IndyTim

I just got back into road biking after a 4-year haitus. I dusted off my old Sigma Sport BC 1200, installed a new battery, and reset everything.

I entered 2155 for my 700x25c tires on my Giant OCR as my first-pass guess (I haven't rolled it out yet.)

I went for a ride and the computer seems way fast: average speed over 19, downhills at 40mph, and total distance covered was 27 miles for 20 mile loop.

I know my tires might be a smidgen smaller than 2155, but not 25-30% smaller!

Does anyone have any ideas as to what's going on? I'm wondering if I'm missing something very, very basic.

deep_sky

Is your computer set to km or miles. At least with my computers, if I enter the circumference of my tire in cm, then the computer is in metric mode, and everything is going to look high for someone who thinks in miles rather than kilometers.
A quick look at my Mavic computer manual that happens to be sitting on the desk says that for 25mm tires, the circumference is 2140. So you are likely in metric mode, and the computer thinks your tires are fatter than they really are, which all adds up to odd numbers.

IndyTim

Ah... I think that's it. Kind of...

I"m in "Miles" mode, which is one of the first decisions that comes up in Setup. However, the default setting for wheel size is 2155 (mm). But upon reading the manual more carefully I was supposed to divide wheel size by 1.61 (kms per mile).

You would think the programmer of the device would enter a reasonable default, depending upon whether you've got the device in Miles or KM mode. Hopefully Sigma addressed that little deficiency in later models...

Thanks for the help, deep_sky!

IndyTim

I was *kinda* hoping I didn't actually do anything wrong, and I in fact was magically carrying a 19mph average speed on my hilly training route today. Bummer.

deep_sky

No problem
Set it to 2140, and see if the numbers are more in line with what you are expecting.

HillRider

A 700x25 tire is close to 2111 mm in circumference so your setting is significantly high.

All of the cyclometers I've dealt with use the circumference settings in mm or cm regardless of whether the distance and speed are set in miles or km.

deep_sky

My Mavic one has me pick miles or km before anything else, so I have to set it to km to be able to set the circumference in mm, then switch it back to miles.
Maybe it's different for different computers, but 2111mm would indicate a 20mm tire, at least according to my manual

mcgreivey

Or you could do a "roll-out" and measure the actual circumference of your tires. If the computer still doesn't seem to be calculating sensibly after you enter the actual circumference, ...

davidad

A friend has a Sigma and it uses a strange number system to set the wheel size. Find the instructions.
https://www.sigmasport.com/us/service...p=bikecomputer

roberth33tiger

to get circumference, sit on your bike, with the tires pumped, have someone measure the distance from
the floor to the center of the axle of the wheel with the sensor, in mm, multiply that number times 2 pi
(3.1416). that will be very close, and you don't have to go outside.

canopus 

The Sigma sets the circumference in mm regardless of display settings. Do a roll out test. 2155 seems kinda of large to me for a 700X25. That's what I have on some of my 700X32's

HillRider

Cat-Eye basis their circumference values on assuming a bike tire is circular in cross section, i.e. the height is equal to the width.

So, the wheel's outside diameter is calculated as 2*tire width+ rim bsd. For a 700x25 the calculation is 2*25+622 = 672mm and the circumference is Pi times that or 2,111 mm. The true circumference as measured by a roll out will be quite close to that value.