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Conference noted::bicycle

Title:	Bicycling
Notice:	Bicycling for Fun
Moderator:	JAMIN::WASSER

Created:	Mon Apr 14 1986
Last Modified:	Fri Jun 06 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	3214
Total number of notes:	31946

783.0. "SEAT POST FAILURE" by FRAGLE::RICHARD () Tue Jul 19 1988 13:41

    I ran into a problem last weekend with my mountain bike.  The single
    bolt that fastens the seat to the seatpost sheared off, resulting
    in the saddle falling off.  The whole thing could have been quite
    serious if I had been riding in rough terain and had landed on the
    seat post!  As it was, I was just riding on the street.  
    
    Has anyone else had an experience like this?  The seatpost was/is
    a fairly high quality SAKE one.  A metalurgist in our group here
    said the bolt failure was one typical of an overload.  I am concerned
    that it might happen again when I replace the bolt, this time with
    more diasterous results.
    
    Any comments/sugestions?
    
    Dave Richard

T.R	Title	User	Personal Name	Date	Lines
783.1	How big are your forearms?	BRMUDA::MJOHNSON		`Tue Jul 19 1988 14:00`	6
	In years of using SR posts, the only problem I've ever had is the bolt coming loose, allowing the seat to move back and forth. The only reason I can think of for the sheering you experienced would be excessive tightening of the bolt. MATT
783.2	I have small forearms	FRAGLE::RICHARD		`Tue Jul 19 1988 17:47`	5
	Re excessive tightening......... My first thought was that maybe I had over tightened the bolt. However, our microscopic examination of the sheared surfaces ruled out rotational shear failure.
783.3	Random Defect....	ASIC::CRITCHLOW		`Wed Jul 20 1988 10:56`	6
	How bought lousy QC at the manufacturing site of the bolt? Replace it. I dought you will have another problem. When I think about the abuse I put my $59 Caldor specials through growing up without any problems I can't imagine that it will happen again. JC
783.4	Could still be too tight...	RAINBO::WASSER	John A. Wasser	`Wed Jul 27 1988 16:57`	11
	> My first thought was that maybe I had over tightened the bolt. > However, our microscopic examination of the sheared surfaces ruled > out rotational shear failure. Just because the bolt wasn't twisted apart (Rotational Shear) does not mean that the bolt wasn't over-tightened. Most of the torque you put into tightening a bolt will end up putting TENSION on the bolt. Once you hit its tensile limit... PANG! a broken bolt. Try not to tighten the bolt any more than necessary to keep the saddle from slipping.
783.5	Ouchhhhhhhh	MALLET::BROOMFIELD	I'd rather be mountain-biking	`Thu Aug 18 1988 08:47`	14
	I have had exactly the same experience with a 'strong' seatpost and vetta saddle - unfortunately I was jumping a log at the time (just thinking about it brings tears to my eyes.... and yes I was out of the saddle). I have put the fault down to over-tightening as I have moved the seat back to get the bike set up properly and was worried about it coming loose...... Please let me know if you find a good reason for your failure or a permanent fix as I feel that sooner or later it will happen to me again (oh god not again please...) Paul