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The physics of stopping: Engine Braking, does it help in an emergency?

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The physics of stopping: Engine Braking, does it help in an emergency?

Old 07-12-2013, 08:24 AM
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Default The physics of stopping: Engine Braking, does it help in an emergency?

This topic came up on another forum, and I thought I'd address it in more detail here. A debate has been raised as to whether the combination of engine braking in addition to using the brakes is a better method of stopping in a crisis.

Anyone who has downshifted while approaching a stop knows that engine resistance is capable of slowing a vehicle down more than coasting or light brake pressure would do on their own. (And the effect will be more pronounced when engaged with higher RPM's.)

However, this apparent relationship between engine braking and the ability to stop quicker is not a universal absolute. The physics concept at issue here is: what is actually stopping the motorcycle? In truth, neither engine resistance NOR the brake pads against the rotor are directly stopping the forward progress of a motorcycle -- while they both reduce the rotation of the wheels in relationship to the vehicle, IT IS THE CONTACT PATCH OF THE TIRES which is performing the work of stopping the motorcycle in relationship to the stationary ground. Once a tire has reached its friction point with the ground, no matter what method is used to do it, no additional braking force will make a difference (and would in fact be counter-productive to a controlled stop.)
Once you think about it, this should be fairly obvious: squeezing the brakes harder once you've locked up the wheels does NOT bring you to a stop any faster, or with any better control.
Given the rate of deceleration available from the proper application of standard brakes and attention already required to modulate the pressure near but not past the friction point of the tires, it is likely that the additional complexity of trying to introduce engine braking into the equation would only serve to interfere with the efficiency of making maximum use of the brakes.

So long story short, the MSF and other experts already have it right when it comes to crisis braking: your best bet is to squeeze the brakes and grab the clutch.
Old 07-12-2013, 11:28 AM
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Engine braking slows the bike some, but won't stop the bike like brakes are designed to. Furthermore, it uses only the rear tire to slow the bike and is more effective than the rear brake initially. Just letting off the throttle for an instant will change the attitude of the bike, shifting some weight forward. This will set the front springs into some compression, thus making the front brakes smoother, more effective, and less chance of breaking traction on the initial jolt of grabbing the brakes. In a panic situation, you do want do pull the clutch and disengage the drive once the brakes have taken over. It's important to down shift to ensure that if you need to accelerate quickly, the bike is in the appropriate gear to do so.
Old 07-12-2013, 01:17 PM
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On any modern sport bike, assuming good traction conditions, applying "maximum braking capabilities" to the front wheel will put the rear wheel in the air. Use whatever type of braking (engine, foot, find a reverse, who cares) on a wheel in the air and it doesn't matter. This isn't true of all bikes, but on sport bikes it is.

As demon mentioned, it is important that you give the suspension enough time to react, and don't go grabbing and stabbing. Squeeze. Not grab. It is possible for squeeze to become a habit.

Downshifting is a good point and often goes without notice. A few months ago I was in an emergency braking situation and as I was about to hit the car that had pulled out in front of me, I actually had the opportunity to think about why the clutch was in and why my left foot was clicking through the gears. The response from my left foot was because that was it's job, so I let it go. Turned out to have really helped as a few feet from the car I discovered just enough space to escape. After tipping in and around the back of the car, throttle was needed to help stand me up, and had I not been downshifting I wouldn't have had enough of a roll on to get back up.
Old 07-13-2015, 10:30 AM
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I've read that if you squeeze the brakes to the point of an uncontrolled skid, you should keep on squeezing and just ride it out. Can anyone explain the theory behind this? Seems counter-intuitive knowing what we know about ABS systems.
Old 07-20-2015, 09:31 AM
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I'm going to assume by "uncontrolled" you refer to a state where the back wheel is locked up and the bike is getting sideways. the reason they tell you to ride out the slide is that if you let off the brake and the rear suddenly regains traction again while the bike is not aligned with the direction of travel, you will induce some heavy cavitation usually resulting in a highside.
Old 11-24-2015, 02:45 AM
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The way I read the original post, it sounds like the argument against engine braking is:

The brakes are able to generate enough force to lock up the wheels.

Therefore, the limiting factor in braking is not the amount of braking force generated, but the amount that the tire can effectively translate to the road.

Therefore any extra braking force generated from engine braking is redundant since the brakes can already produce more force than the tires can handle.
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