K2 / Proflex Riders Group
General => Tech Forum => Topic started by: Thunderchild on May 05, 2008, 07:47:16 am
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Saturday while riding back down one of my favorite local single tracks, I was out of the seat and noticed that my seat suddenly moved to the right a bit. I immediately thought that my seat tube had become de-laminated. Upon examination, my seat tube was still in the frame, but had sheared off at the top where the seat post clamp attaches!!!! :'( The remainder of the seat tube is firmly attached to the frame. I will post pictures tonight. I didn't know the tube had a shoulder. The thickness of the tube inside the frame is thinner than the top portion that the clamp attaches to.
O.K. I hope your minds are all working on how I am going to fix it. My Thoughts: Have a machine shop carefully ream out the remaining tube in the frame and have a new tube insert made, stainless or aluminum? Use appropriate epoxy to hold the tube in place.
Any other thoughts. Has this happened to anyone else. Lastly check your seat tubes!
Thunderchild
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What a bummer. I recall Simon said something about this. I have heard of the seat tube delaminating. I also have noticed that the insert wall is paper thin at the bottom.
If the insert was reamed out, could an insert be made for a 26.8 post? That would thicken up the insert. I would make it from 7075 alum, which the machine shop would enjoy, and the strength is comparable to 304 SS. McmasterCarr has a great deal for small lots of 7075 alum. A 12 inch piece, 1.25" (31.8mm)diameter, part # 90465K15, is book listed at $23.87. Most others have a minimum charge of at least $50. I make sure I ask McMaster for a certificate of conformance for your machine shop to verify.
Terry
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Sounds like you need an insert with a fairly substantial shoulder on it. From your description it sounds as though the original ones are .3- .6mm walls inside the frame, with 1mm - 1.5mm of metal outside the frame the clamp fixes to. Then the 1.5mm section has fatigued and detatched from the thinner lower section. The lower section being bonded in with Loctite / Araldite epoxy, so removing it will be a right P.I.T.A.
I have a no ideas on an easy fix. MAchine down a large billet of ally to make a flanged section, with a 27.2mm??? insert then bond the whole thing in. you might need to source some thick walled tube and get the thing turned down, I would increse the length of the clamping area if you can .
Willbe checking mine closely, aamoi , did you run your seatpost the full lenght of the insert, so it poked out the bottom ??
Mine doesnt, and it only goes about 2inches into the frame , and I'm sure its not right
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Okay this is an easy fix.
I have a drawing available that I drew up (all measured from an origonal) take this to a machine shop and have
this made then just re-bond it back in.
It all very simple you'll soon be back on the trails.
Okay not so simple now as all my drawings in the in the files section and my gallery have all gone ???
That appears to be everything, seat insert,sealed bearing conversion,floating brake system and aluminium s/arm conversion etc, not sure I have any of this backed up >:(
send me your email address and I'll send you the drawing this I know I still have :).
Simon :(
Forgot to mention I just got a hacksaw blade and put that through the stuck in tube then re attached the blade to the handle, then I very carefully cut a series of slots (about 4) down to the thin insert (go very carefully). Then I kept working penetrating fluid around the insert and with a flat bladed screw driver and a hammer carefully tapped the top edges of the inserts in towards the centre until they broke away from there bonding,it didn't take long.
Make sure you clean up everything and de grease before bonding in the new insert.
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Here you go,just had to host it somewhere else first.
Obviously make it from Aluminium :)
Simon.
([url]http://img241.imageshack.us/img241/9633/lastscanseatposttubeki0.jpg[/url])
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Great information all. Thanks for the drawings and the removal idea Simon. Obviously, I am not a metallurgist, but tell me why aluminum would be better than stainless steel? Wouldn't stainless steel be more durable and fatigue resistant.
IMO, I had the seat post all the way through the tube with just a bit of seat post showing at the bottom until last month. Last month my friend re-fit me to the bike and I raised the post up a bit so the bottom of the seatpost was just a 1/4 to 1/2 inch from the bottom of the tube. I also have close to 9000 miles on this frame.
