Please explain hill grades to me
 

Okay, I don't understand hill grades. 10% grade, 17% grade, it makes no sense to me. Please explain.

WikiPedia is your friend for Grade (geography).

Grade is NOT the same as and angle. Grade refers to the amount os increase in height relative to the horizontal distance. Example...a 5 foot rise over 100 ft is a 5% grade. A 100 ft rise over the same 100 ft (aka...45 degree angle) is a 100% grade

So? This is like the 'pitch' or slope of a roof?


Thaks,

Tim

This is, apparently, not the international standard. In some countries the grade is the increase in height over the actual distance traveled. In other words, considering a right triangle with the angle of interest from the horizontal to the slope, we use the opposite over the adjacent multiplied by 100, in some other countries they use opposite over hypotenuse times 100. It doesn't make much of a difference, but could be important to know.

...And in some countries they apear to run out of one sign, so they seem to just use another.

As an example: in Kyrgyzstan, a LOT of what were posted as 6 & 9 % grades were easily 12+. I believe they put up the signs just so the truck drivers know there is a climb/descent ahead of time.

Yes. But let me clarify some things.

Strictly speaking, the 'grade' of the road is the vertical RISE for every HORIZONTAL unit of travel. Thus if you climb 264 feet in a horizontal mile, that is a 5% grade because 264/5280 is .05, or 5% Geometry students would refer to this grade as the TANGENT of the angle of ascent.

Now, to be honest, this is not a realistic way to describe a grade. Instead, most people view the grade as the VERTICAL RISE for every unit of raodway travel. That would be the hypotenuse of the triangle you are envisioning in your mind. Thus in this measure the grade is the SINE, not the tangent, of the angle of ascent.

But as any 9th grade geometry geek will proudly tell you, for small angles the difference between the TANGENT and the SINE, is very very small.

Anyone out there..correct me if i have screwed this up, as I don't have my geometry book in front of me just now. But I think I am correct.

Hopefully I am not playing fast and looses with my hypotenuses!

roughstuff

grade vs angle vs pitch
 

Grade could be considered the same as angle and pitch only it's a very small increment.

For example a 25% grade (unheard of)would be a 3/12 pitch or an 11.25 degree angle...right?

well, if you use tangent, the 'rise over the run,', then the pitch is always correct. If you use SINE, which is the rise over the hypotenuse (the length of the roadway), then yes, it is accurate for only small angles.

roughstuff

Actually, a 25% grade is not "unheard of". I cycled up a 24% grade in Wales a few years back, and when I proudly told a Brit what I had done I was told that there were roads in Yorkshire as steep as 33%.

Those grades just freak me out when I see them, even if they are only short 'pops' of a few hundred feet at most. And going DOWN...you keep looking for where the roadway is as you come over the lip of the incline. I have walked down more hills that I have walked up.

roughstuff

I would have been thoroughly disappointed if I hadn't got a response to that! :D