23c tyres: what circumference have you got in your 'puter?

[Sergeant Pluck]

2095 seems to be the mean, I think.

What 23c tyre are you using and what circumference have you entered in your computer? Have any of you done it anally  - as in measured it out along the floor?

[Gus]

2097

If it's to high then it's only about 0.3% and I don't care about that.

[Sergeant Pluck]

The Sigma manual gives 2133 mm 

[Sergeant Pluck]

Looks like there's only one way to be sure 

[border-rider]

It depends on the exact tyre size, of course.  23 is a nominal size. If it were exactly 23 mm then it'd 2098

=(622+2*23)*Pi

[Sergeant Pluck]

Yes, my question was too general.

Any info for Michelin Pro3, specifically?

[border-rider]

Yes, my question was too general.

Any info for Michelin Pro3, specifically?

TBH it also depends on the rim a bit  - wider rims give a lower profile  - so the best bet is to measure it with the chalk/rolling the bike method.  That takes account of usual tyre pressure, weight and front-back weight distribution too.  It doesn't take long.

[Biggsy]

Have any of you done it anally  - as in measured it out along the floor?

Yes, but I don't trust my measurement.  How anal is that?  Don't answer that!   So I just enter what it says in the Catyeye manual (which is available online; look it up).  My computer is only precise to 10mm anyway.

To be super-anal, you need to inflate to usual pressure and sit on the bike while advancing one revolution.

[Jurek]

It depends on the exact tyre size, of course.  23 is a nominal size. If it were exactly 23 mm then it'd 2098

=(622+2*23)*Pi

Just spotted this thread and I'm really sorry but I would like to digress.

Mal.
Avatar.
Mark II Viva GT (I think I can see the badge) with a matt black bonnet, or is it a Brabham Viva?

2096 according to my Cateye cyclometric data.

[border-rider]

 

You know, I have no idea.  I nicked it from b3ta.

[Jurek]

You know, I have no idea.  I nicked it from b3ta.

[OliverBendix]

To be super-anal, you need to inflate to usual pressure and sit on the bike while advancing one revolution.

I used two revolutions to make the measurement error at the ends a smaller proportion of the total. 

Then I changed the tyres to a different brand, several times!

2094, I think.

[JonBuoy]

1/  Lash GPS to handlebars with insulating tape
2/  Find long straight bit of road
3/  Create user waypoint
4/  Select GOTO waypoint
5/  Cycle away from waypoint
6/  Compare distance and speed figures on GPS and computer
7/  Stop and fiddle with computer setting
8/  Goto step 3 until satisfied with comparison

Someone will probably tell me that this is inaccurate because of GPS errors etc etc but it satisfies the geek in me.

... and in answer to the original question  -  I haven't a clue as it wasn't a 23c but you are welcome to borrow the gps, tape and favourite bit of road.

[Salvatore]

1/  Lash GPS to handlebars with insulating tape
2/  Find long straight bit of road
3/  Create user waypoint
4/  Select GOTO waypoint
5/  Cycle away from waypoint
6/  Compare distance and speed figures on GPS and computer
7/  Stop and fiddle with computer setting
8/  Goto step 3 until satisfied with comparison

Someone will probably tell me that this is inaccurate because of GPS errors etc etc but it satisfies the geek in me.

... and in answer to the original question  -  I haven't a clue as it wasn't a 23c but you are welcome to borrow the gps, tape and favourite bit of road.

Or use a Garmin Edge with the cadence gadget (which also can be used to measure speed distance etc like a non-GPS wireless computer) and 'auto-calibrate' turned on.

[JonBuoy]

1/  Lash GPS to handlebars with insulating tape
2/  Find long straight bit of road
3/  Create user waypoint
4/  Select GOTO waypoint
5/  Cycle away from waypoint
6/  Compare distance and speed figures on GPS and computer
7/  Stop and fiddle with computer setting
8/  Goto step 3 until satisfied with comparison

Someone will probably tell me that this is inaccurate because of GPS errors etc etc but it satisfies the cheapskate geek in me.

... and in answer to the original question  -  I haven't a clue as it wasn't a 23c but you are welcome to borrow the gps, tape and favourite bit of road.

Or use a Garmin Edge with the cadence gadget (which also can be used to measure speed distance etc like a non-GPS wireless computer) and 'auto-calibrate' turned on.

Clarification inserted.

[FatBloke]

2133 as advised in my VDO computer's manual.

Seems bigger than everyone else. 

[Greenbank]

Sheldon says 2097 for 700x23c.

http://www.sheldonbrown.com/cyclecomputer_calibration.html

But yes there's a lot of things that can affect it. Rim width, manufacturing tolerances, total weight, tyre pressure, etc.

The difference between 2097 and 2096 is about 100metres over the course of a 200km Audax.

[Pingu]

Measure it, preferably with you on the bike.

[Pingu]

2133 as advised in my VDO computer's manual.

Seems bigger than everyone else. 

That's true in so many ways 

[pdm]

2110 for my particular tyres and rims...

But then I measured 5 revolutions of the wheel with a 15m tape measure.
I get well below 1% error on rides compared with mapping software and GPS

[Paul Smith SRCC]

Link to Cateye manuals, assume you need it accurate so you now how to make it read 5mph quicker?

I knew a wife who did that once, not as much as 5mph, but enough to make sure her husband would get back from a bike ride in a better mood 

Paul_Smith
www.bikeplus.co.uk

[Sergeant Pluck]

I think I'll settle for 2099.

[Valiant]

As measured 2099 with ProRace2 on CPX22 wheels and 2097 with ProRace2s on the Gipiemme Tecno T30s (The wheels I took to Malta).

[Pete]

Mine is set at 2070.  Which is lower than anyone else.  I'm pretty sure my tyres were at working pressure (110psi) when I measured - and re-measured - but I suppose I'm rather heavy and adopt a fairly 'forward' position.  They're definitely 23c's.

Or perhaps that's why I think I'm slower, and get fewer miles in, than anyone else? 

I recall that when I was living in Bradford (1970s - long before the days of GPS - or computers) there was a 'measured mile' marked out - simply two small signs on posts by the side of the road, stated to be exactly a mile apart.  I think in the opposite direction there was a 'measured half-mile'.  They were on Thornton Road, leading west out of the city.  Anyone know if they're still there?  Of course, in those days I had nothing to calibrate...

[Deano]

2118.  I arrived at this number by measuring the wheel at 2149 when I was using 28mm tyres.  When I changed, I couldn't be bothered to re-measure it, so I subtracted 10 from the diameter (2149 / Pi = 684.4) and calculated the wheel size from that (674.4 x Pi = 2117.6). 

I would have used whatever tyre size is in the manual, but I lost that ages ago.

Anyway, I checked it using the mileposts on the A167, and it was gaining about 0.01 of a mile every three miles, so that's close enough for me.