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  #61  
Old 02-16-2014, 10:09 PM
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Wow--that's kind of weird--I come up with 57 and a smidgeon, and have ended up preferring 57 cm TT frames.
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  #62  
Old 02-17-2014, 02:37 AM
rustychisel rustychisel is offline
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So what I glean from this, and looking through 5 pages of responses, is that the method seems reasonably accurate for many of us (there'll always be exceptions) and that the error from what we actually ride is nearly always in favour of a slightly longer top tube. That would seem to be a fair assessment.

Whether the majority should be riding a slightly longer TT bike than they do, or whether there is a slight miss in the calculation is open to quite some debate.

I ride a 55cm TT, the formula suggests a 56cm. I used to ride a 56cm TT and my first road bike was a 23in TT (57cm). Perhaps the sample on this forum is skewed by the mean age of respondents, I know there's a number of us not as ahem, supple, as we once were.

Comments?
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  #63  
Old 02-17-2014, 07:35 AM
fuzzalow fuzzalow is offline
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This calc is one of those fun-facts numbers that works superficially because it isolates on TT and ignores everything else. So any number that is close enough is good enough. It is useful? Dunno. Would anybody trust the number from this calc to spec a frame purchase with it? Not likely. It is too simple - just having one piece of the frame size puzzle is not enough. And over simplified methods all fall flat with the questions "And then what? What happens next?".

Some of the older bike fitting methods (i.e. pre-stack &reach) rely on torso length as one necessary measurement for frame sizing. Genzling used it although CONI does not.

For me the more important number is head tube length. No matter - the long pole in the tent is what drives each cyclist's own solution priority. And most cyclists know what that is and fix that first.
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  #64  
Old 03-14-2014, 12:52 AM
esldude esldude is offline
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Well after some 14 months by my count the Pi formula was dead on for 21 people, and quite close for 16 people. Off for 8 people. So it looks like it is close for more than 80% of those posting their results. I am surprised it was that close for that many people.

No one number is right for everyone, and customs exist for oddly proportioned people among other reasons. It also seemed the formula was off mostly for taller or shorter than average people which isn't surprising.

Seems like for neophyte cyclists asking what size frame to get, if they are not terribly tall or short this would be about as good as other simple methods. Nothing will replace a proper fitting, but this might get them in the ballpark. Certainly simpler to tell someone divide your height by pi than it is to tell them find your pbh (or inseam with explanation it isn't the one for your pants). Also easier to measure.

Several mentioned shortening top tubes as they age. I have found a bit more head tube height to do the trick for me. As another mentioned, that is actually a more important measure in regard to whether a frame will work for me or not when they have identical top tubes. I will note I am no frame designer or fitter so maybe I am making the adjustment for age all wrong.

Bike fitting still seems more an art than science. Numbers like this which just happen to work are interesting fun.
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  #65  
Old 05-18-2014, 07:55 PM
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Stupid top tube tricks

This is nice trick! Is this effective top tube? And does this work with mountain bikes? Seems pretty close (within a cm) for me!
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  #66  
Old 05-18-2014, 08:15 PM
Cicli Cicli is offline
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Works for me i am 174 and ride a 55. Pretty cool.
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  #67  
Old 05-22-2014, 01:31 AM
esldude esldude is offline
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Quote:
Originally Posted by split View Post
This is nice trick! Is this effective top tube? And does this work with mountain bikes? Seems pretty close (within a cm) for me!
Yes, effective top tube length.

Haven't given any thought to the Mtn Bikes though I own a couple.
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  #68  
Old 05-28-2014, 08:43 PM
linkedrecoverie linkedrecoverie is offline
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Is there a sweet spot?

Admittedly, I haven't read all five pages so far, but is it perhaps working better for people in a certain range? I'm 180cm, and it's dead-on for me. I used to ride 56s, but I've come to see the error of my ways.
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  #69  
Old 06-01-2014, 08:24 AM
bewheels bewheels is offline
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Quote:
Originally Posted by esldude View Post
...

No one number is right for everyone, and customs exist for oddly proportioned people among other reasons. It also seemed the formula was off mostly for taller or shorter than average people which isn't surprising.

.....

Several mentioned shortening top tubes as they age. I have found a bit more head tube height to do the trick for me. As another mentioned, that is actually a more important measure in regard to whether a frame will work for me or not when they have identical top tubes. I will note I am no frame designer or fitter so maybe I am making the adjustment for age all wrong.

.....
I fall into this camp. Height = 190.5 cm which works out to a 60.5 using Pi. However I am riding a 56.5 with a taller head tube. I am an older inflexible ex-racer.
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  #70  
Old 06-20-2014, 08:46 PM
RMcRee RMcRee is offline
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Works for me, but I'm average height

Well,
It would seem that this formula works mostly because the original poster is, like myself, of average height. We are in the middle of the bell curve.

But still, its a nice mnemonic, height-over-pi equals top-tube.

HOPETT? That's my inner engineer trying to make yet another acronym. Sorry.

Randy
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  #71  
Old 06-28-2017, 04:29 AM
esldude esldude is offline
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Yes a necro post. Been long enough maybe some never read this thread.

So wondering if more people would see if the simple Height/pi formula fits their preferred top tube length?

Previous results were around 80% dead on or close.
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  #72  
Old 06-28-2017, 06:56 AM
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Nope for me. That formula puts me on a 57, and my happy place is 55, with 56 being acceptable. flat top tube 57 bikes are ball busters for me as I need a little more standover height..
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  #73  
Old 06-28-2017, 12:54 PM
Clean39T Clean39T is offline
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Formula gives me almost a 61cm TT, which would only work for me with a 100mm stem - maybe that's okay, but it seems like it'd produce some weird handling...
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  #74  
Old 06-28-2017, 02:00 PM
mhespenheide mhespenheide is offline
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Yah, my height of 195cm / pi would put me at a 62cm top tube length, which ain't happening.

I also think it's kind of bogus to look at top tube length in isolation, because (in my opinion) reach is more important. At the length of my seat tube, given trigonometry (heh!), anywhere from a 60 to a 58 top tube works for me depending on the STA. A 60/72 TT/STA is about the same as a 58/74.
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