Minimum post insert, was I wrong?

[rain dogs]

So, my partner has a local-brand frame, which was tig welded out of Tange infinity Cro-mo tubing, crack at the seattube cluster. She weighs about 120lbs btw. See pic below.

We were running it with a Dura-ace aero post, which are short, which was inserted a bit more than 1cm below the bottom of the junction between the tubes, specifically below the bottom of the top tube.

I contacted the original manufacturer and they said they would "sympathy warranty" it.... sell me a replacement 2009 frame for $300 (the complete bike is on sale now for about $1200 for a 2012). They also informed me that it was my fault because the seatpost needs to be inserted the minumum of 2" below the cluster.

Is this BS? I've never heard anyone say that it needs to inserted this far at minimum. That's 4.5" from the collar. I understand that longer is stronger, but I wanna know minimum... cause it's kinda hard to do more with aero posts.

I've almost always run my posts min 1cm below the junction, and Thomson in their literature says it only needs to be even with the bottom of the top tube, and have this picture:

http://cimacoppirides.files.wordpress.com/2012/11/crack3.jpg
http://cimacoppirides.files.wordpress.com/2012/11/crack2.jpg

[echelon_john]

Sorry--didn't read post closely enough. Was/is it a CX frame? That seems like the most likely scenario where a 120-pounder and a too-short seat post would cause this damage.

[rain dogs]

touring frame. She weighs about 120 with rocks in her pockets.

[bicycletricycle]

well,it doesnt really matter what the frame manufacturer thinks you should run it at unless they had told you/have it printed somewhere. You cant just be expected to know what they wanted you to do.

was the post above the limit line?

seems to me that the limit line on the post is the only thing a consumer can be expected to follow unless the frame manufacturer has something else to say about post insertion.

[Ken Robb]

why does the metal at the crack look so rusty?

[esldude]

Yeah, interior of the seat post looks kind of rusty, and the break looks to be all rust. My guess is that had something to do with it. More than where the insertion point was possibly. I wonder if I want a frame from those people whomever they are?

[572cv]

I am curious about the crack being at the back of the tube. Seems to me that the leverage on the seatpost would be in the opposite direction, to wit, tension on the front , compression on the back. If so, what would cause a compression crack?

[cachagua]

I wondered that same thing... also, that's not the crack I'd expect to see form a too-shallow seatpost insertion; I'd expect to see a crack at the front of the seat tube, under the top tube (or lug, if present).

The seatpost would have to be exerting force forwards to open a crack at the back like that. Not impossible, if the saddle's adjusted way, way forward, and the rider sits right on the nose... but an unusual circumstance.

You might consider taking the manufacturer's offer of a replacement, selling it, and getting a frame with a more robust seat cluster. I don't want to say that's a "bad" design, but it's pretty evident that a peak in stress coincides with the heat-affected zone where the stays are welded to the seat tube.

If you're particularly attached to that frame, and if they'd give you the next larger size, maybe that wouldn't ruin the ride but would give you just enough additional insertion to avoid a repeat problem... I don't know of any extra-long aero posts either. And yet -- while it might just have been bad luck, that arrangement looks scary to me. A little more metal seems like it would withstand the stress better.

[cachagua]

Couple of people snuck in. It was the rust I was wondering about first, but the location of the crack too...

[vjp]

First of all, about an inch below the bottom of the seat tube is kosher, it really has nothing to do with how long the seat tube extends above the top tube.

Secondly, the marking on a seat post is for the seat post manufacturers limits, not the frame.

Thirdly, the crack looks like a fracture from the seat stay welds being undercut and causing a stress riser, exacerbated by the shallow insertion of the post.

[Black Dog]

Looks like the crack was following a stress riser on the weld. The repeated flexing from a post that sits above the weld would caused this. The builder should have specified that posts sit below the cluster.

[bart998]

why does the metal at the crack look so rusty?


The rust is because the paint went first. This was a process that took time...

[parris]

i'm just trying to picture how far the post was in the frame as i'm a bit thick headed right now.

was the seat post below the min mark? and even if it wasn't the bottom of the post was inserted past the bottom of the toptube? is that fairly accurate?

it almost appears that whoever welded the frame could have been running to hot with the tig torch given how the break follows the lines of the joint.

could you post some more photos? thanks

[ultraman6970]

A few details....

1 Tange infinity Cro-mo, sincerely, tange tubing is not bad but is not what used to be, specially the inifnity that are new series of tubes compared with tange 1, 2 and 5 that were the tubes tange was famous for (champion tubes).

2 somebody mentioned rust, that thing has been cracked for a while, and maybe nobody noticed until now.

