How precise are seat post measurements?

[allen-uk]

Advice sought on this problem.

a) I have a device (needed for a disability) called a Gravitydropper. It fits in place of the seatpost of your bike and controls the saddle position, on the move. The manufacturers say it is made to very fine limits, and offer it for a variety of seatposts, for example 26.5, 26.8, 27.0. 27.2, 28.6, and so on.

I want a different bike. The new bike has a 26.5 seat tube, but the RETAILER says that this means in reality that you could work plus or minus 1.5mm either way, and that my precisely engineered 27.2 seat post would fit into his 26.5 tube without any problems.

c) The device is $300+, so buying a new one is an option, but obviously I'd rather not.

d) The Gravitydropper is clamped just as tight as a normal seat post (as it takes its place supporting the saddle), so logically nothing INSIDE the device is going to be 'squeezed'.

Opinions on this sort of precision, please. If it was ten quid I'd say sod it and give it a go!


Allen.

 

[Dan B]

I think your retailer is stretching the truth further than you will be able to stretch your seat tube. Certainly in my experience the 27.0mm seatpost was too small to stay clamped inside the 27.2mm I moved it to, and 0.2mm is a lot less than 1.5

 

[Landslide]

The new bike has a 26.5 seat tube, but the RETAILER says that this means in reality that you could work plus or minus 1.5mm either way, and that my precisely engineered 27.2 seat post would fit into his 26.5 tube without any problems.

Seriously? Name + shame!!!

 

[4F]

I agree with Coruskate, there is no way that a 27.2 will fit in a 26.5

 

[Hilldodger]

You can get away with making a small shim - not ideal but it will save you buying another one.

And whoever the retailer is - avoid him in future.

 

[Dan B]

Don't think a shim is going to help if the post is fatter than the tube it's supposed to go into ...

 

[Landslide]

You can get away with making a small shim...

Not for a seatpost with a larger external diameter than the internal diameter of the seattube you can't.

 

[4F]

You can get away with making a small shim - not ideal but it will save you buying another one.

And whoever the retailer is - avoid him in future.

A shim wont work that way though. The op's seat post is 27.2 and the bike is 26.5

 

[Hilldodger]

Sorry, didn't read the thread properly

Of course, at Cyclemagic we make these magic shims that reduce the size of the tube.

 

[allen-uk]

I don't think I'll name and shame, as in fact he really has been helpful, trying to do me favours (and of course sell his bike). The manufacturer in Montana has offered to machine a one-off to 26.5 (full price, but pretty good offer), which is worth a nod.


Allen.

 

[Rob3rt]

Im confused, your seat post is 27.2 (but manufacturer makes them in 26.5 as in the list of sizes you state) and the bike has a seat tube of 26.5.

What manufacturer is machining something to fit here (excuse me if im just being dumb and missing something), the seat post manufacturer or the bike manufacturer?

If the bike manufacturer is saying they will bore the bikes seat tube to fit your seat post, I wouldnt touch it.

If the seat post manufacturer is saying they will make you one in 26.5mm, isnt that already on the list you said that they make, why do they need to machine you a one off at premium cost?



All in all, dont ever try getting a larger diameter than specified seat post into a seat tube, else you risk either damaging the frame, or not being able to get it back out.

 

[allen-uk]

Sorry, Robert. Clarifications:

My current bike has the 27.2 seat tube, and the 27.2 Gravitydropper.

The US manufacturer of Gravitydroppers is willing to machine a one-off 26.5 unit, just for me, at his usual price for his 'standard' units.

The bike that I want has a 26.5 seat tube, so I COULD get one manufactured at around $300, but in order to save that money the bike retailer is telling me that I don't NEED a new one, and that my existing 27.2 unit will fit nicely into his 26.5 seat tube.

It did sound a bit dodgy, and the answers (including yours) have convinced me to pursue a different route!

Hope this clears things up.


Allen.

 

[Rob3rt]

I understand now.

The bike shop is definatelly either telling you fibs to sell you a bike or genuinelly dont have a clue, either way they should be avoided.

If you try putting your current seat post in the seat tube it will either not go at all and require some brute force. This will deform the seat post hence having a bulge as deep as the seat posts goes in, or.... the seat post will crack and the frame will be buggered.

Even if you could get it in using some brute force, you would have major issues trying to adjust the height or get the seat post back out.

 

[MartinC]

Just to re-iterate what the other guys have said - don't try and put the 27.2 seat post in a 26.5 seat tube - it won't work and forcing it will damage things.

26.5 is a strange size. I've seen 26.4 before. Normally frame manufacturers ream their seat tubes to fit the more common seat post sizes (27.2 and 31.6 now). How big the seat tube can be reamed depends on the wall thickness of the tube - you have to leave enough metal to avoid compromising the strength of the tube. It's possible that you could get the seat tube reamed out to 27.2 if the manufacturer says it's OK (unlikely) and you can find someone with the skills and an adjustable reamer. Or you could get a different bike - 27.2 is a very common size.

 

[allen-uk]

Thanks Rob3ert, Martin.

It is uncommon (an electric bike), and there are quite a few better-known brands which already have 27.2 tubes. I was just pursuing a possible option, but I am now convinced otherwise.

A.