Leah
You asked: "
Maybe the 650b wheels will be less floppy with this rack? Maybe the smaller, lighter bike won’t seem as penalized by the heft of this rack?"
The 650B wheel have a minimal effect on Wheel Flop, since the wider tire results in a tire radius close to a 700C x 32 tire (650x48 radius = 344mm vs a 700C x 32 radius = 347mm). Using Jim Youngs on line trial/flop calculator
Bicycle Trail Calculator | yojimg.net with the following frame geometries from RBW for a 50cm Platy with 650B wheels:
HTA, 69.5°, from RBW geo charts
Fork Rake, assumed to be 55mm. RBW does NOT publish fork rake and the 55mm is from a 50 Cheviot with 650B wheels.
Wheel/Tire, assumed to be 650B x 48, RBW states 50mm as the largest tire.
Trial and Flop are solely determined by Wheel Radius, HTA, and fork rake:
Flop = Trail x sin(HTA) x cos (HTA), and Trial = (Wheel Radius x cos(HTA) - Fork Rake) / sin (HTA)
Here are the results for a 650x48 and a 700x32 tire for the 69.5° HTA and 55mm Rake
650x48 700x32
Trail 70mm 71mm No surprise, RBW models are known to have high trail, due to slack HTA and a nominal 2" fork rake
Flop 23mm 23mm High Flop is direct result of the high trial
So the Trail and Flop are essentially the same for a 650x48 and a 700x32.
If you use 650x42, Trial and Flop are slightly reduced
Trail = 68mm & Flop = 22mm
Frame weight or bike weight does not affect trail or flop per the above equations. Your lighter custom will handle the same with the basket as your other Platy. IF you reduce the weight carried in the basket, you will notice less side to side motion at low speeds, due to the downward force produced by the lower weight (mass).
I hope this helps
John Hawrylak
Woodstown NJ