|effective top tube length?||Becky|
May 19, 2003 4:50 AM
|Could somebody please explain to me how to calculate effective TT from the standard frame measurements (STA, TT, etc)? I'm attempting to compare the geometry of several frames to my current frame and, due to my short torso, effective TT appears to be the most important number for me.
Pictures are most helpful, "plug-n-chug" equations are even better (if such a thing exists). As a science geek, my understanding of most math is pretty good, but I rarely use geometry and trig these days, so I'm a bit rusty as a result.
Thanks in advance!
|re: effective top tube length?||tazdag|
May 19, 2003 6:47 AM
|TTr = TT-(cos STa x ST)
TTr = top tube reach (horizontal)
TT = top tube, center to center (horizontal)
STa = seat tube angle
ST = seat tube, center to center
May 19, 2003 7:06 AM
|The term "effective" TT length is used to compare the fit of two frames with different seat tube angles, with the rider in the same position relative to the bottom bracket on both frames.
The relevant formula is: saddle height x (cosA- CosB) where A and B are the two different seat tube angles. This value is added to the TT length of the frame with the steeper STA.
The amount ranges from 1.0 to 1.5cm per degree, depending on the saddle height.
For most comparisons, using a value of 1.2cmm (or 1/2 inch)per degree is sufficiently accurate.
For example, to compare a midsized frame with a 74 degree STA and 54.3cm TT to a frame with a 73 degree STA and 55.5cm TT, add 1.2cm to the TT length of the first frame. The result is the same 55.5cm TT length. These two frames would fit the same once the rider was positioned the same relative to the BB. The only difference would be the position of the saddle on the seatpost.
|don't you have to take HTa into account?||kenyee|
May 19, 2003 9:05 AM
|Smaller by one degree means subtract 1cm in comparison?|
|not very important...||C-40|
May 19, 2003 9:56 AM
|The difference in reach from a 1 degree change in the HTA would be 1-2mm. It would rarely be of any significance considering that stems are mostly available in 10mm increments.|
|I'm surprised it's that little a difference||kenyee|
May 19, 2003 12:59 PM
|A 1 degree change in HTa only makes a millimeter difference in TT? Does it assume you're going to leave your bars close to the top of the HT instead of bringing them within an inch of the saddle height? Why does STa make such a huge difference (1 degree is 1 centimeter)?
And if this is all true, why do people always say Colnagos have short TTs? If the STa makes a bigger difference than the HTa, it seems that a standard Serotta "square" geometry has a shorter TT than a similiar Colnago (after convert C-T to C-C because you have to subtract 2cm from Colnago ST sizes to get to C-C measurements)...
May 19, 2003 1:59 PM
|The formula that I posted tells the story. Both amounts are calculated by multiplying the difference of the two cosines times a length. In the case of the STA the length is the saddle height.
In the case of the HTA, the length is the distance from the intersection of the HT and TT and to the top of the stem (if the stem is horizontal). This distance is generally a small fraction of the saddle height. Also, it's rare to see that much variation in the HTA of standard frames of equal size. In today's world of threadless headsets, not many folks raise the bars very high with spacers.
With regard to what people say about Colnago geometry, it's said by people who don't understand geoemtry. Colnagos have fairly long TT lengths in the smaller sizes up to about 55cm, where they gradually change to being shorter than some other brands in the larger sizes.
|HTa length effect half height of saddle?||kenyee|
May 19, 2003 5:28 PM
|On average (assuming half the people follow the Rivendell fitting guidelines of 1" below saddle and the rest are true racers who have a 3-4" drop), it seems folks seem to have the top of the step at roughly half the height of a saddle, so it should be more an effect than 1mm (?)...maybe more like .5cm to 1cm depending on what your bar height is and assuming an HTa difference of 1 degree.
BTW, w/ your C40, did you have to get one size smaller than you would other bikes? I've always been curious whether that Bike Sport Michigan article was right about Colnagos needing setback seatposts and longer than normal stems to fit properly (they claim to have had help in fitting by the Trialtir importers). When I tried a MXL, it didn't particularly feel like it had a short TT (it was a 55).
p.s., Becky, sorry for hijacking your thread. The only short tube bikes I know of are the Trek WSD and Aegis Swift. At least now you know the Colnagos probably don't have short TTs in your size (I think you mentioned a small frame size in a previous thread)...
|I give up...||C-40|
May 19, 2003 7:12 PM
|You need to take some high school level math.
You can't read and understand a simple mathematical explanation.
You size a Colnago like any other bike. They are measured c-t just like a dozen other brands. There is no magic.
|My math teacher always said diagrams helped describe equations||kenyee|
May 19, 2003 7:49 PM
|I was actually trying to understand why HTa isn't in the eff-TT calculation.
