(Apologies in advance for the length - I'm wordy.)
So a Google search will lead to plenty of history on this topic on this and other forums going back well over a decade, and the general advice seems to be summarized as thus:
1.) Get a good fit by a pro at a qualified shop.
2.) TT bike size needed will probably be smaller than your road bike fit.
A great many of those asking in posts have typically not studied the geometries in question, and are usually armed only with their road bike frame size (such as 56cm). My stance is actually a bit more specific and detailed, I'd like to think, so let's please take it as a given that I _will_ be involving a professional fitter in making a final choice, and let's skip to dissecting and comparing geometries so I can approach that fit experience armed with some greater understanding. :)
For context, I'm a mid-40s guy who's been racing short-course (sprint and olympic) for nearly a decade, and while not terribly age-group competitive I've continued to improve year-on-year. (This is for justification of my budgetary limits - spending several thousand dollars on a bike will not likely make me competitive.) I'm not commonly proportioned - I stand about 5'6" (168cm), but with a short torso and long arms (35" or 89cm from rotator cuff to fingertips) and legs (32" or 81cm is standover on my bike, barely touching while flat-footed). For what it's worth, weight is just under 125 lbs (less than 57kg).
My current bike is a 1993 Trek 2300 (carbon tubes, aluminum joints - unloaded weight about 11kg), 54cm size, which I rode with drop bars for several years before adding a pair of Profile Design T3+ clip-on aero bars a few years ago. Measuring the bike for geometry like a TT bike, I get the following:
Top tube = 55.9cm
Reach = 38.7cm
Stack = 54.0cm
Seat tube = 54.6cm (c-c), 57.1cm (c-t)
The seat tube and head tube angles I've only estimated from the measured triangles (as I didn't have a protractor, only a t-square), but those I calculate at ~76° (seat) and ~73° (head). I have not measured rake or trail at this point.
The saddle is an ISM Adamo Breakaway split-style, with the top surface of the saddle 14.6cm above the top tube; the armpads on the aero bars are 10.8cm above the top tube, so just a bit lower than the saddle.
I should _also_ note that the aero bars themselves are at the furthest extension point for clamp, and the armpads I've installed forward of the aero bar clamp points by 4.4cm, ands forward of the head tube by 12.7cm. This is in contrast to the majority of clip-on installs I've seen, where the armpads are behind the mount points on the aero bars.
This bike is a fairly good fit when riding on the hoods, but simply seems too short in reach when I'm in aero by a few cm or more - my hands rest at the very ends of the bars, and my weight on the armpads is almost directly over the front wheel hub, making things less stable than I'd like. If I move the fit back (saddle and aero bars) to improve stability, I'm seated too far behind the crank for comfort and power.
At this point, you're likely shouting "WHAT'S YOUR POINT?!" If you've read this far, thank you - I'm getting there.
Looking over most manufacturer sizing guides, my height would suggest I need a small or perhaps a medium frame. Once I start studying the geometries more closely, I see otherwise. For instance, Trek suggests a small or medium frame, and other manufacturers seem to do the same. Looking specifically at geometries, though, Trek's large (42.6cm reach) seems to be the better choice for me, as would, say, Cervelo's size 56 (42.5cm reach), each with an equivalent stack (54cm) to my current ride in aero position. These are sizes that are suggested for riders quite a bit taller than I am, which has me second-guessing my reasoning.
Should I be considering the larger-size frames, or am I completely off in my thinking? I want to have some justification for which direction I go when it comes to frame-shopping, and have some basis for push-back to the fitter when they see I'm the size of the average fifth-grade kid and want to put me on a 51cm frameset. Thoughts?
Scott
So a Google search will lead to plenty of history on this topic on this and other forums going back well over a decade, and the general advice seems to be summarized as thus:
1.) Get a good fit by a pro at a qualified shop.
2.) TT bike size needed will probably be smaller than your road bike fit.
A great many of those asking in posts have typically not studied the geometries in question, and are usually armed only with their road bike frame size (such as 56cm). My stance is actually a bit more specific and detailed, I'd like to think, so let's please take it as a given that I _will_ be involving a professional fitter in making a final choice, and let's skip to dissecting and comparing geometries so I can approach that fit experience armed with some greater understanding. :)
For context, I'm a mid-40s guy who's been racing short-course (sprint and olympic) for nearly a decade, and while not terribly age-group competitive I've continued to improve year-on-year. (This is for justification of my budgetary limits - spending several thousand dollars on a bike will not likely make me competitive.) I'm not commonly proportioned - I stand about 5'6" (168cm), but with a short torso and long arms (35" or 89cm from rotator cuff to fingertips) and legs (32" or 81cm is standover on my bike, barely touching while flat-footed). For what it's worth, weight is just under 125 lbs (less than 57kg).
My current bike is a 1993 Trek 2300 (carbon tubes, aluminum joints - unloaded weight about 11kg), 54cm size, which I rode with drop bars for several years before adding a pair of Profile Design T3+ clip-on aero bars a few years ago. Measuring the bike for geometry like a TT bike, I get the following:
Top tube = 55.9cm
Reach = 38.7cm
Stack = 54.0cm
Seat tube = 54.6cm (c-c), 57.1cm (c-t)
The seat tube and head tube angles I've only estimated from the measured triangles (as I didn't have a protractor, only a t-square), but those I calculate at ~76° (seat) and ~73° (head). I have not measured rake or trail at this point.
The saddle is an ISM Adamo Breakaway split-style, with the top surface of the saddle 14.6cm above the top tube; the armpads on the aero bars are 10.8cm above the top tube, so just a bit lower than the saddle.
I should _also_ note that the aero bars themselves are at the furthest extension point for clamp, and the armpads I've installed forward of the aero bar clamp points by 4.4cm, ands forward of the head tube by 12.7cm. This is in contrast to the majority of clip-on installs I've seen, where the armpads are behind the mount points on the aero bars.
This bike is a fairly good fit when riding on the hoods, but simply seems too short in reach when I'm in aero by a few cm or more - my hands rest at the very ends of the bars, and my weight on the armpads is almost directly over the front wheel hub, making things less stable than I'd like. If I move the fit back (saddle and aero bars) to improve stability, I'm seated too far behind the crank for comfort and power.
At this point, you're likely shouting "WHAT'S YOUR POINT?!" If you've read this far, thank you - I'm getting there.
Looking over most manufacturer sizing guides, my height would suggest I need a small or perhaps a medium frame. Once I start studying the geometries more closely, I see otherwise. For instance, Trek suggests a small or medium frame, and other manufacturers seem to do the same. Looking specifically at geometries, though, Trek's large (42.6cm reach) seems to be the better choice for me, as would, say, Cervelo's size 56 (42.5cm reach), each with an equivalent stack (54cm) to my current ride in aero position. These are sizes that are suggested for riders quite a bit taller than I am, which has me second-guessing my reasoning.
Should I be considering the larger-size frames, or am I completely off in my thinking? I want to have some justification for which direction I go when it comes to frame-shopping, and have some basis for push-back to the fitter when they see I'm the size of the average fifth-grade kid and want to put me on a 51cm frameset. Thoughts?
Scott