Kona Explosif Titanium Seat post

[TD75]

I'm currently building up my Kona Explosif Titanium. The seat post size is 31.6, but it's really loose. Before tightening the seat clamp up it will literally drop straight into the frame. Worried about over tightening the seat post clamp and causing cracks etc

Is there any nicer way of shimming a seat post other than with a coke can? Or which 31.6 seat posts come up on the larger diameter size?

 

[bikeworkshop]

A very slightly loose post is a common cause of seat tube fatigue - often the top section will crack along the toptube weld.
Ti is particularly prone to this.

Inch- and -a quarter is 31.75mm
- this is probably the internal tube diameter - many tubes are still supplied in inch sizes, even now.


You could:
A)try a selection of 31.6 and 31.8 posts to find a snug fit.

B)Buy a 31.8 post and sand it down until you get a nice fit

C)Ream the frame to 31.8 - many manufacturers do this, but I'd be wary of taking metal off this crucial location.

D) Shim the tube down to a 27.2 post - id use a plastic USE shim because i think they spread the loads better.
My vitamin t hasn't broken...yet, and is very well used. It's 28.6 down to a USE 25.4 post

You haven't got enough space to shim with your current post.

 

[mk one]

Were these frames shimmed from the factory? And what post are you using now, have you measured the post? Thomson are pretty good with size, though I have had those too being different size than stated, could be an expensive job finding a 31.6 post that fits.

 

[2manyoranges]

I'm currently building up my Kona Explosif Titanium. The seat post size is 31.6, but it's really loose. Before tightening the seat clamp up it will literally drop straight into the frame. Worried about over tightening the seat post clamp and causing cracks etc

Is there any nicer way of shimming a seat post other than with a coke can? Or which 31.6 seat posts come up on the larger diameter size?

Yes I had this on a ti Marin. In the end I solved it by using a USE post and played with USE shims until I got one which was snug. I did need to sand the (oversize) shim a bit though to get it just right. Where in the UK are you? I have many 31-6 seatposts and as other have said you should get some on loan from friends to check whether it's just the seatpost or an unfortunate co-incidence of slightly-smaller-then-should-be seatpost and slightly-larger-than-should-be seat tube....

 

[TD75]

Were these frames shimmed from the factory? And what post are you using now, have you measured the post? Thomson are pretty good with size, though I have had those too being different size than stated, could be an expensive job finding a 31.6 post that fits.

I think the steel version was shimmed on the full bike. I'm using a Kona seat post 31.6. I haven't measured it properly, I could do with getting some digital measuring calipers so I can get an accurate reading. Anybody recommend any, before I get a a pair from Screwfix etc.

Yes I had this on a ti Marin. In the end I solved it by using a USE post and played with USE shims until I got one which was snug. I did need to sand the (oversize) shim a bit though to get it just right. Where in the UK are you? I have many 31-6 seatposts and as other have said you should get some on loan from friends to check whether it's just the seatpost or an unfortunate co-incidence of slightly-smaller-then-should-be seatpost and slightly-larger-than-should-be seat tube....

I'm in Ipswich Suffolk, so I'm probably not close to anyone. I've got a 27.5 Thomson seat post so I'll order a Use shim and try that.

Is it normal for titanium frames to need a shim on the seat post so they fit properly? This is my first titanium frame so I've got no frame of reference.

 

[bikeworkshop]

The screwfix verniers will be more accurate than the seatpost manufacture or even the seat tube dimensions.

Framebuilders choices of tube for ti meant the I.d wasn't perfect for quality posts, and the flexibility of the material meant larger tube diameters, so shims solved the problem of getting a good fit for a quality post.

 

[2manyoranges]

The vernier will be very good for measuring the outside diameter of the seatpost and absolutely hopeless for measuring the inside of the seat tube. Seattubes can be temperamental at 0.05 difference and the width of the internal jaws on a vernier means that they just don't measure to that accuracy.

 

[2manyoranges]

The screwfix verniers will be more accurate than the seatpost manufacture or even the seat tube dimensions.

Framebuilders choices of tube for ti meant the I.d wasn't perfect for quality posts, and the flexibility of the material meant larger tube diameters, so shims solved the problem of getting a good fit for a quality post.

yep - and the shim needed to be long enough to go past the join of the top tube and the seat stays...shorter than that junction and it's a terrifying delayed-action disaster....

 

[bikeworkshop]

yep - and the shim needed to be long enough to go past the join of the top tube and the seat stays...shorter than that junction and it's a terrifying delayed-action disaster....

Well past that point.
100mm - is that even long enough on a kona?
I prefer the thermoplastic USE shims because I believe they squish a little and so spread the load more.

 

[TD75]

Just went out to shed for a quick measurement: