Sigma sport wheel size 18”

[belgiangoth]

I am setting up my cycle computer, sigma sport 509, with 18” marathon racers (40-355).
Does anyone know what size to enter in the computer? The chart covers 16” and 29” but not 18.

[thirdcrank]

Going by forum experience, there's plenty of scope for some controversy here.

With a tyre inflated to your normal pressure, measure a complete rotation of one of your wheels in centimetres - some may recommend sitting on your bike while doing it - and that should give you a figure to enter

The Cateye chart suggests 134 cms

https://www.cateye.com/data/resources/T ... 151106.pdf

[belgiangoth]

Yeah, I know you can do it properly - but a handwave is all I need. Saying this, I entered the sigma sport data into XL, used it to plot a graph of etrto_diameter vs their computer size and got the equation of the line... because that's simple (to me).
Computer reckons 1390 - as long as the etrto is true.

[thirdcrank]

Click for a handwave

[simonhill]

Do you have a GPS device?

You can set your Speedo up roughly using one of those figures, then compare it to what your GPS says. Tweak it till you get it right.

Best on a straight road.

In NZ I have seen a road which had a measured distance ie a measured mile or whatever distance/units. Never seen in the UK.

[Barrowman]

Traffic Police in the UK used to have various measured miles to calibrate the speedo on a regular basis so as there was a record to convince magistrates the equipment was working properly. The traffic lads use to know where they were ,don't think the locations were in the Public Domain. Believe ther was one on the Newton Road in West Bromwich, the stretch leaving West Brom and passing over the motorway.

[thirdcrank]

Here's the start of a measured mile on the Otley Road in Baildon, just along from what was Ellis Briggs. I've previously posted how I once rode to and fro here, adjusting my Mitie2 after each measured mile and after spending some time doing so, I found I was at the factory setting for a 700C wheel.

https://www.google.co.uk/maps/@53.84357 ... 384!8i8192

Here's the end of another measured mile on the A650 near Bingley

https://www.google.co.uk/maps/@53.86315 ... 384!8i8192

[Mike Sales]

Here's the start of a measured mile on the Otley Road in Baildon, just along from what was Ellis Briggs. I've previously posted how I once rode to and fro here, adjusting my Mitie2 after each measured mile and after spending some time doing so, I found I was at the factory setting for a 700C wheel.

https://www.google.co.uk/maps/@53.84357 ... 384!8i8192

Here's the end of another measured mile on the A650 near Bingley

https://www.google.co.uk/maps/@53.86315 ... 384!8i8192

I know these roads, they are where I restarted cycling. I never knew about these measured miles. In fact I never knew that such a thing existed ashore.
There are some for boats, for the same purpose of calibration, and the are marked on charts. The one I know is on the Anglesey shore opposite Bangor Pier.

[thirdcrank]

Barrowman explained measured miles higher up. I've no idea if they perform any enforcement function these days but once upon a time the calibrated speedometers in patrol cars - those used for speed enforcement - were checked on measured miles. All speeding reports began with evidence of that being done. They weren't going to travel all over the country to do this so there were plenty of measured miles. I don't know how well they were all marked or how.