[Bob In Ct]

This topic has been beaten to death over the years. It's the tires, they're not really round. Are they BFGs perhaps?

Even a square tire can be balanced. Many tires leave the factory with either (or both) a dimentional or dynamic out of round. If you have a set of those you get a harmonic at the speeds you mentioned.

Get the tires "trued". There are machines to do this.

Major Racing Traction in Bridgeport. They're at 20 Union Avenue, 203-367-7223, talk to Dave.

Bob