Seat post length - 250mm or 210mm?

[Oxford_Guy]

Hi - am wondering what seat post length would be best to get for my Mercian Strada frame (see this thread), based on the geometry below - except I've now increased the frame size from 52cm c-t to 53cm c-t after a re-fit, so there'll be 1cm less seat post showing. Top tube is horizontal.

I certainly wouldn't need more than 250mm, but would 210mm be long enough? IIRC  I've read that something like  something like 90mm is meant to be the minimum amount of seat post you need in the seat tube. I'm about 74Kg if it makes any difference.




Currently tempted by a Nitto one...

[Brucey]

250mm. Cut it a bit shorter if it really bothers you.

  BTW a slash cut (longer at the front) provides the most support at the least weight.

cheers

[Oxford_Guy]

250mm. Cut it a bit shorter if it really bothers you.

  BTW a slash cut (longer at the front) provides the most support at the least weight.

Thanks, am not a mega weight-weenie, just don't want to have loads more length than necessary. Slash cut is a clever idea, though.

[Efrogwr]

Please excuse my adding to this question. Oxford Guy mentions a minimum insertion into the seat tube. Where does this start? The very top of the seat tube? The top of the top tube? Some lower point in the seat cluster? And is 90 mm correct?  My bike has a seat tube >30mm diameter, but takes a 27.2mm seat post; I assume that the seat post is sleeved inside.

[Brucey]

it varies.  Seat pins have markings on them but these assume that the frame has a certain strength and loading.  Needless to say the length of the seat pin out of the frame and the weight of the rider makes a difference too. 

So if the frame is small, the seat pin is out miles (and perhaps doesn't fit that well either), and a heavy rider is using it offroad (or has a pedalling style that causes a lot of bobbing) then the leverage in the frame can be huge.  [By contrast in the 1950s folk had their saddles barely above the top tube on their road bikes and I have seen such bikes used with as little as ~1-1/2" of seat pin in the frame; indeed seat pins were often made very short to start with; 6" long (including the 7/8" bit that the clip fits to) wasn't uncommon.]

The former situation is pretty common these days and even 90mm insertion isn't always enough for complete reliability; ideally you want the seat pin to fail before the frame does, but it can easily happen the other way round if the seat pin isn't far enough into the frame. If the seat tube projects above the top tube, in most cases the arrangement is unlikely to be really robust unless the seat pin extends into the frame well below the top tube. I have seen plenty of aluminium frames where the projecting part of the frame has cracked.

If the frame is sleeved internally or a shim is fitted, then you can't usefully have a larger overlap/insertion that does any good.  Usually this is thought through fairly well, but not always. If you have a shim then the best thing is to delete it and fit a seat pin that is really the right size for the frame. It isn't a bad idea to run a reamer into a welded frame to be sure that the seat pin fits the tubing accurately, rather than merely be sized to slip through the distorted welded area, and be a slack fit in the tubing.

If you want a simple rule of thumb, I'd suggest at least 60mm insertion for a  frame with a horizontal top tube and a seat pin less than 250mm long, at least 75-90mm insertion in a compact frame with a longer seat pin, and more again if the seat tube has a projection. But basically the more the merrier, up to a point.  BTW when the seat pin goes into the frame a long way, it isn't sufficient to just grease the seat pin;  the grease is wiped off by the process of insertion. It is a much better idea to grease the inside of the frame and the seat pin; that way there is more likely to be grease for the full insertion depth.

cheers

[Oxford_Guy]

In my case, there's not going to be a large amount of seat post showing - frame has a horizontal top tube (and the seat tube is as long as I can go and still stand over), I'm likely to be using a leather saddle* (e.g. Gilles Berthoud) with saddle rails the a quite deep (compared to modern "road" saddles), I'm not especially heavy (74Kg) and don't bob about much when pedaling.  The frame walls are Coke can-thin 853 Pro Team steel, though, so probably best not to over stress these with too short a seatpost.

* one of the reasons I will also be going for about 20mm setback, due to the shorter clamping area on the rails

[Efrogwr]

Brucey, thank you very much for your detailed and helpful reply.

[rogerzilla]

The saddle rail to top distance varies between about 2" and 3" so you need to allow for that (or measure it first, if you have the saddle).  You want the post 1" below the top tube join on a steel frame so 210mm should be ok allowing for all that and a worst case scenario 2" saddle rail to top height.

All those old Campag aero posts are too short if you are not Italian sized and have your saddle quite high (mine's 95% of inside leg).  The size of frame they require mean the reach to the bars is too great unless you fit a very short stem and, even then, there's no nad clearance when you dismount.  They only work if you have a lowish saddle height and you're still happy to ride a 1970s-fashion large frame with just a fistful of seatpost showing.  Your Mercian is quite a small frame and I suspect you don't run the saddle very high for your height, so you can get away with it.

[Oxford_Guy]

The saddle rail to top distance varies between about 2" and 3" so you need to allow for that (or measure it first, if you have the saddle).  You want the post 1" below the top tube join on a steel frame so 210mm should be ok allowing for all that and a worst case scenario 2" saddle rail to top height.

All those old Campag aero posts are too short if you are not Italian sized and have your saddle quite high (mine's 95% of inside leg).  The size of frame they require mean the reach to the bars is too great unless you fit a very short stem and, even then, there's no nad clearance when you dismount.  They only work if you have a lowish saddle height and you're still happy to ride a 1970s-fashion large frame with just a fistful of seatpost showing.  Your Mercian is quite a small frame and I suspect you don't run the saddle very high for your height, so you can get away with it.

I'll likely be using a Gilles Berthoud Aravis or Aspin (or possibly the thinner, lighter Galibier or Soulor) saddle, as find the Aravis on my touring bike exceptionally comfortable to ride on. I've now measured the saddle rail to top distance, it's just over 2 inches. On the 53cm c-t frame I think I have *just* enough clearance to straddle the frame dismounted without putting pressure on my gentleman's bits. Think there'll be basically just over a fistful of seatpost showing - think a little less than this picture):

[giropaul]

The saddle rail to top distance varies between about 2" and 3" so you need to allow for that (or measure it first, if you have the saddle).  You want the post 1" below the top tube join on a steel frame so 210mm should be ok allowing for all that and a worst case scenario 2" saddle rail to top height.

All those old Campag aero posts are too short if you are not Italian sized and have your saddle quite high (mine's 95% of inside leg).  The size of frame they require mean the reach to the bars is too great unless you fit a very short stem and, even then, there's no nad clearance when you dismount.  They only work if you have a lowish saddle height and you're still happy to ride a 1970s-fashion large frame with just a fistful of seatpost showing.  Your Mercian is quite a small frame and I suspect you don't run the saddle very high for your height, so you can get away with it.

I’m pretty sure that Campag made 2 lengths of seat pin, particularly of the early “ aero” , I.e. flattened model.

[Brucey]

...I’m pretty sure that Campag made 2 lengths of seat pin, particularly of the early “ aero” , I.e. flattened model.

I think you are right, velobase mentions this (for victory/triomphe, 130 or 180mm length) but has no photos of the shorter type. IIRC at least one campag catalogue had a photo of both types side by side, but I can't find it.
[edit; I think this is the short version;


More here;http://www.velobase.com/velobase.com/ViewComponent.aspx?ID=D4B9D11D-DBE8-416E-A8A6-BD8C4B45DC75&Enum=105&AbsPos=18
]



BTW neither length is even remotely compatible with the modern penchant for long seat pins. Also you have to be careful with some versions; the diameter of the round part is sometimes reduced for a distance below the aero section, so you need to have more of it poking out of the frame than you might at first suppose.

cheers