Grade and Slope

Grade and slope (or angle) are not the same thing. The graphic above illustrates how a 90-degree angle is infinite. Typical U.S. interstates and major highways are capped at an 8 percent grade. This is a 4.57 percent slope. When you compare this to a protractor it barely registers. Much different than your driving experiences in the mountains. And nowhere near the road sign depiction of a 50 percent grade.

The grade to slope numbers have been translated for your use below, since they represent such a small area of the ‘protractor’ image. Just be aware, when we talk about slope it is a correlation of rise to run, representing the elevation change over a given distance. Most railways don’t exceed a 3 percent grade, and we can all agree those are very gentle slopes at best.

A 45-degree angle is 100 percent grade, meaning for every foot you travel you will gain a foot in elevation. As you can see, comparatively slopes and angles are much lower than the corresponding grades.  We never deal with slope of the road in automotive engineering, its just a correlation of what you may expect compared to how engineering treats the data.

Grade (percent)    Slope (degrees)

3                                1.72

4                                2.29

5                                2.86

6                                3.43

7                                4.00

8                                4.57

9                                5.14

10                             5.71

11                             6.28

12                             6.84

13                             7.41

14                             7.97

15                             8.53

16                             9.09

17                             9.65

18                            10.20

19                            10.76

20                            11.31

Grade is commonly analyzed for axle approvals, and it is a trade-off with weight. This means the more weight you pull, the lower the grade percentage that will be permitted.

Engine/transmission capability takes grade into account as well. There are acceptable limits for starting grade (Startability) as well as grade at desired cruise speed (Cruiseability). As we increasingly deploy downspeeding to tune engines for the EPA regulations, these numbers move to the forefront of performance. Look at one of our postings on this, or check out our tutorial: https://www.youtube.com/watch?v=EWUOV6ElOWQ

And now…

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Believe It OR Not

The reason elections are always held on Tuesday is a relic of America’s agrarian lifestyle in the mid-1800s. Sunday was for church; Monday was a travel day; and on Tuesday you cast your ballot. Congress could change things by simply passing a new law.