Determine Set Back, beyond KOP

[classtimesailer]

Your saddle set "properly" will have the middle of its rails in the seat post clamp.
I'm no expert, but I think it looks the best that way and when I take my position and match it up with all the experts, guess what? I'm within a centimeter and guess what else? My butt slides back and forth more than that depending on the road and my pace and whatever.

[moonhoo]

KOPS does not work well for folks with short legs for their height.

I can't say how many times bike shop fitters with plumb bobs have said things like I need a 76d seat tube and a zero offset seatpost.

I think, "How do you still have a job?," but politely say, "no, please don't write that number on the spec sheet."

[zap]

Originally Posted by classtimesailer

Your saddle set "properly" will have the middle of its rails in the seat post clamp.
I'm no expert, but I think it looks the best that way and when I take my position and match it up with all the experts, guess what? I'm within a centimeter and guess what else? My butt slides back and forth more than that depending on the road and my pace and whatever.

What seatpost...0, 5, 15, 20, 25 offset?

Let's not forget seattube angle.

[zap]

.

[Mark McM]

Setting saddle setback based on position the center of gravity over the pedals clearly makes more sense than position the knee over the pedals (KOPS). But how do you measure the position of the center of gravity? Simply eyeballing might not be very accurate.

Perhaps one of the reasons that KOPS has remained popular is similar to the reason that judging components by their weight is popular - it is very easy to measure (even if it doesn't mean much).

As for setting setback relative to center of gravity - using this also makes bike fitting more time intensive. Using KOPS for setback makes bike fitting a simple sequential proces - first you set saddle height, then you set saddle setback, then you set handle reach, and finally you set handlebar height - done!

But center of gravity is dependant on both setback and upper body position. So after setting an initial setback and then setting handlebar height and reach, you might find that the center of gravity isn't where you'd like it to be. So you have to adjust the setback, which in turn means you have to find a new handlebar height and reach. Keep iterating until the center of gravity finally ends up where it needs to be. I can see why some bike shops and fitters might prefer the simpler, more direct approach of using KOPS.

But that still leaves the question - how do you actually measure the position of someone's center of gravity?

[MikeD]

Originally Posted by Mark McM

But that still leaves the question - how do you actually measure the position of someone's center of gravity?

Bike on trainer, level, bathroom scale under the front wheel.




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[jamesau]

Originally Posted by Mark McM

But that still leaves the question - how do you actually measure the position of someone's center of gravity?

See post #4 here: https://forums.thepaceline.net/showthread.php?t=163401

[Ti Designs]

Originally Posted by Mark McM

Perhaps one of the reasons that KOPS has remained popular is similar to the reason that judging components by their weight is popular - it is very easy to measure (even if it doesn't mean much).

So it's not right, but it's easy to teach...

Originally Posted by Mark McM

As for setting setback relative to center of gravity - using this also makes bike fitting more time intensive. Using KOPS for setback makes bike fitting a simple sequential proces - first you set saddle height, then you set saddle setback, then you set handle reach, and finally you set handlebar height - done!

But center of gravity is dependant on both setback and upper body position. So after setting an initial setback and then setting handlebar height and reach, you might find that the center of gravity isn't where you'd like it to be. So you have to adjust the setback, which in turn means you have to find a new handlebar height and reach. Keep iterating until the center of gravity finally ends up where it needs to be. I can see why some bike shops and fitters might prefer the simpler, more direct approach of using KOPS.

But that still leaves the question - how do you actually measure the position of someone's center of gravity?

The fitter needs to understand two things. The first is range of motion, which is the limiting factor on hip angle for most of us. The second is where the center of gravity is, but it's not really center of gravity, it's the body's reaction to it's own position. If you sit on a chair and lean forward, weight transfer is to your feet on the floor and the glutes hold you up. Once your center of gravity gets past where your feet are on the floor, the quads come into the action. That's not really your center of gravity, but I'm more than happy to call it that to explain position.

The fitting shouldn't be based on where the handlebars are, people get very involved with where their bars are because their eyes are mounted in the front of their head - it's what they see, it's what they want to adjust. The fitting is all about saddle to pedal, your position should work with your hands behind your back. If it doesn't you have too much weight on the bars.