Originally Posted by
noglider
OK then, so why does having a light bike help in an un-ideal environment?
A lighter bike requires less power to accelerate, and mass-start races aren't steady-paced rides. If you intend to hang with a pack, you can't just *not* accelerate when that pack accelerates, because drafting is so crucial.
If you're in a group that's going 25mph, and in 5 seconds you all accelerate to 30mph, 2 extra kilograms on the bike is going to cost you around 10 extra watts during that acceleration. For 5 seconds of intense effort, you're having to do something on the rough order of 1-2% more power than you'd need to do on the 2kg lighter bike. It's certainly less significant than a lot of other differences you see between bike setups, but it's a cost that it doesn't hurt to not pay nonetheless.