02GF74
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 posted on 18/11/08 at 09:32 AM |
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OT seat post stress anaylisis
Any one have access to one of these stress anaylisis packages to do a quick anaylsis? (sorry its in here as AE does not allow attachments)
I would like to see the stresses on the seat post of a mountain bike and to find out how long the post must continue insdie the seat tube. Aslo and
if drilling holes in the seat post furthest from where it enters the tube has an effect on the strength.
I guess I am gonna have to supply dimensions, materials, riders weight etc taht I don't have at hand.
Ta.
 
Rescued attachment sp.JPG
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dinosaurjuice
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| posted on 18/11/08 at 09:39 AM |
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all seat posts are marked with a 'minimum' line when they leave factory. usually based on rider weighing 16 stone. some fancy carbon posts
have a weight limit though.
as a general rule for aluminum posts, always have at least 100mm inside the frame. 
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Mr Whippy
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| posted on 18/11/08 at 09:41 AM |
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use a carbon tube rather than cutting holes in it, as if it snaps it will be very painful and you will be in the papers as some weirdo who has sex
with a bike 
Fame is when your old car is plastered all over the internet
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balidey
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| posted on 18/11/08 at 09:58 AM |
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I have cosmos express and in theory I 'could' do it as the software is more than capable of it, but as I have said many times before on
here, using FEA to get a result and using FEA correctly to get the correct result are two very different things. Its really a job left to someone that
is fully trained and uses FEA all day everyday.
So I would be wary of anyone offering to do it.
Also (again I'm sure I've mentioned on here before) you must remember what FEA is for. It is NOT to get a result that you use. You must
back it up with testing. Anyone that knows anything about FEA knows this. All FEA is, is a tool to help you reduce the number of tests you do. ie you
could have 20 seat posts with different hole sizes and patterns. FEA will tell you which 15 to disregard and then you can test the other 5. Then you
compare the results with what the FEA predicted on those 5.
Hope I haven't put a dampener on things. But my company has subbed out lots of FEA work and some of the results are no where near what we
calculated in house. Some of the test results were close, some weren't.
The hardest part about doing FEA is knowing when to not do it, know your own limitations.
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Mr Whippy
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| posted on 18/11/08 at 10:01 AM |
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^ very sensible
Fame is when your old car is plastered all over the internet
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02GF74
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| posted on 18/11/08 at 10:03 AM |
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^^^ I hear you.
I'd still be interested to see the stresses though.
Let's make it simpler; a post with 10 mm, 20, 40, 80 mm insertion into the seat tube. A rider weight of 100 kg
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Syd Bridge
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| posted on 18/11/08 at 10:16 AM |
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Do you know of Joe Higgins? He's well known in mountain bike circles and specialises in this stuff, and did his M.Eng with my eldest. Now
working for a bike co. in Switzerland I think.
FEA will show you the max stress at the post/tube interface/break line, but not much else. Simple bending and shear calcs will give you an accurate
figure, without having to resort to FEA, which is as put by Balidey. Just a helpful tool, but not ultimately definitive.
Cheers,
Syd.
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Mr Whippy
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| posted on 18/11/08 at 10:21 AM |
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quote:
Originally posted by 02GF74
^^^ I hear you.
I'd still be interested to see the stresses though.
Let's make it simpler; a post with 10 mm, 20, 40, 80 mm insertion into the seat tube. A rider weight of 100 kg
Prob I see is unless you are on a road bike that has no pot holes you are not going to be able to predict the shock loads the post will see. On such a
critical part I'd never dream of taking it anywhere near the limits that it might fail for the sake of what? A few minuscule grams saved. IMO
not even close to being worth looking at and at best just plain dangerous.
If the bike seems to be at the limits…have you considered butt liposuction or shaving your hair? Finger nails serve no purpose for cycling and weigh a
few grams as does your teeth, they are very heavy (especially if you have lead fillings) and just obstruct airfow when breathing harming performance.
Simply get them removed and see your times improve
[Edited on 18/11/08 by Mr Whippy]
Fame is when your old car is plastered all over the internet
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nick205
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| posted on 18/11/08 at 10:33 AM |
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Also consider the fact that the seat post is not a perfect fit with the frame's seat tube. i.e. the post is pinch bolt clamped where it enters
the seat tube and inevitably there will be some flex at that point. The end of the seat post will move slightly within the seat tube causing further
stress and shock loading.
