Seatposts, layback, and a daft idea

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reohn2
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Re: Seatposts, layback, and a daft idea

Postby reohn2 » 10 May 2013, 5:34pm

531colin wrote:
Talking of confusion, is there an accepted way to measure seatpost setback? Engineers usually measure stuff centre to centre, but if you measure seatpost setback from the centre of the post to the centre of the clamp, then I think that will vary depending on the seat tube angle, and you could have 2 posts with 25mm setback, but the one with a 20mm wide clamp will put the saddle 10mm further back than the one with a 40mm wide clamp....?? :?



I've always measured layback from the front of the seatpost to the front of the clamp.Its the only way that makes any sense to my mind.
As you say any other method can be variable.
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CREPELLO
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Re: Seatposts, layback, and a daft idea

Postby CREPELLO » 11 May 2013, 9:07am

531colin wrote:
Talking of confusion, is there an accepted way to measure seatpost setback? Engineers usually measure stuff centre to centre, but if you measure seatpost setback from the centre of the post to the centre of the clamp, then I think that will vary depending on the seat tube angle, and you could have 2 posts with 25mm setback, but the one with a 20mm wide clamp will put the saddle 10mm further back than the one with a 40mm wide clamp....?? :?
crepello wrote:A related question. Is there any way of accurately working out seat post layback? If the measurement is merely taken seatpost centre to clamp centre, it ignores the width of the clamp (looking sideways on). So a wide clamp like on a Campag post (40mm IIRC) gives poor layback - the clamp hits the limit of the saddle rails very easily. Whereas the narrow clamp on the Grand Cru (29mm) ensures what is already a good c-c layback. The shape/position of the front of the clamp is critical for determining layback.

So what's the most reliable way of making the measurement? I can compare most of my seatposts which are all 27.2mm, by taking the measurement from the front of the post to the front of the clamp. But it's not a universal way of getting the measurement. Measure a wide seatpost and the figure given will be false in relation to the 27.2, because of the extra thickness of the seatpost.
SNAP! From viewtopic.php?f=5&t=75623&start=15
As you say, too many threads running. Funny, this question has been going round my head for ages - I never read your post until now!

reohn2 wrote:
I've always measured layback from the front of the seatpost to the front of the clamp.Its the only way that makes any sense to my mind.
As you say any other method can be variable.
It's not quite foolproof though - the measurement is going to be different between 25 and a 31.6 posts of the same design. But for getting the best measurement of the post I've got in front of me/on the bike, it's the better measurement to take.

reohn2
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Re: Seatposts, layback, and a daft idea

Postby reohn2 » 11 May 2013, 11:39am

CREPELLO wrote:It's not quite foolproof though - the measurement is going to be different between 25 and a 31.6 posts of the same design. But for getting the best measurement of the post I've got in front of me/on the bike, it's the better measurement to take.


Point taken,hadn't thought of that,mainly because most bikes I've owned were 27.2 with only one 31.8 and I used a reducing sleeve to 27.2 for that on.
A much better way would be centre line of s/post to front of clamp.
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Brucey
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Re: Seatposts, layback, and a daft idea

Postby Brucey » 11 May 2013, 6:45pm

I vote for centre to front as a measurement.

Mind you this still isn't unequivocal because;

1) some clamps have oval entry holes and allow the saddle further back
2) some seat pins vary setback with saddle rail tilt, and not always in the same way e.g. hockey stick vs typical serrated clamp type. Saddle rail angles vary with setup and saddle design.

cheers
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breakwellmz
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Re: Seatposts, layback, and a daft idea

Postby breakwellmz » 14 May 2013, 6:05pm

Did(?)KOPS today.

I was 10mm back from where i`should`be,moved the saddle forward by the same amount.
Will i notice this amount of change i wonder,will my knees have an easier time?

Cheers

reohn2
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Re: Seatposts, layback, and a daft idea

Postby reohn2 » 14 May 2013, 6:20pm

breakwellmz wrote:Did(?)KOPS today.

I was 10mm back from where i`should`be,moved the saddle forward by the same amount.
Will i notice this amount of change i wonder,will my knees have an easier time?

Cheers

I'd be surprised if you didn't notice a difference.
No your knees won't have an easier life because they're used to original position 10mm further back.
IMHO10mm is a lot to move a saddle in one go if you've been riding like that comfortably for a long time.
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NATURAL ANKLING
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Re: Seatposts, layback, and a daft idea

Postby NATURAL ANKLING » 14 May 2013, 7:44pm

Hi,
10 mm forward will equate to 3mm lower saddle.
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breakwellmz
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Re: Seatposts, layback, and a daft idea

Postby breakwellmz » 14 May 2013, 7:55pm

Thanks chaps.

That was on the bike with the 71 degree seat tube.
IF it stops raining,i might check the same on my other bike(74-5 degree ST)

This is the first time ever that i have done this remember! :oops:

Cheers

torqueless
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Re: Seatposts, layback, and a daft idea

Postby torqueless » 19 Oct 2013, 11:31am

Image
DIY seatpost layback- Bending a plain 27.2mm steel seatpost- 2" of layback (centre of post to centre of clamp). I can get the nose of the Brooks Pro almost 4" behind bottom-bracket centre on this 23" 73deg. parallel frame. About 3 3/4" in the photo.

reohn2
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Re: Seatposts, layback, and a daft idea

Postby reohn2 » 19 Oct 2013, 12:02pm

TBH that doesn't look safe and I'd suggest the seatpost's strength has been severely compromised,if it breaks it'll bring tears to your eyes :shock:
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torqueless
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Re: Seatposts, layback, and a daft idea

Postby torqueless » 19 Oct 2013, 1:34pm

It probably looks flimsier in the photo than it actually is. It's a new steel seatpost. Fairly thick mild steel afaict. If the seam had failed or the metal cracked in the bending process I'd have abandoned the idea. I concede there is room for improvement in my bending technique! Maybe not for heavy loads or heavy sitters, but a very cost-effective solution to this problem.

I take your point though, rest assured I'll be keeping an eye on it. Thanks.