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-   -   Folders and Shearing of Seatposts... (http://www.bikeforums.net/folding-bikes/137670-folders-shearing-seatposts.html)

v1nce 09-10-05 06:28 PM

Hey,

Just wondering if you guys had any experience with seatpost breakage or bending. Quite a few folders require that you have a rather long (not always superstrong or especially beefy) seatpost sticking out of your frame, usually much longer and exposed than a regular bike. Particularly if you like riding high like me. I for example have a a Alu seatpost that is not folder specific, it sticks out a lot. I was talking to someone the other day and she had actually seen someone shearing their post and this wasn't even on a folder/a super long post. Sounded scary.

Of course i stick to the minimum insert recommendation and don't do anything too extreme on my folder.

There is a lot of leverage involved in a long post. Shearing your post sounds pretty messed up, but maybe one could even damage the frame that way?

Would it be better to mount a steel post or...?

Any opinions or experiences?

CaptainSpalding 09-10-05 08:27 PM

The problem you envision is typically avoided by one of two methods: either the seatpost is a larger diameter than found on non-folders, or the wall thickness of the seatpost is greater. No need to go to steel.

As far as stress cracks in the frame near the top of the seat tube, it is not unheard of on some bikes.

jasong 09-11-05 01:30 AM

Quote:

Originally Posted by v1nce
There is a lot of leverage involved in a long post. Shearing your post sounds pretty messed up, but maybe one could even damage the frame that way?

It does seem strange that the only minimum spec given is that from the post mfctr. The frame guys should also give a max force idea as well (torque = f x l) through specifying a max seat post for a given rider weight.

CaptainSpalding 09-11-05 11:48 AM

Quote:

Originally Posted by jasong
It does seem strange that the only minimum spec given is that from the post mfctr. The frame guys should also give a max force idea as well (torque = f x l) through specifying a max seat post for a given rider weight.

My bet is that there is no frame manufacturer that conducts destructive testing to establish a such a specification.

Just a more general word about alloy vs. steel frames: Aluminum and steel have very different fatigue characteristics. When talking in terms of flex cycles, aluminum fatigues many times sooner that steel. Even minor flexing can contribute to metal fatigue which will over time lead to the failure of aluminum components. It is for this reason that airplanes are "retired" after a given number of hours in the air. The odds of a structural failure grow and grow. So, with regards to alloy frames and seat posts, it should be considered that they have a finite lifespan. If ridden long enough, even without abuse, they will fail. Steel has a lifespan too, of course, but it is probably longer than the lifespan of the rider.

v1nce 09-11-05 12:16 PM

Hmm interesting! Thanks for the info. Makes me even happier that i own a Raleigh Twenty (steel!) folder. And makes me somewhat reconsider ever getting a Alu Swift. The weight savings are somewhat moot if durability is so heavily affected. Hmm.

folder fanatic 09-11-05 02:17 PM

My personal favorite components happen to be a good quality mix of steel along with a few aluminum ones for certain parts (like fenders/mudguards, racks, some handlebars.) My frames are always a good quality steel. I never need to consider "retiring" anything about the bikes since the times I might need to, I might not have the cash to do so.

af895 09-11-05 07:31 PM

Quote:

Originally Posted by CaptainSpalding
My bet is that there is no frame manufacturer that conducts destructive testing to establish a such a specification.

Just a more general word about alloy vs. steel frames: Aluminum and steel have very different fatigue characteristics. When talking in terms of flex cycles, aluminum fatigues many times sooner that steel. Even minor flexing can contribute to metal fatigue which will over time lead to the failure of aluminum components. It is for this reason that airplanes are "retired" after a given number of hours in the air. The odds of a structural failure grow and grow. So, with regards to alloy frames and seat posts, it should be considered that they have a finite lifespan. If ridden long enough, even without abuse, they will fail. Steel has a lifespan too, of course, but it is probably longer than the lifespan of the rider.

You said it better than I could have.
I'm a heavy guy (losing weight fast thanks to biking - YAY!) but I'd be genuinely worried about destroying an aluminum folder's frame on account of my height (6'0") and weight (238lbs) putting a lot of torque on that seat-tube. (glad to have a CrMo frame!)

BTW: the seat-POST on my KHS is NUTTY beefy. It's a 29.2mm diameter (that's not a typo) and the wall thickness must be 3mm. It weighs something like 2lbs. (!) I'd be more worried about the frame than the seat post.

Good thread - this kind of issue should be well understood by people selecting folders to avoid nasty surprises.

jasong 09-12-05 11:53 AM

I don't think destructive testing is going to be a necessity to determine guidelines or recommendations for those components. These forces are well understood and well analizable and should not require empirical testing.

Agreed on material selection. Steel rarely should result in a frame weight difference of more than 2lbs if using decent tubing. Lifetime of the frame and maintainability (ie. easy to repair or add things like brazeones) easily justify that difference.

Quote:

Originally Posted by CaptainSpalding
My bet is that there is no frame manufacturer that conducts destructive testing to establish a such a specification.


hubs 09-12-05 02:41 PM

34mm carbon on my Speed Pro. It seems thick, but I don't have another carbon to compare to ... so maybe it's just carbon thickness.


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