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visdelou

Hi

I've just bought a 25.4mm seatpost for my Gitane Grand sport
see: http://www.gitaneusa.com/forum/viewtopic.php?t=796&highlight=

It's too large, is there a chance the bike could have a smaller seat post than 25.4? can you get smaller seatposts than that? there are no markings on the original post

i know i should probably have taken the bike into the shop, but been a bit strapped for time!

any advice cheers
James

PBR Streetgang

You can definitely get seatposts that are smaller than 25.4. Do you have a set of calipers to measure the diameter the seat tube? Another possibility: The top of the seat tube has been clamped too tightly, thereby shrinking the diameter of the seat tube opening.

visdelou

don't have calipers but will have to take it to the bike shop i guess- i tried sanding the seatpost down a bit, but to no avail

thanks for your advice

verktyg

I think that Gitane Grand Sport seat posts were 25mm not 25.4mm.

If I remember correctly the tubing wall thickness on Gitane Grand Sports was 1.5mm. Metric seat tubes are 28mm in diameter.

28mm minus 3mm (two times 1.5mm wall thickness) = 25mm.

The seatpost bolt hole "ears" on the seat lug frequently got deformed from over tightening especially from using an undersized seat post. Also, brazing the seat stays onto the seat lug creates a lot of excess heat and can cause the tube and lug to warp. They can be out of round and/or the tubing can be bulged inside the lug.

Because of these factors, it's difficult to get an accurate measurement of the seat lug hole with calipers. Some better bike shops have a graduated aluminum gaging tool for checking seat post diameters. It doesn't check roundness, just what size seatpost will fit in the lug.

Scroll down to the bottom message in this link:

http://www.gitaneusa.com/forum/viewtopic.php?t=765&highlight=ream

Sanding .4mm (.016") off of a seat post by hand it a LOT of work. I recently spend at least an hour removing .1mm (.004") from an alloy seat post! Shocked

One other point, seatposts are not always round or accurately sized. The old chrome plated steel seatpost were frequently oval shaped - possibly from over tightening.

Good luck,

Chas.

visdelou

Thanks Chas, as always you've got the good info!
I'll have to search out a 25 post i think
i'll keep the 25.4 - will come in handy when i finally find a nice 62cm gitane!

All the best

verktyg

Here's a set of Stein Seatpost Sizing Rods.

Most of the ones I've seen have all of the sizes on one rod.

As I mentioned, these don't measure roundness just what size seatpost fits the existing hole in the seat tube.

I recommend opening up the "ears" of the lug so that the slot at the rear is equal width. This will tend to round out the seat tube so that a larger size (or the correct size) seatpost will fit.

Here's a frame with the wrong size seatpost installed. The ears are almost touching.

The rear of the seat lug should look like this. Carefully spread the ears with a seat post installed to prevent damaging the tubing inside the lug.

I use a brake cylinder hone to smooth out the inside of the seat tube. I hone the inside of the steering tube too. This allows the seatpaost and stem to be easily inserted and adjusted. I recommend a good coat of grease inside the seat tube and steering tube, also on the seatpost and stem.

Here's a Lisle 10500 Brake Cylinder Hone that you can buy for $10-$15.

Here's another style of brush hone.

Chas.