# What seat tube for 30.1 seat post



## MDEnvEngr (Mar 11, 2004)

I have someone that want to use the fox dropper post. They come in two sizes. I think 30.1 and 31.2 maybe? What tube are you guys using for these big posts?

Thanks. Have a good weekend! B


----------



## pvd (Jan 4, 2006)

What are you taking about? They come in the two standard diameters, 30.9 & 31.6. BUT only a fool would build to 31.6. 30.9 is the only size to target.

D.O.S.S. Seatpost Bike Shock | FOX


----------



## TacoMan (Apr 18, 2007)

I only use the* very* common seat post size of 31.6, anything smaller is just too wimpy when running a 400mm+ post length. I use a 1.375" seat tube with a custom machined sleeve on the end about 2.75" long. Works great. After bending a Thomson seat post on the 1st ride, I made my own steel post and the flex is perfect for absorbing ruts and unlike the aluminum post, it springs right back into shape.


----------



## Walt (Jan 23, 2004)

For 31.6 I just use an 1 3/8 x .065 plug turned down to press fit/fuse into whatever 34.9 seat tube I want to use. 

Reynolds makes a 30.9 seat tube if you want to go that route. There isn't a super convenient size of 4130 so without a decent amount of lathe work the plug system doesn't work. I have been bugging True Temper to make one as well but they are not really in new-product mode lately.

You could also sleeve some 31.7mm butted tube with a .4mm thin section, probably, and count on getting a little distortion to help hold things together (lots of those .4mm sections are thicker than spec which would help). Probably not worth doing for most bikes that would use a dropper, though.

Edit: Just noticed that Paragon makes a plug/sleeve (in ti) for 30.9. I imagine they will do a steel version soon as well. 

-Walt


----------



## pvd (Jan 4, 2006)

30.9mm is far more common than 31.6.


----------



## todwil (Feb 1, 2007)

*31.6*

MDE I would go with 31.6 a lot more seat post options. Walt described
the best way to do it. I do mine the same .065 down to .035.

PVD all I can say is.......stop self medicating and be a little nicer to
Folks!!:thumbsup:

Todd


----------



## MDEnvEngr (Mar 11, 2004)

Thanks all! I knew it was trouble posting without rechecking the post sizes! I think PVD let me go pretty easily! But I was lazy...

I see that there is no easy off the shelf solution. No problem. 

Anyway, take it easy. B


----------



## 18bikes (Jan 15, 2007)

Just incase no-one else has noticed, reynolds now have 631 and 853 seat tubes to take a 31.6 post in various lengths. Combine this with the preexisting 30.9 tube (which only allows around 180-190mm of insertion with a 30.9 post but will take a smaller post in a shim full depth) and you have loads of choice without needing to have a lathe to make special sleeves

Matt


----------



## Walt (Jan 23, 2004)

Now they just need a competent US distributor... gawd I hate Fairing.

-Walt


----------



## 18bikes (Jan 15, 2007)

Have you tried talking to Reynolds over here? If your order is big enough and you have good enough reasons to not use Fairing then they may process it, don't see what you lose by asking

I find dealing with Reynolds themselves to be very easy and once you are on the lise you get info about news stuff automatically emailed through.

I've just built a frame with the shortest of the 853 31.6 tubes (I can get part numbers for all of them if people want) and it was a big improvement over the 30.9 tubes. This frame would have left virtually none of the bb butt on the 30.9 tube so having a choice of lengths is great but the main thing is that the new tube was much straighter. I think that because it is swaged down rather than made bigger with a butting bar the end result is a straighter tube. A couple of the 30.9 tubes I've got will only get used on bikes for myself. You still end up with a weird seat clamp size but 2 minutes work on the lathe and a bit of silver and you're back up to a 34.9 clamp. Not the best pic but you can see it on this frame


Finished Frame (Side View) by 18bikes, on Flickr

Matt


----------



## 18bikes (Jan 15, 2007)

Just realised I have this pic which shows the seat tube top better


Seatclamp sleeve slotted and shaped by 18bikes, on Flickr


----------



## wort (Jan 9, 2013)

18Bikes, 

Is that frame built for a Pinion gearbox? That is sweet.


----------



## jay_ntwr (Feb 15, 2008)

wort said:


> 18Bikes,
> 
> Is that frame built for a Pinion gearbox? That is sweet.


Yeah, I was wondering the same thing. Bonded aluminum to steel huh? Pretty cool--would love to see a thread on that one.


----------



## 18bikes (Jan 15, 2007)

Yes it is for a Pinion gearbox, there is some more info here: http://forums.mtbr.com/frame-building/welding-aluminium-821279.html
if you want me to do a separate, proper thread about it then just nag me and I will

Matt


----------



## jay_ntwr (Feb 15, 2008)

18bikes said:


> Yes it is for a Pinion gearbox, there is some more info here: http://forums.mtbr.com/frame-building/welding-aluminium-821279.html
> if you want me to do a separate, proper thread about it then just nag me and I will
> 
> Matt


Hey, there is the thread! I stopped following that because it was all about aluminum and never saw the Pinon stuff there. Very cool. I've been following them for a while and it looks like a cool system. I can't wait to see how that bike turns out. Be sure to post some pictures either there (or in another post at least). Looking forward to hearing how it rides.


----------



## wort (Jan 9, 2013)

Thanks for the link. This is a pretty unique build -the steel/aluminum bonding reminds me of the Raleigh Technium bikes of the 80's (except the other way around; Techniums had Al tubes and steel lugs). I can't wait to hear your ride report - I've been looking at Pinion for a while and think that the Pinion would be great for a trail/enduro/freeride bike.


----------



## Feldybikes (Feb 17, 2004)

If you're foolish enough to make a bike for a 31.6mm post, it seems True Temper makes one now. Last time I looked, Henry James suggested it was for a 32.4mm post, but I think they have it upside down. Something like 34.9x1.6,0.8 single butt.


----------



## TacoMan (Apr 18, 2007)

When you build frames with 1.75 DT and 1.5 TT, anything smaller than a 1.375 ST is foolish. 

Pound for pound, a large diameter tube is stiffer than a small tube, so for equal stiffness, you can build a lighter frame with larger dia. thinner wall tubes. Using thick small tubes is easy, anyone can do it, tube fit can be sloppy, welding can be sloppy, heat input can be out of control etc... 

Frames with a short ST and long seat posts would require a heavy small diameter seat post. The seat post is the one tube on a bike that can not be triangulated, so it must be large in diameter if you want it to be light weight and stiff.

Also, plenty of aluminum frames use 31.6 seat posts, which require large dia. tubes for stiffness, otherwise the frames flex and crack.


----------



## Feldybikes (Feb 17, 2004)

<<<Since a prior post to which I was referring was edited,>>>[Ignore that]* I feel compelled to clarify that the "foolish" comment was being sarcastic and I don't see anything wrong with making a post in that size. I will say however that I find it curious that many manufacfturer's posts go up in weight with increasing diameter whereas some drop. Since Thomson is in the latter category, I'd tend to believe this implies that some companies do more engineering than others.

*I mixed something up when using my phone to write the above message. The perfect storm of a small screen and general idiocy


----------

