More math: braking distance - Saab 9-5 Bulletin Board - Saabnet.com
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More math: braking distance
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Posted by Wilbur Pan [Email] (more from Wilbur Pan) on Thu, 30 Jun 2005 06:51:30 Share Post by Email
In Reply to: Left lane confessionals - how fast is "fast"?, SaabSpeed, Tue, 28 Jun 2005 07:39:28
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Here's the other issue with increased speed. Braking distance goes up exponentially with increased speed. For those of you who would like the formula, d=k*v^2, where d is the braking distance, v is your speed, and k is some proportionality constant.

Now, for the Saab 9-5, most reviews I have been able to find put the 60-0 braking distance at 115-125 ft. I picked 120 ft as a good average. Using 120 ft at 60 mph, I was able to calculate the proportionality constant.

Now, here is what real world braking distances are like:

Total braking distance = distance traveled during reaction time + braking distance

An average reaction time (the time it takes you to get your foot off the gas and onto the brake) is about 0.75 seconds. At 60 mph, the car will travel 66 feet during that time. Add that to the 120 feet braking distance, and you get a total braking distance of 186 feet if you are driving at 60 and slam on the brakes.

Here's a table using these calculations. I'm not sure how well things will line up:

Speed (mph) Reaction distance (ft) Braking distance (ft) Total (ft)
10 3 11 14
20 13 22 35
30 30 33 63
40 53 44 97
50 83 55 138
60 120 66 186
70 163 77 240
80 213 88 301
90 270 99 369
100 333 110 443

So for someone going 80 miles an hour, it would take them the length of a football field to come to a complete stop. Kind of a sobering thought when you are cruising down the highway.

The other interesting thing is that with these calculations, changes in the braking distance measured by car tests really has a minor effect on how your car will perform, as the braking distance depends on the square of your driving speed.

posted by 12.160.185...

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