Time Trial / Time Trialling / TT

Ayone know if an untrue disc effects, aerodynamics/ride or just breaking?

How much runout does it have? More than 1mm radial or 2mm axial will probably be noticeable to the pilot, you'd really want it to be less than half that. Your aeros won't be measurably affected by anything that's ridable, but a lot of runout will cause energy loss in the tyre as it flexes to damp the attempts of the wheel to throw the rest of the bike about.