DRLski

I have been researching on setbacks and found something written by a user that seems to describe it all, but I'm wondering if other have the same opinion in this matter...he suggests not actually measuring your knees over the BB and using a zero setback seatpost to match teh frame's intended geometry. He talks about cross bikes as well but I have both. Also, he says that you can tell how a bike will ride just by it's setback, how can you tell the setback of a frame? I've always looked at seat angles and etc.. Anways, here's the post:

Just my opinion, FWIW (mebbe not much). Assuming you are racing cross (using it for commuting, touring etc is a different story...) you want to be set up for as much power and speed as possible. This is different than setting up for long distance comfort and efficiency. You want to maximize the power from your quads. So you want your seat to be positioned further forward relative to the BB. Cross bikes designed primarily for racing have somewhat steeper seat tube angles and small "setback" than touring or many all-rounder road bikes for this reason. Setback as part of frame design here is the distance between the center of your seatpost to the imaginary vertical line drawn through your seat cluster. More setback = more use of hamstrings and glutes, less quads. More efficient, but less high end power. Think spinning vs. big gear stomping. Classic stage-race road bikes are designed that way. In fact, you can have a clue about how a frame will fit and feel to you just by measuring the setback distance, and compare it to a bike that's familiar to you- no need to know seat tube angle. By the way, you should also consider this factor, and not just the top tube length when choosing a cross frame. Different "cross" bikes have different setback, some really have touring bike geometry and some are even steeper than road race bikes, and the right top tube length for you can vary by a few cm's accordingly, so beware when buying a frame/bike through the mail, as there are more variations with frames sold as "cross" than stock road bikes. Crit bikes, and cyclocross bikes (designed for racing, I'm not talking Surleys and other all-rounders) have minimal setback- built for power and speed, not comfort and efficiency. So choose your poison- in general frames (except maybe tri and time trial bikes, which I think actually have NEGATIVE setback sometimes- and something about saving the legs for running in tri's) are designed for straight seat posts with no additional setback. Unless you know that you want to change the position intended by the frame designer (whether Richard Sachs or Giant, inc) I would go with the straight post, as your better bet. In general, I don't get why someone would buy a setback seatpost unless they have atypical body dimensions on a stock frame, or want to make a steep race bike into more of a tourer, or have the wrong size frame, or are just clueless about position. Just my opinion, sorry if I miffed anyone with a setback post.

ivan_yulaev

Agreed. Also, if I had a setback seatpost on my bike, I probably wouldn't be able to reach the bars!

DannoXYZ 

Uh... not sure who posted this or how to approach this... perhaps point out some mistaken assumptions...

"You want to maximize the power from your quads. "

In simplistic terms of maybe 100-120 degrees of crank rotation maybe yes. But once your quads are exerting 100% of their force, you're NOT gonna be able to push any harder. More power can only be generated when you apply force to the other 240-260 degrees of the crank-rotation that you've been ignoring. Applying just a little more force on those 240-260 degrees will make a much bigger difference than applying twice that increase to the narrow rnage that the quads are pushing.. There's 9 other muscles in the legs that can be used. Anyone who's fast and at the top of the cycling arena is a spinner....

"More setback = more use of hamstrings and glutes, less quads."

Completely twisted. Hamstrings are best used with high and forwards set (less setback). Hamstrings are critical part of the pull-back and pull-up part of the stroke. This helps you spin faster and smoother. Quads are used evenly and can be maximized with a forward or rearward seat, doesn't matter. Glutes are used more with high and rearwards set position. Actually, it's the distance from the pedals that's important as glutes are used most at the very end of leg-extension when your legs are almost fully straight.

Here's a list of muscles used in cycling: what are the muscles involved?. Any 1st year zoo or bio book will show you where those muscles are and what motions they generate. Then you can figure out where on the pedal strokes they work.

The neat thing about KOPS is that it integrates ALL of the factors involved, seat-angle, saddle-length, crank length, seat-rail position and "setback". There's A LOT of variables, you can't just focus on a single one as the "holy grail" of cycling-performance.

rjtokyo

This is more of a non-technical response, but it seems to me that so much is dependent on your: a) body type, b) preferred riding position, and c) preferred riding style. For me personally, I'm long-trunked and short legged, prefer a fairly aggressive riding position (in the drops 50%+), and prefer about an 85 cadence, fairly-circular pedal stroke. To get the most power and efficiency with my short-femured legs, I've also found that with my 74 deg.-seattube-angled race bike, I do best with a zero setback post and the saddle slightly forward in the rails. On my training bike that's got a 75 deg.-seattube-angle (on the steep side), I'm fine with a 23mm setback post and the saddle mid-rail. In either case, probably less setback than average.

- RJ

biker7

+1
Well stated Danno.
George

biker7

Yup...that's right and the OP's broad reference about purchasing the wrong frame size is too sweeping. Every frame out there is compromise. I am built the opposite being long femured and look for the opposite frame geometry. I therefore prefer a small seat tube angle and set back to attain KOPS. Many new bikes are sold with set back seatposts for that reason and will put a rider on a smaller frame accordingly.
George

DRLski

So does this mean that you could have a frame that fits you great but have a huge setback with the saddle all the way back depending on your body geometry? i.e. if you have loooong thigh bones as compared to your tibia?

DRLski

...

DannoXYZ 

It's an oxymoron. The part about "a huge setback with the saddle all the way back" does not equal "a frame that fits you great". There issues of weight-distribution to be concerned with. In order to fit someone with a long femur, you need a frame with laid back seat-tube, say... 72 degrees. This also ends up pushing the rear wheel back as well to maintain weight-distribution. So if you have two different scenarios:

1. frame with 74-degree seat-tube using set-back seat-post with seat pushed all the way back to get KOPS or
2. frame with 72-degree seat-tube with normal seat-post and seat in middle of rails and KOPS also the same

I'd say that #2 is "a frame that fits you great".

However, you have to consider all the other components of fit as well:

1. seattube length / standover height (which I consider least important)
2. toptube length (which I consider most important)
3. headtube angle
4. BB drop
5. wheelbase

These are also factors that will dictate how the bike feels to you and how well your body will work at making power.

DRLski

k, that I understand...thanks Danno, wish everyone else could've been that simple