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 Post subject: Re: 27.6mm seatposts
PostPosted: Sun Jun 23, 2013 11:44 pm 
Gold Trader / MacRetro rider
Gold Trader / MacRetro rider
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Joined: Sun May 25, 2008 11:03 am
Posts: 18206
Location: Sunny Glasgow
Rampage wrote:
Oh, same a a ring gauge for wedding rings etc.
The problem with the it that they measure at the to where it can get distorted :)


Follow the link. look at the picture of the gauge :wink: what do you see?

A ring gauge is effectively a cone. The 'ring' is very narrow so you dont need long stepped graduations. However. those boffins,knowing that the bicycle seat tube changes diameter slightly and have taken that into account.

Pro tool. Pro giving the advice. Pro uses gauge himself :wink:


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 Post subject: Re: 27.6mm seatposts
PostPosted: Sun Jun 23, 2013 11:55 pm 
retrobike rider
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Joined: Thu Apr 17, 2008 1:24 am
Posts: 7561
Location: Manchester
I mean it measures it at the top of the seattube, where it's most likely to not be round!


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 Post subject: Re: 27.6mm seatposts
PostPosted: Mon Jun 24, 2013 1:45 am 
Gold Trader / MacRetro rider
Gold Trader / MacRetro rider
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Joined: Sun May 25, 2008 11:03 am
Posts: 18206
Location: Sunny Glasgow
Rampage wrote:
I mean it measures it at the top of the seattube, where it's most likely to not be round!


It measures it over about 40mm which is more than enough to measure a seat tube
You're not sitting there believing this is some sort of magical thing. A tube is a tube and even if its distorted that doesnt matter in the slightest. The job of the gauge is to measure the tube and that is does.

Sigh :facepalm:
OK we'll start again
Take a tub and cut a slot into it the same as you see on a seat tube
Done that?
Next take a hammer and bash the end of the tube slightly where the cut has been made to narrow the entry slightly
Next take the gauge and push it into the tube. If it wont go in then push harder.
Lo and behold the tube reforms and it gives the exact size of the inside of the tube and THATS IT

:facepalm: Now feeling exacerbated and in dire need of a ciggie :lol: :lol: :lol: :roll:


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 Post subject: Re: 27.6mm seatposts
PostPosted: Mon Jun 24, 2013 11:48 am 
retrobike rider
retrobike rider

Joined: Thu Apr 17, 2008 1:24 am
Posts: 7561
Location: Manchester
Oh, so it isn't rigid? That wasn't obvious!
Gauges aren't usually made of plastic!

As I said above above a cheap telescopic bore gauge will give you repeatable accuracy of 0.02mm if you use it just below the seatclamp slot.

It would avoid this:

Quote:
The ring gauge put it at 27.4 mm but if the wrong size has been in there for so long its safe to assume the top is bent in slightly and the 27.6mm would be the better size even though it might take a little persuasion to make it fit.


Which is what I meant by this:

Rampage wrote:
I mean it measures it at the top of the seattube, where it's most likely to not be round!



But yes, if the cycle mechanic know the limitations of that 'ring gauge' and what to check before using it, then it's probably simpler to use.

I'm out.


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 Post subject: Re: 27.6mm seatposts
PostPosted: Wed Jun 26, 2013 9:32 pm 
Old School Grand Master
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Joined: Mon Jun 14, 2010 9:06 pm
Posts: 4422
Location: Herts UK
I suppose snother way woul be to stick abung down the tube and accurately pour in s known amont of liquid thrn measure the height ....
Obvioudly there sre some prsctivsl isdues to overcomr....


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