One frame laterally deflects 2.5 mm and another 5 mm .....what does this
mean? For me the bottom line is handling. I've ridden the whole spectrum
from a Trek Carbon that was really stiff and tracked through high speed
turns like it was on rails to a lightweight steel track frame (now
relegated to my stationary trainer as a permanent fixture) that becomes
unrideable in the turns at 30 mph on the track.
What exactly is going on to make one frame handle better than another.
IMO its the lateral deflection resulting in the rear wheel
alignment/tracking going out of sync with the front wheel. Smaller sized
and compact frames, even with longer seatposts and stems, tend to track
better because the lateral deflection within the frame is proportionally
less so the wheels stay aligned better.
I also rode a Softride for a couple of seasons where lateral deflection
of the seat was huge yet it had no affect on the handling
characteristics of the bike (which were pretty good).
My 2 cents
Phil H