Stopping Distance: gravity effect, uphill VS downhill - Stromtrooper Forum : Suzuki V-Strom Motorcycle Forums
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post #1 of 31 Old 10-29-2013, 11:19 AM Thread Starter
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Question Stopping Distance: gravity effect, uphill VS downhill

Effect of gravity on stopping distance riding uphill VS downhill.
I had a discussion with my son who is a scientist with a background in physics; needless to say I got lost in the math.

I've always been curious as to how much longer it takes to stop when riding down a steep grade compared to going uphill. Generally I lose some of my nerve when going down a steep mountain grade in second or third gear approaching a hairpin turn. Going uphill I have gravity working in my favour to help me stop; but going downhill it works against me. A recent ride in West Virginia fully loaded and 2-up prompted me to get an answer. We rode 2 days on rain wetted roads sometimes covered with slippery leaves and the rest of the trip was cold, sunny with perfect road conditions.

Anyway with some research I've concluded a simple rule of thumb:
Riding grades of 15% uphill the stopping distance is approximately 15% shorter than on the same type of surface, at the same speed on a level road. less. When riding down the stopping distance is approximately 30% more.

It is very complicated with coefficients of friction and speeds and the exact grade This is a great linky to all sorts of mathematical calculations if you're up for that sort of stuff:

On the Drawing Board: Braking distance and time on level road and on an incline

Comments and suggestions are most welcome, maybe someone has a better answer.

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post #2 of 31 Old 10-29-2013, 11:52 AM
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Very through but maybe more than needed to answer your question. You just want to know how much further relative to a flat surface. Most of the factors in your analysis don't vary whether going uphill out down. Coefficients of friction are unchanged.

Potential-kinetic energy changes with elevation seem like the dominant factor but should have an equal and opposite effect going uphill or downhill on the same grade at the same speed. That suggests you'd see the same but opposite effect, not 15 and 30 percent.

The weight distribution front to rear is much different, but I don't know how significant that is or how to account for it. You have more weight on your front brake going downhill, which should help?

Anyway, like you I'm much more cautious downhill.

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Last edited by dkayak1; 10-31-2013 at 03:26 PM.
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post #3 of 31 Old 10-31-2013, 03:07 PM
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My rule of thumb is simply; set entry speed for corners as early as possible. Then it doesn't matter if I'm going up or down.

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post #4 of 31 Old 10-31-2013, 03:38 PM
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My personal favourite is;nail the brakes, get in the right pulling gear before the bend and gun it on the exit, make it all 'softer' if the weather/roads dictates.
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post #5 of 31 Old 10-31-2013, 04:30 PM
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Quote:
Originally Posted by dkayak1 View Post
Very through but maybe more than needed to answer your question. You just want to know how much further relative to a flat surface. Most of the factors in your analysis don't vary whether going uphill out down. Coefficients of friction are unchanged.

Potential-kinetic energy changes with elevation seem like the dominant factor but should have an equal and opposite effect going uphill or downhill on the same grade at the same speed. That suggests you'd see the same but opposite effect, not 15 and 30 percent.

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Did I miss something here?
If the stopping distance is 15% greater on an uphill grade than a level surface. And sopping distance is 15% more on a downhill grade than a level surface. Thereby equaling a 30% difference in stopping distance between an uphill and downhill grade, is this not the same but opposite effect?
Just trying to follow the math and logic here

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post #6 of 31 Old 10-31-2013, 04:47 PM
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Originally Posted by trapperdog View Post
Did I miss something here?
If the stopping distance is 15% greater on an uphill grade than a level surface. And sopping distance is 15% more on a downhill grade than a level surface. Thereby equaling a 30% difference in stopping distance between an uphill and downhill grade, is this not the same but opposite effect?
Just trying to follow the math and logic here
Anyway with some research I've concluded a simple rule of thumb:
Riding grades of 15% uphill the stopping distance is approximately 15% shorter than on the same type of surface, at the same speed on a level road. less. When riding down the stopping distance is approximately 30% more.


I interpreted the above to say 15% less than flat when going uphill and 30% more than flat going when going downhill. It all depends on what he intended after "30% more." More than flat or more than uphill? Not sure now that you mention it.
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Last edited by dkayak1; 10-31-2013 at 04:52 PM. Reason: Hit "enter" and omitted my reply!
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post #7 of 31 Old 11-03-2013, 09:39 AM Thread Starter
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to clarify - stopping distance going uphill VS downhill

Your calculated stopping distance is a complex mix of many physical factors and some very interesting mathematics. A full appreciation of the physics and calculations is over my head; so that 's why I asked my son.

To simplify what I said; what ever the stopping distance is for a certain road surface and speed, ( if all other variables are the same) it will be less if you are going up a steep hill and more if your are going down a steep hill.
Gravity is your friend going up and not so much when you are going down a steep grade entering into a tight hairpin turn.

I was curious to determine a rule of thumb, based on some true science, that would answer my question. If the grade happens to be about 15% uphill then you can use that to your advantage and stop ( as it turns out if you crank out all the math to it's conclusion) then low and behold you can stop in a distance about, yes coincidentally, 15% less. When riding downhill, the effect of gravity makes it such that you will need a longer distance in which to stop, so on a 15% grade you'll need about 30% longer to get stopped.

Similarly on a 6% grade; 6% less riding up and 12% more going down.

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post #8 of 31 Old 11-03-2013, 02:47 PM
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Here in the mountains the going downhill effect is a common phenomena. The only thing I can say is downhill watch the rear brake. Use front almost exclusively. going uphill the rear brake has more weight and can be used more aggressively. Going downhill take it slower if it's unfamiliar. You can get into trouble quicker.

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post #9 of 31 Old 11-27-2013, 04:45 PM
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Originally Posted by dkayak1 View Post
Most of the factors in your analysis don't vary whether going uphill out down. Coefficients of friction are unchanged.
If you don't mind, I'd like to respectfully challenge that statement. Total coefficient of friction (COF) is the sum of the available friction of the front and rear contact patch of the tires. Braking forces result in the center of mass (COM) weight transfer to the front of the bike, placing most of the weight on the front tire and negating rear wheel braking power. The unknown in the equation is how much the added weight on the front tire increases the contact area and COF during hard braking.

This effect along with a shift in the COM along the wheelbase would be exacerbated if the bike was moving downhill, but actually lessened if the bike was moving uphill. Braking while moving uphill would result in a more balanced weight distribution on the front and rear, maximizing contact patch area and friction, IMHO.

But yer right, it takes longer to stop going downhill than up. Gravity is a harsh mistress! [from The Tick]
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post #10 of 31 Old 11-27-2013, 06:02 PM Thread Starter
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Challenges are most welcome. I was trying to get a handle on "that harsh mistress" gravity. I've always felt ok riding aggressively uphill into a corner, but loose my nerve on the tight downhill corkscrew turns. My 30% rule of thumb wrt braking distance is all I can find.


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