Tyre width discrepancy - why ?

[Chess]

Hi all.
Having bought new tyres for my commuter, fitted them only to find upon examination they're narrower than stated.
Tyres are schwalbe citizens, it clearly states on the sidewall that they are 700x42c - which is what I wanted. But when I measured across the width with my digital vernier gauges it reads 38. It's the same with both tyres.
Why is this ? Has anyone else had this problem ? Is it a factory mistake ?
Advice appreciated.
Thanks.

 

[marzjennings]

Tyre width is dependent on rim width. The rim you’ve fitted the tyre to will be different to the one the factory used to get 42.

http://engineerstalk.mavic.com/en/the-right-tyre-width-on-the-right-rim-width/

 

[ColinJ]

Ha ha - marz beat me to the Post Reply button - the phone rang just as I was about to click ...

(PS Rim size - seconded!)

 

[adamhearn]

It’ll not make any difference in performance but it’s fair to ask the question. Didn’t know about the rim width being the cause.

 

[Venod]

It’ll not make any difference in performance

It will alter the contact patch of the tyre, some good reading here.

http://inrng.com/2013/04/reinventing-the-wheel-25mm-2/

 

[adamhearn]

The op said he’s after a wide tyre for his commuter thus I started with what was most relevant. Though again, good read.

 

[Pale Rider]

There are some very narrow minded citizens on here, so I'm not surprised.

 

[Chess]

Good reading, but still not convinced. The marathon plusses I had on before were 42s and measured 42. But these citizens say 42 on sidewall but measure 38 at Max pressure.
Anyway, thanks for the replies, I'll use them until I can afford something nearer the originals. I only bought these because they were alot cheaper than the marathons, but they don't seem as comfortable.
Thanks again.

 

[ColinJ]

Good reading, but still not convinced. The marathon plusses I had on before were 42s and measured 42. But these citizens say 42 on sidewall but measure 38 at Max pressure.
Anyway, thanks for the replies, I'll use them until I can afford something nearer the originals. I only bought these because they were alot cheaper than the marathons, but they don't seem as comfortable.
Thanks again.

I never run tyres at anywhere near their maximum rated pressure. I am not surprised that they are not comfortable. Why not try them halfway between maximum and minimum?

 

[Smokin Joe]

Very few tyres are exactly at the stated width, most of the ones I've used over the years do come up narrower than described.

 

[Ian H]

They're to be treated at best as nominal.

 

[Ajax Bay]

Some tyres measure wide. Others measure narrow. With 'racing' tyres there's a marketing issue too. By making them narrower they can quote a lower weight for that quoted width, as that influences some potential purchasers.
https://www.bicyclerollingresistance.com/specials/schwalbe-marathon-32-37-40-47
https://www.bicyclerollingresistance.com/specials/conti-gp4000s-ii-23-25-28
Quoting from the first link (you'll see that these tyres in 47 come out at 43mm):
Weight, Width and Height Measurements
Schwalbe Marathon
32-622 37-622 40-622 47-622
Spec'ed Weight 640 grams 730 grams 810 grams 985 grams
Measured Weight 722 grams 729 grams 809 grams 986 grams
Measured Width 32 mm 35 mm 38 mm 43 mm
Measured Height 32 mm 36 mm 39 mm 44 mm

As well as the rim width affecting actual width (as identified above) the pressure you run them at makes a significant (8+%) difference.
See the attachment (graph goes to 37mm 'actual' width).
If your total all up weight is 100kg, this will be split 60kg rear and 40kg front (hybrid) and my reading of the graph suggests pressures of 45psi rear and 38psi front. TyreInflationPressures.pdf
What's a tad annoying is that manufacturers seem to manage to have the same model of tyre measure wide in some "specs" and narrow in others. As @Ian H says treat the ETRTO description as nominal.

 

[Bodhbh]