Bicycle Mechanics - reducing diameter of my seatpost

Well as some of you may know i recently aquired a schwin homegrown frame. The current seatpost i own is 27.2, and this calls for 26.8. THis is just a crappy aluminum seatpost and its served me well, im too cheap to buy a new one and right now i dont have any money. Soooo i was wondering, since I work in a shop @ school everyday and they have a handy dandy lathe, would it cause any catastrophies if i maybe shaved off .4mm :D

Well as some of you may know i recently aquired a schwin homegrown frame. The current seatpost i own is 27.2, and this calls for 26.8. THis is just a crappy aluminum seatpost and its served me well, im too cheap to buy a new one and right now i dont have any money. Soooo i was wondering, since I work in a shop @ school everyday and they have a handy dandy lathe, would it cause any catastrophies if i maybe shaved off .4mm :DDepends on how much meat is in the post,how much meat on your posterior,and how much post is sticking out of the frame. If it breaks it could be sticking some place you don't want it.Not my idea of a good time.

I would say its close to 4mm thick. Im pretty light myself at around 125. Its a 12" long post, probably about 6 will be sticking out of the frame.

I'd save up for the right diameter post. You can find 'em for less than $20.00 US.

well i do plan to eventually buy a thompson elite or some nice seatpost of the like, so this would only be a temporary thing

I agree with nick burns ... the wall thickness is pretty close to the same for all seatpost; just the outside diameter changes depending on seat tube size. So you'd be creating a thin-wall tube for a seat post. Even considering the inherent factors of safety, I wouldn't chance it. I've got an old seatpost on my fixie that I'm still using instead of a spare (good) one that's slightly too big. Just my 2 cents ...

Turning the seatpost down will not be a catastrophe at all.

That jagged piece of aluminum perforating your testicles may be a catastrophe. Or a castratestrophy.

all that over .4mm?

I've reduced the diameter of the seat post many times. .4mm is not a big deal. I've done it with just file. With a lathe this should be no issue at all. If you are concerned, just reduce as much as you need to fit into the seat tube.

I'm going to play devil's advocate here and say go for it. It's just .4mm, and as long as you don't weight too much, or try to do any crazy stuff on it, then I think you'll be fine.
You only plan on riding it for a short while, until you can buy a new, better seatpost, so I'd say go for it!
You may be able to score a nice used seatpost, or a cheap one at the LBS for around $10-15, if you are really concerned.

I've reduced the diameter of the seat post many times. .4mm is not a big deal. I've done it with just file. With a lathe this should be no issue at all.
Thats what i was thinking, I mean the lathe will do a nice even job. With 4mm of tubing .4mm shouldnt be THAT big of a deal. Im only 125, and its only to get me buy until i get a thompson

I agree with nick burns ... the wall thickness is pretty close to the same for all seatpost; just the outside diameter changes depending on seat tube size. .Wrong...How do you think they make the real light ones?

Just go SLOW and keep on measuring that seatpost as you go, as not to shave off more than .4mm.

Yea i know the instructor always says to go very slowly when using that thing so slowly i will go. This things a fairly heavy seatpost so its got plenty of meat. Im going for it somethime this week. FIrst i have to make a screw for the project

Plus that beast has digital read out :D :D so im pretty sure it will be fine

The current seatpost i own is 27.2, and this calls for 26.8

If I were you... "Safety uber alles" - I'd sell the 27.2 on e-Bay and buy a "real" 26.8. That way, I wouldn't have to worry. Small price for peace of mind. There may be no hazard in turning the one you've but, but I'd hate to find out the hard way that I was wrong!

yea but this is just a no name alloy post. I might get 99 cents, and it costs me 60 cents to list the auction

dont forget that it will be .4 mm total.that only is .2mm to take off. if that makes sense.if not i'll try to explain.I used to mount and sell car wheels and tires and had to explain lift/lowering by the tire highth(sp?)

Should be no problem at your weight. Just make sure the post is well centred in the lathe chuck before you start cutting metal.

I'd say go for it. It's brave people like you who resist the self-spun theories and find out in practical terms how things work. Anyway, it's not for an all-out race bike.

IF the seat post fails while you are riding, it is likely to be something that will give you a warning because it is metal. When my carbon seat post broke, it wasn't a sudden, jagged edge up my backside process. Regular inspection for cracking might be a good idea, at least in the first couple of months.

IF the seat post failed while you are riding, you can report back on the subject and let us know that trimming 5% off the wall thickness was not such a good idea.

IF the plan works, we probably won't ever hear about it because it won't break and you won't bother posting in a year's time to say so.

IF the plan works, you'll have saved yourself a buck a two and learned some things along the way.

I sometimes wondered if I should get an old alloy seat post with flutes along it machined down to fit another bike. Your experience may influence me to get it done. :D

As spanky said, you only need to take off .2mm to make it fit.

Since you have access to a lathe, and I assume other machine tools, have you thought of fabricating your own custom seatpost? You'd be assured of a one-of-a-kind post. All you'd really need to fabricate is the post itself, you could use the seatclamp hardware from your existing post, so long as you fabricated the top to match.