Sorry for trying.
|here's an exact analysis....||C-40|
May 20, 2003 5:03 AM
|Since you're trying to understand (not just arguing for fun?) here's a thorough explanation.
You certainly can consider the effect of HTA in the calclulation for the efffective top tube length, it's just that's it's rarely large enough to make any difference.
A typical bike with no head tube spacers will have a distance of 9cm from the top of the bars to the instsection of the TT and HT. Multiply 9cm times the difference between the cosines of 72 and 73 degrees and you get 1.6mm. A bike with this setup would have a bar to saddle height difference around 9cm.
If you want to raise the bars up real high, like 3cm below the saddle, you would NOT put 6cm of spacers under a threadless stem. You might use as much as 3cm of spacers and then use a 100 degree stem angle to gain another 3-4cm. In this case, the reach difference due to a 1 degree change in the HTA would still only be 2mm.
You have greatly exagerated the length that is of relevance when comparing the effect of changing HTA. With the STA, the saddle height would be 60-85cm, while the length from the TT to the top of the stem is only 9-12cm. The saddle height is 6-7 times longer and the effect is 6-7 times greater.
|C-40, Do I dare ask...?||TNSquared|
May 20, 2003 8:19 AM
|Before I do ask, let me acknowledge that I am of the most mathematically challenged brains, and way too old to enroll for college, much less high school remedial courses.
So, in fairly non-mathematical, rudimentary language: Why is it preferrable to add bar height via increasing the stem rise and not by adding spacers? You seem to imply that 3cm of spacers is an outside limit. I'm not at all questioning that this is right, like Kenyee I'm just trying to understand.
My bike came with 5 cm of spacers and a -7 degree stem, which had the bars way too high. Rather than fiddle with the spacers, I simply bought a -17 degree stem which put the stem parallel to the ground and at the bar at the right height, but still sitting on 5 cm of spacers.
However, sounds like I should have removed quite a few spacers and stuck with a stem with a little bit of rise. However, to do this I would either have to cut the steerer tube (which I'm not real keen on at this point) or move most of the spacers above the the stem. Maybe nothing functionally wrong with that, but it sure would look dorky.
If I am affecting performance with my current 5 spacer/-17 degree stem setup, I'd like to know.
I feel your blood pressure rising at this point, so thanks for your patience! :) Seriously, any explanation is much appreciated.
May 20, 2003 8:45 AM
|Bikes that use forks with carbon steering tubes are restricted to a maximum 2-3cm of spacer, as recommended by the manufacturer. This limit is recommended to insure adequate rigidity. The less spacers, the more rigid the setup. Personally, I won't use over 1cm or spacer. If I need more bar height, I would flip an 80 or 84 degree stem to a 100 or 96 degree angle (respectively) and gain an additional 2-3.5cm in height.
Forks with Al or steel steering tubes technically have no spacer limit, other than what looks presentable. Using a big stack of spacers is equivalent to using a very long quill with the old quill-type stem. Looks dorky.
In your case, it would have been simple to flip the original -7 (80 degree) stem over to 100 degrees, which would have allowed the removal of more than 3cm of spacer. Cutting the steering tube is an easy job. Once the stem is at the desired height, use a sharp tool to scribe a line along the top of the stem, remove the fork and use a hacksaw to cut the tube 2-3mm under the scribe line. It's not more than a 15-minute job.
What some folks fail to consider when using lots of spacers is the effect on the reach to the bars. Adding 3cm of spacer brings the bars about 1cm closer to the rider, requiring a 1cm longer stem. Flipping a stem also shortens the horizontal reach and requires a longer stem, but at least it yields a more rigid setup.
If you flip a stem and then remove spacers to restore the original bar height, there should be little change in the reach.
|Thanks. I'll probably make some changes once I'm sure...||TNSquared|
May 20, 2003 9:46 AM
|Thanks C-40, that was quite helpful - and no math! :)
I'm a relative newbie and still playing with my ideal position, so I didn't want to cut the steerer tube until I'm sure on the height. My current setup does look a little dorky, but not as dorky as putting 3-4cm of spacer above the stem until I confirm I've got the right position and commit to cutting the steerer tube, IMHO. (My son assures me I'm a dork irregardless, so I could be wrong on this.)
My steerer is alu., so I guess the only problem for now, other than said dorkiness, is a small loss of rigidity.
Once I ride with this setup a little longer to confirm my position is correct, I will follow your advice and go back to the 100 degree stem, remove the proper amount of spacers to mainatin current height and reach, and cut the steerer tube.
Gotta lose that dork factor - you never know when the stem police might be lurking around the next bend in the road! :)