Seatpost - Seat Tube Clearance - How much is too much?
 

Hello,

This is with a new titanium frame with (I am assuming) an aluminum insert that was supposed to be honed to match the seatpost. Well, the builder (I think) got a bit overzealous with the hone. I have another titanium bike, a Litespeed, where the seatpost is tight enough to be held by friction which is sort of what I was expecting for this new frame.

Your thoughts?

Thanks,
Steve

How is the insert held in place? If it’s removable it can be replaced with a tighter fitting one. But more importantly, how loose is the seatpost in the insert? As long as the seat tube clamp holds it in place, it doesn’t have to be super tight when putting it in or taking it out. I have bikes with varying degrees of seatpost tolerance; none affect the stability of the post once installed.

a) if it is new check with the builder as to what size seatpost is required
b) get the calipers out and measure the seat post and the inside diameter of the seat tube that should tell you a lot

I am not a frame builder but I assume that these are pressed in place and then honed/reamed to the proper fit.

The nominal seatpost diameter is 31.6mm. The hole diameter is .020" (0.5mm) oversize.

I am mostly concerned with possible fatigue on the seat tube over time causing frame damage.

I had a Kona Sutra Ltd with such an insert, and it was an aftermarket unit that came in various seatpost diameters. Something like these. And if yours is a pressed-in unit, maybe a similar shim could tighten things up?

The correct fit is a snug running fit BEFORE tightening the clamp bolt. If you insert the post and can rock it, it's too loose.

Another indicator is the cut slot. On a perfectly sized post it'll barely narrow when the clamp bolt is tight. If it narrows by over 1mm, it's too sloppy.

Good fit is important beyond simply keeping the post from sliding or twisting. Since it's only secured at the clamp, an undersized post will be free to rock as you shift your weight, causing added stresses.

BTW - seatposts are sized to the tubing ID as shipped from the mill. Reaming serves 2 purposes: correcting for undesized tubes, and/or correcting for ovalizing during welding or brazing. An overly large ID usually means that tube was too oval, and now has thin walls in places.

Give the frame builder a call and see what they have to say about it. I would suggest a pop can shim and some friction paste, but if it's a new frame, I'd ask the builder, they should solve it for you.

It is on my list. trouble is that the frame builder is in Colorado and I'm in North Carolina.

Here are a couple of photos. Another issue is that the seatpost clamp is within .015" of completely closed when tightened to the 5nm torque spec.

https://cimg7.ibsrv.net/gimg/bikefor...bf217e7d7f.jpg
https://cimg9.ibsrv.net/gimg/bikefor...04885d85a8.jpg

This is not good at all. The post is definitely sloppy within the frame.is

Of course I have no idea whether the post is below spec. or the frame over.

I can't tell from the photo, but if the seat tube uses an insert, it's an easy fix with a new insert. OTOH - if the post is supposed to fit without an insert it's more serious.

Assuming the post is on spec (measure to confirm) I could consider the frame unacceptable.

FWIW have you considered the possibility that you're using the wrong size post and the right size is 0.2mm larger?

The best silver brazed steel frames had a clearance of 0.1 mm, i.e., 27.4 mm post in 27.5 mm ID tube. In my experience the seat post should not move under its own weight with the clamp open and there should be no visible gap. As above, I'd try a larger seat post.

The Thomson post shown in the photo is 1.245" (31.62mm) The frame measures 1.265" (32.13mm). The seat tube is clearly over by .020" (0.5mm)


The post should fit without an insert. The insert is built into the frame.


This is supposed to be for a standard size 31.6mm seatpost. Is there a standard 31.8mm seatpost? Another thought... Is there a standard 31.8mm reamer? Could the wrong reamer have been used?

Last week I was setting up new-to-me frame and found the 27.2 Super Record Campy seat
post wouldn't go in but an inch. Meanwhile, another recent build bike had another 27.2 Campy post that has a loose fit. Measuring both posts and both frames I found the solution was to swap posts between the frames for a perfect fit in both. The larger post actually measured 27.35mm and the other measured 27.1mm. Glad I figured all this out before I began the process of sanding material off the larger one. Lesson learned: measure the hole and the post.

