mr. vegas, retro seatpin fitment

[wallymann]

finally got my reasonably-healed-and-somewhat-fatter-ass motivated enough to polish a SR seatpin i had in the parts-bin and fitted it up to my gazelle. i knew the gazelle was on the threshold with suitability for the stubby old-school seatpin.

after getting the saddle position dialed-in, look like i'm 5mm or so above the limit-line. given how overbuilt old stuff is, i figure i'm still in no-big-risk territory. gonna head out to the 7-11 for some lotto tix next!

fwiw...i forgot how nice a SR seatpin is to set the angle, despite being a 1-bolt design. on modern posts i prefer a 2-bolt for precise setup. but with a SR there's no appreciable difference when fitting up in terms of convenience or micro-adjustability.

[merckx]

It is not the post that is at risk, it is the frame.

[wallymann]

Originally Posted by merckx

It is not the post that is at risk, it is the frame.

5mm doesnt seem like a big shift to "less safe"...think a lugged 531 frame built for big-boned dutchmen is at risk?

[christian]

I like to see at least 30mm insertion below the bottom of the top tube. If you have that, I think the frame is safe. So total of ~60mm inserted.

[merckx]

There are a lot of "depends" in question here, of course. The basic physics suggest that the longer the lever above the seat lug, and the fewer mm's inserted below the seat lug, the greater the risk for things to go pear-shaped. I've seen some of those "depends" become realities.

[oldpotatoe]

Originally Posted by merckx

There are a lot of "depends" in question here, of course. The basic physics suggest that the longer the lever above the seat lug, and the fewer mm's inserted below the seat lug, the greater the risk for things to go pear-shaped. I've seen some of those "depends" become realities.

I agree. It may be fine, 'may' not. I like twice the thickness of the top tube, below the bottom of the top tube...in terms of SP insertion. So, if top tube is 3cm, 6cm below bottom of top tube. General rule of thumb.

[wallymann]

15mm below the bottom-edge of the TT; call it 45-50mm total insertion.

so splain to me what the failure-mode is we're guarding against?

found this thread: "Insertion is just to protect the frame from the leveraged loads exerted by the seatpost." http://forums.thepaceline.net/showthread.php?t=88084

has anyone ever seen a steel frame where the failure was caused to the frame by insufficient seatpin insertion? i have no doubt it has happened, i'm just curious to see what the failure-mode looks like.

[wallymann]

i did some googling, every single seat-tube/seat-lug failure i've seen was on a TIG'd frame (steel, Ti, Al). not a single one on a lugged steel frame. while not conclusive it does suggest something.

in any case, here's my bike's situation...while i wouldnt mind another 10mm of insertion i'm comfortable with this amount, it goes all the way to the lower tip of the seat-lug.

[Black Dog]

Originally Posted by wallymann

i did some googling, every single seat-tube/seat-lug failure i've seen was on a TIG'd frame (steel, Ti, Al). not a single one on a lugged steel frame. while not conclusive it does suggest something.

in any case, here's my bike's situation...while i wouldnt mind another 10mm of insertion i'm comfortable with this amount, it goes all the way to the lower tip of the seat-lug.

I would ride it like that at my weight of about 155lbs. If you are more of a clydesdale then there could be an issue. 531 is solid stuff and if the seat lug was 5mm higher you be at the insertion mark and probably not have even thought about this.

[merckx]

Looks like a cool machine. Can you post a full profile?

[wallymann]

Originally Posted by merckx

Looks like a cool machine. Can you post a full profile?

here ya go!