Can anyone suggest a solution to this problem? I have tried the following in the last couple of days: tightening the seatpost nut (a little), cleaning both the seatpost and the plastic bushing with isopropyl, partial wrapping of the seatpost with packing tape where it contacts the plastic bushing.

Seatpost still slips... note that I weigh 72 kgs/158 lbs. The bike is approximately 2 weeks old... is there a break-in period for the seatpost? Should I get a replacement plastic bushing?

The perennial folding bike issue. It happened with my Dahons until I greased the contact area between shim and frame. I assumed that Bromptons wouldn't suffer from this problem as they're talked about as though they're engineering marvels, hand-built to perfection.

Nope, same problem occurs and there's some sort of crappy plastic shim that's glued to the frame, so no opportunity to grease between the two for my go-to Dahon fix. Looking at the contact area between seat post and shim when clamped there's a clear gap in certain areas, i.e. the shim doesn't contact the post around it's entire circumference (as would happen if the shim had some freedom to move independently from the frame, and to slide as the clamp were shut). The Dahon design is superior.

Same problem with my Downtube Mini. I bought a rear rack that has a quick release and clamps to the seatpost. The two clamps solve the problem. Next time I order online I will try to order a seat post clamp that will clamp around the seat post so I don't have to be dependent on the rear rack.

Another second clamp solution, I got a 32 mm QR frame-seat post clamp band,

At my LBS, to answer question below.

took off the lip from around the the inside edge, that is there
to shoulder on top of seat tubes.
so It would Just slip over the seat post, then set it upside down on top of the frame,
top is square , as is the top edge of the frame's bushing lined seat tube.
and cranked it's QR shut.. Metal to Metal around just the seat post.
and stacking on top of the top of the seat tube.

I think having a second clamp is the real solution. Fiestbob where did you get this QR clamp? I am thinking of temporarily getting a radiator hose clamp and use that in the meantime. I don't need to completely fold the bike at work anyways and it still fits in my cubicle with the seatpost sticking out. I will need a second QR clamp in the long term for when I need to use my bike for multi-modal commuting.

The seatpost clamp is a quick release. I would tighten the nut until you feel significant resistance when actuating the QR clamp lever. Don't worry, the clamp, seatpost and frame are all steel.

ok
before you make the seatpost permantly non foldable ( adding a clamp on top )
clean with regular soapy water
THAN grease the contact area of the quick release.. the cam in other words... NOTHING on the seatpost or near it..lol
After that turn the adjustment knob a little tighter and clamp that sucker down

it will hold
thor

Absolutely no problems with slippage of my seat-post on my S-Type Titanium Brompton I wonder if that fact that my seat-post is made of Titanium has anything to do with it?

From what I have researched, it appears aluminum, titanium and carbon seatposts are less likely to slip than the glossy metal ones.

Mine is tight in the middle and loose at the usable extension.. ,
and so was the extended new one.

so not a bent post issue..

How easy is it to replace the shim/remove the original? How firmly is it stuck in place? I'm thinking it'd might be better to glue a new shim in only on something like a quarter section of it's circumference, allowing some movement between shim and frame as the clamp is shut.

Perhaps rather than gluing a new shim in it'd be better to use a narrow strip of double-sided adhesive tape, and to put a tiny bit of grease on the rest. That way it could be easily removed if necessary. Of course you have to make sure the grease won't migrate to any area that the seat post contacts.

Saw this:
https://www.flickr.com/photos/door2su...8115211/page2/

I guess the brompton shim is 34.9 OD?

OMG! why would anyone want to put a Dahon seat post on a Brompton...... that would be like putting Skoda seats into a Rolls Royce

I think that shim on the photo is a good idea... metal to metal probably works better than plastic to metal.

I'd have a nice Dahon SDG or Kore I-beam seat post any day, rather than this crappy lump of steel.

I see there are two types of shim for frames from different periods:



One's got an inny belly button and the other an outy. . . so it looks as though there's some protrusion that stops the shim from sliding out of the frame whether it's glued or not. Perhaps it'd be worth trying to install one without any glue at all.

now carefull boys and girls .... no heckling ..mine is better than yours....
of course I do chuckle as well if I hear " glossy" metal .... lol

Thor

exactly the same and I am 210lbs.... Likewise, never an issue on my speed TR. I would go with Thor's advice, rather than immediately think it is an inevitable problem of folding bikes.

does brompton has a plastic shim ? then the question is - why would a rolls royce put a plactic shim on their car...

I had the SDG / Kore I-beam on that Dahon MU SL and I must admit it wasn’t too bad.... not as comfortable as my TITANIUM railed Brooks Swift sat on top of my Brompton TITANIUM seat post though

Ahhh but it's not just plastic, its Bromptons own Special plastic..... I'm sure that Rolls also use plastic but probably call it by some other fancy name

I actually cleaned and oiled my seat-post clamp the other week (I don’t run with any mudguards ) and set it to be slacker than it was from the factory..... still no slippage whatsoever.... I weigh 170 lbs

Here's one reason..

Yeah, but I got an SDG seat post off ebay for a tenner and it weighs less than 400g.