When your car wheel rubs against the axle head it can cause the wheel to wobble because the corner of the axle head is not a perfect 90 degrees. Cutting a groove directly under the axle head removes this source of wheel wobble. Cutting this groove is difficult to do by hand, but it can be done with an axle file. Our grooved Speed Axles and Pro Super Speed Axles have this groove, exclusive Pinewood Pro, cut directly under the axle head.