The draft effect for me is noticeable not huge but noticeable, you dont feel a sudden resistance drop but more of a relaxed and easier peddling if that makes sense, you will notice more of a change in the w/kg on the right in the riders nearby panel when riding, bigger the group the more draft
so for example i was spat out of the back when riding with zsun and bob last night in the pack it was around 2.5-2.7 w/kg to get back on i had to do 5-5.5w/kg soon as i got near i could ease off the power some times i would go straight to the front as the draft effect took hold, at times when i was on the front i was doing 3.3-4 w/kg.
I think what people are expecting is a sudden drop in resistance and hardly any peddling and get sucked along, as we know bkool and zwift are very different in how they write there software for other trainers so let's not go down that road

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How far are you from Nottingham carl your more than welcome to try mine