Some years ago I scratched a tiny horizontal line on my Brompton seat post so I could find the right saddle height easily. I figured since it is steel there would be no problem. Riding home yesterday I felt the saddle slipping away beneath me and discovered the post had broken (torn) perfectly evenly. The steel seat post walls are very thin and I suppose over the years that weakness just allowed the metal to fail. Of course, I can only blame myself for this and a new post is already ordered and on its way. Still, if anyone else is tempted to mark their post with anything that affects the metal it would probably be best to avoid making my mistake..

That scratch is what is referred to as a "stress riser". Basically, it is a region where normal stresses on the tube act at a weak point and eventually cause failure. A gold or silver sharpie pen work well on a black seatpost to mark your preferred position.

It will be sharpies henceforth.

Yes, learned this the hard way on a different bike. Now I use a Sharpie.

The very bottom of the seat tube is flared slightly which stops a theif from just pulling your seat/tube out of the quick release. Gorilla tape a ring at the appropriate spot above that flare, and it will stop your seat at the correct height every time. This is basically what the 'Brompton Seat Height Insert' accessory does.

Great idea. I will do it.

The Kamoya Seat Post Keeper also assures that the seat is oriented properly. The photos there can be used as a guidance how to make one oneself.

If you are curious, I know a forensic metallurgist who could tell us why it failed.

Paul, I think I know why the post failed, it was the idiot who filed the little mark. What I found surprising was it tore at the same line all the way around. Couldn't have been much neater if I had tried to do that. I had changed the seat position on the Pentaclip saddle holder so the mark was about an inch above the frame, which makes that even tearing a little more surprising. I also think, given the way the post stresses that had I made a mark on the side or back it may have gone longer without failing or maybe not failed at all.

That is a very elegant solution and it is unfortunate it is hidden in the frame since it looks so good. I think a little duct tape or gorilla tape would work very well. Does the little finger protrusion go into to seat post slot as well to keep the saddle straight ahead as well?

I made one of those from aluminium tube which I had lying around.

Yes indeed.

In making a keeper yourself it gets tricky to make the sheet metal stick to the seatpost tube close enough so that it slides fine into the interior of the seat tube extension. Using a piece of another tube to make the keeper should indeed help in getting the keeper stick close to seatpost surface.

Hard to believe a scratch can lead to complete structural damage. Is it High-Ten steel like the rest of the bike?

FWIW, the Brompton saddle height insert is $9.99 (USD) and installation instructions are at:




-HANK RYAN-
Norman, Oklahoma USA

I made one of those from aluminium tube which I had lying around.
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Do you have a picture?

Hard to believe a scratch can lead to complete structural damage. Is it High-Ten steel like the rest of the bike?

I think it must be like an eggshell....if intact it is fine but a weak spot will grow over time. As noted before, I had the mark on the front which is the worst place, I think. DK what kind of steel, but it seems very thin, much thinner than other steel posts of this type which i have accumulated over the years and those are much shorter.

At those prices and weight, kinda inexcusable.