Bicycle Mechanics - Seatpost questions
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12-26-10, 09:11 AM
I'fe just finished building up a mid-80s Raleigh frame into a singlespeed, and eveything's working so far except the seatpost. Basically, the post itself is bent about halfway along its length, which means it won't go far enough into my frame and my saddle's too high. I have a plan, however. I intend to cut off the bent section and use the remainder of the post, which should be long enough, as the frame is HUGE and I only need about 3-4 inches of post. Before I break out the hacksaw, is there a way to straighten the seatpost? It's made of fairly thick aluminium, 27.2 mm OD.
While I'm on the subject, is the "minimum insertion mark" on a seatpost there because a seatpost inserted less distance than that will be insufficiently clamped into the frame, or because a seatpost clamped below that point is in danger of bending just above the seat lug due to the large turning moment about the point where it clamps in? I've often wondered about this...
12-26-10, 09:34 AM
1. First, do ho harm. Your highest priority here is not to damage your bike's frame. You need enough seatpost length to extend past the top tube lug.
2. If you can't use the seatpost the way that it is you have nothing to lose by trying to shorten it. I doubt you'll be successful unbending it because it's important to maintain near perfect roundness.
3. 27.2 mm is a common seatpost size. If you can't find a used one a brand new Kalloy shouldn't cost much money. If it was my bike that's what I would do.
12-26-10, 09:53 AM
Airburst: If it was my bike I'd spring for a new seatpost. If you insist on cutting it please measure how far from the bottom the minimum insertion mark is and be sure there is at least that much inserted into the seat tube when you mount it. It would be a shame to wreck a frame (or injure yourself) over a $20 part.
12-26-10, 09:58 AM
+1 on the above and I would only add that I generally figure adding 4" past the insertion line that fits me, so if you think 3-4" of exposed post will fit, cut it to 7 -8" minimum. If you can't manage that much, pitch it and buy a new one.
12-26-10, 10:27 AM
Almost all minimum insertion marks are 3" -4" above the end of the seatpost so if you can shorten the post and still be able to insert at least 3" into the frame you should be good. But, as noted, 27.2 mm is very common and brand new seatposts in that diameter can be had very reasonably.
12-26-10, 10:43 AM
--and who among us does not?--straightening
and reusing a bent alloy seatpost is not a good
idea. Metal fatigue and all that stuff interferes.
+1 on the others comments about proper length
of insertion. If your diameter is 27.2 and the
frame material is steel, it usually means you have
a frame made of DB tubing, thus worth preserving
for future generations (this goes back to crotch
in point #1).
Do us all a favor and spend 15 bucks on one of these:
so we don't have to worry about you. Plus you will
not have to explain the whole thing someday to the
on duty M.D. in whatever ER you end up in.
12-26-10, 10:50 AM
the inserted portion needs be far enough in to be below the bottom of the join of the top tube and the seat tube..
Kalloy as mentioned,are a bargain, Plain sort to mount a saddle clip even more so..
Belt and Braces approach...
unless you read the markings on the botom of the post saying size..
Take the seatpost you have to the bike shop, to make sure the size is proper.
too small damages the frame trying to grip it..
Going upmarket a ways you get a nice stepless adjustable saddle mount mech..
The general rule for seatpost insertion is 2 diameters. That's to keep the post from camming out. If the seat tube extends above the toprube, you also have to protect the frame by making sure the bottom of the post extends 1" below the bottom of the top tube. If you can meet both tests with your newly shortened seatpost go ahead and cut it.
12-26-10, 11:59 AM
I'm sorry......a new post AND personal safety are both cost prohibitive?
12-26-10, 11:59 AM
OK, I cut it down, I saved about 5 inches of the actual tube in the end, plus the height of the clamp and saddle. With the frame I've got, which is the biggest I've come across in a while, I need very little height from the seatpost, which means I've been able to meet both of FBinNY's tests, so I figure I'm safe.
I hate to quibble with 2 diameters, but remembered reading somewhere that 2-1/2 diameters was the safe figure! All I can say is I've actually ridden briefly on far less, not that I recommend that, and keeping in mind I weigh all of 155lb.
As far as I know there's no absolute rule. It also depends on the rigidity of the various parts. A solid steel peg in a block reaches full strength sooner than a wooden peg or thin walled tube would. I use 2 diameters, or 1 diameter below the bottom of the top tube (which ever is deeper) as my rule of thumb minimum, and it's served me well for years.
If we look at an analogous situation, the insertion of the fork's steerer into a stem, we see that they work with even less overlap, somewhere in the range of 1-1/2 diameters.
Years ago, I read an engineering analysis that discussed this, but I couldn't find it at a free source this evening.
I should note that many, or most, steel posts taper off at the bottom, with a generous radius. That might easily account for the extra 1/2 diameter.
I agree that two diameters looks like plenty when the post is cut off square, and as long as the seat tube doesn't extend past the top tube.
Over the years I've seen a small number of seatposts cam out of frames, but they've all been cases of very shallow insertion depth, usually as little as one diameter or less.
Much more common failures in my experience were posts that were fitted blindly following the marked minimum insertion line, but not deep enough with regard to the frame design. It's become very common to extend seat tubes well above the top tube joint creating a new type of possible failure. Some frame designs reinforce the extended section, but most don't, so a post can be inserted 2-1/2 inches and still end at or above the highest weld.
With all the possible variations so seat tube design, it's more important than ever that end users understand the basics and think about how it applies to their frame.
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