I did hear back from the builder about the fabrication steps. Apparently the relief slot through the insert is not cut until insert is reamed to the correct dimension. The reamer apparently can't ream through a discontinuity (cut) in the sleeve. So the insert is reamed to size and then the slot is cut. According to the builder when the slot is cut in the sleeve it can cause the seat tube to expand. However take a look in the photos at how much the slot closes up when the clamp is tightened. Too bad I don't have a cylinder bore gauge to check the depth of the bore further down the seat tube.

The builder claims... "The additional .020” of tolerance is within the acceptable margin that is pretty common with hand reaming metal bikes from what I have experienced from fabricating bikes...". He did mention that the seat clamp is "less than optimum" and will be sending a different replacement.

Bad news. Good news.

The bad news is that the frame is definitely defective.

The good news (for both you and the builder) Is that it uses a liner, so the tubing itself is OK. The solution is for the builder to remove the bad liner and fit one sized properly.

Depending on how the liner was fitted (what kind of glue, if any) this is something that might be fixed locally, saving the cost of round trip shipping.

It's easily solvable ad long a you and the builder can come to terms. CALL THEM.

Pretty sure 31.8mm is a standard seatpost size, just not terribly common.

A quick look on eBay shows a listing for a used Thompson 31.8mm alloy seatpost. So Thompson at one time appears to have made them.

There seem to be multiple listings 31.8mm seatposts from other manufacturers.

Since the frame is new, I'd try to get satisfaction from the vendor first. But if the insert is indeed reamed oversize and a 31.8mm will fit properly, that might also be an option.

The specs on my frame drawing say 31.6mm. I don't think that there is a standard 31.8 seat post. I think a lot of folks get confused as 31.8mm is standard clamp size for handlebars.

I agree that there was some confusion here and there, but while not "standard" (whatever that means) there were 31.8mm posts, and bikes that used them. This is first hand info, since I sold plenty of them 20-25 or so years ago.

However, the fact that you can find a post that might fit better, doesn't excuse the builder from delivering a frame built to his own specs.

It's not your job to make do to compensate, it's the builders job to do right in the first place.

Well the builder has responded and claims that it is within "tolerance" so won't be reworking the frame. The bike has been paid for long ago and I don't have any leverage other than the builders good will. The only way I could see them possibly doing anything is that if the post won't hold position while riding. Yes, eventually I hope to be able to ride this bike. It has only been 15 months so far.

I'd like to know who built the frame.

What good will? The builder doesn't care about what he sells, a long as he gets paid for it.

In any case, there's always small claims court. For my part, I can't believe the builder is refusing to deal with it, since replacing the insert is an easy fix for him.

I can't be you, but if it were me, I'd do whatever I could to turn up the heat.

If you are near a reputable builder, or decent pro shop, ask for a consult, then let the builder know you're prepared to sue (and, of course go public).

BTW to be clear, I don't know what this jerk thinks "within tolerance" means, but there actually are standards for this. The frame is to be reamed to 0.05mm +0.00 -0.02mm, over nominal, or between 31.63 and 31.65 for a 31.6 post.

Anyone who can't produce within those tolerances fails as a builder. Denying the facts means he also fails as a human.

If the builder has kicked you to the curb, you should be willing to name him/her/it. It's useful info for your fellow bf'ers.

Sometimes the threat of something is more powerful than the action.

The OP should hold off satisfying people's curiosity, and preserves tactical options.

I was asking out of curiosity, and because some bf'ers might benefit from the info. I would not, under any circumstances, advise the OP to threaten the frame builder...You know, tell him "Fix my problem or I'll trash you on the internet." That's never the way to deal with such things.

The threat doesn't have to be, and shouldn't be stated. However, in the age of the internet. It's implicit.

OTOH implicit threats cut both ways. Until and unless the OP is sure of his position, naming the maker can put him at risk.

I agree that threatening someone with bad PR in order to coerce them is tantamount to extortion. However, there's, no foul in simply reminding people they live in a goldfish bowl.

Despite seemingly not caring about quality, the builder may be very concerned about brand image. He may be motivated to avoid bad PR, but once the genie is out of the bottle, he has nothing to gain by resolving this problem.

For example, the OP might send the builder a link to this thread. In doing so, he can state that he's kept the maker's out of it out of respect, but will post the outcome once there's closure.