Seatpost setback, effective ST angle

Discuss light weight issues concerning road bikes & parts.
eyedrop
Posts: 102
Joined: Wed Jan 27, 2016 5:33 am
Location: Prescott, AZ

by eyedrop

Hello everyone. Im a 5'2" guy who rides a 1999 Trek 2000 WSD 47cm. Im in the market for a carbon seat post that will take my seat tube angle from 75 degress down to an effective 72 degrees. My seatpost diameter is 27.2mm and seatclamp diamater is 35.0mm.

The reason for this is my Retul fit coordinates and fitter said that ideally, I should be on a 72 degree ST. Most racing/training bikes in my small frame size are 74 or 75, So buying a different frame might be difficult to find and costly.

I also dont want to overspend. Im not too concerned with weight or brand. I just want something that is a good value and will relax my ST angle. Going from aluminum seatpost to carbon on an alloy frame should be the way to go, right? Any suggestions? How good are the cheap chinese eBay seatposts?

by Weenie


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kkibbler
Posts: 840
Joined: Tue Sep 09, 2014 9:30 am

by kkibbler

I've never heard of a bike fitter recommending a different seat tube angle. It's more straightforward to recommend more saddle setback, which is what it sounds like you need. If your saddle is already as far back as it can go on your current seat post, find out how much setback your current seat post offers, and buy one that offers more. There are potentially hundreds of options out there.

rheosibal
Posts: 321
Joined: Fri Jun 14, 2013 10:26 pm
Location: LA

by rheosibal

Your fitter should have all that info, as well as suggestions to achieve the fit you need..

Basically, what the dude said above. Find out your seat height from the center of your bb, then calculate and adjust the amount of setback to achieve around a 72STA. Basic trigonometry, SOH,CAH,TOA will help. I'd go for a cheap alloy until your fit is perfected, then go carbon.

Also consider that while your frame's "reach" won't change, your "seat to bar" reach will change depending on the amount of setback you use, so you may need a shorter stem.

Fiery
Posts: 420
Joined: Sat Mar 16, 2013 9:21 am

by Fiery

You will need a seatpost with the setback in the 30-35 mm range, if the ideal 72 degree angle is projected with a zero setback post. The one seatpost I know with this sort of setback is FSA K-Force.

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Calnago
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Joined: Sun Nov 07, 2010 9:14 pm

by Calnago

How much confidence do you have in the fitter you went to? Have you always needed such extreme setback? Seat tube angles in the 72 degree range usually will accommodate the 6'+ kind of guys, whereas a seat tube angle in the 74-75 degree range is pretty normal and generally works well for someone your height. So unless you've got some real abnormal dimensions I might just want a second opinion about your fit.
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fa63
Posts: 2274
Joined: Mon Feb 19, 2007 7:26 am
Location: Atlanta, GA, US

by fa63

Fiery wrote:You will need a seatpost with the setback in the 30-35 mm range, if the ideal 72 degree angle is projected with a zero setback post. The one seatpost I know with this sort of setback is FSA K-Force.

This. The rule of thumb is roughly 1 cm per degree, so 3 degrees would require a 3 cm more setback seatpost.

GT56
Posts: 571
Joined: Fri Jun 05, 2009 9:40 am
Location: Switzerland

by GT56

fitters should just determine where the contact points should be, and stay away from frame design

wingguy
Posts: 3950
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

eyedrop wrote:The reason for this is my Retul fit coordinates and fitter said that ideally, I should be on a 72 degree ST. Most racing/training bikes in my small frame size are 74 or 75, So buying a different frame might be difficult to find and costly.