Thunderchild
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Stainless would probably be fine as long as it compresses okay without to much effort enabling you to clamp your seat post,
if it doesn't then you've gone to all that work only to find your unable to clamp your post without having to use excessive force on the seatpost clamp.
Aluminium is what was used from stock and I think you may of answered your own question why yours failed,
maybe you need to purchase a slightly longer post so you can ensure you have it going all the way through.
Good luck
Simon.
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Great fix Simon - appreciate all you do -
OP
(good karma stroke for simon)
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Great information all. Thanks for the drawings and the removal idea Simon. Obviously, I am not a metallurgist, but tell me why aluminum would be better than stainless steel? Wouldn't stainless steel be more durable and fatigue resistant.
IMO, I had the seat post all the way through the tube with just a bit of seat post showing at the bottom until last month. Last month my friend re-fit me to the bike and I raised the post up a bit so the bottom of the seatpost was just a 1/4 to 1/2 inch from the bottom of the tube. I also have close to 9000 miles on this frame.
Thunderchild
What seat post are you using. I find that carbon ones flex the top part of the clamp.
In fact I had to clamp extra hard on carbon to prevent slippage. Have since stuck with my Thompson on my Oz.
Terry
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Great fix Simon - appreciate all you do -
OP
(good karma stroke for simon)
I agree. Would never have thought of peeling from inside.
What gets me is the super thin wall. ???
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Shovelon, I have a used a K2 alloy seat post since I built up the Oz.
A couple of more questions.
1. Anyone care to comment if it would be an improvement to increase the height of the thicker section of the seat tube above the frame?
2. Any down sides to machining the tube from existing tubing?
Status of Oz surgery.
My nephew is a Die Machinist and is dropping by tonight to look over the severed part. He might be able to make the part for me for material costs only. 8) He was impressed with the drawing and the suggestion for alloy 7075. Thanks Simon and Shovelon. If he makes it, we should be able to turn a few more if anyone wants an extra. Just expect to pay for material costs and maybe shipping. My nephew is one of us as he owns a Disco Monkey that I helped him build.
On another note, I started to build up my old 5000 frame to ride until the Oz is all better and found the already discussed cracks on the back of the seat tube by the water bottle mount. The search for a local aluminum welder begins. Simon, didn't you post a few pictures about welding a 5000 seat tube? I searched but couldn't find it. Anyone know the title or where it is?
Thanks
Thunderchild
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A couple of more questions.
1. Anyone care to comment if it would be an improvement to increase the height of the thicker section of the seat tube above the frame?
I wouldn't of thought so, the extra length may give more leverage,just my opinion no actual proof either way on this.
Simon.
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Shovelon, I have a used a K2 alloy seat post since I built up the Oz.
On another note, I started to build up my old 5000 frame to ride until the Oz is all better and found the already discussed cracks on the back of the seat tube by the water bottle mount. The search for a local aluminum welder begins. Simon, didn't you post a few pictures about welding a 5000 seat tube? I searched but couldn't find it. Anyone know the title or where it is?
Thanks
Thunderchild
That was mine. :o
http://idriders.com/proflex/smf/index.php?topic=2850.0
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Pictures of the broken seat post are posted in my gallery. My nephew machinist came over tonight to examine the Oz. Here is the plan. I am going to move to a 25mm or 25.4mm diameter seatpost and increase the wall thickness of the seat tube. I will have the part made per Simon's drawing with the inside diameter adjusted for the 25mm seatpost. Thomson's lists a 25 mm on their website; I will find out tomorrow if they have any in stock.
(http://idriders.com/proflex/coppermine/albums/userpics/10042/seat_tube_piece.JPG)
(http://idriders.com/proflex/coppermine/albums/userpics/10042/seatpost_and_seat_tube_top.JPG)
(http://idriders.com/proflex/coppermine/albums/userpics/10042/remaining_seat_tube.JPG)
Thunderchild
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Oz seat tube update. I have my machinist lined up. Once the 25.0 Thomson seat post is in, he will begin.
