U-Joints change speed as they rotate off from a center axis. They are NOT constant velocity (CV) joints. CV-Joints are a different design and can tolerate larger angles. If you have a single U-Joint in the driveline, the angle should be kept to a minimum. I think 5* in that case. If you have two in the driveline that are angular opposites, then I think the max is 15* (for longevity).