Quote Originally Posted by Scrappy View Post
I'm going to guess geometry. Your road bike may have a steeper seat tube angle than your commuter, causing you to use your quads more than you have been on your commuter. An example of this (to the extreme) is a Triathalon bike - the seat tube angles are very steep, and the head tube angles are very slack putting you into an agressive aerodynamic angle. In a Triathalon bike, this serves the purpose of isolating your quads for the bike portion of the race, and your hamstrings for the running portion of the race.

Your commuter may have a more slack seat tube angle than your road bike, especially if your road bike is Women Specific Design (WSD.) WSD bikes have a steeper seat tube angle in order to shorten the top tube length because many women have long legs and short torsos, and therefore shorter reach.

Without more info I'm just guessing here. But a more set back saddle/seatpost may help engage your hamstrings and create less fatigue.

Scrappy
I think hers is the unisex model, if I remember correctly.