Damn hard tail has reminded me that I am not as young as I used to be. Like I needed another reminder. I miss my Oz!!
Thunderchild
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Hi all,
I seem to have arrived a bit late, but I’ll add my 0.02 Euros
Exactly the same thing happened to me in 2002.
As I have NEVER lowered my seat for technical riding I simply used acrylic sealant (like bath silicone but a bit stronger) and just glued the post into the insert.
A good quick fix.
Only in January of this year the top of my seat post split and the head fell out!
So I cut the post to 5 cm, bonded a suitable piece of solid bar into the end (to stop the tube collapsing) and went at it with a BIG pipe wrench and lots of silicone lube.
Turns out the acrylic sealant is stronger than the original bond between tube and frame, and after a lot of sweating and swearing out came both parts.
I was interested to discover that there is another aluminium part bonded into the carbon.
I measured up and traced a drawing for a new post.
2 things I improved on:
That shoulder is the perfect stress concentrator; I replaced it with a nice long taper.
Inner diameter 26.8 because I happened to have a post that size kicking around.
I discovered a third improvement when the machinist left out the slot
It clamps fine without the slot, as long as your tolerances are respected.
To reply to some of the suggestions/ideas above:
Bigger shoulder = bigger concentration of stress in one zone = bad idea.
Moving the SHOULDER away from where the tube leaves the frame gives the thin part of the tube the opportunity to flex with the post (and at 25mm it WILL) = good idea.
Taller thick section = less flex in the thick section = more flex where the diameters change = bad idea.
The thin seat post will really flex, it will work the tube MUCH more. See above for stress concentrations.
Materials
Stainless + alu + water = battery (Google galvanic corrosion or galvanic couple)
7075 is stiffer, has a higher yield strength BUT the step between yield and rupture is very small. In other words if it bends it will break; not good for a part that you have to preload just to clamp the post…
I had mine made from 2014 alu, lower yield strength and the same rupture. Lower grade alu generally has better fatigue characteristics.
Sometimes it’s better to let the parts work together rather than try to stop the flex with exotic materials.
And it’s a damn sight easier to machine.
If I can work out how to upload files I’ll post the Autocad or PDF file of my insert.
In the mean time PM me for a copy.
I’ve just re-read that; sorry if it sounds a bit authoritative, it wasn’t my intention. Part of my work involves forays into stress evaluation/calculations (albeit in hydro-electric turbines rather than bicycles)
Happy trails
Luke
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Hi all,
I seem to have arrived a bit late, but I’ll add my 0.02 Euros
Exactly the same thing happened to me in 2002.
As I have NEVER lowered my seat for technical riding I simply used acrylic sealant (like bath silicone but a bit stronger) and just glued the post into the insert.
A good quick fix.
Only in January of this year the top of my seat post split and the head fell out!
So I cut the post to 5 cm, bonded a suitable piece of solid bar into the end (to stop the tube collapsing) and went at it with a BIG pipe wrench and lots of silicone lube.
Turns out the acrylic sealant is stronger than the original bond between tube and frame, and after a lot of sweating and swearing out came both parts.
I was interested to discover that there is another aluminium part bonded into the carbon.
I measured up and traced a drawing for a new post.
2 things I improved on:
That shoulder is the perfect stress concentrator; I replaced it with a nice long taper.
Inner diameter 26.8 because I happened to have a post that size kicking around.
I discovered a third improvement when the machinist left out the slot
It clamps fine without the slot, as long as your tolerances are respected.
To reply to some of the suggestions/ideas above:
Bigger shoulder = bigger concentration of stress in one zone = bad idea.
Moving the SHOULDER away from where the tube leaves the frame gives the thin part of the tube the opportunity to flex with the post (and at 25mm it WILL) = good idea.
