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Seatpost Minimum Insertion?

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Seatpost Minimum Insertion?

Old 08-04-14, 09:04 AM
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jyl
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Seatpost Minimum Insertion?

140 lb boy on a Columbus SLX road bike frame. Stock seatpost is a short 26.6 mm (which seems narrow to me, but it is the stock post), it is about 200 mm long.

His legs are long and the seatpost has to be raised quite a lot - past the minimum insertion line. About 2 inches of post is inserted, the end of the post is appx 1/2" below the bottom of the top tube (to clarify: where the underside of the top tube enters the seat lug).

What do you think - safe, or do I need to find a new 26.6 mm post? It isn't the easiest diameter in which to find an interesting vintage post, especially when shopping the coop used parts bins, so the bike would probably get a cheapo modern post.

Last edited by jyl; 08-04-14 at 12:42 PM.
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Old 08-04-14, 09:41 AM
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I think the safe amount is what's printed on the post. It would be irresponsible to suggest anything less is "OK." The judgment is yours to make.
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Old 08-04-14, 09:51 AM
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Think of the consequences of failure of the post; you could potentially damage the post, frame and rider all in one go.

If it were my child/friend riding that bike I would look for a less "interesting" and more "safe" seat post.

Or perhaps a frame which does not require the seat to be raised so much would be safer and more appropriate.
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Old 08-04-14, 09:53 AM
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extended seat tube frames over extend the marked line .. and frames break, [Look in past posts here on that]

in that case the bottom edge has to be below the bottom weld where the top tube joins the seat tube.

old style frames where the seat tube is no longer than the top of the seat tube may be OK following the marked line as given.


btw jyl, kalloy makes posts in all diameters , and a 1" post + a sizing shim will make it fit 26.6 frames.. too
(Redline has a 1" Chromoly post that is long and strong)

its smaller ID because seat tube walls are thicker.

Last edited by fietsbob; 08-04-14 at 09:58 AM.
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Old 08-04-14, 09:54 AM
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If the bottom of the seatpost is lower than the lowest weld -- top tube to seat tube and seat stays to seat tube -- you'd stand a better chance of not breaking the frame and injuring your kid...

A bike shop should be able to order you a post in this size, especially if it has one of the older seatclamp assemblies which basically just bolt onto the end of the round seatpost top.

Or he's on the wrong size bike. Get a bigger bike and swap out a stem to fit.
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Old 08-04-14, 10:40 AM
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Safe and unsafe aren't digital. There's a large gray area between what is safe for sure (the minimum mark) and what is unsafe for sure (insertion of less than 1" or so). So having the minimum mark 1/2" up in this case is definitely pushing the limit, but IMO not earthshakingly so.

The problem with safety margins is that all the factors add and subtract, so it's not only the insertion depth, but the rider's weight and road conditions which determine how far into the gray area one might go.

I never advise that things are safe, only when they're unsafe. But at 140#s and 2" of post inserted, I'd probably go with it if it were my bike and son, and it was only an interim thing. If this is long term, or if your son is growing and will need a longer post anyway, let him ride it while you start shopping for the replacement now.
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Old 08-04-14, 09:45 PM
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you dont need a longer seatpost


you need a bigger bike
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Old 08-05-14, 04:41 PM
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Originally Posted by xenologer View Post
you dont need a longer seatpost


you need a bigger bike
You haven't seen the bike. It's a beauty, with vintage Mavic parts. I say get a shorter son!
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Old 08-05-14, 05:39 PM
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Originally Posted by gaucho777 View Post
You haven't seen the bike. It's a beauty, with vintage Mavic parts. I say get a shorter son!
It's always good to see all the sides of an issue!
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Old 08-05-14, 05:55 PM
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My shop rule is no less than 3".

However, that's because of my liability insurance. IMO, it depends on the rider's weight, riding style, and what material the bike is made out of.

I've seen 120lbs. lady riding a bike with 1.5" inserted post on a steel frame without an issue.
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Old 08-05-14, 06:11 PM
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A pic of the bike in question would be nice to help judge if the bike is just two small or you need a longer post. I would recomend you go with a better quality 26.6, 300 to 400mm post there are quit a few vintage and new avaible in the right size. On a bike a a seat post that is not the right size or length needed is basicaly a no no. Also a longer correct size post will tend to solidify the ride of the bike often times a lot. Suck it up and get the right post you can order fairly nice one in black or silver from 200-400 in nearly any size for $30-40.

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Old 08-05-14, 09:01 PM
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The seat post should be inserted at least to the bottom of the top tube if its above the top tube you risk cracking the seat tube and welds, or use the seat post guide line that's stamped on it.
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Old 08-05-14, 10:09 PM
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Thanks everyone. I'll fit the bike to him and see precisely where the post ends. Then I'll assess.

There is a "game" aspect here. I'm trying to keep the completed bike under $200. It's just an arbitrary goal for fun - I can, fortunately, spend $40 on a post and if need be I will.
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Old 08-05-14, 10:18 PM
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kalloy are $20
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