Newsgroup: it.sport.ciclismo
Titolo: A formula for frame size (x-post)

Some time ago I deviced an easy formula for frame size.
At close scrutiny it proved rather accurate, albeit simple, for people
of 'normal' height (say 160-185 cm).
Here it is, h being the body height and L the frame size, both in
centimeters, for a standard (vertical=orizontal) geometry as a first
approximation:
                                     L=(h/3)-4.
Now, someone pointed out to me that, in fact, it is grossly wrong for
much taller guys.
Would you, please, tell what your best bet would be for people in the
range 200-215 cms?
Thanks
Sergio
Pisa

sergio ha scritto:
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I tell you something about my last consultancy service for a friend of mine.
Last week we went to the shop in order to buy a used bike, he is 176 tall.
After several measurements the right frame was 56, exactly the same size 
of my steel horse, but I'm 180.
Using this formula the results would be:
57 instead of my 56 for me
54,5 instead of his 56 for him
Not a big differences but it's enough... :-)
Marco

 Not a big differences but it's enough... :-)
Thanks, Marco, for  letting me rejoice.
We live in a statistical world, don't we?
Sergio
Pisa


180 / 3 - 4 = 56 

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What does L stand for?  Center to center seat tube?  Center to top
seat tube?  Top tube length?  For me top tube length is all that
really matters.  I'm roughly the 180cm described in the other post.
So your formula puts me onto a 56cm bike.  Too small if c-t.  About
right if c-c.  But too short a top tube if your formula supposedly
represents top tube length.  If any bike shop tried to put me on such
a short top tube as 56 I'd take the frame and bash his head in with it.

 a short top tube as 56 I'd take the frame and bash his head in with it.
Thanks.
That was meant to be c-c. For a 'conventional' geometry I take one
that has equal lengths of vertical and top tubes (both c-c), though I
am aware that this usually underestimates the horizontal one.
Problem is that quite gross departures were pointed out on frames for
people in the 200+cm range of height.  Nothing that could be
reconciled by rescaling the
subtraction of 4 that I had mentioned.
Sergio
Pisa
Sergio
Pisa

 180 / 3 - 4 = 56 
oops!

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I think the assumption of equal tube lengths breaks down for taller
(and smaller) riders. At 190 cm this formula suggests 59.5cm; that's
okay for top tube but too short on seat tube. 59TT/64ST would be
better for my frame.
-pm


Agree. I'm also 190 cm, and prefer a 62 or 63 cm ST (c-c). 59.5 TT
would be fine.
But folks come in all proportions, so I don't think frame size can be
reduced to an equation. Plus, lots of people see to prefer smaller
frames these days.
Art Harris

 would be for people in the range 200-215 cms?
We don't make no steenkin frame sizes no more!  Just 18" seat posts
and bar extensions.  Building a frame for a specific size is
out... far to expensive ans demanding of skill.  We make a big one and
a small one the rest is all in the seat post and bar stem.
Pisa?  Is there another Sergio bikie in Unipisa?
Jobst Brandt

 Pisa?  Is there another Sergio bikie in Unipisa?
Not by my name, as I gather.
Sergio
Pisa

 would be for people in the range 200-215 cms?
We don't make no steenkin frame sizes no more!  Just 18" seat posts
and bar extensions.  Building a frame for a specific size is
out... far to expensive and demanding of skill.  We make a big one and
a small one the rest is all in the seat post and bar stem.
Pisa?  Is there another Sergio bikie in Unipisa?
Jobst Brandt

 Not by my name, as I gather.
Jobst Brandt

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I'm 208cm. Your formula gives 65.5 for me. I prefer at least 68cm 
frames. I might like 70cm, but I've never had the chance to try, my 
guess is that it might be the ideal size for me.

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indeed.  with the advent of high quality cfrp seat posts that can be 
reliably used at greater extensions, and because smaller frames make for 
a stiffer more torsion-resistant ride, small is the way to go - provided 
the top tube length is sufficient.  besides, as a guy that rides mtb's 
as well, i can tell you, you never know when you might need the top tube 
clearance.