Not sure what you're considering, but IMHO you shouldn't be modifying a manufactured part which "should" have been designed
and tested to work within acceptable saftey limits in it's intended use.
USE, Easton and many others produce some fairly trick carbon and alloy seat posts which weigh very little.
BTW - do you still have that Whyte MTB lurking in the garage. I've alsways been fascinated by the front suspension design.
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balidey
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| posted on 18/11/08 at 10:33 AM |
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Done it, the answer is 39.9
http://s204.photobucket.com/albums/bb270/balidey/?action=view¤t=POST.jpg
Just to show you how quick and easy it is to 'do FEA' and because of this it is considered to be easy to do. But obviously its all
b011ocks. I know that, but someone with out self restraint could easily have done similar and told you a different answer.
Does look pretty though when you get the nice blues, yellows and reds.
And thats another misconception. Red is NOT areas of failure.
[Edited on 18/11/08 by balidey]
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Mr Whippy
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| posted on 18/11/08 at 10:37 AM |
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quote:
Originally posted by balidey
Done it, the answer is 39.9
http://s204.photobucket.com/albums/bb270/balidey/?action=view¤t=POST.jpg
Just to show you how quick and easy it is do 'do FEA' and because of this it is considered to be easy to do. But obviously its all
b011ocks. I know that, but someone with out self restraint could easily have done similar and told you a different answer.
Does look pretty though when you get the nice blues, yellows and reds.
And thats another misconception. Red is NOT areas of failure.
[Edited on 18/11/08 by balidey]
go and stick some holes in it want to see more red
Fame is when your old car is plastered all over the internet
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balidey
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| posted on 18/11/08 at 10:42 AM |
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What am I, your FEA biatch?
Sorry, not much red around the hole (sure thats a punchline to a joke  )
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Mr Whippy
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| posted on 18/11/08 at 11:02 AM |
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wonder how much weight did that saved?
Fame is when your old car is plastered all over the internet
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balidey
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| posted on 18/11/08 at 11:05 AM |
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On my model with 2mm wall thickness (no idea what it should be) in plain carbon steel (again, no idea if thats correct) with a 12mm hole (again....
ditto) then it saved about 5 grammes.
Its probably better to have a nice big poo before you go riding. Not that I've ever weighed my own poo before 
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Mr Whippy
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| posted on 18/11/08 at 11:40 AM |
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the wind has much more effect, its quite fierce really. I get up to around 40mph on mine and it feels like I’m going though treacle the winds so
strong and I was considering that a recumbent bike would show the greatest efficiency, especially if a wind defector like this was fitted – not sure
if I’d feel happy on such a strange machine though as I’m use to normal frames?
Fame is when your old car is plastered all over the internet
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nick205
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| posted on 18/11/08 at 01:38 PM |
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quote:
Originally posted by Mr Whippy
the wind has much more effect, its quite fierce really. I get up to around 40mph on mine and it feels like I’m going though treacle the winds so
strong and I was considering that a recumbent bike would show the greatest efficiency, especially if a wind defector like this was fitted – not sure
if I’d feel happy on such a strange machine though as I’m use to normal frames?
FWD too, now that might take some getting used to on a pedal bike!
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Mr Whippy
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| posted on 18/11/08 at 01:48 PM |
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might explain his expression
this looks a bit more normal but should still be good at cutting through the wind
[Edited on 18/11/08 by Mr Whippy]
Fame is when your old car is plastered all over the internet
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balidey
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| posted on 18/11/08 at 02:24 PM |
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MEB
Man Engined Bike ?
Also, 02GF74, what are you trying to achieve, are the holes for adding lightness? Or for mounting something ?
[Edited on 18/11/08 by balidey]
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02GF74
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| posted on 20/11/08 at 12:19 PM |
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I've had two seatposts snap and not surprisingly is is always at the point where they leave the seat tube, the red area in the diagrams.
Posts do have the minimum insertion mark but I was wondering what effect does the amount of post within the tube have as far as it snapping.
Not having a post nearby, it is someting like 100 mm; I cannot see the end ofthe post (furthest away from the clamp) contributing a lot so drilling a
few holes in the sides of it should not compromise it.
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