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  1. #1
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    Calculating hill gradient

    I've been trying to use mapmyride to try and get a general idea of hill gradient for the hills that I ride. One of the county engineers hosts weeknight rides during the summer and always talks about the hills in the valley area that we ride, and just about every hill out of the valley he says is 20%. Also, there are signs posted on some hills that I ride that warn trucks to use low gears because of 17% hills. When I go to mapmyride.com or similar sites that show distance and elevation, I get no where near these results.

    For instance this stretch of road here is where the hill states 17% gradient and warns trucks to use low gears.

    http://www.mapmyride.com/route/us/oh...d/135096614057

    But when I do my calculation I get like 4% grade? Am I doing something wrong? Math has never been my strong point.

  2. #2
    08 Madone 5.2 Rider
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    20% grades? Give me a break...I have been designing and building roads and driveways since 1971 in the Atlanta area and throughout the Southeast. The maximum grade is around 13%.

    If indeed you have 20% grades, I do not want to climb them!

    Where are you located?

    In answer to the math, the grade is simply calculated by rise/run...in other words, the elevation difference divided by the distance between two points.

    Example:

    Point 1 Low Point = Elevation 1000.00
    Point 2 High Point = Elevation 1250.00

    Distance between points = 5000'

    % grade = (1250 - 1000) / 5000' = (250 / 5000) = 0.05, which is 5%

    Hope this helps.

  3. #3
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    forgive me, but isn't the distance between the two points the hypotenuse, and therefore not the run?

    admittedly, the difference would be small with these angles; your example would yield an angle just a fraction under 5%.

  4. #4
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    Grade = Elevation gain / length of the road, not the hypothetical horizontal distance you traveled.

    I couldn't get the map program you're using to give me really good info, but hill steepness is often a point of some confusion. First, make sure you're talking about the same thing. Your friend might be saying the road has a maximum grade of 20%, the sign may be talking about a 17% grade over a quarter mile or so, and you may be getting the 4% number by averaging the grade over the entire length of the climb.

    I don't think I've ever climbed anything with a 20% grade, but I have seen some around 17-18%.
    Formidable Pharmacologically

  5. #5
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    So by this calculation, the killer hill on the back side of me that is 4km long is 10.8% grade.

    435m/4000m = 10.8%
    Once I thought I was wrong, but it turns out I was mistaken.

  6. #6
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    I appreciate all the answers. The only riding I ever do is in my area, but have been interested in doing a week long ride in Colorado where they have some long 10% climbs, and one climb that was 17% for 7-10 miles or so. I want to know if I train on some hills around here that aren't quite that long, but supposedly that steep, that I'll be prepared for something that long and steep when I get out there.

  7. #7
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    I can almost guarantee that the road doesn't average 17% for 10 miles. That would result in an elevation gain of almost 9,000 ft, and the difference from the lowest and highest road in Colorado is 11,000 ft. I'm guessing those spots are not 10 miles apart.

    It's more likely that the climb is 7% for 10 miles.
    Formidable Pharmacologically

  8. #8
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    I'm guessing there is some "fuzzy" math when people claim these hill grades. I just double checked the brochure I have for the ride and indeed they claim 17% for 7 miles as the killer part of the ride. That comes out to a 6200 ft climb, which sounds pretty insane. I'm guessing that they probably have a 7 mile stretch with some sections that are as steep as 17%.

  9. #9
    Carbon Fiber = Explode!
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    Garmin never lies. And if it does, well it doesn't lie a lot.
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  10. #10
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    Quote Originally Posted by pw1972
    I'm guessing there is some "fuzzy" math when people claim these hill grades. I just double checked the brochure I have for the ride and indeed they claim 17% for 7 miles as the killer part of the ride. That comes out to a 6200 ft climb, which sounds pretty insane. I'm guessing that they probably have a 7 mile stretch with some sections that are as steep as 17%.
    Yeah, that's probably right. Fuzzy math indeed.

    Just did a quick google and found that the Pikes Peak climb has a gain of 7,400 feet, the biggest gain in Colorado, and is over 19 miles of road.
    Formidable Pharmacologically

  11. #11
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    A sign like "17% grade" doesn't mean the whole hill is that grade; there could only be a short section with that grade. For instance, the Manyunk "wall" is posted as a 17% grade, but the section between the 200 and 400 block is much steeper than the rest. A half mile of 17% grade would be a lot worse...

