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stahlmangc
 
Feeling metrically challenged here. I want to build a bike with a 1 1/8" seattube. What diameter seatpost would work? I've seen 28.6mm, 27.2mm

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s-lafleur
 
1 1/8= 28.6mm subtract 2x wall thickness from the tube diameter and voila, you have your seat tube diameter. So if your seat tube had .7mm walls you would need a 27.2 post.

stahlmangc
 
Thanks.

Nessism
 
1 1/8= 28.6mm subtract 2x wall thickness from the tube diameter and voila, you have your seat tube diameter. So if your seat tube had .7mm walls you would need a 27.2 post.

27.2mm is typically right but most seat tubes have 0.6mm walls. The extra 0.2mm is for clearance.

Peterpan1
 
Actually I think it's 26.8 if you are using straight wall chromo 4130, which is as likely as not what is involved in following the non-metric straight wall math route. but if you are ordering single butted seat posts it's 27.2, since that is the size they are built to, or perhaps even pop open to from the contouring. Actually I haven't ever ordered a straight wall seat post fron a bike supplier, used lots of 4130 though. The 27.2 won't have any slack and you may need to ream it or not depending on heat distortion and how it fit in the first place.

s-lafleur
 
27.2mm is typically right but most seat tubes have 0.6mm walls. The extra 0.2mm is for clearance.

But still the point remains that it is outer dia. minus 2x wall thickness and if he was using straight gauge .028 the wall thickness would be approx .7mm

Nessism
 
But still the point remains that it is outer dia. minus 2x wall thickness and if he was using straight gauge .028 the wall thickness would be approx .7mm

The most common seat tube dimensions for better steel frames is 1-1/8" diameter with .8/.6 or .9/.6 butts. The 0.6mm butted section is at the top of course. These seat tubes take a 27.2mm seat post.

The math: 28.6mm - (2 x 0.6) = 27.4mm The 27.2mm seat post comes in when you realize that without a little clearance the seat post will not slide in the frame. Also, the seat tube is not perfectly round and will distort when it's brazed/welded and need to be reamed. A 27.2mm seat post takes these real world details into consideration.

s-lafleur
 
The most common seat tube dimensions for better steel frames is 1-1/8" diameter with .8/.6 or .9/.6 butts. The 0.6mm butted section is at the top of course. These seat tubes take a 27.2mm seat post.

The math: 28.6mm - (2 x 0.6) = 27.4mm The 27.2mm seat post comes in when you realize that without a little clearance the seat post will not slide in the frame. Also, the seat tube is not perfectly round and will distort when it's brazed/welded and need to be reamed. A 27.2mm seat post takes these real world details into consideration.

Wow thanks for enlightening me. All this time I thought it was common practice to hammer the seatpost in. In fact I've even been telling my customers that because their seatpost was jammed in so tight they didn't need to use a binder bolt thereby saving them a few grams.:D

Nessism
 
Wow thanks for enlightening me. All this time I thought it was common practice to hammer the seatpost in. In fact I've even been telling my customers that because their seatpost was jammed in so tight they didn't need to use a binder bolt thereby saving them a few grams.:D

In your original post you referenced a 28.6mm seat tube with .7mm walls taking a 27.2mm seat post. Sorry if I sound negative but this is not correct. In that instance the proper seat post size would be 27.0mm.

My subsequent posts are for the collective here to make them aware that they need to include some clearance when performing their calculations. Sorry if you feel slighted by my clarifications. These posts go into the archives and some people perform searches so I like to see the proper information out there.

Good luck.

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