3 No idea if the tubes can be tigged, columbus and reynolds have series of tubes that can be used specifically with tig and can't be brazed. So who knows, not that you can't tig but so far all japanese tubing can be brazed.

4 no idea what frame is but if the seat tube is cut at the wrong side or cut wrong then you have issues like that. If that was part of the problem then when tigging the seat stays was going to create a weak spot (rust is prove of that IMO)

5 Agree with the seatpost length comments and probably that was 50% of the problem, being the other 50% the rust.

6 300 bucks for a replacement frame is a good deal. I would take it and next time your friend has to look more at the bike, who knows since when the tube is cracked, maybe has been like that for months now.

7 get a longer seat tube now :)

good luck :)

[572cv]

This has been a very interesting thread.:) I hadn't considered the stress riser situation...

[Peter P.]

The Thomson seatpost instructions pertain to the safety of the seatpost, not the frame.

I agree with some of the other responses; if other than the standard seatpost markings was an acceptable insertion, the manufacturer should have mentioned that as it would be an exception to standard practices.

To alleviate unusual loading of the seat cluster/joint, a seatpost should be inserted such that it extends BELOW the BOTTOM of the top tube a distance equal to the diameter of the seatpost. Why? Because (let's hope I'm saying this correctly) at that insertion length, the seatpost will provide maximum support/resistance to flexing and minimum stress to the seat cluster. Any more insertion is wasted in strengthening resistance to flex at the joint. Any less and the leverage on the seatpost from the rider's weight becomes a torque load on the joint, which gets even worse as the bottom of the seatpost sits shallower and shallower in the seat tube.

While all I have to go in is the original poster's word, I question whether the seatpost was actually inserted as deeply as they say. I can understand the OP expecting a full warranty of the frame, but I'd say the $300 replacement is a very fair offer and as has been suggested, if you've soured on the brand after this warranty experience, you can always sell the warranty replacement and buy something else with the proceeds.

[Louis]

seems to me that the limit line on the post is the only thing a consumer can be expected to follow unless the frame manufacturer has something else to say about post insertion.

This is the key. Most traditional seatpost Min Insert lines were probably located assuming a standard, reasonable ST extension above the cluster. As the ST gets higher and higher above the ST / TT / SS joint the min insert line is probably still fine for the post, but the ST by itself is no longer able to handle the moment. To get that transfered to rest of the frame you really need the post to extend far enough past the joint, as the Thomson info indicates.

[rain dogs]

Thanks for all the replies.

1. That's not rust on the crack, it's just dust from riding on a gravel path. It's not rusty at all there. I wiped it off to take the photo and it left some residue, I wiped it just now with a damp rag and now it looks perfectly clean.

2. The Thomson instructions for min insertion marks on their post are for the strength of the post, BUT that diagram is for the frame. It says: " The Thomson seatpost is perfectly safe to run at it's max line in any frame. However, please be warned that if the seatpost is not sunk below the top tube of the frame, there is a risk of damaging the bicycle frame"

3. Her seat post is a Dura-ace setback aero post and her saddle rails are centered, so there is little chance her weight is too far forward. It was inserted down beyond the max marking on the post.

I personally think it's stress risers from the weld as some others have said. You can see that the weld is undercut into the frame, but I wanted to see if others thought the same. The dust that is in the photo is in the undercut... and it bothers me a little that the mfgr is passing it off on my seatpost install vs the mfg quality.

Lastly, the post was certainly inserted the depth that I have said, part of the reason I am asking is because my Thomson 240mm Masterpiece seatpost is inserted basically the same distance below the top tube on my CAAD8... and now I'm a bit paranoid about that one as well, but I've always done it this way and haven't had any problems. In Fact, I think my EL Basso and Record ti post are similar. I've always liked a lot of bar drop, and on a lot of old posts they just don't have the length. I guess I could have bought a 330 Thomson, but I figured that depth "rule" was fine... so why?

[Louis]

Whether or not you get a failure can be a function of a number of things, and in this case I bet it was two factors. Here I'd say it was both the loading (the way the seatpost combined with the frame to create a load path) and the ability of the local structure to handle that load (local thickness, stress concentrations, etc.). Either one alone, and you probably would not have had a failure. Especially with a 120 lb rider.

[cachagua]

Looking at the photo again, I see another indication the post insertion's not the problem: if it had been, the joint would have failed catastrophically, with as little of the tube left uncracked as that.

In other words, if the force backwards on the post had been enough to start that crack, it would have been enough to finish it!

If you're not feeling too much affection for the manufacturer anyway, --yeah. Take the deal they're offering, sell the new frame, get something you really want (or the rider does). This could be the opportunity for some very enjoyable shopping!

