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Old 09-06-09, 05:08 AM   #1
Glenn1234
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Seat Post Length

I was working on my seat post yesterday and something occurred to me. The bike I was working on, along with every other one I've ever had, have had seemingly short seat posts and stems.

Now I'm wondering, what is common for seat posts and stems when it comes to length? Are they all like what I've encountered, where you get very little play with them in terms of how much you can adjust them? Or do you get more than an inch or two commonly between the minimum insertion and maximum insertion line (what I'm finding on these bikes)?

I would think these bike companies would want their bikes as adjustable as possible to accomodate as many common body types as possible, but maybe I'm wrong?
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Old 09-06-09, 05:54 AM   #2
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Unless you're working on exotic racing frames or high end aerodynamic posts, a typical "short" seatpost will have about 4 inches of usable height - 6-1/2" overall, less 2-1/2" minimum insertion. Mountain posts of 300mm or longer have 10"and longer working ranges.

I don't know what you're referring to when you say "maximum" insertion line. Unless there's an obstruction in the frame the post can be inserted as far as it will go.
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Old 09-06-09, 05:58 AM   #3
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Hello I am sure all us will have different opinions on this but here is mine. first of all (going a bit backwards) bike companies do realize that people come in different sizes that is why frames comes in different sizes. the frame is bought to fit your general proportions and then the seatpost and stem is adjusted to fine tune the fit.

seat post come in basically road 180mm to about 200 lengths and mountain about 350mm. generaly when adjusting a seat post only a few inches is needed if the frame size is correct. MTB seat post are longer because you need more stand over height but need to keep the seat to pedal distance.

quill stems pretty much can standard as to how much up and down adjustment, again if the frame fits only fine tuning is needed. as for the "reach" (lenght from 'quill' to the bars) this is proportional to size of the bike. and generally a -71d angle. mtb bikes again the reach being proportional but had a more positive angle to allow tou be in slightly more upright position. many companies made 'aftermarket' stems of varing heights and reaches the most comon being the tall Nitto 'technomic'

threadless stems again being proportional to the bike size also come in a wide variety of lenghts and angles. to adjust the fit. however unlike a quill stem gaining height with a threadless stem is dependant on the lenght of the steerer. that is whay most threadless bike seem to have an inch or more of spacers so you can finetune the height of the bars to some extent.

I hope this helps rather than adding further confusion.
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Old 09-06-09, 06:18 AM   #4
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Adding a bit to BG's good summary, road seatposts have had lengths as much as 240 mm (Thomson Masterpiece) to around 280 mm (American Classic). Since 1977 (I think) they've been made with MINIMUM insertion marks, around 3 inches above the end of the post. If your saddle adjustment results in this line showing, you need a longer post. Decent ones are not $$. The "best" ones are $$$$$$$$$$$$, and in a few cases worth some extra money.
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Old 09-06-09, 06:54 AM   #5
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Bikes used to be fit to the rider better than they are today. . along the lines of what BG said. But, with the advent of the mountain bike came longer seat posts . . and with longer posts . . .suddenly the line of proper fit became blurry. Now it's very blurry. . . with very long seatposts on road and mtb's.

Proper bike fit takes time . . .trail and error . . . and experience. This is something our "modern" world often sees little value in.

Even paid mechanics in bike shops these days are sometimes shockingly poor. My neighbor told me he was looking for a new bike because his current one was "a bit small". Yeah . . he's 6' and he was riding about a 21" frame with a 7cm. stem. The mechanic who fit the bike for him raised the bars and seatpost to within 1" of their bottoms. . . not the min. insertion mark . . . the very bottom! I told him to not ride it like that, as those min. insertion marks are there for an important reason! He's a firefighter . . so I told him if he wished to keep his body intact to fight more fires . . . find a larger bike ! The mechanic who raised his bars and post should be banned from working on bikes.
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Old 09-06-09, 09:40 AM   #6
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That, ^^^, is inexcusable.
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Old 09-06-09, 12:15 PM   #7
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That, ^^^, is inexcusable.
Agreed.

Even as a kid back in the 70's, the rule of thumb we all used was "never above the lowest top tube weld".



Since BMX bikes then and now are sold by top tube length, not seat tube length, a riders legs had to be accommodated by seatpost lengths. As long as you didn't exceed that insertion rule, you may break a post but you wouldn't break the frame.

Considering that again bikes are being sold by top tube length, or by vague Small-Medium-Large-XL sizes, this rule still applies. Wedge-style, compact and other geometries have really messed with the traditional sizing methods. A large selection of seatpost lengths is the answer to accommodate variations in "people geometry".
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Old 09-07-09, 01:46 PM   #8
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Originally Posted by FBinNY View Post
I don't know what you're referring to when you say "maximum" insertion line. Unless there's an obstruction in the frame the post can be inserted as far as it will go.
A couple of seat posts I've seen have maximum insertion marks.
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Old 09-07-09, 01:56 PM   #9
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Originally Posted by Glenn1234 View Post
A couple of seat posts I've seen have maximum insertion marks.
I think you are refering to "minimum" insertion marks. That is they have a minimum amount that HAS to be inserted into the frame. I've never seen a "maximum" insertion line.
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Old 09-07-09, 10:23 PM   #10
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I think you are refering to "minimum" insertion marks. That is they have a minimum amount that HAS to be inserted into the frame. I've never seen a "maximum" insertion line.
It's hard to get a good picture of a shiny metallic object, but hopefully you can see it on this seat post. If not the line itself, it should be apparent that there's some lettering there as well.
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