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PostPosted: Sat Nov 17, 2007 11:46 am 
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Location: Mansfield, Notts
Just a quicky how close does seatpost size have to be to your seat tub size, does it have to be spot on or say for instance can you get away with so much either way, e.g. my seat tube takes 30.6 and have found a nice one but is 30.9 would it fit does anyone know?, and if so how far out can you go without causing problems?. Cheers


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PostPosted: Sat Nov 17, 2007 11:47 am 
retrobike rider / Gold Trader
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i tried to fit a 27.2 in a 26.8 as an experiment. it wasnt tight, it just didnt physically fit.


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PostPosted: Sat Nov 17, 2007 12:53 pm 
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26.6 into 26.8 needed a coke can to shim.

It would, I imagine, become less important as the diameter increaces, but I am no engineer.


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PostPosted: Sat Nov 17, 2007 1:04 pm 
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Location: No brakes? Way to commit soldier.
.3mm is actually a fair bit of difference. I would be pretty confident in saying that you won't squeeze a 30.9mm post into a 30.6mm tube

Having said that, I don't know what tolerance seatposts/tubes are made to but I would imagine its less than +/- .05mm? Rody would probably know.

Better to buy small and shim it up than buy big and not have it fit at all.

Sorry :(


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PostPosted: Sat Nov 17, 2007 1:12 pm 
BoTM Winner
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i think its recomended that its within .2


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PostPosted: Sat Nov 17, 2007 1:26 pm 
retrobike rider / Gold Trader
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It will never be 100% right if it's anything other than stated.
Remember that different metals and grades expand and contract at different rates and temperatures. They can cause in extreme circumstances, unecessary stress.
An engineer will always tell you to fit the size it asks for, or you're looking for problems.


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PostPosted: Sat Nov 17, 2007 2:11 pm 
Old School Grand Master
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Location: Tredavoe, Cornwall
I would never put the wrong size post in.

Its not a clever thing to do, well not on a well built frame anyway as they are usually spot on and either forcing it or over tightening the clamp can do some bad things.

Although i once came across a numpty who had beat an oversize pin into a Merlin ti. Coz his mate said it would be ok!

The frame had to be heated to remove the pin. Whats the saying some people dont deserve nice things.

Cheers,Al


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PostPosted: Sat Nov 17, 2007 2:37 pm 
BoTM Winner
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true , because you wont fit a 27.2 in a 27.0

without said hammer :shock:


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PostPosted: Sat Nov 17, 2007 2:57 pm 
retrobike rider / Gold Trader
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Got a 27.2 in my kona which is a 27.0. Admittedly its a bit tight but it goes in by hand!


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PostPosted: Sat Nov 17, 2007 3:13 pm 
BoTM Winner
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Kinda like time spent with the woman you love...

you can always shim up a small post to fit but try and put one in that's too big, and you'll stretch it out forever.

And that's no good for nobody :wink:

cheers,

rody

Seriously though, I would not try to put a post that is larger than the inner diameter of the seat tube. Material will stretch to accomodate, however, it does weaken the frame.

When shimming a smaller post to fit a larger seat tube, use a shim that is long enough to extend below the intersection of the top tube/seat tube. This will help distribute the stress (think long lever) to the strongest portion of the seat tube and not bend/break the st extension. Be sure to lube both the shim and the post, as pressure and time will cause corrosion and make removal a painful process.


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