Senior_Rider

What would cause a bike seatpost to shear? Bike is approximately 4 years old and has been ridden 8-10 miles most days.

fietsbob

Cheapest cost part selected? Ignored the minimum insertion line?

blamester

Depends where it broke.
If shear is the right word I assume you are talking about where the saddle clamp joins the seatpost. It could be poor installation or riding conditions. 10 miles a day for 4 yrs is fair enough use but there are fit and forget types.
A lot them are glued and I have had them break before. You get warning tho.
Seatpost tubes are strong and difficult to break and the frame will often break beforehand.

HillRider

Make and model of seat post?
Material of construction: steel, aluminum or carbon fiber?
Exactly where did it break?
How was it installed in the bike, how much post was exposed?
What type of seatpost clamp on the bike?

Senior_Rider

Responding to all previous responses at once, Definitely could be a cheap seatpost but don't know how to check model. However, the bike is a Sun Ruskin Sport with original equipment (except twist shifters were replaced by lever shifters before I ever rode bike). There is one at Bicycle World on web with general specs that list the seatpost as "Aluminum, suspension" . I am not allowed to post a picture (considered as URL?) but the shear is close but not directly below the seat clamp. Not sure of seat clamp terminology but rails of seat are secured to seat clamp by nuts on each side of clamp using (socket) wrench, not from underneath with an allen wrench.

blamester

You need a new one. Go to an lbs and buy if you can. Or measure and order one.
​​Get one of another scrap bike if your lucky.
Make sure it works with your saddle.

grizzly59

Without being sarcastic, you most likely have a chicom alloy seatpost on that bike. You are lucky you didn't have an injury accident when it broke. Don't replace with the exact same thing. Best bet would prob be for you to take the broken post + bike + seat to a bike co-op or bike shop and see if they have a replacement. Doesn't have to be new or fancy, but not dangerously cheap like the one that broke. Regular post, not a suspension would be fine. The handlebar and stem may be sketchy as well.

cyccommute 

Looking at the bike, the seat tube is at a very relaxed. Most bikes have seat tube angles of in the 72° to 74° range. This one looks like it's down in the low 60°, possibly high 50°. That puts a lot of strain on the seatpost because the load is far behind the leverage point. Put in a cheap post and add suspension to it and the load varies, straining the post even more.

blamester

I should have added it's not unlikely that other parts will soon start to fail.
So look over it good.

Senior_Rider

I assume you have seen the image of bike and the post sheared just above top of rubber covering suspension mechanism. I had just stopped waiting for traffic to clear when I put my foot on ground to balance and my weight shifted back causing seat back to take a big dip backwards. Any suggestions on replacement to make this event less likely?

WizardOfBoz

Suppose it looks like below.

Get a very accurate measurement of the seatpost diameter. Use a micrometer or wrap a piece of paper around it tightly and make a pencil mark across both ends of the paper. Measure the distance between marks and divide by 3.142. The you can shop for a replacement that will fit.

But cyccommute is correct, as usual. The design of this bike cants the seat tube back a lot. This puts a lot of stress on the seat post. You may want to swap out the weaker suspension type for a stronger non-suspension type.

Senior_Rider

Thanks for posting an image of the bike. I do understand about the exaggerated angle, but assuming the seat angle was adjusted correctly or nearly so, wouldn't you expect the shear to begin at the back near the seat clamp and not the front if the angle is creating some stress?

Miele Man

It'd be interesting to see an image of the two sections where the break occurred. Perhaps there was a hidden crack that eventually sheared . That is quite an angle on the seat tube and thus as others mentioned quite a load on the seatpost. Are you a heavy rider? Not all seatposts are created equally.

I think I'd get a non-suspension seatpost for that bike if it were mine.

Cheers

Senior_Rider

I Have a good picture, but I need 10 or more posts to attach a picture to post. When I replied to message with quotes which included bike image, I had to delete bike image in the quote due to my low number of posts. I weigh about 155 lbs tops

Iride01 

You can put the pictures up on a pic hosting site and then put the URL for them in the plain text of your post. You likely will have to replace "." with " dot " and maybe a few other things if it gets shot down by the scrubbers when you preview or post it. Just don't try to use any of the image, file or link tools. Just write it in the plain text.

