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Look Frame Geometry question(5 posts)

Look Frame Geometry questionflying
Jun 15, 2002 12:11 AM
I am looking at a KG241 frame & wondering about size.
I currently ride a 53 c-c or 55 c-t Colnago MXL. It has a 54.3 TT

I was going to order a 53 c-c Look then I noticed the TT is 53.8

I am wondering if I should get the 55 c-c which has a TT closer to what I am use to. (The 55 Look KG241 has a 55.6 TT)

What would be the better choice?

For what it is worth I run my saddle way back on my Colnago ( climbing ) so I use a 10cm stem.
10cm with my TT = 64.3

A last Q for those with this frame is....
I see the head angle changes right at this 53-55 frame size junction. From 71 to 73. How will this affect things?
The colnago I ride is 71.7

Thanks for any help.
geometry analysis...C-40
Jun 15, 2002 8:21 AM
To correctly compare the TT lengths of these two frames you have correct for the significant difference in seat tube angle. The LOOK has a 72.5 while the Colnago has a 74 STA. The slack seat tube angle effectively shortens the top tube length of the LOOK frame by 1.8cm. The effective length would be; 53.8-1.8=52cm, which is 2.3cm shorter than the Colnago. You should expect to use a 120 stem on the 53cm LOOK.

Why jump all the way to a a 55cm? Unless your current frame is guite small, a 2cm increase is a lot. Your saddle may be too low or youmay have insufficent standover clearance with a 55cm.
geometry analysis...flying
Jun 15, 2002 8:48 AM
Thanks C40 that helps.
Let me ask though as I never quite understand this.....
When you say......

""The slack seat tube angle effectively shortens the top tube length of the LOOK frame by 1.8cm""

I always think if the seat tube is slack or more tilted back away from the bars...isn't the top tube effectively longer?

Your right about the 55cm as a 12 stem on a 53 would be fine. I just wondered because on my existing Colnago I do have 3 spacers under the stem now ( about 1 1/8" total stack )
I have 5.5" seat post showing which is about normal I would think.
My inseam ( book between legs against wall method of measure) is 83cm which should put me on a 53 c-c or 55.6 c-t
This Colnago is comfy I just wondered about the Look having a shorter TT overall.

Thanks again for any info on this effective TT stuff ;-)
effective top tube lengthC-40
Jun 15, 2002 10:26 AM
The effect of seat tube angle (STA) on top tube length can be expressed simply. The idea when comparing frames is to maintain the same body position relative to the bottom bracket while comparing any difference in the reach to the handlebars. If two frames have the same STA, the actual TT length will be an accurate comparison. For instance, if the both frames have a 74 degree STA, but one has a 55cm TT and the other has a 56cm TT there is a difference of 1cm in the reach to the bars. If the STA is not the same, this comparison would not be accurate. Here's why:

On a frame with a 74 degree seat tube angle, the saddle must be moved further back than a frame with a 73 degree seat tube angle, to achieve the same position relative to the bottom bracket. This movement of the saddle must be accounted for. Since the TT length is the standard reference value for comparing the reach to the handlebars, the difference in SADDLE POSITION is added or subtracted from the actual TT length to produce an "effective" TT length. The effective TT length will accurately predict the difference in reach to the handlebars between any two frames and allow you to determine the difference in stem length that would be needed to maintain the same reach. When comparing frames with different STA, add TT length to the frame with the steeper (74) angle or subtract length from the frame with the shallower (73) angle, using one of the following formulas: 1.32 x (cosA-cosB) x frame size, or an alternate formula, saddle height x (cosA-cosB). An average amount is 1.2cm per degree for a midsize frame.

The explanation for the 1.32 constant is simple. It's merely a way of expressing (approximate) saddle height in terms of frame size. Frame size (c-t) is .67 times inseam and saddle height is .883 times inseam. Expressing saddle height in terms of frame size, you get .883/.67 =1.32 frame size.
effective top tube lengthflying
Jun 15, 2002 4:55 PM
Very Very Good explanation!
Thank You Very Much!!