Interesting bike fit article for you tall Clydes (6'6" types)
At least i thought it was interesting and demonstrates the way that tall people fall outside the "norm" for bike fit wisdom.
It's a good overview. I broadly agree with him that tall frames are made with too slack a seat angle, not enough top tube, and too short chainstays.
I do disagree on a couple points. First I think that "knee over pedal spindle" is not what a designer should be chasing. KOPS just coincidentally happens to approximate balance for average sized people in a road bike position, at least well enough for the purposes of a bike shop sales flooor. People take KOPS as an article of biomechanical truth and then run with it and come up with nonsense about how seat tube angle should be determined to your femur to shin ratio.... which I've never found a basis for in physics.
I think what really matters is something more like rider center-of-gravity and weight in relation to crank, saddle, and typical pedaling force, something like Keith Bontrager talked about in his "myth of KOPS" article (http://www.sheldonbrown.com/kops.html).
For instance, I have long femurs, am decently strong, but also have a light upper body / low CG, so it turns out I feel "well balanced" with a saddle position that's significantly forward of KOPS; with less upper body mass able to react against the pedal, I need to get forward to put my CG over the cranks so that I can pedal with force and not be pushing myself off the back of the saddle. But that means that seeking a decent weight distribution and arm extension with a normal length 100-110mm stem would put me on a frame with a 64cm top tube (given 74 degree seat tube). The biggest mass produced road frame I see, 66cm Soma Smoothie ES, has a 62 cm top tube, but it also has a 72.5 seat tube --- It's tall enough with a 350mm steer tube, but it's fully 4 cm too short in the reach and that's too big a difference to make up with a longer stem.
Second point of disagreement, a longer crank does put the saddle position lower, but it also puts the well-balanced saddle position further forward, eating up reach. So it enables a smaller frame size, but not really.
The issue of KOPS, it's like anything else in bike fit, it's a starting position, by establishing common starting points, it's easier to make the required final adjustments. A saddle has about 3" of travel fore and aft, so where do you start, front back, somewhere in the middle? The KOPS position is a starting point, that should be reasonably close to correct for the majority of riders. Each individual rider then can adjust fore slightly or aft slightly until they reach a position that is best for them.
Originally Posted by zzyzx_xyzzy
Perhaps this is the right place to ask my question then. I'm 6'5" and a few years ago I ordered a bike online. One of the disadvantages of that approach is that you can't be sure you're getting the right size. However, I thought I'd be at the high end of the size no matter what. Well, the bike was 64 cm, which is getting kind of rare.
It turns out it feels like it was too big for me. I had lower back pain until I went from a 120 mm stem to an 80 mm stem. Now it's pretty comfortable. Do I just have odd dimensions? I thought I had a long torso and short arms and legs for my height, if that matters. But I seem to recall the fitter saying the opposite. Oh, and I got a bike fit that confirmed my stem length, it was almost spot on the recommended horizontal distance from seat post to stem. It turns out that for most bikes it corresponds to about a 590-595 length top tube (with something like a 120 mm stem), which often isn't even the largest size. Doesn't that seem odd to other people that a 6'5" guy would be getting a 60-61 cm bike instead of the largest offered size of 64cm? I'm really at a loss as to what to do if I ever get a CAAD9 :-)
Not at all. I am 6'4 and ride a 58cm frame.
Originally Posted by cooleric1234
Last edited by clink83; 06-20-10 at 07:24 PM.
Originally Posted by carleton
As many of you may know - I've got a custom built Zinn Titanium bike.
Eyed it for years - was worried about the money - was worried about buying a bike from the other side of the world - was worried about fitting via email discussions - was worried that I was putting a lot of faith into Lennard Zinn's different design ideas - was worried that I was buying a bike without ever having tried let alone seen one of his bikes.
While I was worried about his design ideas - they did make theoretical sense and were backed up by his and his customers experience. But It was still a worry to jump outside of conventional wisdom.
I shouldn't have worried!! (except about the money - I still worried about that - but that's true of any expensive bike - but you can justify it in hindsight )
I've got an amazing machine that performs unbelievably.
I remember after I'd ordered it - showing the plans to a guy in my cycling club - who worked in the cycling industry for years and years - and he thought I was mad - and that Lennards ideas about long cranks didn't hold up (obviously this made me more worried at the time!). So it was funny when after a few months of him witnessing the large jump in performance that I've had - that he was questioning whether long cranks were legal as they were obviously giving me an unfair advantage.
7months after buying it - I'm the winner of our summer racing series - and the winner of the "Most Improved" rider in our club.
I climb incredibly better - I accellerate better - and ride way quicker than I could before.
When I tell people that my bikes got 210mm cranks - they still look at me funny. But they can no long deny the effectiveness.
My 2 cents worth...
It sounds like a familiar story. I started riding on a 60cm frame, 59cm top tube, which came with a 120mm stem, and I actually swapped it out for a 100mm stem and that was an improvement.
Originally Posted by cooleric1234
But it turns out what really fits me best is a 68cm frame with a 110mm stem and a 64cm top tube. How'd I get to there from my original bike with a 59cm top tube and 100mm stem?!?
The smaller frame with the longer stem had me aching because I had so much saddle-bar drop that I was reaching way DOWN... had to bring my hands in to make my back happy. Think about keeping the angle of your back constant, and reaching out to different places with your arms: you can either reach low and close, or high and far. [eta: http://www.rivbike.com/article/bicyc..._top_tube_ruse has a cartoon of a Clyde demonstrating this] If your back is too bent you can lessen the reach, or raise the bars, either will let you sit up more. For me it turns out the "high and far" fit is much better, for reasons I don't fully understand, but I suppose it could be different for different folks.
A puzzling (and I would argue wrong) fact of contemporary road bike design is that while frame height varies a lot, frame length doesn't very nearly as much. If you look at a typical geom table here, http://salsacycles.com/bikes/chili_con_crosso/ you will find that the "stack" changes by a far greater proportion than the "reach." This means that keeping the saddle height and setback constant and keeping the same stem length/spacer stack, but changing the frame from a 47 to a 51, would raise the bars 4 cm but only shift them forward by 8 mm.
This means that the conventional wisdom of "if it's too far a reach, go to a smaller frame size" is all wrong. Keeping the saddle position and stem the same and substituting a "larger" frame of the same model road bike will bring the bars up higher and let you sit up higher. The adjustment for people who want longer or shorter reach seems to be all in the stem length and not in the frame size. In your case, if you have short arms you might need a short stem to make the fit work, but a smaller frame would probably put the bars even further away from you.
Note this is peculiar to road bikes: I've looked at a few tables for mountain bikes, and there the reach varies a lot more than the stack! So if you want a mountain bike I suppose you pick the reach you want and then change the stem angle or bar rise to get the height you want.
The only geometry tables I've seen so far with really proportional geometry where the bikes get taller AND longer in harmony are for some Rivendell models, oddly enough.
Last edited by zzyzx_xyzzy; 06-22-10 at 01:04 AM.