As stated, the easiest thing would be for you fitter to talk to you like a normal person and tell you how far back in mm your saddle needs to go, then you can just work it out. Although obviously if your seatpost already has some setback it simply won't be possible to achieve that much change.

rpenmanparker
Posts: 216
Joined: Wed Dec 29, 2010 1:47 pm

by rpenmanparker

I agree with others that 30 mm of setback will correct the effective seat tube angle to 72 degrees...more or less. It is easy to find posts with 25 mm setback. That is where the greatest choice lies. It is much harder to find any selection in 30-35 mm setback posts. Assuming your fitter meant that with a 72 degree STA you could center your saddle rails on the seat post clamp with a zero setback post, I would just go for a 25 mm post. So in that case you would still have to push your saddle back another 5 mm in the clamp, but so what. Being off the center that little amount is totally insignificant.
Robert

kulivontot
Posts: 1170
Joined: Sun May 16, 2010 7:28 pm

by kulivontot

Seat tube angle typically goes hand in hand with the length of the top tube of the frame. A steeper seat tube angle counter-intuitively counteracts a shorter top tube to provide a longer reach for a given top tube length. That said, achieving a specific seat tube angle is determined by both the frame AND the seatpost, so to know what is an appropriate angle is requires the specifics of the frame your trying to fit it to. So is the 72 degree requirement specific to the frame in question that you've mentioned? Or is there a specific stack/reach requirement that you're trying to make?
If you have a particularly short torso and long legs, you might find that women's specific frames may fit you better because the larger women's frames typically have a shorter reach for a given stack.

wingguy
Posts: 3950
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

You're thinking about it backwards. Smaller frames have steep STAs so that you can get a shorter saddle/bar reach for a given BB/HT reach. There's a limit to how far the headtube can come back while leaving clearance for the front wheel (unless you use a super slack HTA or 650c wheels) so to make the bike feel shorter the STA comes forwards. The trade off is that the saddle may end up too far forwards for weight distribution and pedalling efficiency.

RussellS
Posts: 738
Joined: Wed Feb 03, 2010 1:31 am

by RussellS

kulivontot wrote:you might find that women's specific frames may fit you better


The original question asker already has a " I'm a 5'2" guy who rides a 1999 Trek 2000 WSD 47cm." This is Trek's Women Specific Design. WSD. I think the 47cm may be the smallest one they made/make. My mother has one of these from the mid 1990s I think. Just overhauled it a week ago. It came with 650C wheels. Not sure about the bike the question asker has. 650C wheels?

For the original question. look for an Easton EC70 or EC90 seatpost. They are from 10-20 years ago. I measured the center of the seatpost to the center of the clamp at 4 cm. Lot of setback on this post. Carbon too.

shimmeD
Posts: 395
Joined: Thu Jun 28, 2012 10:52 pm
Location: eNZed

by shimmeD

I love this site! Everyone's chimed in with their knowledge, and the OP keeps quiet. Some one has actually answered the question, but if anyone else has a similar situation and trigonometry isn't their forte I find this website really helpful http://www.csgnetwork.com/righttricalc.html
Input your seat-height (for me, I measure centre of bb straight up to the centre of saddle) into Box C, and frame seat-tube angle Box a.

And yes, I'm short (5'6") and find that new frames have too steep a seattube at 74-75deg. My 2003 Look 381i is 72 or 72.5.
Less is more.

kulivontot
Posts: 1170
Joined: Sun May 16, 2010 7:28 pm

by kulivontot

@wingguy,
check any frame geometry chart out there. Between frame sizes, the top tube length will decrease by around 15mm, yet the reach will only change by less than 8mm or so. How does this make sense? It's because the seat tube angle will become about 0.5 degrees steeper, meaning that a lesser percentage of the top tube will be behind the bottom bracket (where the reach is calculated). For an individual to obtain the same knee angle between frame sizes (TT fitting excluded), the height and horizontal offset from the bottom bracket should be the same between frame sizes, meaning that even if the seat tube angle is steeper and puts you closer to the handlebars for closer reach, you have to sit back with further horizontal offset to set the knee angle back where it should be.
Check geometry for specialized Tarmac:
You'll note reach does not change between 47cm-54cm, meaning that a smaller frame does not necessarily improve the fit.
Now compare to geometry for specialized Amira
same top tube lengths, different reaches.

tl;dr,
frame sizes are really weird and single measurements in isolation can be misleading.

wingguy
Posts: 3950
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

Yeah, so just like I said they didn't want to reduce the reach / front-centre any further so they cheated it with STA - by a full 1.5degrees between a 49 and 52.

by Weenie


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