why a difference between seat tube length and frame size

[terry_gardener]

i have been looking at road bikes recently but when i have checked. why is there a difference between the seat tube length C-T too the frame size, normally about 2cm difference.

i have noticed this on trek and verenti bikes.

thanks

 

[jimboalee]

Getting the correct frame size for a person is quite complex.

The seat angle needs to be correct to get the hip ball centres at the correct distance from the pedal spindle ( depending on the cycling discipline the bike is intended for. Roadrace geometry is usually the most ergonomically correct ).
The top tube should not be too high so it clashes the rider's reproductive bits when they stand over the bike.
And the hip ball centres to the handlebar clamp should be correct to get the lumbar vertebrae at the correct angle when the handlebars obscure the front axle.

Move this one to 'know how'.

 

[MacB]

Terri, it will be frame angles, how a specific manufacturer measures and design choice. Seat tube length is relatively unimportant fit wise due to the wide variety and length of seatposts. Sheldon says it better:-

http://sheldonbrown.com/frame-sizing.html

for a quick and easy answer then Dave Moultons little table on page 2 of this link is useful:-

http://www.prodigalchild.net/Bicycle6.htm

 

[raindog]

The "old" way of measuring the seat tube to give the frame size has become more or less meaningless since frames with sloping top tubes appeared. Alot of manufacturers simply label their bikes small, medium, large and extra large these days.

 

[fossyant]

The 'top tube' length remains important - depending upon the 'compact' geometry used, then the seat tube length's vary. Most manufacturers give a 'virtual seat tube' measurments as well, which mirror's the 'old standard' of horizontal top tubes.

 

[jimboalee]

I've spend all lunch time trying to describe how to arrive at 'Toptube length'.



The long and the short of it is if you calculate the seat angle you need, the major manufacturers have done all the design work to dimension the frame accordingly.



To determine the seat angle.



Measure your thigh length ( a ) and your knee height ( b ).

Divide a by b and divide this by 4.



That is the Cosine of the seat angle. So Google 'arccosine 0.xxx in degrees'.



FOR ME, a = 24", b = 21.25"

24/21.25 = 1.1294

1.1294 / 4 = 0.282



Arccosine 0.282 in degrees = 73.6.



My SWorks is 73.5 seat angle and happens to be Spesh's size 54cm.

 

[jimboalee]

In the very old days when gent's bikes were enormous, these two measurements ( a and b ) would be added together and this sum would be divided by two.
So I was sold a 22.5 inch ( 57 cm ) Peugeot racebike.
By today's standards, its a tad too big, but it still rides beautifully.