I had 750R16 on my GQ and never noticed the shakes except a rough ride because the shocks weren't the best. Today, changed to R15 with 30 x 10.5 tyres (Kumo). Between 75 and 90 kph she shakes a lot but then settles down. I was wondering what I had done wrong. Jumped out and checked the wheels nuts were still there and tight. I haven't had a chance to check it out yet. Having read the previous posts, I can see I may have a challenge ahead and an empty wallet.