Question about position setback / straight seatpost
 

Hi! Question about position when using a seat post with 20 mm setback compared to a straight seat post if I use the same saddle and maintain the previous seat position forward/backward and height from the bottom bracket or pedals. I myself don't think there will be any change but I'm not a bicycle geometry specialist or bicycle fitter, so I'll take the liberty of asking on the forum as I know there are many here with great knowledge of bicycle geometry.

Thanks & Regards
hsea17

A setback seatpost of 20mm will need to clamp the rails of the saddle 20mm further to the rear of the saddle than the zero post to achieve an identical seat position. Sometimes, that is not possible because the rails don't go back that far.


Zero setback posts were created for MTBs, and are often misused on road bikes that are designed to be paired with setback posts. Make sure you have appropriate setback to your position.

To add to Kontact's post - position rules. If the post you have allows setting your seat in that position, it works. (Well, if you have the seat completely slammed and the rails are titanium or perhaps CF and the start of the bend is in the clamp and that clamp has sharp edges, the rails may break.)

That said, I try hard to get seatposts where the clamp sits roughly centered on the rails. Best for rail long life though that usually doesn't matter, probably best for suspension and shock absorption for your butt and my real reason - it allows any and all tweaking to the most critical fit adjustment on the bike. People pay hundreds for their bike fitting sessions. A very expensive seatpost of that centered setback might cost a single hundred dollars and be a continuing fit blessing the next decade or two.

That said, I paid far more for two custom made seatpost of 160mm setback for my two titanium customs - because I had those bikes made with 74 and 75 degree seatposts - to allow very short chainstays, decently large tires and fender. (My butt wants a normal 20mm setback post on a 72.5 tube.) Posts were made by Ti Cycles with Thompson clamps and titanium tubes. Thompson so a joy to work with. Total keepers and that clamp allows me probably 2 cm adjust in each direction. (But if I were smart, I would have asked about bending the seat tube and using a normal post! Next life!)

I've known about the short chainstay, steep seat tube and therefore big setback needed for my best fit since I raced nearly 50 years ago. Slammed seats were my norm. Broke my first ti railed seat. (I'm light. My butt really likes ti rails on those steep angled bikes.) Rode the compromise of not enough slam for fit to keep the rails intact on several bikes. Then started paying for good big setback posts. Yeah!! (Customs, the lugged steel Nitto and the SR 100 something adjustable MTB posts. Those SRs will be old but probably not expensive. Max setback is huge. The steel Nitto has a lot of setback, is beautiful, really nice to work with and much lighter than it looks. Visually an upgrade to any bike you put it on.)

Thanks for replying.
So, what you say is that even if i place the saddle in identical position and that exactly from bottom bracket and stem to saddle nose I will have to push it backwards on the straight seat post to get similar position as with the 20mm setback seat post?
hsea17

If the saddle is in identical positions, the straight post will clamp 20mm further forward than the setback post. That's what setback is.

Sometimes, a particular saddle position won't work with both types. My saddles, because I have mostly bikes with steep 74 degree seat tube angles, need to be clamped toward the front of the saddle rails to get into correct position. That is on setback type seatposts. If I tried to do that with a zero setback post, I would have to mount the saddle about 20mm too far forward to get the rails to the post clamp.

This is a rather extreme example, but if this is your position, how could you possibly use a seatpost where the clamp is even further forward?

https://cimg5.ibsrv.net/gimg/bikefor...33285574ef.png

I'm opposite, this picture is with the straight seat post and it's still possible to move it a bit more forward and back plenty of saddle rail. With the setback seat post then saddle is pushed to the limit forward inside my standard measure saddle height and saddle nose distance from mid intersection stem / handlebar.
hsea17

https://cimg2.ibsrv.net/gimg/bikefor...f328b09009.jpg
https://cimg3.ibsrv.net/gimg/bikefor...65cbab2b5c.jpg

If this is a road bike, you are likely sitting so far forward that you have to use your hands to keep from falling forward. It is an unusually far forward position.

Yes its a road bike. It may not be exactly Greg Lemonde's bike position, but then again, I competed in a fair number of triathlons at a younger age, so maybe I've adopted some bad/good habits from that time! But if I use the now "dead KOPS" and measure from the tibial tuberosity I'm not far off.
Back to my question, does setback or straight seat post make any difference in overall geometry for me personally when I maintain the measurements I've used for at least the last 12-15 years?
hsea17

What do you mean "overall geometry"? The posts allow different setback positions, and it is often difficult to switch between them because saddle rails aren't long enough to accommodate either style.

But they exist entirely to create setback - nothing else. Is that why you're asking?

How much set back a seat post gives just changes how far back or forward from the bars or how for behind the BB your saddle can go. If your saddle can't be moved to the position you wish it to be in, then you get a seat post with the appropriate amount of set back to put its rail clamp where you can adjust it to the position you want.

It's your body proportions and your preferences that determine where your saddle should be.

Thanks to both Kontact and Iride01 for the answers. Iride's answer / conclusion below is well in a way what I was looking for and even if it's not a usual road bike position it has functioned for my body proportions. I was a little unsure if the straight seat post would change my position even with the same distances to the seat/handlebar etc. It's your body proportions and your preferences that determine where your saddle should be.
hsea17
:)

There's more to position than having your knee over the pedal. It is unlikely that you are balanced on the bike being so far forward over the cranks. But I'm glad you got your answer.

Yep. Zero set back seat posts are made because some cyclists need them to get their saddle in the correct position.

I'd say you are over generalizing. Straight seatposts have their application on road bikes too. Everyone's body is different. Nearly every bike has different geometry.

Road bike geometry is not so variable, especially in the direction you're talking about - having such a slack seat tube angle that you need a zero setback post to fix it. Often the opposite is true - small frames with ridiculously steep seat tube angles that even a set back post can't fix. I have an old Bianchi like that. The normal spread of road race bike STAs is 74 to 72.5, which means that a tall rider only needs to move his saddle forward 6mm to get the same position as the guy riding the 56cm. Touring bikes have slacker STAs, and that is because you are supposed to sit up more on those with your seat further back.

KOPS was never a fitting goal - it was a datum that seemed to put many people in about the right set back. Good set back is really just an angle - a line from the BB to the hip bone that isn't so steep that you aren't rotating your pelvis forward to reach the pedalling orbit. Just because you have short femurs or long feet it doesn't mean that you are better off sitting at a steep angle over where you pedal, even if KOPS suggests that is where you should be. KOPS doesn't work on everyone.

Regarding balance on the bike, do I have no problems sitting up without my hands on the handlebars while riding to take my phone out of the saddle bag or feed myself with a gel when needed. However, when it comes to putting my hands on my back and bending forward without supporting with my arms, I come only halfway compared to what I did in younger days.
hsea17 :)

If you aren't having any issues from riding, then don't imagine that your fit to the bike needs anything. Possibly if you change your riding habits, then issues might show. Especially if you have been doing fairly short rides of around a hours time and then start doing longer rides of 2 to 3 hours or more. Those will show you that things you thought were not much bother, really are a bother.