Sigma sport cycle computer - what tyre size

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PaulaT
Posts: 159
Joined: 20 Dec 2018, 6:41pm
Location: Staffordshire

Re: Sigma sport cycle computer - what tyre size

Postby PaulaT » 1 Aug 2020, 10:54am

reohn2 wrote:Yep as suspected,the OP needs to measure the circumferance of the tyre to get an accurate reading.


I've read that even that wont give totally calibration because of how the tyre deforms when you're sitting on the bike. I think it makes the effective circumference smaller that you'd get by measuring around the wheel. I bet it's also dependent on tyre pressure too as the lower the pressure the more tyre deformation you'll get.

tatanab
Posts: 4234
Joined: 8 Feb 2007, 12:37pm

Re: Sigma sport cycle computer - what tyre size

Postby tatanab » 1 Aug 2020, 10:55am

Manual --- https://www.sigmasport.com/en/images/pd ... en_SP3.pdf taken from https://www.sigmasport.com/en/produkte/ ... nu=manuals
Item 12 at the bottom shows you how to measure the wheel diameter. Item 7 towards the top shows you how to enter this value into your computer. Or, you could use the generis value for 700 C (shown in 12) but that will give errors since the tyre could be anything from 23 upwards.

Like many of us, the answer is --- read the manual.

Mike_GMT80
Posts: 30
Joined: 23 Jul 2020, 8:37am

Re: Sigma sport cycle computer - what tyre size

Postby Mike_GMT80 » 1 Aug 2020, 12:36pm

Done...measured the circumference and input into the computer. Will see if this improves accuracy.

Thanks
Mike


I'm a trendy consumer. Just look at my wobbly bog brush using hovercraft full of eels

reohn2
Posts: 39974
Joined: 26 Jun 2009, 8:21pm

Re: Sigma sport cycle computer - what tyre size

Postby reohn2 » 1 Aug 2020, 2:04pm

PaulaT wrote:
reohn2 wrote:Yep as suspected,the OP needs to measure the circumferance of the tyre to get an accurate reading.


I've read that even that wont give totally calibration because of how the tyre deforms when you're sitting on the bike. I think it makes the effective circumference smaller that you'd get by measuring around the wheel. I bet it's also dependent on tyre pressure too as the lower the pressure the more tyre deformation you'll get.

You could sit on the bike whilst carrying out the measurement.
Though as you so rightly say tyre pressure can make a difference,as can riding up hill when less weight is on the front wheel*,or even riding down hill when more weight is on the front wheel.That said if you chose each ride with with equal ups and downs it could even out any discrepancies.

*unless you climb out of the saddle which puts more weight on the front wheel

Ho hum life's problems :?
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sjs
Posts: 874
Joined: 24 Jan 2010, 10:08pm
Location: Hitchin

Re: Sigma sport cycle computer - what tyre size

Postby sjs » 1 Aug 2020, 2:40pm

I've always just started with a random or default setting for tyre size, ridden a known distance, then applied a correction factor to the original size setting.
ie
correct size = random size X (known distance)/(incorrectly measured distance)

Jdsk
Posts: 3020
Joined: 5 Mar 2019, 5:42pm

Re: Sigma sport cycle computer - what tyre size

Postby Jdsk » 1 Aug 2020, 2:42pm

That's an excellent approach because it removes a whole pile of possible errors.

(And the way I recommend for car speedometers, using GPS speed as the reference standard.)

Jonathan

igauk
Posts: 81
Joined: 8 Nov 2016, 2:12am
Location: Glasgow

Re: Sigma sport cycle computer - what tyre size

Postby igauk » 2 Aug 2020, 2:39am

Moulton TSR 30

thirdcrank
Posts: 29741
Joined: 9 Jan 2007, 2:44pm

Re: Sigma sport cycle computer - what tyre size

Postby thirdcrank » 2 Aug 2020, 7:52am

Unless something is obviously way out, I'd say it's not worth bothering about, on the world-wide scale.

I must have posted before that I bought a Cateye (Mity 2?) and decided to check it on a measured mile. To give a bit of local detail, I chose the one in Baildon, just along the road from where Ellis Briggs shop used to be. I manually set it to what it said in the manual for my wheel size, then road backwards and forwards (always going forwards of course) along the measured mile, fine tuning the computer at the end of each mile ridden. After doing that for some time, I checked the setting and I was back to the setting recommended in the handbook. I suspect one of the things that this shows is that the slight deviations from a straight line when riding, especially in busy traffic, have more effect than a minute difference in wheel circumference.

There are better ways of getting the miles in.

Pebble
Posts: 287
Joined: 7 Jun 2020, 11:59pm

Re: Sigma sport cycle computer - what tyre size

Postby Pebble » 2 Aug 2020, 10:13am

thirdcrank wrote:Unless something is obviously way out, I'd say it's not worth bothering about, on the world-wide scale.

I must have posted before that I bought a Cateye (Mity 2?) and decided to check it on a measured mile. To give a bit of local detail, I chose the one in Baildon, just along the road from where Ellis Briggs shop used to be. I manually set it to what it said in the manual for my wheel size, then road backwards and forwards (always going forwards of course) along the measured mile, fine tuning the computer at the end of each mile ridden. After doing that for some time, I checked the setting and I was back to the setting recommended in the handbook. I suspect one of the things that this shows is that the slight deviations from a straight line when riding, especially in busy traffic, have more effect than a minute difference in wheel circumference.

There are better ways of getting the miles in.


A MITY 2 I'm on the Mity3, its well battered and glued and soldered in place but still working.

Annoyingly the setting of the wheel size on the mity3 leads to inaccuracy, calibration is to whole centimetres and wheel circumference is 2136 mm - that could knock my yearly total out by 15 mile (and 15 mile is 15 mile especially into the wind)

I do calibrate all mileometers with bike-hike http://www.bikehike.co.uk/mapview.php, very important to do so with vehicles if you are interested in true mpg figures. cars and vans are notorious in over-reading distance, (my van is 4.5% out and the car 1.3% out). In comparison my bike is pretty near spot on 99.7% accurate.

However when it comes to my weekly average of 150 mile, this is done purely through bike-hike and no ride can be less than 10 mile. So little rides to the shops don't count :-( take the SUV instead.