I just did a bit of an experiment to see what size TT frame I might buy were I to buy one... I want to get some feedback and see if this sounds insane. I comfortably ride a 58+cm road bike typically, and I was a little surprised, as my results yielded a 54cm TT bike, were I to get a Cervelo (which was the easiest to assess against as they give stack and reach for their framesets).
So:
I took, from seat tip, seat height, vertical seat to BB and seat setback from BB, to form a triangle. I checked for errors by doing pythag on this right angled triangle and re-measuring until (i) my measurements were consistent and (ii) the triangle was pythagorian.
I measured: Length from greater trochanter to acromion process, length from acromion process to bottom of elbow with 90 degree bend between forearm and upper arm, vertical height of greater troch off seat and distance behind seat tip of greater troch. The greater troch measurements formed another known triangle off the seat tip. I then ran a line parallel with the ground from greater troch to acromion process. I dropped off this line vertically the length from acromion process to elbow. I also calculated the slight difference that 10 degrees raise from troch to process made. Not much. So, long story short, using these triangles I ended up with maximum stack and reach measurements for a size frame, assuming negligible rise from bars to pads. Basically, I ended up with the reach and stack of a 54cm frame. This kind of makes sense - I have always thought that TT frames should be a little smaller. I guess I have short-ish legs, so maybe that is why I need a small frame to achieve an appropriate drop. But it seems like a lot. Obviously my method has lots of assumptions:
(i) I am flexible enough to achieve the position
(ii) Seat angle change (fore-aft) is fairly negligible
(iii) I can get low rise pads
(iv) The vertical upper arm and 90 degree angle with forearm is a good idea
(v) I sit similarly on a TT bike to my road bike
All these seem reasonable, and if not, I can always fiddle with headset spacers and stem length until I actually achieve sufficient flexibility. Many of the assumptions concern only small margins of error.
One number seems big: ~24-26cm vertical drop seat to elbow pads.
So what do people think?
So:
I took, from seat tip, seat height, vertical seat to BB and seat setback from BB, to form a triangle. I checked for errors by doing pythag on this right angled triangle and re-measuring until (i) my measurements were consistent and (ii) the triangle was pythagorian.
I measured: Length from greater trochanter to acromion process, length from acromion process to bottom of elbow with 90 degree bend between forearm and upper arm, vertical height of greater troch off seat and distance behind seat tip of greater troch. The greater troch measurements formed another known triangle off the seat tip. I then ran a line parallel with the ground from greater troch to acromion process. I dropped off this line vertically the length from acromion process to elbow. I also calculated the slight difference that 10 degrees raise from troch to process made. Not much. So, long story short, using these triangles I ended up with maximum stack and reach measurements for a size frame, assuming negligible rise from bars to pads. Basically, I ended up with the reach and stack of a 54cm frame. This kind of makes sense - I have always thought that TT frames should be a little smaller. I guess I have short-ish legs, so maybe that is why I need a small frame to achieve an appropriate drop. But it seems like a lot. Obviously my method has lots of assumptions:
(i) I am flexible enough to achieve the position
(ii) Seat angle change (fore-aft) is fairly negligible
(iii) I can get low rise pads
(iv) The vertical upper arm and 90 degree angle with forearm is a good idea
(v) I sit similarly on a TT bike to my road bike
All these seem reasonable, and if not, I can always fiddle with headset spacers and stem length until I actually achieve sufficient flexibility. Many of the assumptions concern only small margins of error.
One number seems big: ~24-26cm vertical drop seat to elbow pads.
So what do people think?