As spanky said, you only need to take off .2mm to make it fit.

Since you have access to a lathe, and I assume other machine tools, have you thought of fabricating your own custom seatpost? You'd be assured of a one-of-a-kind post. All you'd really need to fabricate is the post itself, you could use the seatclamp hardware from your existing post, so long as you fabricated the top to match.
Yea i was thinking about making my own seatpost. Im in this class for 2 more years so why not. THe lathe does have the capability to bore out whatever. I have access to many of these good tools so i figure why not. Im conident .2mm will not effect. THanks all

~$10 (-10% with coupon) plus another 5 or so for shipping doesn't seem that unreasonable http://www.nashbar.com/profile.cfm?category=117&subcategory=1187&brand=&sku=2460&storetype=&estoreid=&pagename=.

Just the opinion of someone who bears the scar (>100 stitches just below the man-bits) from a cycling component failure.


PS: You're aware of the fact that if you rework the seatpost you've altered it's heat treatment, right?

Well as some of you may know i recently aquired a schwin homegrown frame. The current seatpost i own is 27.2, and this calls for 26.8. THis is just a crappy aluminum seatpost and its served me well, im too cheap to buy a new one and right now i dont have any money. Soooo i was wondering, since I work in a shop @ school everyday and they have a handy dandy lathe, would it cause any catastrophies if i maybe shaved off .4mm :D

Since you're still in school I'm guessing that your family isn't complete yet. If that's the case, I wouldn't recommend taking any chances on weakening your seatpost.

Let's see, $19.00 or the potential of a secondary route for the removal of your body's waste. The choice seems obvious to me.

http://www.titec.com/items.asp?deptid=4&itemid=130

PS: You're aware of the fact that if you rework the seatpost you've altered it's heat treatment, right?Gimmie a break here. :rolleyes:

All the people who say dont do it sound like the people who dont ride a bike because "its too dangerous".

I was so afraid, I rode 8000 miles last year (or 12900kms.)

You see, I have this silly notion that the engineers who design bike parts do so with certain parameters in mind. You know, things like a certain wall thickness to support certain weights, insert minimums to prevent collapse, dumb crap like that.

Modifying a structure that supports your weight in order to save <$20 seems just plain stupid to me.

To others, it's innovation.

I was so afraid, I rode 8000 miles last year (or 12900kms.)

You see, I have this silly notion that the engineers who design bike parts do so with certain parameters in mind. You know, things like a certain wall thickness to support certain weights, insert minimums to prevent collapse, dumb crap like that.

Modifying a structure that supports your weight in order to save <$20 seems just plain stupid to me.

To others, it's innovation.
Maybe more like Evil Keneivel(sp?) syndrome?... ;)

All the people who say dont do it sound like the people who dont ride a bike because "its too dangerous".

Risk/Benefit Analysis - If I take the risk and ride my bike, I get multiple returns. If I turn my seatpost and it fails, I've saved $10 and maybe killed myself. DOH....

Gimmie a break here. :rolleyes:


Please enlighten us with you're indepth knowledge of machining, heat treatment, product design and properties of aluminum.

So your saying a reduction of .4mm will do all that? Ive already got at least 4mm thick of tubing, this is not a light seatpost in the least. Some people are not reading the entire thread before posting. I said this is only a temp thing until i get money to buy a thompson elite. Like it was said, its only going to be .2mm off the tubing! Thats not a lot....

Please enlighten us with you're indepth knowledge of machining, heat treatment, product design and properties of aluminum.
Well i doubt my seatpost has heat treating or anything, i mean it its really cheap, heavy and has thick tubing

I'm saying that materials engineers who actually know what they're doing put a lot of time and energy into setting the design parameters for their products. They have to, because they can be responsible for product failures due to their mistakes and misuse by consumers.

Who knows whether .4mm is going to make any difference at all? You don't, and neither do I. But I suspect that the intention of the people who are telling you that it's a bad idea is not to cause you grief, but rather to suggest that messing around with something like this has inherent risks. No one here is going to be able to tell you whether it will last forever or break the first time you go over a jump.

The point is really very simple - some people don't think that designers wake up in the morning and decide to make a seatpost out of certain materials just for grins. There is a reason why the wall thickness is what it is, and why they didn't use 3.6 or 5.8 or 9.2 or 4.1 the first time around. Might be that that tubing was all that was available, might be that it's overkill. Or, perhaps being a cheap post in the first place, they used the bare minimum. Now, with something aiming at your soft tissues, do you really want to find out the hard way that it was already the bare minimum safe thickness before you modified it?

Spend $20 and get something until you can get the Thomson - cost is no excuse.

..There is a reason why the wall thickness is what it is..

Trogon's gotten it right - the fact that your post was a cheap one is the reason that the walls are so thick - higher quality material wouldn't need that thick a cross section. Yes, there is some safety factor built into the product, but your cheap post probably has less safety factor than an expensive one.

Cast aluminum, unlike steel or titanium (which tear before they fail) has a tendency to crack suddenly and without prior warning. You literally take your life in your hands if you mess with cast parts such as stems and seatposts. Aluminum handlebars are another item that should never be reused after a crash - you don't want a failure in the steering while you're riding.

