speed both brakes or engine braking work. To emergency stop engine braking offers nothing.

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DasMotorsheep t1_ja9v5fg wrote

>Right, but engine braking reduces the excess energy leftover once the tires would start skidding,

Again, it doesn't. Think about where the energy for revving up your engine comes from when you're engine braking... It's the wheels rotating against the road, which via the driveshafts make the gears in your gearbox rotate, which via the clutch will turn your engine over.

If you have a motorbike, you can try this out relatively easily - apply rear brake until just before you lock the rear wheel, and then downshift. You'll see that your rear wheel will lock. It has happened to me.

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DasMotorsheep t1_ja9uj9d wrote

>Engine breaking transfers the energy of you moving into engine rotations instead of via the wheels to the road

This is the part you're getting wrong. When you're engine braking, you're still using the friction between the wheels and the road to transfer kinetic energy - only into the engine instead of the brake discs.

The wheels and road play the same role in both cases - the road is your frame of reference for how much kinetic energy you need to shed, and the wheels are your contact point with it. So in both cases, the friction between wheels and road is the initial limiting factor. And if you can break that limit with your brakes alone, there's no use for any engine braking.

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nmxt t1_ja7f0ht wrote

The fastest way to stop a car is to drive it into a wall. Also when you are braking with brakes you are already doing engine braking as well, since that takes place whenever you are in gear and not pressing on the accelerator.

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zaphodmonkey t1_ja7ij9l wrote

Brakes. The gear differences between manual and automatic aren’t meaningful to slowing tire rotation. Limiting factors will be tire traction and brake strength, obviously including mass or vehicle.

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bbqroast t1_ja7ealf wrote

Hypothetically yes, engine breaking might* give you a small amount of additional breaking force.

In practicality you should focus only on slamming down the breaks as far as they'll ago (assuming you're driving a vaguely modern car). Fyi the breaks on a car go a lot further than most people ever try.

*Might not because the limiting factor could be how much traction your wheels get, at which point adding additional breaking via the engine won't help.

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pseudopad t1_ja7hlyd wrote

I don't think so. The inertia of the engine and the rate of engine braking means a set of brakes will slow the wheels way faster than the engine would lose speed, which means you'd theoretically brake slower, because the engine still wants to rotate at a higher speed.

However, the brakes are also going to be strong enough to lock the wheels even when the motor's still trying to turn the wheels, and ABS will kick in anyway.

Typically, a set of good brakes will have many times more braking power than the same car's engine has motor power. Your brakes could take an engine from full power to stalled in a second (and probably ruin your drive train in the process).

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Atmosphere-Terrible t1_ja7wb40 wrote

I am not sure I understand the question, but yes hitting the breaks is THE way to go.

Of course in case of ice/snow it's not recommended or actually preferred to do engine break but that's not at all quick.

As someone who drives manual daily and have only driven automatic 10 times, why did you specifically ask about the manual shift breaking?

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navenarf OP t1_ja7xhqj wrote

I am trying to figure out if a car (say running at 100km/h on asphalt pavement) would stop faster 1. slaming on the brake, or, 2. slaming on the brake plus rev match downshift, say if you can execute it at 100% accuracy. Basically I am trying to approach this question from the matter of physics rather than real life situation. Hope that helps clarify.

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Mental_Cut8290 t1_ja7zix7 wrote

From a physics standpoint, you can only brake with 100% of your tires' grip. If your brakes are capable of locking up the wheels (or triggering ABS) then you already have more than enough braking power. Engine braking won't help unless your brakes are weak.

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tjn182 t1_ja851xr wrote

Your brakes and ABS slow you down at the maximum amount without your tires losing friction with the pavement (sliding / slipping). When you are already at the maximum braking-power:friction ratio and you dump more stopping force by downshifting, you will have more stopping force than friction and you will start to slide, taking you longer to slow down.

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Atmosphere-Terrible t1_ja7y10c wrote

Got it! Ok in an ideal and I am talking textbook scenario probably, but it would be so marginal that hitting the breaks would be the most efficient.

Because what you are suggesting is:

You are in 5th or 6th gear at 100km/h you start hitting the breaks and you simultaneously shift to 3rd, or something? It might be more efficient, dunno.

