can anyone tell me, around what driving speed will you start to feel the kayak start shacking ? or what is the typical speed you guys/lady drive when is loaded with kayak

I never feel it shaking, up to Interstate speeds of 70-75mph. The two straps grip the kayak securely, and when I grab the kayak and shake it to make sure it is secure, the whole truck shakes instead of just the kayak. The kayak is on solid. 100% confidence

I also have had my kayak securely transported at Interstate speeds. I try to keep it under 75, but even at those speeds, it is solid. It's mounted to the Thule XSporter 422XT using a 16.5 ft kayak. No issues whatsoever.

I have a 16' poly tandem boat, about 75lbs and it is rock solid in this rack. I've had it around 70mph and it doesn't move. If there is much of a cross wind while driving you will feel that since it acts as a large sail when the wind hits the cockpits, but i don't worry about the boat moving. One time I put the rack on a friends Grand Cherokee and I didn't get the rack tightened tight enough to the cross bars and the wind caused it to shift a bit, but it wasn't going to go anywhere. So make sure you tighten the rack securely to your crossbars as it can be a bit of a challenge. Bottom line: you won't be disappointed with this rack.