Wondering about seatpost setback and proper fit

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Catagory6
Posts: 399
Joined: Thu Feb 09, 2017 2:36 am

by Catagory6

is the need for a 0-setback seatpost an indicator that the frame is too large?
is there a proper amount of seatpost setback?

by Weenie


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fa63
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Location: Atlanta, GA, US

by fa63

The answer is; it depends :-) Some people indeed put zero-setback seatposts on frames that are too big for them, to shorten their reach to the handlebar. Others may legitimately need a zero-setback seatpost because they have short femurs, or the frame is designed around a zero-setback seatpost.

The proper amount of setback depends on your physiology, and your fit preferences. For example, I run a good amount of setback (~9 cm) because I am on the tall side (183 cm) and have long femurs.


2lo8
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by 2lo8

No, it's an indicator that the seat tube angle is too slack for your saddle and position of choice, and/or you really don't care about clamping at the ends of the rails. Seatposts are traditionally setback because of those seatposts you find on cheap beach cruisers that have a seat clamp with serrations that attaches to a round seat pin, and Brooks-type saddles have the clamping area very far back (and a whole bunch of nuance and minutiae I will skip over) so that's the way frames have been designed for a century.

If you do a fit the modern (and proper) way centered around the BB, the only thing about the frame that affects seatpost setback is the seat tube angle (assuming the seat tube radiates from the BB).

I for example ride small size frames. If I have a frame with a 76 deg ST and a ~51cm TT, that's basically the same as a frame with a 74 deg ST and ~53cm TT, except I need a 20mm SB post on one, and can use a 0 SB post on the other.
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Wookski
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by Wookski

fa63 wrote:
Mon Dec 10, 2018 3:25 am
I run a good amount of setback (~9 cm)
Which seatpost provides 9cm of setback?

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Calnago
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by Calnago

^ He’s talking about saddle setback, not seatpost setback, or “offset”. Saddle setback is the horizontal distance from a vertical line through the BB to the tip of the saddle. Seatpost offset is the perpendicular distance from a line along the center of the seatpost to the clamp center at the rails.
Saddle setback is only transferable between bikes if you’re using the same saddle and fit. For example, a short stubby saddle (Specialized Power for example) might have a very different saddle setback number for essentially the same fit as a more traditional saddle of 275mm in length. I run a saddle setback of ~95mm, with an ideal seatpost offset of 20mm, and an ideal seatube angle of between 72.5 and 73.0 degrees. That ensures that when properly fit (for me), the saddle is clamped pretty much in the center of the rails, allowing not only for a nice aesthetic, but also a good distribution of force on the rails. Versus a saddle that is clamped at one extreme end of the rails or the other, which hurts my eyes. And if the saddle rails could talk, I’m sure they would ask for the clamp to be moved more to the center.
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diegogarcia
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Joined: Sat Apr 24, 2010 7:31 pm

by diegogarcia

I always consider if a bike supports or can take more inline options as not blessed with the longest legs out there. Not short by any stretch but seem to get more out of a bike in an inline position and longer top tube for me. I think it a great option.

kafreeman
Posts: 119
Joined: Thu Jul 23, 2015 8:46 pm

by kafreeman

I see lots of frames at 73+ degrees of seat tube angle now, so setback would need to increase as the tube gets steeper.

My frame is 72.5 degrees, I have my saddle at ~77cm from the bb. That puts the center of the post at ~23mm behind the bb. I have run a 12mm and 0mm setback. I wound up with the saddle jammed up to the front with the 12mm and poor angular adjustability, so went to a 0mm setback, now I am centered on the rails.

Cross bike is at 73.6 degrees. Same distance to the top of the saddle, center of the post is at 21 cm behind the bb. seatpost has a 12mm setback. Its a bit towards the front of the rails, but a 20 would be at the back.

I can get both of them in the same spot with a drywall square centered on the bb.

When I first bought my road bike, it had a 25mm setback, and I still shoved it all the way forward. Stem was a 110mm +8 degree on a full stack of spacers. Which one is better though? I dont know. I had better fitness two years ago. 5 years ago i started messing with the setback and was at 12mm. Now im heftier and slower. Cant blame that on the setback though.

zefs
Posts: 438
Joined: Sat Aug 05, 2017 8:40 pm

by zefs

A good method to find the correct setback is to do the balance test by Steve Hogg.
Also another indication is if you are slipping on the nose of the saddle when applying power to the pedals, that usually means you need more setback to hold your center of gravity. Could also be caused by saddle tilt down.

Bigger Gear
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Location: Wet coast, Canada

by Bigger Gear

https://cyclingtips.com/2018/07/tour-te ... crankarms/

I had to dig up this article from last summer. It was an interesting read, seems as though in an effort to become more aero riders are adopting positions with less setback. And also the shorter cranks, it totally makes sense if you have a very agressive position the shorter crank will keep the hip angle a little less extreme at the top of the pedal stroke. However, I feel these trends in the World Tour peloton do not have as much application to the average cyclist, the main reason being that most average cyclists cannot ride positions as aggressive and low as the World Tour riders.

mattr
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Location: The Grim North.

by mattr

TBH, it does happen, mostly shit fitters or DIY from customers though, they change the bit that is easiest to swap so they don't feel stretched out or cramped, push the saddle all the way back (or forwards) and/or swap the post. Then don't have to muck around with bar rotation/centring/headset preload/spacers and so on.

Also on MTBs, you simply have to have an on trend 50mm stem, so to stop feeling cramped, you get a seatpin with 30mm of offset and ram the saddle all the way back. :/

Maybe the next frame size up would have helped! (Or a 70 - 80 mm stem.)

Mostly fixed by understanding what you are doing in the first place.

zefs
Posts: 438
Joined: Sat Aug 05, 2017 8:40 pm

by zefs

It's a compromise, aero over fit. If they were doing 25km/h average instead of 40km/h none would use those positions I guess.

basilic
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Location: Geneva, Switzerland

by basilic

framebuilder Dave Kirk has this advice:
http://kirkframeworks.com/2009/06/19/riding-tip-3/
you'll end up in a balanced position that reduces strain on arms/shoulders, easy to hold.

zefs
Posts: 438
Joined: Sat Aug 05, 2017 8:40 pm

by zefs

Yes, and you will probably notice that when you set the bike so that you are balanced you are behind kops, which is the reason it is not recommended by good fitters. What is the point of using kops if it makes you slip out of the saddle.

by Weenie


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