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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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