Forum Index » General Lightweight Backpacking Discussion » weight vs. time calculation


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Paul McLaughlin
(paul) - MLife
weight vs. time calculation on 02/12/2011 18:34:10 MST Print View

I'm looking for a way to estimate how much a given amount of added pack weight would slow you down if you put out the same amount of effort with either load. Or, how much further you would go in the same amount of time if you reduce your packweight by a given amount. My first thought is to add body weight and weight of gear (from skin out, of course) to get a total weight. Each pound would be a certain percentage of that total. Then take the amount of time spent actually walking, say 6 hours as an example. If you add 1 pound to the total weight, does it seem reasonable that you would add that same percentage to the total time - assuming that the amount of effort remains the same? I realize this will never be a precise calculation, but I'm thinking of a way to put some numbers to the comparison of light weight vs. simplicity - for instance, if a shelter is very easy to set up, how much extra weight is that worth?
Anyone else have an idea how to estimate this? Any biomechanical experts out there?

David Chenault
(DaveC) - BPL Staff - F

Locale: Crown of the Continent
dial sez on 02/12/2011 18:49:25 MST Print View

http://www.backpackinglight.com/cgi-bin/backpackinglight/calculating_range_in_lightweight_backpacking.html

Paul McLaughlin
(paul) - MLife
Re: dial sez on 02/12/2011 22:05:22 MST Print View

Well, that's certainly complex! But running a basic calculation using the calculator built into that article yields numbers that don't reflect my experience. I tried a 120 mile trip with a base weight of 10 lbs versus a base weight of 12 lbs, and the difference it gave me was a full day, or a difference of 3 miles per day. Seems high to me. Which may just mean that his formula work better for a top athlete than for an average joe, or that the idea of equal effort is harder to determine realistically than I might think - it may be that I wouldn't slow by 3 miles per day by adding 2 lbs to my base weight, but I'd be putting out a higher effort to do it, without being aware that I was putting out a higher effort.

a b
(Ice-axe)
Freaky on 02/12/2011 22:59:00 MST Print View

The freaky thing works! I went back into my trail data from the PCT and CDT and started pluggin numbers into the calculator and it came back with times very close to what I actually had.
For instance: I am(was) an Animal factor 45 with a 10lb base and 2lbs of food per day. I put the final 88 miles stretch from Stehekin to Canada in and got 3 days exactly what it actually took me.
The weird thing is i never really plan where i am gonna be that way.
Usually I look at a 100 mile section as 3 days of high 20's and a nearo in to resupply.
Like 3 days @ 28 miles per day leaves 16 miles on the fourth day, perfect for a quick lunch, laundry, beer, resupply, and get back on trail by late afternoon.
Despite this strategy that calculator works out pretty dang close.

Math is scary!

Edited by Ice-axe on 02/12/2011 23:01:55 MST.

David Chenault
(DaveC) - BPL Staff - F

Locale: Crown of the Continent
scary math on 02/13/2011 07:40:52 MST Print View

There are very few folks with the same authority as Roman, and even fewer (none?) who are also math professors.

Paul McLaughlin
(paul) - MLife
Re: scary math on 02/13/2011 12:22:33 MST Print View

I'd like to be able to discuss with Roman what the limitations of his calculation are. It doesn't seem to work with what would be my typical ski tour base weight, for instance - whether that is because that higher base weight takes the answer out of the relatively linear section of the graph, I don't know. Maybe I need to run the full calculation using all of his equations rather than the version used in the included calculator. He seems to imply in the wirteup that as packweight goes down, each added pound makes a larger difference - which makes sense since it would be a larger percentage of the total weigh that way.

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: Walking vs Skiing, Biking, on 02/13/2011 12:54:23 MST Print View

It is not surprising that the approach comes up short for skiing. It's pretty hard to glide when you are hiking. I doubt if it is applicable to cycling. Or to packrafting. But it might come close to snowshoeing.

The application target is hiking over open ground.

"Apples to apples" and "The right tool for the right job" come to mind.

Paul McLaughlin
(paul) - MLife
Re: Re: Walking vs Skiing, Biking, on 02/13/2011 14:21:08 MST Print View

Well, I'm not expecting his approach to work for skiing. I just mentioned skiing because the packweight is higher due to winter gear. And at that packweight his calculator will not return an answer. Anyway, what I am looking for is somewhat different than what the calculator is set up to do. His premise was to determine how far he - or anyone else - could go unsupported given a known starting baseweight, known weight of food per day, and known level of fitness. My interest is in working out how much more time (expressed as a percentage) I would expect to travel each day for each pound added to my packweight, when covering the same distance. Or another way yo look at it would be how much slower will I go in miles per hour (again, as a percentage) for each added pound. Which is data that I expect could be extracted from the calculations Mr. Dial uses to get the answer he is after if I rearrange them appropriately. He's interested in miles per day, base packweight, and total distance in his answers, where what I am after is a percentage of time on the move - which is unaffected by mode of travel. At a given level of fitness and a given level of output, I can ski or walk for the same amount of time - my distance traveled will be different, but that is irrelevant to what I am after, which is time.