Because this ride will be designed for pedal only and/or electric assist and belt drive (which can’t cheaply be changed like a chain bike), I will be limiting 3rd gearing to about 15mph at 80rpm cadence. That’s plenty fast for the flats off-road, beach cruising and general bike trails. I’m no powerhouse single speeder, so I want a low enough 1st gear to be able to climb, yet a high enough high gear to hit about 15mph when doing pedal only.
Front Gates belt drive sprockets seem to start at 46T and I believe that size bolted to an adaptor will clear a BaFang motor housing (the well known Lekkie Bling-Ring is 42T), so that is what I will use if it clears. Running 27.5/650B X 2.25 tires with 46 X 28 gearing yields 10.7mph in the 1:1 2nd gear, and yields about 8.03mph in 1st and 14.23mph in 3rd. This should work well for pedal only (01.23/1.64/2.19). My local beach boardwalks are limited to 10mph, although they are frequently ridden much faster in areas where people are not present, so limiting to 15mph will keep me under control. Hmmm…
I wonder why I happen to be zeroing in on Europe’s maximum assist speed. Maybe there’s some logic in their seemingly slow speed madness after all…
Another reason I have zeroed in on this gearing (besides the fit for my pedal only abilities and intended use) is that my 350W/37V BaFang mid-drive eMTB had a low gear of 34T and a Lekkie 42T up front (8.1mph/0.809), which will match 1st gear with this 3spd IGH. That ratio allowed me to climb just about any steep hill on my local off-road trails with leg assist. I’d still get a workout on those mile long steep climbs, but was able to complete them without having a heart attack. I know a 0.810 ratio is very low for most single speeders, but hopefully I can improve enough to make that work when needed without electric assist for the majority of my planned usage over time. I know it is low enough for my ability general use, including short freeway over/under passes on the street and bike trails. Remember, this ride will be general recreational use; that includes beach cruiser duty.
After much consideration on frame selection and size, I think a large will be best even though I’m 6’2” with a 34” pants inseam. There are a couple reasons I’m going to size down from my typical XL on this build. First, a more upright sitting position is desired with sweep-back handlebars for the type of riding to be done. If reach proves tight for me (it shouldn’t be), I can install a setback seat post and this will move the pedals a little further in front of the seat as well; good for cruising. Second, this ride will also serve double duty as a loner for others that want to join me on a ride, while I ride my carbon CS Hightower. The shorter seat post length on a large frame (18.5” on the Soma B-Side) allows for a very low sitting position if needed for other riders. Geometry won’t have to be perfect for them, because those without their own bike probably won’t be highly skilled riders anyway, and the electric assist will make up for any minor inefficiencies in fit if they are smaller than optimal for a large frame.
That’s where this build stands at present. The wheels and hubs are at the LBS getting laced up and surprisingly my prior eMTB is still for sale at far less than it can be built for. I’m hoping I don’t have to repurpose the BaFang off that ride to this one, as it is a great ride as is. I hate to break that sweet ride up, but only time will tell…