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12-21-13, 09:06 AM
#22
Walter S
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Join Date: Jan 2013
Location: Atlanta, GA. USA
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Bikes: Surly Long Haul Disc Trucker

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Originally Posted by valygrl
I couldn't read all of this, but i think part of what you were wondering is how to calculate the difference in speed based on weight, taking into consideration the incline.

Here's a calculator:
http://bikecalculator.com/veloUS.html
That's interesting. Thanks. I'd like to see the math. Not that I'm totally qualified to evaluate it but...otherwise the validity of the calculations are in question. I looked at the http://en.wikipedia.org/wiki/Bicycle_performance link too which spells out all the math but there's a good bit of disagreement there about it too. So I'm less sure how important it is to see the math really! But in a broad sense I am seeing confirmation about my belief that weight does not affect speed "very much".

If I believe the calculator, the effect of weight is small indeed. Probably less than I would have guessed. I increased the weight of the bicycle from 22 lbs. to 120 lbs. That dropped the speed from 15 mph to 14.07 mph. Then to decrease the weight of the cycle from 120 lbs. to 100 lbs., the speed improves from 14.07 mph to 14.26 mph. So to shave 20 lbs. off my pack improves my speed by so very little.

Then on hills the effect is small too. For a 150 lb. rider with a 120 lb. bicycle on a 7% grade at 150 watts the speed is 3.79 mph. Then this rider shaves 20 lbs. off the load and can go 3.52 mph. That's a bigger hit percentage wise on the hills, but still not much to complain about if we're not racing. That tells me that pouring over my pack to find 20 pounds to get rid of is not doing a lot for me (according to this calculator).

I suppose the place this hurts the most on hills though is when you get onto the smallest cog. It's hard to ride much slower than 3.5 mph so if 20 lbs. makes the difference between 3.25 and 3.5 then you may be talking about riding vs. walking (or of course the need to temporarily increase effort a lot which is no big deal unless it's super-long hill).