I'd suggest you start with the stem length that gives you the same overall reach as your road bike. Then get on the track and see what you need. Three different aspects of your riding will determine changes to your stem length: 1. At speed at the pole, you'll find there's an optimal speed (not necessarily your fastest) at which you'll be glued to the pole, but faster than that you will feel increasingly unstable. This is a result of the optimum speed your track is designed for. To go faster, you can get a bit of an improvement with a different stem length (for most people, a longer one). 2. If you have to dive down to the pole at speed, you'll be doing some radical direction changes where the track wants to throw you. You may find that a different stem will give you better balance -- for me a longer stem helps a bit. 3. Your event also determines upper body extension -- you may get the fastest acceleration in a kilo or 500m with a shorter extension, and may find the same for sprints as well. Pursuit may need a longer stem and I definitely find that for madisons I like to ride a centimeter or so longer because it helps stability during handslings, while also opening my chest and diaphragm a bit for long chases. Overall, I used to head toward a shorter upper body position, but with a power meter and a lot of motorpacing I've found that I was giving a lot away. Now, riding a longer position, I do better and I feel stabler as well, which also permits me to work closer to my limit.