I tried, I am not a physicist.

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MyNameIsRay t1_ja8864v wrote

Modern brakes are more than powerful enough to lock all 4 wheels up with ease, and have the thermal capability to stop a car without issue even if you're going twice the speed limit. The traction of the tires is the limiting factor, not the power of the brakes.

In an emergency stop situation, just slamming the brakes is the fastest way to stop.

Downshifting and using engine braking doesn't stop you more quickly, it lets you continue more quickly. Less heat goes into the brakes (less fade over time), and the engine is in the correct gear to accelerate out (faster exits), which means you can achieve faster lap times in a race.

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53bvo t1_ja7g5ew wrote

Almost all cars are limited by the tyre grip for how fast they can de-accelerate. Putting bigger breaks or adding engine braking power won't have impact on how quickly you come to a stop.

I'm not sure how well regular cars are at brake balance between the front and rear, so in theory if the rear wheels do very little braking you could stop a bit slower if you drive a rear wheel drive car and do some additional engine braking. But around 80% of the braking is done by the front wheels so it won't be that helpful.

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IAmJohnny5ive t1_ja7hmw0 wrote

I've ridden bikes for a long time and without an ABS system it's a very delicate balance between engine braking, front brakes and rear brakes to maximize your braking.

But the difference with ABS is ridiculous. ABS is pretty much when you hit those brakes you stop and do so without without locking up. Fully pulling on ABS brakes on a bike is pretty scary even though it's relatively safe.

The same applies to cars except you have less danger of locking up or swerving. Without ABS you can combo down gear, brake pedal and emergency brakes for maximum effect.

But with an ABS system you really only need to jam down on the brake pedal.

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Viper_JB t1_ja93vp8 wrote

>Without ABS you can combo down gear, brake pedal and emergency brakes for maximum effect.

I was always thought to pump the breaks without ABS to stop the fastest. Break hard till you start to loose traction release and break hard again.

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IAmJohnny5ive t1_ja986b5 wrote

Depends on the circumstances - pumping brakes is more for a steep downhill

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WraithCadmus t1_ja7yn2n wrote

20 years ago I was told to hit the brakes hard and to be perfect push the clutch in just as you're about to stall, so you get engine braking all the way down. In a real emergency stalling is not the worst outcome.

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UltraCoolPimpDaddy t1_ja8382k wrote

Had to do this a few weeks ago. In an emergency there's 0 time to try and downshift. Hit the brakes, there's time to hit the clutch, then put it in the appropriate gear.

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Stiggalicious t1_ja8vs5s wrote

Your engine braking will only contribute several dozen horsepower, since it's just drivetrain loss and vacuum pulling from the throttle body causing the drag.

Even on a typical Toyota Camry, its brakes are capable of over 1500 hp of braking power. The brakes will easily overpower the maximum traction of your tires, which is when ABS kicks in and keeps the brakes at maximum braking power (because sliding friction is lower than static friction, ABS helps to minimize tire slipping).

So just push the clutch in, slam the brakes, and let your car do its thing.

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buildyourown t1_ja8k412 wrote

The limiting factor is tire traction. If the brakes have enough force to match the tire traction, then nothing can be gained with engine braking.

On a heavily loaded truck with hot brakes? That's when a jake brake is used.

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DeadFyre t1_ja989gw wrote

Disengage the clutch, depress the brake. Any effort and thought put into downshifting or engine braking is time taken away from stopping the car.

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boytoy421 t1_ja8w7qz wrote

I mean the FASTEST way to stop any car would presumably be to hit something that's well anchored like a wall or a tree.

Barring that I'd imagine immediately throwing a car into park and slamming the brakes and throwing the e-brake would do it but also destroy your car

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KAWAWOOKIE t1_ja90nij wrote

In almost all situations, brakes can exceed the traction the tires have to the road, so applying additional braking force to the wheels will not allow you to slow down more quickly. To put it another way, apply brakes "too hard" can already exceed the tires ability to slow the car down through friction with the road, as can be felt when the car slides while you're trying to brake. ABS is a system that keeps brakes from locking up by lessening the braking force applied to the tire to the point where it has traction again or is within it's braking threshold.

