Flinstone

Any discussion of the competitive cyclist fit seems to have most people saying it's pretty ok and a few folks reasonably pointing out that tube lengths are meaningless without seat tube angle (STA). Normally you would need both to calculate stack and reach.

However, what these criticisms apparently failed to account for is that CC also provides nose to handle bar (rear edge of bar), stem length, and saddle setback.

Reach=nose_to_bar +1/2 bar diameter - stem_length - setback

and maybe plus a couple more mm for the tilt back of the steerer as it rises from the headset bearing to the center of the stem.

Once you have reach you can work out the STA by working back to the x position of the top of the seat tube using the TT length, then combined with C-T ST length you get the angle. The vertical of that triangle then gives you stack.

From there you can also get the seat to bar drop although their seat height recommendations seem a couple of cm too long, so might need modifying. And then you can get the true nose to handlebar distance in 2d (not just the horizontal distance), which in principle could be the most useful bit of information it provides. Basically how far between you rear and your hands, to make sure your shoulders are at a good angle, something that probably can actually be calculated reasonably well. In my case it did seem to do pretty well at that, which gives me some reassurance on that aspect of my fit.

The question is does CC really use, under the hood, a detailed self-consistent geometrical model so that it even makes sense to read this much into it mathematically? Or do they just have some simple rules they've found that tend to guess each of the stated tube lengths about right on average for some common bike geometries?

Now, and this is my opinion (I think not too controversial), I don't see how a calculator can possibly know what setback or seat to bar drop you need, and thus how it could know even approximately what stack and reach you need anyway.

Does it know your weight distribution? Does it know how much you're willing to compromise aero for comfort with a forward position? Does it know how much you favor your quads vs glutes or how much stress you like to put on your forward saddle bits?

It's telling me I need 5cm of setback and to me I feel like I'm falling forward off my seat at 5cm. It's not a very generic all-around number either, although it might be close to KOPS for me. I can hold that position even without touching the bars because I have enough core strength, but it certainly takes more effort/energy and I'm not a competitive racer.

If you need more setback you're surely going to need a relatively shorter reach to compensate, or a shorter stem at least. I don't see how you can even ballpark this. I guess if you put everyone 2.5cm behind KOPS you'd get within 2.5 cm plus or minus for most people, and if you calculate for a 110 or 120 stem they'll have 2cm of reach adjustment either way left up front, barely, or compromise a little on both setback and reach, but almost anyone can probably guess a bike size by eye if that's good enough.

And clearly a calculator can't tell anyone what stack they need.

Still, Competitive Cyclist technically does determine more than it's usually given credit for, if you consider it as a calculator that leaves much of the calculating left for you.

philbob57

I don't see how the nose -bar measurement has much meaning at all. It doesn't take into account newer saddles by ISM (noseless, though it appears to have a nose) or Selle SMP (on which important bits ride forward of the nose); nor does it seem to take into account saddles with long noses (Brooks) vs saddles with shorter noses.

Yeah, you can compute things using the measurement. It's just that the calculations are likely to be invalid.

I very much appreciate CC's providing the tool at no cost, but you need to remember it's just a guide. For example, I rode a bike with 'competitive' measurements for for several years, but I didn't ride it much, because it felt wrong. I got a bigger bike, and my mileage went up about 300% ... that data is a hell of a lot more significant than the numbers the tool provides.

berner

I also don't understand a measurement from the nose of the saddle since saddles can be different dimensions front to back. I measure from where the sit bones would rest on the saddle which I take to be the widest part. My sit bones are a lways in the same place.

Flinstone

Yes, I've also noticed that source of error. However, I don't know how you get around that. Depending on the curvature of your sit bones , the tilt of your pelvis, and the shape of the seat, the exact contact point on the seat and your body can shift a bit regardless of the length measurement. For me, I read up on my particular seat and it's said to tend to set you back about 1 cm more than the measured value, compared to other "average" seats.

I'm not sure where the sit bones contact is the widest part. It depends what you mean by sitbones, and what you mean by contact. What most people think of as sit bones (the protrusions you sit on on a bench) don't carry weight in my normal position, and the part of the pelvic bone that does carry weight certainly isn't at or near the widest part. However, the "sit bones" probably are approximately over the widest part (maybe slightly in front), and if I do sit up straight to rotate back onto my sit bones, then they do contact approximately there. I'm not sure this approximation does much better than the nose though. Maybe a little.

I'm afraid some errors do in fact just have to be fine tuned in the fine tuning and you better size the bike with a little wiggle room in all directions, like with a predicted stem of 110 to give a little room for play either way.

Still, I think nose to handlebar is actually the most meaningful number a calculator can probably produce. It's probably slightly more meaningful as a 2-d distance rather than a horizontal, but unless you have 20cm of bar drop the difference isn't huge, and I'm assuming the same arm angles are optimal regardless of how tilted you are, which might not even be true. In the extreme case of doing a hand stand, I don't think arms at 90 degrees works very well. So it might make sense to stretch slightly anyway if dropping down that far? That's beyond my expertise.

While a calculator can likely tell you about how stretched out to be, I don't see how it can tell you how setback you should be, and that directly impacts frame reach and/or stem length. I also don't see how it can tell you how leaned forward you should be.

Flinstone

Don't get me wrong, stack and reach are still the best way to choose a frame, it's just that I don't think a calculator can determine your needed/desired stack and reach very well at all, while I think it can determine your nose to handle-bar reasonably well. You have to figure the rest out on a bike. Once you know your setback and handlebar drop too, then you have your stack and reach and can go shopping.