Energy used when cycling hills vs flats

June 1, 2010 7:17 AM

How does a bike ride on flat terrain compare with going up a hill and back down, in terms of energy use, exertion, etc.?

When I enter a ride into a service like DailyMile, it says 'you burned xxx calories'. I assume that's based on [distance] x [speed] x [weight]. But here in Asheville every ride has lots of hills, and I started wondering if that sort of calorie estimate is correct, close enough, or completely wrong. From a physics standpoint, they should be equivalent (minus a small amount for friction) right? But that doesn't feel accurate in practice. i.e. I feel like I've worked a lot harder to climb a steep mile and come down, than I have when riding 2 flat miles. How would I quantify this?

posted by Who_Am_I to health & fitness (13 answers total)

I should ask my extremely serious triathlon friend, who rides with a power meter. It will tell you exactly how many watts you've generated during a ride. Hills, coasting, tailwinds, terrain.....these factors will affect speed, but not the amount of power you output. A power meter is the best tool a cyclist can have to objectively measure performance on the road.

With that said, there's a factor you could use to multiply your hilly distance to get an equivalent flat distance, but that's going to vary based on the length and grade of your hills.
posted by spikeleemajortomdickandharryconnickjrmints at 7:24 AM on June 1, 2010

Hills, coasting, tailwinds, terrain.....these factors will affect speed, but not the amount of power you output.

I don't think that's necessarily true. Power = work/time, and work = force x distance. Riding uphill requires more force than riding on flat ground, so it's more work. If you ride uphill at the same speed that you ride on flat terrain, you will generate more power. But if you slow down enough to offset the increased amount of work, power output will be constant.
posted by ludwig_van at 7:41 AM on June 1, 2010 [1 favorite]

Hilly terrain involves at least a minimal amount of energy expended in braking on the downhill parts. So hilly terrain should involve at least a little more energy.
posted by amtho at 7:44 AM on June 1, 2010

You do burn more calories when you're sweating up a hill, riding with more resistance, riding a heavier bike (or transporting a heavier you), or pushing against a headwind, than when you're gliding across the same distance on the flat. You can geek out with a guide like this, then get a cycling computer that will measure watts, although I believe there is some dispute about how accurate even those estimates are.

There's also this formula. When I start feeling lazy, I just look at the difference in caloric expenditure between 20 km/hr and 25 km/hr. This formula is for hills.

If you ride one mile on the flat, then ride exactly half a mile up a hill and exactly half a mile down a hill, it might work out to the same number of calories each mile, but it really depends on how hard you ride both up and down that hill.

If you're cycling to lose weight, just cycle a lot and track your progress. If you lose weight a little faster on your hilly rides than Daily Mile charts would indicate, that should be pretty motivating.
posted by maudlin at 7:45 AM on June 1, 2010 [1 favorite]

The only way of properly estimating this, is using a device to measure some of these parameters. The cheapest way is to use a heart-rate monitor, and then estimate your power output using a formula like this. You could try and crudely calibrate it by wearing it on an exercise bike that has watt measurements. A second, more expensive approach is to use a power measuring device on your bike, but they are very expensive (e.g. powertap).

I think it's necessary to measure this scenario as there are many factors at play (wind resistance, bike-weight, tire-pressure).

There is a significant difference between the energy you expend riding up a hill, and riding down a hill, when compared to a comparable distance on the flat. This is due to the rate at which you need to burn energy on the uphill. On average you are probably producing 100-150W on the flat, and ~250-300W going uphill, and ~50W downhill. But the speed that you are traveling at influences the wind-resistance, which is not a linear relationship and an average speed would not take into account slower-uphill/quicker downhill.
posted by a womble is an active kind of sloth at 8:23 AM on June 1, 2010

Completely different.
Riding flat merely requires you overcome rolling resistance and air resistance.
Riding up a hill requires that you do serious work (Work = Mass * Gravity * Height), and you don't get to recoup that energy on the downslope the way an electric car would (you're like a combustion engine car instead - once you've burned the gas to get moving, you can't reverse the process and put the fuel back in the gas tank in order to stop moving, the way an electric car can)

If you could recoup the energy on the downslope, it would be the same as riding flat, but you can't, so riding downhill you either dump the extra energy as heat through the brakes, or increase your speed until the air resistance (which increases as a square of your speed!) takes it.
posted by -harlequin- at 9:06 AM on June 1, 2010

I don't think that's necessarily true.

I should have phrased that better. What I mean is that a power meter is a better tool for a cyclist than a speedometer.
posted by spikeleemajortomdickandharryconnickjrmints at 9:08 AM on June 1, 2010

As to how to better estimate the calories you're burning, yes, the program is based on an average. For an estimate based on yourself and the particulars of your setup, cycle up a hill. Since you're going slowly, air and rolling resistance is insignificant - the work is pretty much entirely going towards raising your mass. Take it slow enough that you're only putting in about the same amount of effort as you would cycling flat.
So, given the height of the hill and the mass of you and your bike, and how long it took, you know how many calories per hour were needed to do that work. Since you were putting in the same effort as riding flat, you can estimate that that's how much you burn per hour riding flat.
posted by -harlequin- at 9:15 AM on June 1, 2010

I've not found any hard science to back up the claim (granted, I've only googled for about 20 seconds), but popular wisdom has it that automobile fuel economy is worse on hilly terrain than on the flat, and I would assume the same thing would hold for a bike.

I suspect the exponential nature of wind resistance is to blame. You get the benefit of slightly reduced wind resistance riding uphill compared to flat riding (because you are going slower), but you are hindered by significantly increased wind resistance on the way down compared to flat.
posted by drlith at 9:43 AM on June 1, 2010

It depends on how much you weigh and how steep the hill is. A flat ride is just fighting road friction and wind resistance and gravity plays almost no part. This is the reason why when training for races, riders focus on steep hills - it can really intensify the workout.

Wikipedia has an article which will take some of the guesswork out performance.
posted by JJ86 at 9:45 AM on June 1, 2010

fun with physics
posted by caddis at 9:56 AM on June 1, 2010

You can geek out with a guide like this, then get a cycling computer that will measure watts, although I believe there is some dispute about how accurate even those estimates are.

It depends what you mean. The various mainstream power meters (Cyclops Powertap, SRM PowerMeter, etc) are extremely accurate: +/- 0.5% for the costs-an-arm-and-a-leg versions.

Where the inaccuracy lies is in the estimate of how much energy you're burning...i.e. how much fat are you losing. This is a question of technique, individual fitness, individual physiology, etc. Rules of thumb for converting from human power output to human energy consumption could be off by 20%-40%.

For the average person just trying to lose weight though, this is overkill. No-one's weight loss plans ever went off the rails because they couldn't measure their energy consumption to the nearest 1%....it usually goes off the rails because they never got off their couch.
posted by randomstriker at 10:18 AM on June 1, 2010

Of course riding up a hill burns more energy that riding flat.

It's true that power output is the best measure of energy output & calorie consumption, but it is quite expensive to outfit a bike for power measurement.

A good and more economical way is to go with heart rate measurement on a bike computer. A bike computer with heart rate measurement will take into account that direct measurement of your body performance and give a more accurate number for calories burned.

I used to use the Polar CS series computer and it worked very well. Watching my heart rate taught me a lot about my maximum effort levels and how to push myself to get there.

There are more advanced computers, like the Garmin series, that take into account grade of climb and altitude, but to start with the Polar is great. I highly recommend using heart rate training to get the most out of your riding.
posted by Argyle at 11:48 AM on June 1, 2010