WizardOfBoz

Senior Rider, keep responding and you'll have ten posts soon. Or, upload the pics to imgur or google pics and message (to me) or post the tiny url ref to that pic. We can post the pic then.

As an engineer, I can say that the exaggerated angle puts significant bending moment on the seat post. Tension in the front of the post, compression in the rear. Tension and compression cause different types of failure in metals. If I'm seeing this correctly, you also have a forged (better) or cast (worse) top part of that post. Stress is force per area, so you'd have to take the forces at any point and divide by the cross-sectional area. Holes drilled in the part make the cross section less, and if the holes were poorly drilled (rough surface, with gouges and scratches) you could have stress risers. In fact, even a well-machined hole will be a stress riser. I think that the section up near the neck has less cross-section, so higher stress. A high tensile force and low cross section leading to high stress, coupled with a weak casting or an imperfect forging, exacerbated possibly by poor machine work and the possible presence of stress risers. I can easily see this as a likely failure mode.

Of course, hindsight is 20:20. Its a much better engineering feat to predict failures in poor designs before they are made so as to avoid the problem you face. Of course, if the engineer does a fantastic job and the design is robust, reliable, cost-effective and well-suited-for-purpose, then there are no problems. And management says "We didn't have any technical problems: why do we need an engineer?" Sigh.

Good luck.

Iride01 

I'm not sure it's just ten post.... might be ten posts and and a certain time period. If the forum software was just using 10 posts, then any spam bot could get past that.

Senior_Rider2

Can't post more than 5 posts in 24 hour period. Can't message/email anyone with less than 10 posts to my credit (at 5 now). Trying this (just copy/paste):

photos.google.com/photo/AF1QipP4IlgoOYRIXBDifZpqfkllzvjWT2OaSQEH0CLQ

Break is very clean, so forged or cast?

Iride01 

Google comes up with a 404 error on that. Did you use the sharing link they give you?

First part of the URL for sharing google photos will look like:

https://photos.app.goo.gl/

Senior_Rider2

One more time with feeling: drive.google.com/file/d/16FcEp-a37kcSjwjddyvDu1aUR9OJnbHh/view?usp=sharing

WizardOfBoz

Was there an issue requiring you to change your username by adding "2"?

Anyway, the pic is below. Looks like there was a crimped-on tip to which the seat was bolted, and the crimp failed. Or am I seeing this wrong? Was this a piece of solid metal that failed or what? To facilitate a better diagnosis, you might want to remove the seat, and the rubber weather seal/boot.

Senior_Rider2

I think you are correct. I look and feel the edge of the :"tip" and feel nothing that feels like metal that has been crimped, but it appears a piece of that tip has broken off in front allowing the tip to list backwards. That also explains why the exposed metal below the tip feels and looks smooth. Is this normal construction for a suspension seatpost? If so, it seems it is designed to fail.

I have some back issues and needed a bike that I could ride in an upright or nearly upright position for most of my ride. I tried several bikes at several different bike shops and this was the most comfortable ride by far. The Senior_Rider2 was created in order to attempt to post an image of seatpost since Senior_Rider had reached his 5 post limit in a 24 hour period. Even that maneuver was a challenge since the forum didn't like me creating a second account from the same IP address (within 24 hours). So I created the second account from a "different" IP address. I will request second account be deleted tomorrow.

WizardOfBoz

I suspect that the tip of your seatpost was crimped onto the lower part of the mechanism, and the driver for doing that was cost. Not sure how much actual shock absorption the seatpost really did, but if your back is bad and you want some shock absorption I think that there are a lot of options. Or, just buy a straight post. Pull that seat post and measure it and you'll know which seatposts you can buy.

I think that the seatpost design is probabaly adequate for a more upright seat post: the bending moment on the post would be lower. But your lower angle would be expected to put more force on the thing.

fietsbob

Why ? Metal fatigue.. those posts are only $25 , no incentive to make them better @ that price..




need the suspension? the good ones are $200 not 25...



..

...

Senior_Rider2

Thanks for all advice and information. Shearing was definitely not the right word for what occurred. I wonder what would have happened if I had shifted weight backwards on seat at speed rather than at a stop and seat continued tilting backwards. I am not sure I could have recovered unscathed. And so it goes....