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Speed - "In the last 5m of braking you wipe off half your speed."

43K views 38 replies 28 participants last post by  kivyee 
#1 ·
There’s been some recent discussion in another (tragic) thread about speed and accidents. So I thought it apropos to share a video I saw at an advanced riding course last year which illustrates what a huge difference even a few km/h in speed makes in the outcome in an emergency braking situation. Probably a repost but the message is worth repeating.

 
#7 ·
I remember starting to ride and the beginning of every season (now). It's impressive how a little practice or professional training safely decreases braking distance especially when dealing with poor conditions. My first emergency stop involved immediately locking the rear and wasting so much precious stopping distance.

Physics govern the distance 100% braking efficiency covers of course. Great video.
 
#8 ·
And none of us, and I mean NONE of us get even CLOSE to 100% braking efficiency, right Schmii? The average for riders is more like 55% efficiency.

At 55%, it takes a LOT further to stop.
 
#10 ·
Of course you are right. In THIS scenario, you wipe off half your speed in the last 5 metres of braking.

The general idea is though, is that MOST of your decelerating comes in the last short distance of the stop. So if it takes you 5 meters (or more) longer to complete the stop, you are still travelling at a large fraction of your original speed on impact.

Ouch.
 
#13 ·
If you don't pay attention to the message; you will probably scrub off half your speed in the last 5 cm! :eek
 
#19 ·
Good video and reminder. I was wondering though...reaction time is very important as well...

I assume that the speeds in the video are in mph. If the driver's reaction time in the vehicle travelling at 60mph is just a 1/4s slower than what was portrayed in the video, then he (theoretically) goes an extra 22 feet lodging his vehicle under the trailer unit.

Of course a slower speed will allow you more reaction time.
 
#20 ·
Your reaction time remains the same but the distance you cover during that reaction time obviously increases with added speed. So at a higher speed you travel a lot further before you actually get the brakes applied.

The video is in kmh.
 
#29 ·
I had a dump truck with a trailer pull out in front of me once on Westminster Highway. I was going over the bridge and around the corner onto Westminster Highway, near the bike shops. There is a light at the bottom of the corner. It was my first month or two of riding. I looked ahead and saw that the light went green, so I accelerated. The moron in the dump truck on the opposite side entered the intersection to turn left, but his filter went off. Instead of sitting there waiting for a yellow he just decided to turn. The cars in front of me braked hard. I was probably doing about 50 when I tried to stop like a noob. Locked my brakes up and I went down. I slid along the road, but stopped short of sliding underneath the dump truck and trailer. The bike slid off to the left. There are a lot really dumb tractor drivers in the city. I should know I've worked with a lot.
 
#30 ·
I had a 53' truck similar to this make a U-turn across a 6 lane stretch on hwy 5 returning from osoyoos and got uncomfortably close to that guard rail. From several hundred feet back the driver made a move once I was close...

That rail is important for backing up to a loading dock that has standard guards for them.
 
#32 ·
I think, from what I have read in the past, it's almost no contest, ie, the average person, driving a car, equipped with ABS, will be much shorter than a bike.

The motorcycle takes so much more skill to implement a high speed emergency stop, effectively, to use all it can, in the shortest distance....a car, not so much.

I'm sure, if you had a very seasoned racer, doing emergency braking exercises, on a test track, you could get some hugely improved (shorter) distances, but in the real world, where reaction time and skill, and panic come into play, good luck out-braking the average car.

Here is some info to chew on:

http://www.begin-motorcycling.co.uk/elc2.htm
 
#35 ·
For The Disbelievers

KE=mv^2/2

That is the equation for kinetic energy, and as you can see, KE is proportional to the square of velocity. KE is what your brakes are turning into heat when you make each and every stop.

SO, plugging in some numbers for a 1000 Kg object:

v= 100, KE = 5000000
v= 90, KE = 4050000, 81% of original KE
v= 80, KE = 3200000, 64% of original KE
v= 70, KE = 2450000, 49% of original KE -just 30 M/s slower, but only half KE!
v= 60, KE = 1800000, 36% of original KE
v= 50, KE = 1250000, 25% of original KE -half the speed, a quarter of top KE!
v= 40, KE = 800000
v= 30, KE = 450000
v= 20, KE = 200000
v= 10, KE = 50000
v= 5, KE = 12500
v= 2.5, KE = 3125
v= 1, KE = 500
v= 0, KE = 0

If this is not making sense to you, read it from the bottom up and see how much KE you accumulate with each 10 units of speed.

You can see your brakes have a much bigger job to do with each 10 units of speed.
 
#39 ·
Here's some actual and average data for
1. Motorcycle stopping distance
http://www.msf-usa.org/imsc/proceedings/a-green-comparisonofstoppingdistance.pdf

2. Vehicle stopping distance
http://www.passmytheory.co.uk/learningcentre/stoppingdistance.aspx

For motorcycles, the finding was that ABS helped in terms of consistency between stops, but the shorter braking distance was consistently w/o ABS, at the lower speed (30mph), at 80 mph the ABS equipped model consistently had a shorter stopping distance. In most cases it was only <5% difference either way, except for the heavy bikes.

Still, best braking distance of the 4 bikes tested came from the FJR 1300,
30 mph - 10.40m
80 mph - 67.46m

Comparison to the *average* distances in the second link
30 mph - 14m
70 mph - 75m

According to car and driver's 2008 stopping distance test, a 911s they tested had a stopping distance of 93m from 70mph, it was the best of the bunch.

So based on that I would say from real-world testing data, your average motorcycle will be able to out-stop your average car. If the rider screws up and bins it, well....
 
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