In another forum someone posted the image below, which purports to illustrate the impact of a 220-lb rider losing 20 lbs. I've never seen anything like this, though it agrees with my experience/intuition, sort of, and I was wondering on what basis it could be justified. Playing around with some numbers on BikeCalculator-- changing the grade, etc.-- yields a similarly-shaped curve, but I can't explain why. Maybe it is because the last time I took a physics course was nearly 30 years ago, but I figured some of you savvy people would be able to give a cogent explanation.

It's because wind resistance is proportional to the square of the speed.

The curve is for 150 watts. Who in the world can go up a slope at 150? I'd be going backwards.

The chart shows me that if I want to get 10% faster on the flats, I need to lose 200lbs.

Yeah, definitely indicates whomever made the chart was not a 220lb cyclist! I am a 220lber, and getting up anything steeper than 5% is gonna take like 250w, on the slow boat!

No doubt dropping 20lb would be a boon for climbing speed, though. Oh, the dream...

Zipster, the best info I have for you is that you can post a larger picture by double clicking the image and adjusting the size in the menu that opens. Sometimes the adjustment malfunctions and you have to do it by editing the post after it is first posted. That is how I do it.

Bicycle riding consists of riding against two major sources of resistance, wind drag and gravity. There's also tire, chain and bearing drag, but those are very minor in the scheme of things.

On level ground, weight is only a factor when accelerating, so the bulk of your work is against wind drag, which is proportional to the square of your speed.

However on hills, you're working against gravity and the bulk of your effort is simply lifting yourself to higher altitude (why climbing stairs is so much harder than walking on level ground). But most people don't ride as fast when climbing, so wind drag is diminished and becomes less of a factor, and on steeper hills almost negligible. That leaves only gravity which (obviously) is proportional to your weight.

So you get a curve, where being lighter produces only a minor improvement on level ground, and a major improvement climbing.

BTW- with improvements in aero the curve is pretty much reversed, with the most benefit at the higher speeds on level ground (or downhill), and greatly diminished at the lower climbing speeds.

There's your explanation.

Losing 10% of your BW requires 10% less force to lift it. What's so hard to understand about that?

It is a simplifying assumption that permits a 2-D graph to be displayed, instead of a 3-D surface, which almost certainly would be more difficult to understand.

I just dropped 20 lbs over the last few months (I'm drinking less beer), and I definitely notice that the climbing is easier, but I was off the bike for a time as well, so it's hard to tell the effect of weight loss from the effect of increasing fitness.

Many thanks for the clear, concise explanation. Regarding your last comment, it brings to mind a quote I noticed while skimming the latest Velo News (9/2015, page 59, "Unified Front"):

Strap 4 5lb bags of sugar to your bike and go for a ride.

That's 5 X 4lb bags. Sugar hasn't come in 5 lb bags for at least 20 years.

I lost 20-ish lbs last spring, and I did notice a difference in climbing. I noticed it for a few months, then I got weaker to compensate. Same effect as when I lighten my bike.

But all kidding aside, it has made a pretty noticeable difference, though I don't have any way to measure that difference, other than Strava metrics.

The asymptote at 10% comes from:
10% weight loss ==> 10% climbing improvement.
And, as others have mentioned, less benefit on the level, giving the nice curve.

This implies that fitness is the same. No increase in strength/fitness from exercise, or loss of it from loss of muscle.

Well I'll be....thought I was thinking of flour but nope. That is 4.25 lbs. Now I feel robbed.

Wouldn't it be great if we could lose weight like that?

effin A ! I'm stuck at 210. grrrr.

There is another effect that is hard to capture.

During a steady sate effort, say a 10 mile long hill climb, then the speed may be close to the 10% deficit for a hard climb. Still, the heavier person is working for 10% longer.

However, many hills are much shorter, and can be conquered with anaerobic power.

So, say one can ride at a steady state of 100 watts.
Then hits the hill...

Say one budgets anaerobic energy of 150 additional watts (250 total) for 30 seconds, or 4500 watt seconds.

The light person gets to the top and starts cruising back down the other side while the heavy person is still climbing the hill, and either burns out at the 30 second point, or powers down allowing them to extend their energy budget.

So, my guess is they in fact, may ride slower than otherwise predicted.

Just buy your sugar in 10 pound bags

Just need 2 of them, one for each pannier

What the graph fails to consider is the physiological effect of having 20 lbs less fat (especially if riding in warm weather). So while it may not have much effect on how much power you need at the rear wheel to maintain a certain speed in the flats, the efficiency by which your body can produce that power may be increased. The effect will be an increased anaerobic threshold and consequently a greater reduction in fatigue than the 1% power difference might suggest.

I've lost 25lbs this summer, down from 195. Huge difference on hills but also maintaining higher speeds on less than hilly rides (we dont have flat ground here). Nearly every course I ride has been faster. Significant difference for me.

Did you increase your exercise during the weight loss period?

I usually don't notice 10 or 20 pounds of "cargo", but certainly can notice 30-50 pounds or more, especially on steep hills.

I did. My typical ride week would show 3 major rides and maybe a casual here and there. This summer I have made it a point to grab a bike after dinner and tour the town or neighborhoods (always involving hills).

I can feel the difference with every 5lb loss.

Additionally I would think a loss of weight also yields a reduction in wind drag helping on flats as well as climbing.

I don't think it is, at least enough to make a meaningful difference. I could be way off on this, but here's how I understand it.

In a lot of other sports (think running) there's a lot of variation from one person to the next in what you're talking about, how efficient they are at turning stored energy (like fat) into mechanical work. But on a bike, we're all seated for most of the time, turning our feet in circles with our legs extended to about the same length as each other, etc. You might ride in the drops while someone else is on the hoods but it's pretty constrained and we all have pretty similar energy efficiency because of it. When you convert kJ from your power meter into kCals, you guess at how efficient you are at this, and there just isn't a very big range.

You guys are over thinking it to death. There are tons of variables, so graphs like this are based on averages with other variables held constant. This isn't a precise statement of any change, just an approximation of the net effects.