One thing that isn't touched on here is that engine compression applies a passive slow down and can be very important for situations where your brakes might overheat, such as descending a steep and long hill.

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dokter_chaos t1_ja92b3i wrote

apply the brakes progressively until ABS kicks in. stomping the brakes doesn't allow for weight transfer to happen and the suspension to react, which causes an increased stopping distance.

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Jpro325 t1_ja94oj8 wrote

I think anyone who has driven a manual transmission for performance knows/feels engine braking is 100% a thing. Drop the gear to the lowest you can without red-lining (80mph?, drop to 3rd), then 100% brakes. A lot of momentum/energy is absorbed from pushing all the components of the transmission, flywheel and engine at a disadvantageous gearing. Similarly, if you were to put it in neutral and try 100% braking it’d take much longer to stop. That difference is the engine breaking.

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Bensemus t1_ja9qh7l wrote

Stopping distance is determined by your tire grip. Brakes can already lock up all for tires. Modern and even most old cars are over braked. There's nothing gained from engine braking as it all relies on the tires maintaining traction.

To control speed outside of emergency stopping engine braking is very useful.

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JockAussie t1_ja9q2hs wrote

I always assumed you were best off hitting the clutch (like fully) at the same time to de-engage any passive revs from the engine?

I'm now second guessing myself and wondering if you'd be missing the impact of engine braking which would be more significant? I guess you'd not want to use the clutch when going down a hill because it would probably make things slower, but not sure if the answer on the flat.

We are taught in driving lessons here in the UK to do clutch and brake, but I never thought about what actually stops the car faster. I'd be happy if someone wanted to inform me so I can avoid a long internet rabbit hole about engine braking :).

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paralleljackstand t1_jaa2tjb wrote

Engine braking along with brakes would be fastest in theory as it applies the most stopping forces. But humans cannot downshift that fast realistically so realistically, slamming on your brakes and letting ABS do the work is the best choice.

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Volcan_R t1_ja7hm7c wrote

I don't think there is a clear answer. ABS works by pumping the brakes very fast. It doesn't necessarily stop you faster than locked brakes. Instead it provides steering control authority while you are coming to a stop. The questions then become: does a fresh piece of tire brake better than a piece of tire that is getting hot from sliding friction? Does the act of rolling the tire reduce the effective speed of the sliding part of the friction equation (because the slower you are moving the faster you come to a stop)? I don't think both ABS and engine braking in combination would affect the stopping distance because you are already locking the wheels up as much as the ABS system allows but there might be an advantage to agressive downshifting because the tires continue rolling and that might reduce the sliding component force the wheels need to overcome more effectively than the ABS does by pumping the brakes.

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JiveTalkerFunkyWalkr t1_ja8ariy wrote

Fastest way to stop is to lock your wheels up with ABS turned off. ABS doesn’t make you stop faster, it just improves steering as you brake. It sacrifices a tiny amount of braking for that control. ABS would help Keep the rubber side down though which is nice. A car skids farther on its roof. Engine breaking doesn’t do anything if your wheels are already locked up.

For a motorcycle which can tip over easier, abs is definitely going to keep the rubber on the road better. Note: I’m not suggesting that anyone turn ABS off. But if it was a competition to see who could drop fastest, and all identical cars, those who used the least ABS would win.

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wobble_top t1_ja98cvt wrote

Absolutely incorrect that locked up wheels stop faster than ABS. Skidding wheels stop slower due to a lower coefficient of friction.

ABS also controls all four brakes at once, which is something you cannot do with the 1 brake pedal.

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JiveTalkerFunkyWalkr t1_jaagezn wrote

Shit, I am wrong. So confidently wrong too. Cring. ABS does stop faster in almost all cases.

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Max_Tongueweight t1_jaa0m6n wrote

I was sliding down an icy hill in Lake Tahoe once. Locked up the wheels in the Subaru, shifted into reverse, popped the clutch and floored the gas pedal. The car stopped so fast. Let’s just say, that your brain does not want you to do this. For some reason, it just came to me.

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