The few bucks you spend on a correctly-sized post are bucks WELL SPENT!

I felt the need to mention something.

Aluminum vs. carbon vs. steel:

steel and carbon are materials that can bend or crack pretty badly and still hold some weight. Aluminum seems fine for a while and then if it "fatigues" or is damaged it can fail without any warning.

One of the reasons I know this is because I used to have aluminum drop bars. I thought I could get away with pounding them into a handlebar stem that was too small. They went in, all right. I used them that way for a few weeks. And then one day when I was 4 miles from home I hit a bump and... off goes my handlebar. it was sort of disconcerting because, sure, I can ride with one hand, but on a road bike it's pretty uncomfortable. and scary if there are cars nearby (there were) or bumps (there were.)

Well i doubt my seatpost has heat treating or anything, i mean it its really cheap, heavy and has thick tubing

Trust me, your seat post has heat treating.

You should just do the math for it. See how much that reduction in cross section reduces the overall mechanical strength of the post. Something similar to below.

http://www.eng-tips.com/viewthread.cfm?qid=1593&page=31

Please enlighten us with you're indepth knowledge of machining, heat treatment, product design and properties of aluminum.Simply put, taking .4mm off a seatpost without significantly raising it's tempature does nothing to the heat treatemnt !! . Assuming it was even heat treated to begin with. Simple concept.Buy it or not.

Its only .2mm off the tubing anyways, if it was more i would not consider. But i mean .2mm, the human eye has a maximum resolution of .1 i believe

Just go do it, and you'll have the biggest crowd since SuperBowel watching your first ride.

I put money (a coupla dollars) on the fact he'll be fine until he gets the seat post he wants. At least he'll be out riding gaining practical experience while most of you lot are theorising.

The alternative (shock horror) is to hone the seat tube so the post will fit. Hehehehe. Now let's see what comments come from that little suggestion!

</troll>

I am searching for one of my favourite links to some guys who would absolutely send the majority of the doomsayers here into a spiral with what they do with frames, forks and assorted other stuff. They've all survived, apparently, without huge injury to their menhood, too.

I am searching for one of my favourite links to some guys who would absolutely send the majority of the doomsayers here into a spiral with what they do with frames, forks and assorted other stuff. They've all survived, apparently, without huge injury to their menhood, too. Darwinism takes a bit of time....

You old cynic, you. Tell me you haven't done something off-the-wall to something in your life and survived!

You old cynic, you. Tell me you haven't done something off-the-wall to something in your life and survived!Yeah, 20+ years of too much booze, nicotine, gluttony,and other assorted depravity,but I don't tell others to do it.And After 23 years of abstinence,Im still not sure I'm square with Darwin.

Just the opinion of someone who bears the scar (>100 stitches just below the man-bits) from a cycling component failure.

OK, I gotta ask... what happened? ;)

I just cant see .2mm making that big of a difference. Im not removing half of the material there, its .2mm! Plus i believe the instructor has teh head treating equipment. If he doesnt the intern who is from the local college (UNH), can do it for me.

So why did you bother asking?

You're clearly a materials expert with knowledge that far surpasses that of anyone here. Go for it, be sure to let us know that it worked out. I know I'm going to lose sleep waiting for the answer.

Here it is for under $10, including postage.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=56189&item=7143686385&rd=1

The cost of the seatpost should be the absolute least of your concerns. The issue of the seatpost failing out weights the other by so much, that you should just get another post. The price is so low that even if the chance of the post breaking is 1%
why risk it? Just skip the question, and get another post. You may well be safe, but unless you know the alloy, the heat treatment, and the stress loading, and what to do with that information when you have it, you could be wrong. You don't mention if it is a casting or tubing? Or the wall thickness.?

For $20 you don't need to learn or care.

The cost of the seatpost should be the absolute least of your concerns. The issue of the seatpost failing out weights the other by so much, that you should just get another post. The price is so low that even if the chance of the post breaking is 1%
why risk it? Just skip the question, and get another post. You may well be safe, but unless you know the alloy, the heat treatment, and the stress loading, and what to do with that information when you have it, you could be wrong. You don't mention if it is a casting or tubing? Or the wall thickness.?

For $20 you don't need to learn or care.

Where's Raiyn when you need him? He undoubtedly would point out that despite all that wonderful engineering prowess, there was a recent recall of seatposts for one brand of bike because they are subject to cracking.

Just a question also, experts. How deep does scoring on an alloy seat post have to be before you ditch it? You know, the scoring that occurs when moving a seat post up and down? I even hear of people scratching marks around an alloy seat post to mark its position on the clamp! Does 0.2mm equate to a bit of a score or scratch like that? On my ruler, 0.2, a fifth of millimetre, looks pretty tiny to me -- about scratch or score depth.

As I said before, just go do it phantomcow2 and put us all out of our misery. Or not. But stop umming and erring on it and decide!

Go to your local bike shop and see if they have the right size seatpost in their 'old junk' box and swap 'em for your present post.
Just another 'cheapo' idea that may be less of a risk and less $.