[rain dogs]

.... Take the deal they're offering, sell the new frame, get something you really want (or the rider does). This could be the opportunity for some very enjoyable shopping!

I would if the frame was worth $300. I dunno who is going to shell out $300+ for a 2009 tange infinity touring frame without a fork on a bike that's barely over $1000 complete full build for a 2012.

Honestly, we don't care that much about the frame to buy it again, but I care about more frames breaking if I'm doing something stupid with short seatposts on all our bikes...

[Ginger]

Is it an externally butted seat tube?

Not using an externally butted seat tube or reinforced seat tube junction is a common error on budget frames. It's a common failure.

(I've just had the quick and dirty explanation from Dazza on why that will happen if it's not an external butted tube....)

If the replacement frame has a seat tube with a .6 or .5 wall single butted seat tube, it will happen again.

The proper tube is externally butted at least 1.0 -1.2mm wall at the seat lug junction. Or, the builder uses a reinforcing sleeve. (Ewen Gellie has a good photo of one method on post 3 of the thread below. His is of a brazed in, thicker sleeve, but it will give you the idea....
http://www.framebuilderscollective.org/forum/showthread.php?507-Week-44-The-Process)

Basically, as long as your seat post is installed to manufacturer's specifications and it reaches below the bottom of the top tube to manufactuer's specs, the seatpost won't really affect the outcome if this is the cause. Different speeds, different riders, might take longer or less time, but it will happen.

Cheers!
Ginger

[Louis]

In other words, if the force backwards on the post had been enough to start that crack, it would have been enough to finish it!

It depends on when the crack initiated and when they noticed it.

Believe me, if a ST is not designed to take the moment due to the rider, it's asking too much to expect it to do so. If the frame manufacturer tells you that the frame is not designed to have a post not extend below the seat cluster, I'd believe them. This is very basic Statics 101.

[oldpotatoe]

What I use is bottom of seatpost twice the width of the top tube below the bottom of the top tube.

So, if the top tube is 2 cm in width, the bottom of the seatpost 4cm below the bottom of the top tube.

[mike p]

I weld for a living and from those pic's I'd have no idea what caused the failure. I'd have to see the frame in person after the paint was removed and steel cleaned. I do know that you've got almost nothing to lose by having a little more post inside the bike. I love the old DA post's and wanted to use them on a vintage frame I have but chose another post as I couldn't get enough seat post inside the frame. I won't use it unless I can get about two inch's below the bottom of the top tube. To me the frame in question would look better with a Thompson post anyway, not like it's an old frame. Just a matter of taste.

Mike

"Lastly, the post was certainly inserted the depth that I have said, part of the reason I am asking is because my Thomson 240mm Masterpiece seatpost is inserted basically the same distance below the top tube on my CAAD8... and now I'm a bit paranoid about that one as well, but I've always done it this way and haven't had any problems. In Fact, I think my EL Basso and Record ti post are similar. I've always liked a lot of bar drop, and on a lot of old posts they just don't have the length. I guess I could have bought a 330 Thomson, but I figured that depth "rule" was fine... so why?"

[EricEstlund]

Minimum insertion marks on posts and stems have nothing to do with the safe insertion for use. They represent the minimum safe insertion for the part in question in isolation.

For stems, the minimum insertion on production stems must be located a distance of at least 2 ½ times the diameter of the stem from the bottom
of the stem. For seat posts a distance of at least two times the diameter of the seat post from the bottom of the post.

This represents a point for testing bending standards, and is not relevant to the safe depth for operation when considering headset types, fork steerer length, head tube rise from top tube, spacers, etc.

I won't make any comments on the frame in question, but I try to stress conservative insertion depths for all of the components of the system with my clients.

[ultraman6970]

Ginger, tange doesnt have any tube that is externally butted and I doubt they will do such a thing ever, but so far today all their stuff is a copy of the old coulmbus SL or reynolds, one detail tho, their tubes are not seamless from what I understand, that helps them to control the thickness of the tubes better but whats the price?... My old master builder did not like the infinity and he rather use champion 2 or 5 than infinity.

IMO that thing failed for a sum of reasons, I suspect that they maybe cut the tube wrong and the joint area with the seatstays is just in the thinner area of the tube (that tube is 0.9/0.6/0.9), that created a stress area. No idea if brazing would have done the same but who knows, no idea if the tube can be used with TIG, maybe not.

What the op can do is just get the 300 bucks replacement, sell it with the old fork and go with another manufacturer? By the way how old is that frame?? if the thing stood like 7 years w/o problems then maybe it lasted too much and was a thing of time you know.