This is probably a real dumb question and I will hear it.
But if you decrease your stack and increase your drop creating a more aggressive position on the bike how is reach effected?
I assume you mean on a road bike, because this is where I think reach will be more of an issue. it will be increased.
Actually I was talking about a tri bike. P2,P3 bikes.
no fitting expert, but typically the bars and stem are positioned to optimize your aero position. As a result, typically the basebar grips (where the brake levers are) are already pretty close to you, unlike on a road bike.
I guess what I’m saying is, for a TT bike, dropping the bars will just make you bend over more and if using the same stem, it will push your elbows a little further out from your current position.
what do you mean by “reach?” i ask because “reach” is a term of art, and i’m thinking you may not be using that term in that technical way.
what do you mean by “reach?” i ask because “reach” is a term of art, and i’m thinking you may not be using that term in that technical way.
Who is this “Art” guy, and why does only he get to use the term? 
Little more detail.
If you are riding a P2C with 25mm of spacers and 100mm stem. If you took 10mm of spacers off would you typically shorten or lengthen the distance from sitting position to aero bars. Or in other words would you shorten or lengthen the stem or move seat back or up.
I do plan to spend some time with the guy that does my fit this week-end but just curious beforehand.
I thought reach was the distance from seat to handle bars/aero bars or whatever.
Reach will increase. The steerer tube on your fork slopes down and forward (it’s not straigt up and down) as it goes towards the fork, or in reverse, it slopes up and back at the top. So therefore, going lower, effectively goes out just a little bit too.
No matter how you look at it…dropping your stem a few spacers to get lower will increase the length to your cockpit.
“Reach” as defined by Slowman’s frame measurements is a static # assigned to a particular frame per size. It will not change due to other equipment changes.
But the setup of the bike in terms of spacers will change the effective amount of space you have to “reach” to be in your cockpit. If you drop spacers…you will move the effective length between your saddle and the center of the stem steerer clamp a few millimeters. Consult your friendly old pythagorean theorem then…and you’ll see the effective change in your “reach”…
That is perfect. To keep my present dimensions going from a 56 P2 to a 58 p3 was going to require me to go from a 100mm stem to a 85mm. The 85mm stem was a little short for my liking. But if I go lower and with a little seat position change I can probably use a 90mm stem.
This probably also explains the slight difference in reach from a P2 to P3. With the greater drop on the P3 you are going to need larger reach.
Thanks for all the help!
Ok re-read, I think I got it wrong. Probably because of the terms.
As you go lower you are increasing the reach which means to fit on the bike you actually have to then decrease the stem length.
I assume the published reach values are with no spacers. So today on my 56 P2 the reach is actually less than published because I use spacers. If I then go to a 58 P3 without spacers I am going to have to have a pretty small stem. First the 58 P3 has a published 16mm longer reach. But then without the spacers I might need 4mm more. So now I am down to a 80mm stem.
Do I finally have this right?
This is from the bike perspective. But what about your fit. When you bend over more don’t you have to be closer to the seat? If so, why would they have increased the reach going from a P2 to a P3 in the same size? It would seem that you would decrease it. Unless you just assumed that people would be using a lot more spacers on the P3 and when you include the spacers you end up with a more compact cockpit.
Summary:
A Increase drop and cockpit gets bigger
B Increase drop and you will need cockpit to get smaller
C Increase drop and you will need to shorten the stem to account for A and B
Conclusion
Might be difficult to get a 58 P3 to work for me. Instead I might have a giant stack of spacers on a 56 P3. 