d2create

I'm artist, I'm tired and I suck at math. My brain is just not computing at the moment.

So I have a new Cateye computer. I'm setting up the wheel circumference. I measure it. The computer asks for the measurement in centimeters, not millimeters. My measured number does not come out to a round number, so I need to round up or down.

So this got me thinking... either way my computer's calculations will be inaccurate.
If I round DOWN, will that make the computer think I'm going faster or slower than I'm really going?
And how inaccurate are we talking? Let's say half a centimeter... what will that do to the speed measurement? Or mileage measurement?

And before you even have the chance, let me help you out ---> :facepalm:
There.

eminefes

If you round up, the computer will think you're going faster than you really are...but not by much.

RoboChrist

Round to the nearest cm?

fosmith

just put in 210 and go with it.

Reynolds 

700 x 23 = 209/210 cm

eminefes

Or he can put in 211 and get a .5% speed increase!

cooker

For a road bike it's going to be somewhere around 210-211 cm. If you are one half cm off, that is about 1/4 of one percent. Absolutely meaningless. The difference between 20 mph and 20.05 mph.

Tulex

I have my garmin set to auto, and it varies from 2088 to 2100. I don't lose much sleep over that though.

Kurt Erlenbach

Or just measure the diameter of the wheel and tire and multiply by 3.1416.

fosmith

all that number cipherin' is hard.

d2create

Exactly what i needed to know. My 25's were measuring about 210.5
Thanks!

Creakyknees

did you measure your rollout on level ground while sitting on the bike? tires compress a bit ya know...

531Aussie

I've done several 1km on-the-bike roll-outs on marked velodromes, and have found that 208 is most accurate for Cateyes; 209 at the most. I'm 190lb, if that makes any difference. 23mm tyres

Either way, 1cm doesn't make much difference.

camhabib

And that is why you should get a Garmin.

rollin

Because when you really want accuracy to the nearest cm you should use GPS

rollin

OP use 190 to 200 cm. Cycling is about suffering and this will aid your self loathing.

Or use the BF standard range of 250-400cm, then you can post "I cycled 15,000 miles this summer at an average speed of 37 mph" threads.

rollin

Just round pi to 3 - that'll do ya.... :ducks:

blacksquid

That's probably fairly accurate since computers can lose precision on floating point numbers anyway.

iareConfusE

https://www.cateye.com/sites/cateye/u...T400v1_E-1.pdf

Bottom right of the first page.

d2create

Yup, that's why I never just go by the charts listed in the computer instruction manuals.
My 25mm Conti Gatorskins measured out to about 82.875 inches.
So i just put in 211cm which is what the manual recommended anyway.

DiabloScott

What do you really want to know about your distance and how accurate is accurate enough?
I don't really care if I'm doing 21.22 or 21.24 miles an hour at any given moment in time.

For me, I'd like the mileage markers on my century ride sheets to match up so when it says "turn left at mile 73.2" I'd like my odometer to read pretty close to that number when I get there. So setting my computer off of a measured mile or a known LONGER distance makes a lot more sense to me than trying to measure your tire circumference at whatever pressure and whatever weight you want to use down to four digit precision. Ask a dozen people on the ride at that distance what their readings are and you'll see a disparity range of at least a mile and they'll all swear their own reading is the most accurate.

But if you're really concerned about that 0.5 cm precision that you can't get - you can always use two magnets on opposite spokes and divide your circumference reading in half.

d2create

That was my main question/concern... how off will the measurement be, and apparently it is a VERY tiny amount.

I'm with you though, I would like very accurate mileage results.