You are obviously unfamiliar with the WC Testimony of Chief Curry, SA Sorrels, and SA Lawson. All of these law enforcement individuals were inside the Lead Car. Please bone up on the Officer Chaney/ Lead Car Event and the WC testimony of these 3 individuals. Once you become knowledgeable regarding all of this, we can engage in an enlightening discussion. Your current unfamiliarity in these areas renders a meaningful discussion difficult if not impossible.
Piecing together what happened.Muchmore captures the leading bikes as they turn onto Houston, 3 bikes followed by 2 more bikes, then like Zapruder she realized that Kennedy's Limo was not directly behind and stops filming, in the interim we miss Curry's car then when she starts again we see Kennedy's Limo with the 4 escort motorbikes.
Zapruder captures the 3 lead motorbikes, then stops filming....
....and then starts again when he sees Kennedy's Limo. And we know at this point that in front of the Limo is Curry's car and 5 bikes.
In this piece of Nix film it looks like the only motorbike that doesn't stop is the bike closest to us.
In the first part of this gif we see the three leading bikes at the top of the frame with the outside escort bike starting to speed up and the two other middle bikes must be somewhere and if you look closely there seems to be the helmet of one of the two middle motorbike riders on the right side of the tunnel.
Here we see one of the 2 bikes on our left in the tunnel and the outside bike which was escorting JFK's limo is just leaving the tunnel
Conclusion.One of the two bikes following the three lead bikes approached Curry and that rider was "believed" to be Chaney.
JohnM