Taller thick section = less flex in the thick section = more flex where the diameters change = bad idea.
The thin seat post will really flex, it will work the tube MUCH more. See above for stress concentrations.
Materials
Stainless + alu + water = battery (Google galvanic corrosion or galvanic couple)
7075 is stiffer, has a higher yield strength BUT the step between yield and rupture is very small. In other words if it bends it will break; not good for a part that you have to preload just to clamp the post…
I had mine made from 2014 alu, lower yield strength and the same rupture. Lower grade alu generally has better fatigue characteristics.
Sometimes it’s better to let the parts work together rather than try to stop the flex with exotic materials.
And it’s a damn sight easier to machine.
If I can work out how to upload files I’ll post the Autocad or PDF file of my insert.
In the mean time PM me for a copy.
I’ve just re-read that; sorry if it sounds a bit authoritative, it wasn’t my intention. Part of my work involves forays into stress evaluation/calculations (albeit in hydro-electric turbines rather than bicycles)
Happy trails
Luke
So you bonded your seatpost into the frame? Hmmm, I saw an old American classic seatpost with an expander at the bottom. may not be a bad idea to lock it at the bottom too.
You are right on with SS/alum contact and galvanic corrosion. Wonder if there is any coating that can arrest that? Some kind of zinc chromate paste perhaps?
I can agree to a point with your assessment of 7075-T6 having a narrower fatigue band, but 2014 has a higher degree of plasticity which leads to strain hardening to the level of 7075. Upon reaching this level of strain hardening, the yield is much lower than 7075. 7075 has a bigger memory as far as movement is concerned which also makes it less plastic. The biggest issue really is the stress riser positioned right at the stepdown of the 2 diameters.
I certaily do like your choice of 26.8 inner diameter. 8)
Terry
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just a thought. but you guy's have access to the bottom of your seat post's.. what if you used a wedge system like in the old stems?? :'(
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wow... you guys confuse me. Im a carpenter so im pretty sure anything I make WILL break, but look lovely in varnish ;D. But I also have an OZm frame with the same predicament. So can I ask if and when someone decides which is the best way to remake the insert,can you make two and sell me one, please...
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Luke,
That shoulder is the perfect stress concentrator; I replaced it with a nice long taper.
Inner diameter 26.8 because I happened to have a post that size kicking around.
How did you get the taper to fit into the frame; isn't it a constant diameter?
I will also have the machinist put a radius in the shoulder.
Thunderchild
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The frame is a constant diameter, the whole of the taper sits outside of the frame. It sits the collar a little higher but that’s more help than hindrance (see previous post).
Simon, you’re right about the 2014 work hardening problem. The higher the stress for each loading cycle (bump on the trail)the quicker it will harden.
I don’t think the tube will be deform plastically during it’s use.
I’m just hoping (and I’m not going to calculate it!) that my taper and the long tube keep the stress levels low enough for the work hardening to take a couple of decades to reach critical level.
It will also depend on how much the seat post flexes, which is a function of how much it sticks out and how fat I am.
IMHO a 25mm seat post will rip the tube apart regardless of the alu grade used.
But that’s only based on instinct.
Only time will tell…
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A couple of questions for you metallurgist dudes.
#1 Will it matter if the seat tube is made from a billiet of 7075 or should I insist on bar stock?
#2 What are the tolerance limits for seat post to seat tube fit?
Thanks again
Thunderchild.
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The frame is a constant diameter, the whole of the taper sits outside of the frame. It sits the collar a little higher but that’s more help than hindrance (see previous post).
Simon, you’re right about the 2014 work hardening problem. The higher the stress for each loading cycle (bump on the trail)the quicker it will harden.
I don’t think the tube will be deform plastically during it’s use.
I’m just hoping (and I’m not going to calculate it!) that my taper and the long tube keep the stress levels low enough for the work hardening to take a couple of decades to reach critical level.
