Bike fit Q - road vs TT

Generally speaking, how feasible (or unfeasible) is it to take a known bike fit from a road bike and adjust/convert it to a time trial bike? Assuming the geometry of the two bikes is significantly different - seat angle, reach/stack etc - and riding position on tribars is completely different to drops, it’s clearly not a straight swap. Obviously TT bikes tend to be longer and lower than road bikes and have steeper seat tubes, but comparing only reach/stack or top tube length in isolation is comparing apples and oranges.

What I’m hoping for is a reasonably simple rule of thumb to go from a road bike of a given size and fit and use it to work out where to start with a TT bike size/fit. For the record I have a 58cm Trek Emonda (although I should probably be on a 56cm but the 58 is very close to my old 56cm Cervelo RS with the headset spacers taken out), and I’m looking at picking up a Cervelo P3C - 56cm looks like it should be right and doing some maths via BikeCAD shows the distance from the saddle to the top of the head tube is nearly identical with the Cervelo on the 78 deg seat tube setting. I’m not terribly flexible so I don’t want to go too aggressive. I’ve heard suggestions to go down a size from road to TT, but others say this only works if you’re very flexible.

Of course it goes without saying that any position will need fine tuning for aero and/or comfort, but I’m hoping for a simple solution when it comes to picking the right size bike and getting the initial set-up somewhere close to good.

I didn’t really have much choice with the Argon 18; my road bike is 54cm and I opted for M in the Argon 18 (a choice of S/M/L IIRC) and it has seemed right and it was never questioned the twice during the two subsequent TT bike fits. Its hard to tell with the sloping top tube of the road bike but I think the toptube length on each is identical and the the TT seat tube measures 2cm less than the effective road bike seat post (although 2cm more than the actual c/c of top tube). :exploding_head: