'Grabbing a handful'
- what does it mean?
By: James R. Davis
It's my contention that unless a person can consistently achieve a deceleration rate of at least 0.7g's, that person is not 'qualified' to ride a motorcycle on public roads. Further, if a rider can consistently achieve deceleration rates of at least 0.8g's, he or she is a competent rider in terms of braking skills.
Many riders use parking lot practice sessions in order improve their braking skills. They mark out an area on the parking lot with a starting gate and some distance markers. Thus, when they begin a braking test from 20 mph, for example, they can determine what their average deceleration rate was during their stop by merely noting the distance it took to stop. A distance of almost 17 feet indicates that the rider achieved an average deceleration rate of 0.8g's during the stop, if starting at 20 mph.
But very few riders have any idea what their actual deceleration rate was throughout their stopping effort. Knowing what I'm about to share with you can help you avoid a nasty spill on that parking lot, and can lead to much better braking performance in an emergency.
Let's explore the idea of 'rapidly squeezing' the front brake lever during an emergency stop.
We all know that 'grabbing a handful' of front brake lever is dangerous. That is, squeezing the front brake lever too quickly leads to locking up the front brake and the result is a bike that ends up on its side - almost faster than you can blink.
But how fast is too fast?
Well, I'm betting that you've heard otherwise knowledgeable riders and even instructors suggest that you should squeeze hard, then squeeze harder, then harder still until you've achieved your maximum deceleration rate. And when asked how long it should take until you've reached that maximum, you hear suggestions such as 'about one second'. NUTS!!!
It should never take you as long as .7 seconds to reach maximum squeeze pressure on your front brake lever when you are trying to stop aggressively. In fact, most people can easily hit their maximum squeeze pressure in .5 seconds, and with practice you should be able to reach maximum in about .3 seconds. There's a huge difference between reality and the well intentioned suggestion of 'about one second'.
I remind you that on the parking lot, when practicing quick stops, you will reach a given speed such as 20 mph as you approach a starting 'gate', and then begin an aggressive squeezing of your front brake lever, maintaining your maximum squeeze pressure until you've come to a complete stop. Then measure the distance of that stop in order to determine your deceleration rate. And, though this article focuses on the front brake usage, any emergency stop should begin with the application of both brakes, not just the front one.
Assuming you want to achieve a 0.8g deceleration rate, you must be able to stop from 20 mph within approximately 17 feet.
The deceleration rate is the average deceleration rate during that stop. But in actuality, until you reach your maximum deceleration rate (maximum squeeze pressure), your deceleration rate is obviously less than 0.8g's, if the average is 0.8g's. Which means, of course, that the maximum achieved deceleration rate is higher than 0.8g's.
Let's look at what happens to the deceleration rate when stopping from a speed of 20 mph if it takes you only 0.3 seconds to reach your maximum squeeze pressure.
If you average a deceleration rate of 0.8g's, you will come to a complete stop in 1.1 seconds (and take 16.7 feet). The chart above shows that for the last 0.8 seconds you must have achieved a deceleration rate of 0.93g's, because for the first 0.3 seconds you had to have achieved half of that rate.
That is,
.8X + (.3 *.5)X = 1.1 * 0.8
.8X + .15X = 0.88
.95X = 0.88
X = 0.93
In other words, to achieve an average deceleration rate of 0.8g's when stopping from 20 mph and using 0.3 seconds to reach your maximum deceleration rate, that maximum had to be 0.93g's.
Bravo! You now know that a competent rider in terms of braking skills actually achieves a rate in excess of 0.9g's in order to average 0.8g's when stopping from 20 mph.
But this thread is about how long it takes you to achieve your maximum deceleration rate, not what the maximum rate is. Still, it's very informative to explore what happens if you take longer than 0.3 seconds to reach your maximum. What, for example, happens to deceleration rates if it takes you as much as 0.7 seconds to reach your maximum?
If it takes you as long as 0.7 seconds to reach your maximum squeeze pressure, you must achieve a deceleration rate of 1.17g's when you get there in order to average 0.8g's. I showed that in red in the chart above because that deceleration rate is extremely near skidding even on good concrete, let alone asphalt. It should be no surprise that if an instructor suggests that it takes you 'about one second' to reach your maximum squeeze pressure, he doesn't really understand the real world.
What this little example clearly shows is that it is much easier to achieve a deceleration rate of 0.8g's when stopping from a speed of 30 mph than it is if you are stopping from 20 mph, because you will spend significantly more time decelerating at your maximum rate than the time it takes to get to that rate. (It takes 1.7 seconds to come to a complete stop from 30 mph if you average a deceleration rate of 0.8g's.)
And so I suggest that 'grabbing a handful', when meant as a pejorative indicating dangerous behavior, also means taking less than 0.3 seconds to reach maximum squeeze pressure.
I also suggest that when you do parking lot practice braking runs you pay more attention to getting to maximum deceleration rate quickly than you do at the distance it takes you to stop.
You will need the following chart to determine your deceleration rate:
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(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)
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