It will also depend on how much the seat post flexes, which is a function of how much it sticks out and how fat I am.
IMHO a 25mm seat post will rip the tube apart regardless of the alu grade used.
But that’s only based on instinct.
Only time will tell…
whats your definition of a "long" taper.If the step is say from a of(using theoretical rounded dimensions for our seat post insert) say 32mm od for the actual collar and say 28mm for the actual peice that goes into the seat tube...
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The change in diamaters is spread over 10mm, I've glued the tube in so that the taper doesn't contact the frame, in all there's about 35mm of tube outside the frame.
If the glue's ok the whole lot doesn't slip down and work its way into the frame.
Billet, bar, it's all hard to machine in 7075. The clamp will stop it splitting along its length, so it doesn't make much difference. Setting up a non-round part in a lathe to then turn it down is just more of a pain.
Tolerance - I used H8 which works without a slot. I haven't got the tables nearby to tell you + - how much that is (on lunch); any machinist will be able to find them.
Considering the multitude of seatpost manufacturers, the volume at which they produce and the varying quality assurance standards the world over, I'm not sure tolerancing without having measured the seat tube you're going to use is that useful.
After all, that's why there's a slot, to make up for mass production tolerance problems.
Happy machining
Luke
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The change in diamaters is spread over 10mm, I've glued the tube in so that the taper doesn't contact the frame, in all there's about 35mm of tube outside the frame.
If the glue's ok the whole lot doesn't slip down and work its way into the frame.
Happy machining
Luke
Hmmm, 10mm taper. That's a good idea. 8)
That gives me an idea of my own. I can put a road clamp on my Thompson seat post, and add a wedgelock at the bottom of the seatpost. The screw would run up thru the post and exit on top of the seat plate. Then I reduce all the other clamping points to make up for the added mass. That may just length the life of the clamp area.
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just a thought. but you guy's have access to the bottom of your seat post's.. what if you used a wedge system like in the old stems?? :'(
terry!! great mind's think alike.... but didn't i say this some day's ago.. c,mon keep up.. ;D
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just a thought. but you guy's have access to the bottom of your seat post's.. what if you used a wedge system like in the old stems?? :'(
terry!! great mind's think alike.... but didn't i say this some day's ago.. c,mon keep up.. ;D
That's where I got it! :o
Thanks,
Terry
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Update: The seat tube should be done next week. I had already purchased the 25.0 seatpost, so I have to stick to that route with the thicker wall seat tube. It is a Thomson (reinforced on the front and back on the inside, so I hope it doesn't flex too much. I decided to stick with the design with a shoulder with a full radius and no slot for the seat post as the machinist is going to do a tight slip fit with the seat tube.
I forgot to ask what epoxy glue others have used to bond the seat tube into the frame. Seems like a GM type of epoxy was mentioned in the past. Chime in if you have used any. Once I remove the old tube, surface prep with IPA is sufficient, right?
Thanks
Thunderchild
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I forgot to ask what epoxy glue others have used to bond the seat tube into the frame. Seems like a GM type of epoxy was mentioned in the past. Chime in if you have used any. Once I remove the old tube, surface prep with IPA is sufficient, right?
I used Scotchweld 490, an epoxy adhesive from 3M. But only because they were throwing away tubes of the stuff at work after it failed the mass spectrometry test for Shuttle flight qualification.
Any LONG cure two part epoxy adhesive should do the trick, there's so much contacting surface compared to the load that It doesn't have to be the flashiest stuff.
Just a thought, but it's too late for me: perhaps it would be wise to bond it with a glue that has a known and available release agent, just in case you have to repeat the whole ordeal...
Surface prep: I rubbed my bits down with sandpaper and degreased with acetone. What's IPA? (I'm so ignorant)
Good luck
Luke
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IPA = isopropyl alcohol. I like the idea of acetone. I think it is a better all around solvent that evaporates well. A glue with a release agent would be nice; I will post any that I find.
Thunderchild