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Old 08-30-2018, 12:12 PM
NewDFWrider NewDFWrider is offline
Join Date: Aug 2018
Location: Dallas, TX
Posts: 43
Seatpost setback

What is the fitting rationale for getting a seatpost with a setback on a road bike? It seems like you'd be changing two different things at the same time--lengthening effective top tube and decreasing seat angle (but I'm not sure what that does).

I ask because I just bought a used bike frame, and this is my first time trying to piece together a bike by myself (though I'll probably go to the LBS to actually assemble everything, as I don't have a death wish).

It turns out that the top tube to my new bike (55.9cm) is a little shorter than the effective top tube of what I now ride (ETT 57.3). The seat tube angle on the new-used bike is 73, whereas on my current bike, it's 73.5. (Head tube angle is 72.5 on new-used bike and 73 on current bike.

So now I'm trying to figure out whether I just want to increase my stem length to make up for the difference in top tube length, or whether I want to muck around with a setback seatpost too.
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Old 08-30-2018, 01:20 PM
sitzmark sitzmark is offline
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Join Date: Jul 2013
Posts: 1,097
Where do you want your center of mass located between the wheels for the handling you prefer? What leg extension gives you best power output in conjunction with hip angle and flexibility to reduce aero drag?? More simply what feels comfortable?

It can be relatively simple - how you make all the parts you have work together to JRA, or complex if you are seeking to maximize every possible watt from your your efforts.

Sounds like you've got the basics down and want to engineer a fit on the new bike that is similar to the old bike. Unless the old bike was uncomfortable, you're on the right track.
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Old 08-30-2018, 01:47 PM
pdonk pdonk is offline
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Join Date: Jan 2004
Location: 416
Posts: 1,235
Take this from someone who after riding for 30 years, finally got fit and discovered he's likely been riding the wrong sized bike for all that time. I am going through something similar right now as I try to make my current bike fit close to my incoming bike.

If you like the fit of your current bike, then I would see if you can achieve the same saddle position relative to the bottom bracket with whatever seatpost you have on hand. If you can get the saddle in the right location, then get a longer stem if you want the same nose to bar reach. If you can't get the saddle in the right location, then a setback seatpost may be needed and then figure out your stem situation.

I think this is also why people use stack and reach to describe a frame, so that you are comparing apples to apples (distance from bb to headtube both vertical and horizontal).
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Old 08-30-2018, 01:50 PM
defspace defspace is offline
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Join Date: Sep 2014
Posts: 11
The seatpost is just there to connect the saddle to the bike. Get the right offset to put your saddle in the right spot relative to the BB.

As a rough rule of thumb, every increased degree of STA is worth about 1cm of top tube. So figure your new bike would reach about the same as a 55.5 with a 73.5* STA (normalizing STA between the bikes). You're about 2cm shorter on the new bike - adjust that with stem (assuming you use the same bars). Don't adjust that with seatpost.

Whatever your relationship between saddle and BB is, maintain that. Unless you're at the absolute forward limit of your current saddle, you should be fine using the same amount of offset as the seat post as you have now.

The sort answer to your question is "muck around with your stem to achieve your reach goals. Your current straight seatpost should fit fine. You'll probably have to scoot the saddle forward a few mm on the saddle rails to achieve the same saddle-bb relationship with the new bike."

Edit: beaten to the punch/edited for clarity.
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Old 08-31-2018, 05:03 PM
John H. John H. is offline
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Join Date: Mar 2004
Posts: 3,839

From what you have described- Effective reach could be much shorter on new bike (shorter effective tt, slacker seat angle, and slacker head angle).

Starting from the start- Are you ideally positioned on your current bike?
Is that with a setback post?
Where are you on the adjustment range of the post on the current frame?

Answering these questions should help you to decide if you need a different setback post for your new bike.

Lastly- .5 in seat angle does not change the position of the saddle by more than 5mm, so same post may be fine depending on where you sit currently.
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Old 09-11-2018, 01:34 PM
NewDFWrider NewDFWrider is offline
Join Date: Aug 2018
Location: Dallas, TX
Posts: 43
Thanks everybody for your feedback. If this doesn't work out, and if I ever get around to satisfying my 30 post requirement, you might see my bike frame on the classified section of the forum.
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