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alnyland t1_ja8y6j0 wrote

Reply to comment by phiwong in ELI5: What is the fastest way to stop a car with a manual gearbox ? by navenarf

> 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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