Correct inner tube size or not?

[hymasimo]

Hi all,

I didn't realize that inner tube sizes was a science all to itself.

Recently my inner tube split at the base of the valve and thus had to purchase a new inner tube. I checked the details on my tyre and then bought what I thought should match.

Details are:
Wheel: AlexRims DH19
Tyre: Continental Contact 28-622 (700x28C) C192 max inflation 102psi

I bought the following inner tube: Raleigh 700x25-32C - 27x1.1/4

I thought this would fit the tyre as 700 is the diameter and the width 28 was between the range of 25 to 32.

When I place the tube on the wheel I noticed it was a little to large, but continued hoping the tube would spread around the wheel evenly. However, once I pumped the type to 45psi the tyre bulged in one place and popped the tyre from the wheel. Have I bought the wrong size? What figures do I need to take into consideration?

I am a little lost as to what I have done wrong and would very much appreciate some guidance.

Thanks in advance.

[RobAR]

That tube was the correct size, yes. It sounds to me as though the tire was not seated properly all the way around the rims bead and blew off.
I inflate to 20 - check bead line all around, both sides. Repeat at 30 then 40. If you see the bead creaping out, DEFLATE and start over. If you see the bead line stuck below the rim, use thumbs to help ease it out.

[hymasimo]

(11-14-2012, 08:41 PM)RobAR Wrote:  That tube was the correct size, yes. It sounds to me as though the tire was not seated properly all the way around the rims bead and blew off.
I inflate to 20 - check bead line all around, both sides. Repeat at 30 then 40. If you see the bead creaping out, DEFLATE and start over. If you see the bead line stuck below the rim, use thumbs to help ease it out.

Thank you kindly. First it's good to know I got something right and secondly it appears to have worked.

Just out of interest I measured the diameter of my tyre and it's about 680mm. Is it common that the actual figures on the tyre are not accurate?

Thanks again

[Joe_W]

Well, the important number is the bead seat diameter (the part that hooks into the rim), this is what you can see in the ETRTO dimensions (eg. 23-622). This is the more reliable measure. The outer diameter is basically a function of the bead seat diameter, the rim width and the tyre width. A tyre mounted to a wider rim will have a (slightly) smaller outside diameter than the same one mounted to a narrower rim anyway.
Oh, and it used to be very common that the actual width of the tyre was very wrong: basically by producing a 22mm tyre and lableing it as 23mm it would be lighter than the real 23mm one produced by the competitor company...

[Bill]

From another thread to suppoprt Rob's and Joe's posts I do remember some talking about for each manufacturer has their own "close" standards, but technically are supposed to be the size it says. So one does have to watch what they buy.

[hymasimo]

Thanks guys.

So in short, inner tubes are a science to themselves.

[RobAR]

(11-15-2012, 12:20 PM)hymasimo Wrote:  So in short, inner tubes are a science to themselves.

No. More like an Art.

[Bill]

As one tire engineer wise@$$ said to a tire engineer wiseman, "So innertube science is at best, just a lucky guess?!".