Road Bike, Cycling Forums banner
1 - 8 of 8 Posts

·
Registered
Joined
·
199 Posts
Discussion Starter · #1 ·
I got a new Polar CS200 for xmas and took off my old Sigma (non-wireless).
The first thing I noticed is that my average speed dropped by about a mile an hour and the length of a loop I do a couple times a week went from slightly over 21 miles to just under 20 miles. I thought this was strange, but since I didnt know how accurate my Sigma was I didnt do anything bout it.
Now my girlfriend got a Garmin GPS for her bike and has that on their as well as a wireless Sigma. Both hers read pretty close to what my old Sigma did.
Has anyone ever had to tweak the wheel size numbers on their computer to make it more accurate? Is there a good way to calibrate it?
On a 30 mile ride it reads almost 2 miles short.
 

·
Registered
Joined
·
2,799 Posts
I would just adjust it so that it is right and double check the number you put in. I also found that by fiddling with the magnet I got mine more accurate. I think I moved the speed sensor up towards the outer edge of the spoke/wheel.

Also try doing more / different loops at different paces once you do adjust it so that you can double check it in different situations.
 

·
jaded bitter joy crusher
Joined
·
19,723 Posts
hoehnt said:
Has anyone ever had to tweak the wheel size numbers on their computer to make it more accurate? Is there a good way to calibrate it?
On a 30 mile ride it reads almost 2 miles short.
I've checked my Polar CS600 (calibrated with a 1-revolution roll-out measured with a steel tape to 1 mm precision) against both a Garmin and a century course that was carefully measured by the organizers with a Jones-Oerth-Lacroix counter following IOC and USATF procedures. In both cases the discrepancy between the CS600 and the other measurement was less than 2%. My old Polar S710i gave similar results.

What can I say? I'm a measurement geek.
 

·
I ride in circles..
Joined
·
4,645 Posts
I get my exact tire measurement by putting around 30psi in the front tire. I then roll the bike forward while pressing down slightly over a metal tape. I get he total MM value which I'm currently rolling 2078mm up front. My Garmin Edge 705 estimated my front wheel was 2100mm so I think I'm pretty darn close

I don't noticed a different between speeds on this system than my old Polar CS300.. However I get the sensor and magnet within 2mm and always make sure it's perfect. I'm a geek too.
 

·
Registered
Joined
·
21,636 Posts
Higher level of geek

ZoSoSwiM said:
I get my exact tire measurement by putting around 30psi in the front tire. I then roll the bike forward while pressing down slightly over a metal tape. I get he total MM value which I'm currently rolling 2078mm up front. My Garmin Edge 705 estimated my front wheel was 2100mm so I think I'm pretty darn close

I don't noticed a different between speeds on this system than my old Polar CS300.. However I get the sensor and magnet within 2mm and always make sure it's perfect. I'm a geek too.
Actually, a much more accurate way to calibrate is to find a section of road or rail trail that has surveyed mile markers and ride at least 10 miles, comparing odometer readings with the real thing. With most bike computers, 10 miles will give you a 4 significant figure calibration correction, so if your computer has a 4 digit calibration number, you can get great accuracy.

To the OP, yes, it is common to have to change calibration on bike computers.
 

·
I ride in circles..
Joined
·
4,645 Posts
I'd need to find a decent stretch of road that isn't too curvy and marked well for them. Would be interesting to see. But I'm guessing based off what my GPS says.. What my measurement says... and what google maps says my sizing must be pretty close.
 

·
Le Misérable
Joined
·
4,374 Posts
ZoSoSwiM said:
I get my exact tire measurement by putting around 30psi in the front tire. I then roll the bike forward while pressing down slightly over a metal tape. I get he total MM value which I'm currently rolling 2078mm up front. My Garmin Edge 705 estimated my front wheel was 2100mm so I think I'm pretty darn close

I don't noticed a different between speeds on this system than my old Polar CS300.. However I get the sensor and magnet within 2mm and always make sure it's perfect. I'm a geek too.
Why 30 psi? It seems to me that 30 psi plus pressing down slightly would give you a number noticeably smaller than reality, as the tire will compress much more than in the real riding situations you are trying to recreate with a rollout. Then again maybe you know something I don't...

...I've always pumped the tire up to normal riding pressure, made a thin, straight chalk mark all the way across the tire (eyeballing the stem isn't quite good enough for a real measurement geek, IMO), then done a single-revolution roll-out along a straightedge while putting normal riding pressure on the handlebars. When measured against odometer markings on the road, it's always been really, really close, regardless of the computer (that is, when interference doesn't F everything up, which is all too common).

I can't imagine how messing with the magnet could change anything; either the sensor reads a signal at each revolution of the magnet or it doesn't, right?
 

·
I ride in circles..
Joined
·
4,645 Posts
It's just a guesstimate. I've eyeballed the amount of squish from the tire with full pressure and me on it and compared it to the lower pressure in my rollout test. Sure it's not 100% but it's close. I think..
 
1 - 8 of 8 Posts
Top