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i'm 195 and it works roughly for me.  i'd make it -5 though, not -4.

 i'm 195 and it works roughly for me.  i'd make it -5 though, not -4.- Nascondi testo citato
Good.
That sounds like appropriate scaling.
Sergio
Pisa

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Oh, the seat post extension isn't the issue. The height of the seat
cluster is arbitrary and has no impact on fit -- flat top tube,
sloping, whatever, what we really mean by the outdated term "seat tube
length" is how high the top of the _head_ tube is above the bottom
bracket. It's the bikes that come from the factory with all of 15mm of
spacer and the steertube shorn that are another problem.
-pm

 We live in a statistical world, don't we?
eh eh, you're right, but maybe it's more political

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The problem isn't any formula but manufacturers 'sizing' methods. One
manufacturer's '56' and another's '56' are vastly different in terms
of seat tube angle and even basics as to where they measure the seat
tube length from.
The other issue is people's femur lengths vary wildly even if the
rider is the same height as another. Long femurs, shallow seat tube
angle, short femurs, steep, even tho their inseam, height may be the
same.
Formuls are a great place to start but start only. A initial slice of
info for the fit guy to put into the fit machine on the way to a
complete and accurate fit, IMO.

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Measure the crotch height in cycling shoes. Choose the bike with the
nearest smaller standover. It will certainly be a more comfortable
bike than a smaller size selected by seat post size and, with the
increased toe room, turn in as quickly as soon as you become used to
the longer wheelbase. Back angle and reach to the handlebars can be
adjusted with different lengths and angles of stems or (troll, troll!)
spacers on an uncut steerer and an adjustable stem. The exception is
you have an exceptionally short upper body and/or arms in relation to
your legs, in which case you should choose the smaller frame. (Andy
Blance of Thorn designed his Raven to offer at each size two top tube
lengths, one for drop bars, one for straight bars, which also allows
possibilities for various leg/upper body length ratios.)
I like your formula, Sergio, because it works well for me, but I doubt
whether manufacturers can measure. I have three "Dutch city" bikes
that all make the same visual-mass impression, and on all of which I
have made myself equally comfortable with a similar back angle, but
which are in fact 3cm apart in official size. The official policy of
Utopia, the makers of my most recent bike, is that (with caveats about
arm/trunk length ratio outside the median)  one selects the bike to
have the minimum seat tube extension (i.e. the biggest bike on which
one can reach the pedals) and then adjust the seating position with
spacers on the uncut steering tube in conjunction with an adjustable
stem and handlebars of various rise and sweepback. It works: it took
me fifteen minutes to fit North Road handlebars with a central rise
and a much bigger sweepback than the nearly straight default bars --
voila, an exact duplicate of the seating position on my Gazelle
Toulouse. The stem raiser and spacers I thought would be required were
not and were returned to the shop for a refund. (Several people have
commented variants of, "That looks even more comfortable than your
other bikes.")
****
At a wild guess, I would say that for taller riders your formula's
subtractive constant of 4 should be increased but not in proportion.
Here's the logic: Low bars are an advantage, right? So a taller
person, with a longer back can at the same angle as a shorter person
position his shoulders lower and, considering his arms will also be
longer, reach lower than a shorter person. Therefore lower handlebars.
But if one shortens the bike by a constant proportionate to the tall
person's height, the top tube will be too short and the stem would
have to be abnormally long.
****
I understand the classic aesthetic of the horizontal top tube, but in
a properly designed road bike for any but the shortest riders, logic
dictates a downward slope for the top tube, from near the saddle clamp
to wherever the head tube ends with due regard for enough bearing
surface and spacing to assure the integrity of the steering and also
the rider's safety in the event of a crash. Judging by stuntbike
practice, the head tube can be very short and the top tube on a road
bike can therefore angle downwards to the front quite acutely, so that
the diamond frame at the front has a real triangle rather than a
trapezoid depending in part for its stiffness on the external bracing
of the seatstay/chainstay triangles.
Andre Jute
Visit Jute on Bicycles at
 
http://www.audio-talk.co.uk/fiultra/BICYCLE%20%26%20CYCLING.html

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I measure 203cm in height and I prefer a 68cm frame.  That's h/3,
without further modification.  I have two observations about the 4cm
difference with respect to your formula.
1) Perhaps I am somewhat long-legged compared to my overall height.
2) That 4cm may represent how much longer the crank should have grown,
but didn't.  The taller the rider, the smaller a percentage of his leg
extension is taken by the crank length.
Chalo


is a more accurate measurement to use in determining frame size.

 is a more accurate measurement to use in determining frame size.
Seems to me upper body length is what matters in frame size.  Top tube
length is the key bike measurement.  As long as the seat can go high
or low enough for your feet to reach the pedals, leg length/inseam is
irrelevant.

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Sure, but too much seatpost & stem may shift weight too far aft.

 wrote:
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Seat tube height and seat tube angle and saddle position
must put the center of effort over the bottom bracket so
that pedaling does not act to push the rider fore or aft.
When the saddle is not correctly placed, the fore-aft 
resultant force is compensated with the arms.
-- 
Michael Press

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how do you figure that???  73' in a 56cm frame and a long post is the 
same as 73' in a 60cm frame and a shorter post.