How accurate are the readings on a bike computer?

The Noble Nudge

Just wondering how accurate the readings on a bike computer are?
I recently purchased a Garmin 500,
3 of us doing the Sean Kelly and we checked calories burned,
My reading from the Garmin 7700,
Mate number one from a cateye 3500,
Mate number two from another cateye 3000??
I would imagine it varies with the cyclists own weight but as far as I know I didnt tap that info into my Garmin.

I havent even put on the heart rate monitor yet but dread the thoughts of it.
Am I better cycling by cadence or heart rate.
I hope I am not opening a can of worms....

andy69

well, funny you mention that - on my Garmin 500, if I go for a spin without the HR strap on, it'll still give me the figure for calories burned (some sort of estimate they must have a formula for I guess).
This would easily be double the figure I get when I am wearing the HR strap.
I just usually take about half the estimated figure if I was looking at calories burned.

Cyclepath

The Noble Nudge wrote: »

Just wondering how accurate the readings on a bike computer are?
I recently purchased a Garmin 500,
3 of us doing the Sean Kelly and we checked calories burned,
My reading from the Garmin 7700,
Mate number one from a cateye 3500,
Mate number two from another cateye 3000??
I would imagine it varies with the cyclists own weight but as far as I know I didnt tap that info into my Garmin.

I havent even put on the heart rate monitor yet but dread the thoughts of it.
Am I better cycling by cadence or heart rate.
I hope I am not opening a can of worms....

The readings you get from any device are estimates at best. There are so many variables in play that the device could not possibly give an accurate count. Your calorie consumption on any give ride cannot simply be measured by knowing your weight, cadence, and speed. Your device is not aware of a huge number of other factors that affect calorie consumption, for example:

Rolling resistance/friction (your tyre size and pressure affect this).
The true weight of the bike (i.e. including water bottles, and other bits)
The wind speed and direction
Your own muscular efficiency (two people may weigh the same but their ratio of muscle to fat may differ).
Your true heart rate zones (these differ from person to person and change as you become fitter).

Their are ways to calibrate Garmin devices to try to make them more accurate (http://www.dcrainmaker.com/2010/11/how-calorie-measurement-works-on-garmin.html) but at the end of the day it's just a guideline figure. For example, I get different readings from the same Garmin 410 from Strava and Garmin's own Connect website.

youtheman

The Garmin 500, as it is a GPS, can detect elevation, and therefore the slope of any hill you are climbing/descending. This, in my simple mind, will have a great input into the work you are doing, and therefore the calories you are burning. The Cateye just calculates speed and didtance from the wheel revs, so I would not expect it to be nearly as accurate (maybe it assumes you are cycling on the flat all the time).

ashleey

I recently got a HRM with a Garmin FR70 watch and have used the HRM with my Edge 500 now. The estimate for calories burned is about half what it used to be, but much closer to Strava's estimates. I was always suspicious of the Garmin estimates. 7500 calories for the SKT would require some serious gels consumption to avoid collapse.

2 Wheels Good

Cyclepath wrote: »

The readings you get from any device are estimates at best. There are so many variables in play that the device could not possibly give an accurate count. Your calorie consumption on any give ride cannot simply be measured by knowing your weight, cadence, and speed. Your device is not aware of a huge number of other factors that affect calorie consumption, for example:

Rolling resistance/friction (your tyre size and pressure affect this).
The true weight of the bike (i.e. including water bottles, and other bits)
The wind speed and direction
Your own muscular efficiency (two people may weigh the same but their ratio of muscle to fat may differ).
Your true heart rate zones (these differ from person to person and change as you become fitter).

Their are ways to calibrate Garmin devices to try to make them more accurate (http://www.dcrainmaker.com/2010/11/how-calorie-measurement-works-on-garmin.html) but at the end of the day it's just a guideline figure. For example, I get different readings from the same Garmin 410 from Strava and Garmin's own Connect website.

Same as above, the general consensus out there on any of the computers that estimate the calories burning is that it's way off, too many variables to be accurate. Garmin wouldn't have as good a rep as Polar for example in getting the calories burning correct even with the hrm belt being worn.
Using a hrm would be the best option for us non pros, I'm guessing there are more accurate methods once you get into body temp and the rest to measure energy use.