Our racing limo is steel frame, carbon fork. With 25c tires at 120 lbs., it's the smoothest bike on chipseal I've ever been on. I've had fellow riders watch us from behind when climbing in and out of the saddle. As far as they can see, when we're synced the frame doesn't flex. When we're out of sync it's obvious. We're IP, so I mean that our pedal loading is timed the same throughout the stroke when we're synced. I've watched tandems climbing on RAMROD and it can be almost hilarious, while the experienced teams are just beautiful. So I'd say that's a very important thing, maybe the most important thing.