Seatpost Size Conundrum

[Reynard]

The steel seatpost on my Raleigh MTB build needs replacing. It's rusty, too short (I'm at the upper range of height for the frame size and would like more post in the seat tube) and I'd much rather prefer a layback one with a clamp that's adjusted using an allen key. On my brief test ride, I had the saddle as far back as it would go, and I just felt it needed a little bit more to make things truly comfy.

The post is stamped 25.4mm. It's been fitted in the seat tube using a shim - a proper one, not a DIY one.

I've been looking around, but there's not many options available for 25.4mm posts, especially at the budget end of the spectrum, and of the few that I can find, a lot are listed as currently out of stock. Now I don't mind the shim being there, but I'm wondering what the CC collective mind thinks:

a) Is it advisable to keep the shim and stick to a 25.4mm post?

b) If I remove the shim, what size post might I be able to fit?

It's not an urgent problem - probably the last one on the list of "things to do" but one I do need to think about.

 

[Venod]

Remove the shim and measure the ID of the tube with a digital vernier or similar tool, or measure the thickness of the shim multply the measurement by two and add it to 25.4mm, look for a seatpost at your new size.
A seatpost without the shim should be OK.

 

[Reynard]

Remove the shim and measure the ID of the tube with a digital vernier or simlar tool, or measure the thickness of the shim multply the measurement by two and add it to 25.4mm, look for a seatpost at your new size.
A seatpost without the shim should be OK.

OK, thanks

I've had a look at the shim - it's really thin (drinks can territory), so virtually no wiggle room there whatsoever.

I may get a 26mm post in there, but again, I may not. Will borrow a friend's callipers to check.

 

[Ming the Merciless]

Do you know what model it is and or year?

 

[Gunk]

The steel seatpost on my Raleigh MTB build needs replacing. It's rusty, too short (I'm at the upper range of height for the frame size and would like more post in the seat tube) and I'd much rather prefer a layback one with a clamp that's adjusted using an allen key. On my brief test ride, I had the saddle as far back as it would go, and I just felt it needed a little bit more to make things truly comfy.

The post is stamped 25.4mm. It's been fitted in the seat tube using a shim - a proper one, not a DIY one.

I've been looking around, but there's not many options available for 25.4mm posts, especially at the budget end of the spectrum, and of the few that I can find, a lot are listed as currently out of stock. Now I don't mind the shim being there, but I'm wondering what the CC collective mind thinks:

a) Is it advisable to keep the shim and stick to a 25.4mm post?

b) If I remove the shim, what size post might I be able to fit?

It's not an urgent problem - probably the last one on the list of "things to do" but one I do need to think about.

As I said on your other post, I may have one

 

[Reynard]

Do you know what model it is and or year?

Raleigh Max 15, frame number dates it to 1998.

It's a junior frame, btw.

 

[Ajax Bay]

I may get a 26mm post in there, but again, I may not. Will borrow a friend's callipers to check.

Don't faff around: measure it. Then buy the correct seat post (OD and length). Fit it. Sorted.
If the shim was made from a (Coke) can, its thickness would be about 0.15-0.12 mm. So the seat tube ID is more than 25.6mm and likely 25.8mm.

 

[Reynard]

Don't faff around: measure it. Then buy the correct seat post (OD and length). Fit it. Sorted.
If the shim was made from a (Coke) can, its thickness would be about 0.15-0.12 mm. So the seat tube ID is more than 25.6mm and likely 25.8mm.

Which is exactly why I said I'd measure it.

No, the shim isn't made from a coke can. It's a properly-made shim seemingly fitted from new given the scuffs etc on the post and inside of the seat tube. So it looks like it was a Raleigh parts bin special - cheaper to fit a shim and use up surplus 25.4mm posts than to fit the right size post in the first place...

 

[Reynard]

Well, the inside of the seat tube has been measured with digital vernier calipers. Came out at 26.03 mm average after taking multiple readings around the tube.

So it looks like either a 25.8 mm or a 26 mm post. Although the latter may well be a bit of a squeeze, as that's minimal clearance.

 

[Gunk]

Well, the inside of the seat tube has been measured with digital vernier calipers. Came out at 26.03 mm average after taking multiple readings around the tube.

So it looks like either a 25.8 mm or a 26 mm post. Although the latter may well be a bit of a squeeze, as that's minimal clearance.

It's more likely to be a 25.8mm due to the vintage

 

[Reynard]

It's more likely to be a 25.8mm due to the vintage

You're the expert.

25.8 it is, then...

 

[rogerzilla]

Measuring an ID with calipers rarely gives the right result. You have to use a seat tube gauge, basically a stepped tube.

Measuring the existing post and adding twice the thickness of the shim will give a more accurate result if you only have vernier calipers.

 

[Ajax Bay]

Measuring an ID with calipers will never give one a larger dimension than the ID - a 'diameter' init. Any chord (eg with calipers slightly off centre) is shorter.

 

[Reynard]

Well, with the best will in the world, the 25.8 mm seatpost I ordered won't fit.

I can't get it to go past the clamp, even with the bolt out, which makes me wonder whether that is bent. The clamp that is. With everything that was knackered on the bike components-wise, it wouldn't surprise me.

Any thoughts?

 

[Ajax Bay]

the inside of the seat tube has been measured with digital vernier calipers. Came out at 26.03 mm average after taking multiple readings around the tube.

What's the OD of the 25.8mm seatpost (this is not a trick question - measure it)? You have established by measurement that the "average" ID of your seat tube is 26.03mm. What was the range and was one of the ID measurements less than 25.8mm?
My sympathy: bl**dy annoying. Maybe establish where the obstruction is using the original 25.4mm tube (with the shim removed) and take a file to it.