[Halekai]

@ ciquta...your saddle height is a function of your inseam...roughly. If you have large feet then that could add a little to saddle height but it depends on your philosophy regarding pedal stroke and ankle position. Thinking used to be more into ankle extension...these days riders are moving more toward a flexed ankle pedal position. This minimizes your calf muscles and emphasizes your hamstrings and quads (larger muscle groups)

But...If you have long femurs relative to your lower leg...then you will need to move your saddle position rearward to maintain the optimum position of your leg on the power stroke. This is the Plumb Bob method in action.

Long femurs would bring your knee out front of the pedal axle. The thinking is that your knee should be roughly equal with the pedal axle. (this is just one way of thinking).

That philosophy will dictate that you move your saddle back either using the rails... a setback post of decreasing (slackening) the angle of the seat tube. For our purposes...make the changes in the frame and not with components.

If you move the seat tube rearward and keep your top tube the same then that is going to decrease your front center measurement. So now your h-bar position is closer to the BB.

My point here is that you need to solve the issue of your saddle position in relation to your BB first...Then you can begin figuring out the rest of the equasion.

Dis you try BikeCad?

 

[ciquta]

you need to decide where your saddle needs to be in relation to your BB. That is going to depend on what type of riding and your level of experience, flexibility and fitness etc.

STA looks like the key here


I run a exampke with bikegeocalc (I know bikecad but that's quicker)

this is a OEM (drop bar) frame in XL size:

and this is how it would be scaling its medium size proportionally with the height/inseam, keeping the same angles:

lets focus on the rider fit and leave the handling out of the equation for now (wheelbase and trail):

the main obvious difference is with saddle center to BB and reach
the XL version simply shift the rider rearward by a good 20-25mm!

I don't get why the STA should be different, a taller rider has a longer fremur but also a longer saddle height.
A M-sized rider with same Ape Index should be proportionally at the same distance with the pedal (apart for the crank arm lenght).


What do I miss here?

 

[dr.welby]

The slacker seat tube angle would make up for the common cranks lengths not having enough range to scale to rider size.

 

[Halekai]

I guess I lost the plot here.
Are you trying to figure out why a specific manufacturer does what they do?
Or are you trying to find a good fit for you?

If its the latter, I would concentrate on your saddle position relative to the pedals and cranks you use and go from there.

 

[ciquta]

The slacker seat tube angle would make up for the common cranks lengths not having enough range to scale to rider size.

good point, but that would make up for 7-8mm top

 

[dr.welby]

good point, but that would make up for 7-8mm top

If it actually made up for all of it the seat angle would be too slack - either outside the range of lugs, or it would look "too old-fashioned".

You're not going to find a single pure answer that distills current large bike geometry to an engineering concept - it's a mix of silly tradition, marketing, economic restrictions, and limited sample data.

 

[ciquta]

I would concentrate on your saddle position relative to the pedals and cranks you use and go from there.

I use 180mm cranks but I don't buy the KOPS logic.
Modern bikes are so different in STA, ranges 70 to 80°... hard to believe they are so unfit, is there something more proven?

 

[JackOfDiamonds]

I agree that the only way to preserve aesthetics with a giant frame is to get bigger wheels. A giant frame is going to look funny, there's no way around it. For me, though, it's much more important for the frame to fit the human, than for it to look "good". For bike companies that want to sell bikes though, it might be the other way around...better to build a good-looking bike that will sell, than build something with better functionality that people won't understand or looks weird and won't sell. We see this all over the industry after all.

Aesthetics aside, I disagree that wheel size matters much for bike fit. It should be possible to fit the bike to the rider, and work around the wheel size. After all, there is only 1 wheel size and there are different size frames, so we are already doing this.

The fact that companies don't change the chainstay length for larger frames is pure laziness / cheapness. If you can build all your bike sizes with some of the same rear-end parts it saves a lot of cost. Short people end up with CS too long and tall people end up sitting over the rear axle, so it sucks, but consumers seem to accept it. You don't have to.