seat post fit?

I'm rebuilding an 80s step through Orbit Gold Medal as an upright city bike. It's a 531c frame.

The seat post that it came with fits fine in the tube when raised quite high, but if I put it as low as I need it I can't tighten the seat tube binder enough to hold it. It spins quite freely from side to side.

This is the seat post - I think the fluted area is a little narrower than the lower part. The unfluted part of the pots is a very tight fit, in fact even with the binder bolt completely removed I scraped off a load of metal from the post getting it out.

This is the seat tube binder. The slot is about 5mm but the binder lugs have these points that mean it will only tighten by < 1mm, which isn't enough.

What's the best fix here? File down the pointy bits so I can tighten the seat tube a little more, or buy a new seat post? (and if so, what size?)

Personally - I would always go new seat post. Do you have callipers to measure the seat tube or your current seat post?

callipers - no, but I can probably get some tomorrow.

It just seems odd that all the force holding the seat post should be concentrated on two tiny points of metal. Seems like they would wear down all the time. All the photos I can find online have the same gap between the lugs as the seat tube itself.

It shouldn't be just those two points though. Once you tighten the bolt it should shrink the overall diameter to grip the entire seatpost. Like a hug.*

*paging chainbraker

It could be that a post that's too small has been fitted and the clamp had been squashed a bit. easy enough to remedy but probably best to get a set of calipers to measure it first.

I have an Orbit America in 531c and it has always troubled me that it took a 26.4mm seatpost.

I stripped it down at the end of the Summer and was cleaning out the seat tube and found a lip of 1mm or so around the inside of the top of the tube: ie only that lip was 26.4mm and the remainder of the tube was wider (presumably 27.2mm).

I mentioned this at my LBS. Apparently when Orbit first started making bikes they were rather good, but quickly went downhill (something to do with bringing in some below par frame builders) and he remembered having to re-build several of them from scratch.

His best guess is that the seat tube was fitted upside down, so that the butt is at the top!

NB Although Orbit are still making bikes today, the people involved are not the same as BITD, so there is no reason to doubt what they make now.

However, it is interesting that they were quick to quote for a respray of the trademark Orbit paintjob, but there was a deathly silence when I queried the seat tube...

/CSB

Seat post should be 27.2mm, but don't assume anything with an old Orbit.

His best guess is that the seat tube was fitted upside down, so that the butt is at the top!

Isn't the butt supposed to be at the top of a seat tube? (sorry, awful dad joke)

anyway, on to the fettling.I now have callipers and I'm getting

• 26.5 as the inside diameter of the seat tube
  
• a range of 26.4-26.7 as the diameter of the main body of the seat post
  
• a range of 26.25 - 26.4 around the fluted area, varying wildly with the flutes.
  
  
  

all these are approx as the tubes are worn/scratched in places

It makes sense that the seatpost body is actually wider than the seat tube in places, because when I took the seat post out it had to be zig zagged hard out of the tube and scraped a noticeable amount of metal off the post.

I can't find a lip like Scilly's bike had so perhaps these old 531c Orbits really are 26.4. It's certainly not 27.7 unless the Maplin callipers are wildly out of whack, and we got similar measurements using grandad's old brass micrometer screw gauge.

Right now I'm leaning towards

a) it's supposed to be a 26.4 seatpost
b) the existing seatpost is either the right size or a 26.6 which has been forced in
c) if I can find a position where the existing post is ok, I'm probably best off using that while I look for a second hand 26.4
d) there's a good chance I'm not going to ride this bike to work on monday after all :(

Isn't the butt supposed to be at the top of a seat tube? (sorry, awful dad joke)
Well played, well played!

Just seen your post on Bec's sales thread: you have the same model? Doesn't look like a 531c decal on hers, but can't quite read it: is yours defo 531 competition?

If it's regular 531 then the seat tube could well be 26.4mm if it has a butt at each end: as demonstrated by my upside-down tube, that's what the butt measures.

Do you need the post right down to the fluted area? As you've found, the flutes are effectively flats, so reduce the diameter: what you need is a completely round post.

This System Ex seatpost is a rebranded Kalloy SP-248, which comes with the MDCC_Tester seal of approval and at £15 won't break the bank.

it's definitely a 531c *sticker *at least

Mine's very similar to Bec's but the decals are gold not red. But you are right, hers has the 531 sticker. Hmmmm.

I'm 5'1" and it's a 19" frame so the seatpost has to be pretty low. I'll see if I can ride it with the fluted area fully above the clamp.

Also I've just been down to Kinoko cycles who very nicely let me measure a few known seatposts and realised I'm measuring low consistently... the 26.4 came up as 26.3, the 26.8 as 26.7. So, given that I measure my seat tube as 26.5 it's probably a 26.6 I need after all.

Kinoko suggested I bring the bike in so they can measure it, but I need to get it reasonably rideable to do that. They are round the corner from work...but it's a very long corner featuring some of London's most exciting congestion and not one I want to negotiate with a randomly spinning saddle. It adds a certain thrill to the steering when the handlebars go one way and my hips go the other...

Cann't you just make a gromit out of a beer can? Just cut a beer can down and rap it round the seat post it will be hidden away and hold fast. You need tin snips but could us scissors as the aluminum the cans are made from is very thin

I rode from east london to kinoko with a step through mixte frame like yours on my back! Can you wheel it or carry it in?

I think I've got the seat post stable enough to get it to Kinoko - still turns a bit by hand but seemed ok on a quick test ride. I'll give it some welly tomorrow and see if it holds. It's quite a nice saddle position actually - i'm used to riding slightly too big frames so it's no more than that. But one of the reasons for getting the 19" was I was quite looking forward to not having to jump quite so far off the saddle at the lights - there's lot of lights out there.

I can do the beercan shim, yes...but I don't really like bodging until I understand the problem. Once I get what's supposed to happen, then I can make a call on whether to fix it or bodge it :) With this one, I want to work out if I've got a seatpost that's too big, a seattube that's been overtightened, or both. I'm a bit worried that if the tube's been distorted by overtightening it'll never get a good hold on the post and I'll end up cracking up the tube.

buy a new seatpost :)

So, this is how the scenario played out.

I took the bike to Kinoko and the problem is that the seat tube has been overtightened to fit a too-small seatpost. It's now reamed back to circular and comfortably takes a 27.2 seatpost.

<>

Awesome. I love a happy ending. *sniff!