Forbes was 4 of 8 from 3-point range.
Charlie Lee had 16 points and six assists. Devon Long added 10 points. Tim Kamczyc and Mason Marlin had eight rebounds and seven points apiece.
Ball State shot 40 percent from the field for the game and led 34-30 at halftime. The Vikings (5-2) shot just 37.9 percent in each half. But they made 17 points off turnovers and had 19 second-chance points, as compared to 11 and six for Ball State.
Jesse Berry scored 28 points for Ball State. Chris Bond added 12. Majok Majok had eight rebounds.
The loss gives the Cardinals (2-3) a three-game losing streak.