wheel diameter

cyclopsbiker

just bought myself a bike computer and to calibrate it I need to know the wheel diameter. on the tyre it says 26x1.6, but the computer needs it in a figure of something like 2016. helpfully (I say sarcastically) the guide that came with the computer gives me the conversion for a 26x1.5 and a 26x1.75. I made a rough guess in the middle-ish of the two but my regular commute is nearly 10km. the computer put it at 7.79km so I know it's out.
can anyone help me?

crankycrank

Here you go. From Sheldon Browns' site. http://sheldonbrown.com/cyclecomputer-calibration.html I believe 2016 is just the circumference in millimeters so you can fairly accurately get the right setting just by running a measuring tape around your tyre. The "rollout test" is the most accurate since it measures your own actual tyre with your weight and tyre pressure but also the biggest PITA to do.

pud77

Your figure of 2016mm should be close enough give or a take a few mm. I run 26x1.5 on my commuter bike and it's set to 2005mm so you're pretty close.
Setting it to that figure and having an error of over 2km over a 10km run means you have made a big error somewhere. Are you sure it's 10km ? What have you used to gauge this distance previously ? Check and check again.

cyclopsbiker

yep. big error somewhere!

I know its just under 10km for 3 reasons:

1. I've cycled the same route with my Garmin 305 and had readings 3 or 4 times at just under 10km.

2. my boyfriend works about 2 mins closer to home and his computer (the same one) reads 9.33km

3. I've used mapmyfitness to measure the route

I'll be resetting the computer today and giving the rollout test another go.

ps. same route today was 8.28km.....that's a difference from 7.79km on Friday. hmmm. could that just be taking a different line on the road?

desweller

What model computer is it? It sounds like it is missing the odd rotation of the wheel, rather than using the wrong circumference figure. Try adjusting the magnet and sensor a bit.

cyclopsbiker

OK. now I'm starting to get annoyed. :evil:

I have a Cateye micro wireless. so does my boyfriend and his is reading the correct distance. mine still isn't. I've tried various wheel diameters. this morning we did the 'roll out' test and the measurement is 2015, or thereabouts give or take a few mm. so, I adjust the setting and cycle to work.

real distance= approx 10km
computer distance=5.52km

how can it be so far out? it would have to be missing virtually every other wheel turn to get that reading. hmmmm.
the magnet and other bit are in the right positions, it's close - we adjusted that this morning to make sure.

can anyone help?

the only accurate measurement I can get with the darn thing is the time, which to be fair I am pleased with (24 mins being my fastest time so far) but I have a watch that can do that!!!! :x

PBo

Does the speed seem to be accurate? (I know it's hard to guess your speed, but if trip distance is coming in at approx 50% I would expect the speed to be also about half - and you should have an idea about that much discrepancy)

If speed seems right, that would suggest that the magnet/sensor set up is ok, and the signal is not being blocked, so the fault is in the computer - which won't be fixable.

definitely set to km and not miles?

desweller

It sounds like either the computer is faulty or the batteries are duff (does happen with new kit sometimes) or something like that. As PBo says, check the speed. If the problem is faulty pickup, you'll get wildly fluctuating readings.

cyclopsbiker

mild embarrassment. :oops:
it was set in mph not km....as I thought I'd set it.
thanks PBo....I'd have been sending the thing back!!

btw, my av MPH today was 11.8...commuting in London traffic. does that sound about right?