A difference of millimeters?

August 11, 2012 9:53 PM

Idiot-on-a-bicycle-filter: Seatpost edition.

A few months ago I bought a used beach cruiser bicycle from a local bike co-op. I have run into some problems while trying to replace the seat and I could use some advice.

My seat post says 25.0 on it; the internet informs me that a "standard" seat post size is 27.2. The seat post I have looks exactly like this, designed for a cruiser style seat that attaches with a bolt. This is all well and good except that I don't know how to find a seat that will fit this size, as most cruiser seats I've been looking at on Amazon don't have sizes listed (because it doesn't matter? Does it?)

This all started because the seat I bought from the hardware store a few weeks ago will not securely attach to the post no matter how much tightening I do. I realize this might be a problem with a poorly-designed seat but I am really not sure.

So my question is this: would a higher quality seat tighten down to 25mm? Do I need a different style of seatpost/seat that doesn't attach with a bolt?

The old seat (very rusty and squeaky) fit just fine but unfortunately I am no longer in possession of it.

posted by janepanic to health & fitness (13 answers total)

Seat posts come in different sizes to fit differently sized seat tubes. The spacing of the rails on bicycle seats is standard and is not specified. Newer seat posts have an integrated clamp; you have an older style that has a separate clamp. Nevertheless, the clamp should fit any standard bicycle seat.

Maybe you bought a seat that was manufactured to a different standard, or you have an old bike that predates standardization. France, for one example, had their own standards until relatively recently (although I don't know about their seat rail sizes) and there are still some markets that exist outside of the ISO. How do the rails of your new seat compare with the seat that came with your bike? If your seat post clamp is non-standard, a new seat post isn't that expensive.
posted by hydrophonic at 10:50 PM on August 11, 2012

This all started because the seat I bought from the hardware store

Go to a local bike shop. They'll sort you out. The folk at the bike coop should be able to help as well.
posted by sebastienbailard at 10:54 PM on August 11, 2012 [1 favorite]

Hydrophonic has it exactly right. But I wouldn't change the seatpost. The cheapest and most effective way of solving this is probably going to be simply to put a "shim" inside the seat bracket. A shim is just a very thin sheet of metal wrapped around the seat post to increase its diameter sufficiently for the seat bracket to clamp to it strongly.

If you feel so inclined and you have a suitable pair of snippers, you could make your own by cutting one out of a soft drink can. You could try and buy one, though getting the size right is probably easier done by eye than by measurement, and if you've got some antique or at least non-standard parts here, a home-made shim may actually be more successful than a bought one. If you're not confident, just ask your friendly local bike store to to it for you - should be a five minute job!
posted by genesta at 3:21 AM on August 12, 2012

you have an older style that has a separate clamp

I'm confused about all this talk of clamps. I have one of these that works just fine to secure the seat post inside the seat tube. That's not the problem. The problem is that the bike seat I have (which came with its own hardware) seems to be too large for the tapered bit at the top of the seat tube.

What I don't have is a separate part that looks like this (was I supposed to remove it from the old seat? Uh, whoops).
posted by janepanic at 4:43 AM on August 12, 2012

Yeah, you were supposed to remove that seat clamp from the old seat or seat post. Not a big deal, you just need another clamp now.
posted by spikeleemajortomdickandharryconnickjrmints at 4:55 AM on August 12, 2012 [1 favorite]

the seat ... will not securely attach to the post no matter how much tightening I do

Okay... I'm not clear what you were tightening if you didn't have a clamp...

If you haven't got a clamp, you need a clamp. If you have a clamp but you can't tighten it enough, you need a shim.
posted by genesta at 6:09 AM on August 12, 2012 [1 favorite]

If you need to buy a clamp, I'd spend the few extra dollars for a new-style seat post instead, because the integrated clamps are so much easier to deal with.
posted by hydrophonic at 9:58 AM on August 12, 2012

Take a picture of your situation and circle or draw an arrow to the part that won't tighten down enough.
posted by RustyBrooks at 11:28 AM on August 12, 2012

I'm sorry. I didn't mean to make this so confusing for everyone. The new seat came with a clamp. That is the part that I cannot seem to tighten enough. I'll post pictures when the camera battery is charged...
posted by janepanic at 4:59 PM on August 12, 2012

Okay, here is a photograph of the nut that won't get tight enough (I've done a good job of scratching the paint off, clearly). There is an identical nut on the other side. I try to tighten them both evenly but it doesn't seem to make much of a difference.
posted by janepanic at 8:30 PM on August 12, 2012

OK the picture helps (good suggestion RB!).

JP, you've got all the parts that you need. You are absolutely doing the right thing in trying to tighten those nuts to secure the bracket onto the seat post. Clearly, the bracket on the seat that you've bought is not quite a match for your seatpost, but this is fixable!

How you proceed from here is largely up to you. I get the impression that you're keen to fix this yourself at minimum cost if you can and I'm absolutely with you on that. In the UK, a fix of this nature is known as a "bodge". There is great satisfaction to be derived from effective bodging and this is not, as you might think, an insult, but rather a term of praise (unless the environment is a nuclear power station). But if none of the bodging ideas below appeal, then a new saddle and/or new seatpost remains an option.

Firstly, Have you tried simply flipping the seatpost upside down? I'm guessing you're trying to attach the seat to the tapered end of the post (as intended, of course), but there's no reason at all why you shouldn't attach it to the other end if it fits well - you won't compromise safety at all. It could be that simple!

If that doesn't work, proceed to stage 2 by first answering the following question. How loose is the seat when you tighten those nuts fully? Is it still "rattle aound and fall off" loose, or does it grip, but just not tightly enough to be secure?

If it's "fall off" loose, you're going to need a fairly thick shim (see above for definition), probably made from nylon, and probably designed for another purpose but suitable to solve your problem. Your local bike shop is your source for this. Take the seatpost and saddle in with you. It's a good bet that they will have something suitable in one of their "bits that we don't need now but will find a use for someday" bins. Yours for a few cents.

If it's a tight fit but just not tight enough, you could make your own shim. The traditional material is aluminium cut from a soft drinks can. You wrap this around the seatpost once or twice before you put the bracket over it, and then tighten those bolts up onto it. Job done.

Cutting soft drink cans into shims is not difficult and doesn't require any specialised equipment, though it won't do your kitchen scissors any good. But if this is likely to turn your kitchen into a set from a horror movie, then it's probably not a skill that you're going to be interested in acquiring. Ask a friend for help, or revert, again, to your local bike shop.

Good luck, and happy bodging!
posted by genesta at 11:24 AM on August 13, 2012 [1 favorite]

One thing to remember when tightening that style of clamp is that you need to tighten both nuts equally. A little on the right side, a little on the left, a little more on the right side, a little more on the left.

Personally I don't like shims, but I see bikes in my shop all the time with shims, and they usually work just fine. It's better to get a clamp that's meant to work for your seat post, but hey, it's a used beach cruiser. Try a shim first.
posted by spikeleemajortomdickandharryconnickjrmints at 9:42 AM on August 14, 2012

Thank you all for your advice so far; I've learned a lot.

I went to the local bike shop today. The guy there said that the rails are too wide and that issue is preventing me from being able to tighten the nut, not the clamp itself. He didn't have any cruiser-style seats that were less than $200, so he suggested I go back to the hardware store to see if they could swap it out for another one. I think I will try shimming it first, though.
posted by janepanic at 12:22 PM on August 14, 2012