  12. #12
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    Whenever anyone tells you the grade of a hill, they always exagerate. Always. People tend to take the steepest 10 feet of the entire hill and talk about the hill as if it was all that steep. I never trust any such numbers.

    By the way pw, tell us exactly what hill in Colorado you're talking about. There's a pretty good chance that a number of people here have actually ridden it.
    Last edited by John Nelson; 08-31-2008 at 07:41 PM.

  13. #13
    haole from the mainland
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    Quote Originally Posted by CleavesF
    Garmin never lies. And if it does, well it doesn't lie a lot.
    My Garmin isn't anywhere near accurate enough to give me a slope value I'd trust. There have been times when it gives me the wrong slope direction, e.g., says the slope is + when I'm going downhill.

  14. #14
    It's Good For You!
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    who cares!? just ride. why boast?! does your chest hair care?

  15. #15
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    [QUOTE=SilasCL]Grade = Elevation gain / length of the road, not the hypothetical horizontal distance you traveled.

    QUOTE]

    Not to be contrary, but I believe that a 45 degree slope is a 100% grade. That would mean a slope that is 200 yards on the horizontal and 200 yards on the vertical is 100%. The distance traveled would be 282.8 yards. Your calculation method would show this climb as a 70.7% climb.

    I often avoid this climb either way you calculate it! But when I do get good enough, please don't screw me out of nearly 30%.

  16. #16
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    I was tired of the silly discussions of how steep a hill was so I went to the boat store and got this.
    Attached Thumbnails Attached Thumbnails Calculating hill gradient-level-o-gage-013-medium-.jpg  

  17. #17
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    Not to be contrary, but I believe that a 45 degree slope is a 100% grade. That would mean a slope that is 200 yards on the horizontal and 200 yards on the vertical is 100%. The distance traveled would be 282.8 yards. Your calculation method would show this climb as a 70.7% climb.
    You and Cygnus are correct. But for the grades mere mortals typically ride (< 20%) the values are close enough to just use the distance traveled for convenience.

  18. #18
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    You can get reasonable straight-up estimates by just downloading google earth and use the plot function to draw a route between any two points. The elevation will be stated wherever you place your cursor. You'll probably be shocked that those 20% "gear grinding" sections are actually just 10%, so beware if you don't want to deflate your ego.

  19. #19
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    Quote Originally Posted by Dr_John
    You and Cygnus are correct. But for the grades mere mortals typically ride (< 20%) the values are close enough to just use the distance traveled for convenience.
    Yes,
    the grade that was calculated to be 10.8% in the example would only change to 10.94%. Who actually cares about breaking it down that far.

    Like most riders, I always round up (10.94% rounds up to 15%, 11.9% rounds up to 20% and 12.9% rounds up to "no other human could have made it").

  20. #20
    08 Madone 5.2 Rider
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    Again, it ain't rocket science!

    I think you get the idea from the posts here that the 20% is either calculated wrong or, most likely, exaggerated!

    Just think about a 20% grade...that is 20 feet up for every hundred feet you travel. THAT is extreme! I doubt you will find that anywhere on paved roads. Most likely 10-12% for short stretches.

    Whatever, just ride what you can.

    Oh, yeah, how's yer brakes????!!!

  21. #21
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    There is a very short section of Highway 108/Sonora Pass in California that is posted as a 26% grade. And it's above 8000 feet. I have ridden some of this road, (not all of it!) and can tell you that it is steep. How steep? I'll let one of you more tech-successful people do the calculations from Kennedy Meadows to Sonora Pass...and yes, there are some flatter spots along that path!

  22. #22
    wim
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    Obsessed.

    If you really want to know the exact grade of a short stretch of road, do what I did to settle an argument: get a straight piece of tubing, tape a long carpenter's level on it and measure rise and run on location as shown by yours truly here in the photo. Keep your ears open for cars . . .

  23. #23
    Frog Whisperer
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    rise over run, just like a roof
    Of course I'm sure...that doesn't mean I'm right......

  24. #24
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  25. #25
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    if you download googe earth and search for the hill

    than go to tools, click on rulers, and in that window click on paths. you can click all the way along the road in short pieces so you can go around corners, that will get you the distance. Than put the cursor on the start, mark the elevation, and go to the finish and mark the elevation. Divide the total elevation (in feet) by the distance in feet,( 1 mile = 5280 feet) and you will get the degree of climb (close enough)

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