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Stem length question (Paging C-40 or others)(13 posts)

Stem length question (Paging C-40 or others)03Vortex
Oct 28, 2003 11:56 AM
I currently ride a LS Vortex (55) which has a 73 STA and 55.5 TT. I use a 110 stem. I am purchasing a Colnago 55 which is the same fit despite the different geometry (meaning 74 STA and 54.3 TT). Would a 110 stem on the Colnago be what I should expect to need? This is of course assuming same KOP on both.
Oct 28, 2003 12:18 PM
assuming same BB placement, the ~1 inch TT dif is due to the forward STA. If the HTA and barheight are constant, you'll theoretically need a shorter stem.

Really cant answer w/out ALL the dimensions.

Having said that, I'm not one who worries over minutia.
I'll try...........Len J
Oct 28, 2003 1:03 PM
and then the teacher (c-40) will give the marks out & explain why we are wrong.

According to the teacher, every one degree of STA = 1.2 cm of TT change. There for 73/55.5 would equal 74/(55.5-1.2) or 54.3. By my math, Steve O is wrong. assuming the same KOP, the exact same stem would be necessary (assuming the same seat to bar drop & the same stem angle).

I'll be waiting for my mark from the teacher.

len is righttarwheel
Oct 28, 2003 1:31 PM
The same size stem should fit both bikes. Because the Colnago has a steeper seat tube angle, you will have to move the seat rearward about 1 cm to achieve the same knee-over-pedal position. That should make your reach from the saddle to the handlebar nearly identical, and the same size stem should work.
i'll give you that much...Steve_0
Oct 28, 2003 1:39 PM
ok, honestly wasnt considering the movement of the saddle back, I was envisioning a bare seatpost (ouch)

Having said that, we STILL dont know his total geometry to give an answer. whats his HTA? his BTL, his HTL? steering diameter?

again, we're talking minutia. plug the old stem in and ride happy.
your math is wrong.Steve_0
Oct 28, 2003 1:35 PM
First off, the delta due to the modified STA is a function of STA AND STL; not just STA. Also, some builders modify STA by modifying the location of the BB, not the length of the TT. or both.

Like i said, we need ALL the measurements to answer.

I give you a c-.
I beg to differ.Len J
Oct 28, 2003 1:47 PM
"First off, the delta due to the modified STA is a function of STA AND STL; not just STA." I agree with this however in the range of frame size he is talking about this variable is insignificant.

"some builders modify STA by modifying the location of the BB, not the length of the TT. or both"
I disagree with this. If you start a fitting with the location of the BB, the first thing you need to know is the STA. Comparing a 73 degree STA with a 74, on a 73 the seat would be 1.2 cm forward in the rails compared to the 74 to get the same relative position for the rider relative to the BB. The question this poster was asking was about reach. If he has a 73 degree STA with a 55.5 cm tt "That he fits well on" with a 110 mm stem, could he use the same stem with a 74 degree STA with a 54.3 cm TT. The answer is as I stated, yes.

To be precisly accurate, you are technically correct, in the real world, my answer is correct.

we agree to differ...Steve_0
Oct 29, 2003 9:39 AM
"To be precisly accurate, you are technically correct, in the real world, my answer is correct"

I completely agree; as my original post eluded.

Math teachers grade against theory and accuracy; not 'real world' results (that would be stats)
steve, you're wrong....C-40
Oct 28, 2003 3:15 PM
The location of the bottom bracket cannot be "modified". It is the the center from which measurements are taken.
The only necessary dimensions to compare how these two frames will fit are the two STAs and the two TT lengths.

The difference in TT length due to STA has NOTHING to do with the seat tube length. It is the saddle height that is of relevance.

The simple formula for calculating the difference in TT length between two frames with different STA is: saddle height x (cosA-cosB), where A and B are the two seat tube angles the (positive) amount is added to the TT length of the frame with steeper STA.

The resaon for this simple. Saddle height for a given rider will always be the same, regardless of the frame size he might choose to ride. A frame with a steeper STA requires the saddle to be moved further back to achieve and identical KOP. The amount is given by the formula above.

The only other thing of importance with regard to fit is the head tube length. I always insure that the head tube length is not too short. For my frame size, anything less than 150mm (with headset) will require spacers or a stem angle greater than 84 degrees. I don't like to use spacers or stem with a greater angle. My Colnago is a perfect 150mm. I made an exception when I bought my Fondriest. Only had a 140mm headtube length.
Oct 29, 2003 9:38 AM
agree the BB lis the center from which measurements are taken; i was trying to impress that some makers push the ST forward to increase STA (thereby shortening TT), whereas others bring the BB back (thereby lenthening the DT).

I also agree difference it TT legth has nothing to due to STA, my statement was in response to the statement that so many degrees equals so many inches...THATs only true for a given STL. I think your formula verifies this substituting saddle height with STL.

Also, HTA is a consideration as it displaces the stem for or aft; a steeper HTA will move the stem forward, theby (theoretically) necessitating a shorter stem.

thanks for the explanations. as i implied earlier, i dont sweat de minimus changes; and i'm sure it will work; theoretically, i believe you need to consider the whole package.
HTA rarely significant...C-40
Oct 30, 2003 4:43 PM
The difference in reach due to a change in head tube angle is rarely significant. For a given frame size, it's rare for builders to vary the angle more than 1 degree. This angle difference over the very small distance from the center of the top tube to the center of the stem is usually only 1-2mm. I always ignore it, since most stems come in 10mm increments and the HTA difference would not change the selection of stem length.

Believe me, I have considered the whole package.
Oct 28, 2003 3:02 PM
You got it right. Just so happens I've owned a Litespeed and 55cm Colango. Once the saddle is adjusted to an identical KOP position, the reach with a 110 stem will be the same on both.
Oct 29, 2003 11:34 AM
Thank You